NIST NCSTAR 1 full text:Part II

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50 NIST NCSTAR 1, WTC Investigation This page intentionally left blank. NIST NCSTAR 1, WTC Investigation 51 Chapter 5 THE DESIGN AND CONSTRUCTION OF THE TOWERS 5.1 BUILDING AND FIRE CODES Codes for the design, construction, operation, and maintenance of buildings are the blueprints by which a society manifests its intent to provide public safety and welfare. They incorporate the knowledge, experience, procedures and practices of the applicable engineering disciplines, the values of the contemporary society, the experiences from prior successes and failures, and knowledge of the commercial products, services, and technologies available for the tasks at hand. In the United States, building and safety regulations of state and local jurisdictions are most frequently based on national “model” building codes (model codes). Developed under the auspices of private sector organizations in an open process, the model codes include minimum requirements for public health, safety, and welfare. The model codes are traditionally organized into volumes according to the official responsible for their enforcement and include a building code, fire code, plumbing code, electrical code, mechanical code, etc. The model codes adopt by reference voluntary consensus standards developed by a large number of private sector standards development organizations. These standards include measurement methods; calculation methods; data sets; and procedures and practices for design, construction, operation, and maintenance. The model codes and referenced standards do not become law until they are adopted legislatively or administratively by a jurisdiction empowered to enforce regulations, for example, a state or city. These jurisdictions may modify specific provisions of the models codes and referenced standards to suit local conditions or traditional practices. Once legally adopted, the totality of the modified model codes and standards are referred to as building regulations. Proposals to modify the model codes, offered by individuals or organizations, are discussed in open forums before being accepted or rejected by a voting process. Localities adopting model codes update their versions periodically as well, but typically not on the same schedule. To a lesser and decreasing extent, some jurisdictions have generated their own building codes to reflect specialized local conditions and preferences. The Federal government’s role in determining specific codes is minimal and not mandatory (except for federally owned, leased, regulated, or financially assisted properties). There are also key stakeholder groups that are responsible for or influence the practices used in the design, construction, operation, and maintenance of buildings in the United States through the code development process. These include organizations representing building owners and managers, real estate developers, contractors, architects, engineers, suppliers, and insurers. (Infrequently, members of the general public and building occupants participate in this process.) These groups also provide training, especially as it affects the ability to implement code provisions in practice, since lack of adequate training programs can limit the application of improved code provisions. Chapter 5 52 NIST NCSTAR 1, WTC Investigation 5.2 THE CODES AND THE TOWERS 5.2.1 The New York City Building Code The New York City (NYC) Building Code was and is part of the Administrative Code of New York City. Until recently, the various versions of the Code were not based on any model code, but rather were written by local code development committees. However, there are many similarities between the versions of the NYC Code and the model codes of the same time, since they all reflected accepted practice. The NYC Code has been amended from time to time by Local Laws to update safety requirements or to incorporate technological advances. These Local Laws were enacted by the New York City Council. To aid the implementation of and to clarify building code requirements, New York City issued mandatory “rules” that were typically initiated by City Government offices and issued under authority of the Building Commissioner. At the time the WTC project began in the early 1960s, the 1938 NYC Building Code was in effect. In 1960, reflecting growing dissatisfaction with the failure of the Code to keep pace with changes that had occurred in the building industry, the Building Commissioner requested the New York Building Congress to form a working committee to study the problem. On December 6, 1968, Local Law 76 repealed the 1938 code and replaced it with the 1968 code. As is the general custom with changes to building codes, the new provisions did not apply to buildings approved under the prior code, provided they did not represent a danger to public safety and welfare, or until they underwent a major renovation or change in primary use. The 1968 NYC Building Code also included “Reference Standards.” These included standard test methods and design standards published by standards development organizations. Some of these Reference Standards included modifications to the published standards, as well as stand-alone standards developed by New York City. Through 2002, 79 Local Laws had been adopted that modified the 1968 Building Code. The major Local Law affecting the structural design of buildings dealt with seismic provisions. Five of the Local Laws had provisions that pertained to fire protection and life safety that were of interest to the WTC Investigation: • Local Law 5 (1973) added, among other specifications, requirements for: − Compartmentation (subdivision) within upper story, unsprinklered, large floor areas. Its provisions applied retroactively to existing office buildings. − Signs regarding the use of elevators and stairs, also retroactive. − A fire alarm system for buildings more than 100 ft in height. • Local Law 55 (1976) added a requirement for inspection of all sprayed fire protection, effective immediately but not retroactive. • Local Law 33 (1978) added a requirement for trained fire wardens on each floor. The Design and Construction of the Towers NIST NCSTAR 1, WTC Investigation 53 • Local Law 86 (1979), among other provisions, required full compliance with Local Law 5 by February 7, 1988, and exempted fully sprinklered buildings from compartmentation requirements. • Local Law 16 (1984) added requirements for sprinklers in new and existing buildings taller than 100 ft. Since Local Law 5 only required compartmentation of non-sprinklered spaces, this negated the compartmentation requirements from Local Law 5. The World Trade Center (WTC) was located in Manhattan and would normally have been designed and constructed according to the NYC Building Code of 1938. However, the WTC was constructed by The Port Authority of New York and New Jersey (The Port Authority or PANYNJ) on land that it owned. As an interstate agency established under a clause of the United States Constitution permitting compacts between states, The Port Authority’s construction projects were not required to comply with any building code. Nonetheless, The Port Authority instructed its consultants to design the two towers to comply with the 1938 NYC Code. In 1965, The Port Authority directed the architect and consulting engineers to revise their designs for the towers to comply with the second and third drafts of what would become the 1968 NYC Code. The rationale for this step was that the new Code allowed the use of advanced techniques in the design of the WTC, as well as more lenient provisions regarding exit stairs and the reduced fire ratings. To reaffirm a “long standing policy” of The Port Authority that its facilities meet or exceed NYC Building Code requirements, a formal memorandum of understanding between The Port Authority and the New York City Department of Buildings was established after the bombing in 1993. 5.2.2 Pertinent Construction Provisions To gain perspective on the conditions under which the WTC towers were constructed, the rationale for the design, and the building structures themselves, the National Institute of Standards and Technology (NIST) and its contractors reviewed tens of thousands of pages of documents provided by The Port Authority and its contractors and consultants, Silverstein Properties and its contractors and consultants, the Fire Department of the City of New York, the NYC Police Department, the NYC Law Department, the NYC Department of Design and Construction, the NYC Department of Buildings, the NYC Office of Emergency Management, the manufacturers and fabricators of the building components, the companies that insured the WTC towers, and the building tenants. NIST deemed it important to understand how the provisions under which the WTC was constructed and maintained compared to equivalent provisions in place elsewhere in the United States at that time. The Investigation selected three codes for comparison: • The 1964 New York State (NYS) Building Code, which governed construction outside the New York City limits • The 1965 Building Officials and Code Administrators (BOCA) Basic Building Code, a model building code typically adopted by local jurisdictions in the northeastern region of the United States • The 1967 Municipal Code of Chicago, under which the Sears Tower (110 stories) and the John Hancock Center (100 stories) were built Chapter 5 54 NIST NCSTAR 1, WTC Investigation For the most part, the provisions in the various codes were similar, if not identical, indicating that there was a common understanding of the essentials of building safety and that the codes were at similar stages of evolution: • There were only modest differences among the codes in the provisions for gravity loads. • All three of the contemporaneous building codes had provisions for wind loads that were less stringent than those used for the tower design. • None of the codes had provisions for design against progressive collapse. • For alterations or additions to a building, there were criteria to determine whether the whole building or only the alterations needed to comply with the current code requirements. The “trigger” was either the fraction of the building cost involved in the renovation or the fraction of the building dimensions. The 1968 NYC Building Code was slightly less conservative than the Chicago Code and the BOCA Code. The NYS Code required that any addition or alteration conform to the contemporary code. • The 1968 NYC Building Code required inspection of sprayed fire protection, but did not specify if testing was required. • Only the NYC Building Code contained provisions for bracing (lateral support to prevent buckling of columns and walls) and stresses associated with transverse deflections of structural members. NIST examined the 2001 edition of the NYC Building Code to determine the extent to which Local Laws had modified the code provisions between the times of construction and collapse of the towers. The 2001 edition of the NYC Building Code was essentially the same as the 1968 edition, as amended by the intervening Local Laws. 5.2.3 Tenant Alteration Process With hundreds of tenants, The Port Authority realized that many would want extensive modifications to their space, both before they moved in and during the course of their occupancy. In anticipation, The Port Authority: • Set up a special office to review and approve plans, issue variances, and conduct inspections. • Developed a formal tenant alteration process for any modifications to leased spaces in WTC 1 and WTC 2 to maintain structural integrity and fire safety. The Tenant Construction Review Manual, first issued in 1971, contained the technical criteria, standards, and review criteria for use in planning alterations (architectural, structural, mechanical, electrical, and fire protection). Alteration designs were to be completed by registered design professionals, and as-built drawings were to be submitted to The Port Authority. The 1968 NYC Building Code was referenced. The review manual was updated four times and supplemented, in 1998, by the Architectural and Structural Design Guidelines, Specifications, and Standard Details. The Design and Construction of the Towers NIST NCSTAR 1, WTC Investigation 55 The interiors of the towers had been heavily modified over the years due to tenant turnover, same-tenant space usage changes, the addition of sprinklers, and asbestos abatement. 5.3 BUILDING DESIGN 5.3.1 Loads The NYC Building Code specified minimum design values for both dead and live gravity loads and for lateral (wind) loads. • Each tower was designed to support dead loads (its own weight) consistent with the provisions in the 1968 NYC Building Code. The dead loads included the weight of the structural system and loads associated with architectural, mechanical, plumbing, and electrical systems. • Each tower was designed to support live loads (the combined weights of the people and the building contents) exceeding those specified in the 1968 NYC Building Code. • The design wind loads used in the towers were higher than those required by the 1968 NYC Building Code and the three other codes identified earlier. 5.3.2 Aircraft Impact The accidental 1945 collision of a B-25 aircraft with the Empire State Building sensitized designers of high-rise buildings to the potential hazards of such an event. However, building codes did not then, and do not currently, require that a building withstand the impact of a fuel-laden commercial jetliner. A Port Authority document indicated that the impact of a Boeing 707 aircraft flying at 600 mph was analyzed during the design stage of the WTC towers. However, the investigators were unable to locate any documentation of the criteria and method used in the impact analysis and were thus unable to verify the assertion that “…such collision would result in only local damage which could not cause collapse or substantial damage to the building and would not endanger the lives and safety of occupants not in the immediate area of impact.”8 Since the ability for rigorous simulation of the aircraft impact and of the ensuing fires are recent developments and since the approach to structural modeling was developed for this Investigation, the technical capability available to The Port Authority and its consultants and contractors to perform such an analysis in the 1960s would have been quite limited. 5.3.3 Construction Classification and Fire Resistance Rating Building codes classify building constructions into different “Types” or “Classes.” The Class pertinent to the WTC towers was Class 1 (fire resistive). The 1938 NYC Building Code had no subdivisions of Class 1 construction, which required a 4 hour fire resistance rating for columns and a 3 hour rating for floors. The 1968 version of the Code subdivided Class 1 for office occupancies into 1A, with requirements identical to the 1938 Class 1, and 1B. Class 1B specified a 3 hour rating for columns and 8 Letter with an attachment dated November 13, 2003, from John R. Dragonette (Retired Project Administrator, Physical Facilities Division, World Trade Department) to Saroj Bhol (Engineering Department, PANYNJ). Chapter 5 56 NIST NCSTAR 1, WTC Investigation girders supporting more than one floor and a 2 hour rating for floors including beams. There were no height or area requirements that differentiated between Class 1A and Class 1B, and the towers could have been classified as either one. The Port Authority elected to provide the fire protection in the WTC towers with Class 1B standards. Achieving a specified rating for a truss-supported floor using a sprayed fire-resistive material (SFRM) was an innovation at the time of the WTC design and construction. NIST was not able to find any evidence that there was a technical basis to relate SFRM thickness to a fire resistance rating, nor was there sufficient prior experience to establish such thickness requirements by analogy. NIST did find documentation that the Architect of Record and the Structural Engineer of Record had each written to The Port Authority, stating that the fire rating of the WTC floor system could not be determined without testing. NIST was unable to find any indication that such tests were performed nor any technical basis for the specification of the particular SFRM product selected or its application thickness. The NYC Building Code required inspection at the time of application of the SFRM, to be conducted under the supervision of a building inspector or a licensed design professional who assumed responsibility for compliance. This inspection included verification of the thickness of the material, its density, and its adhesion, each using a specific ASTM test method. The Code contained a requirement that SFRM installed in areas where it was subject to mechanical damage be protected and maintained in a serviceable condition. 5.3.4 Compartmentation Both the 1968 NYC Building Code and The Port Authority practice required partitions to separate tenant spaces from each other and from common spaces, such as the corridors that served the elevators, stairs, and other common spaces in the building core. These were intended to limit fire spread on a floor and to prevent the spread of a fire from one tenant space to that of another. • The Port Authority specified partitions separating tenant spaces from exit access corridors to have a 2 hour rating. This allowed dead end hallways to extend to 100 ft (rather than 50 ft with 1 hour partitions), which permitted more flexibility in tenant layouts. Above the ceiling, penetrations for ducts or to allow for return airflow were fitted with rated fire dampers to preserve the fire rating. This 2 hour rated construction was not used in the original design, but was specified later by The Port Authority as tenant spaces were altered. • For walls separating tenant spaces to achieve a 1 hour rating, they needed to continue through any concealed spaces below the floor and above the ceiling. The Port Authority chose to stop these demising walls at the bottom of the suspended ceiling and use 10 ft strips of 1 hour rated ceiling on either side of the partition. There was no precedent for this approach and, after a warning from the general contractor, the tenant alteration guidelines required that tenant partitions have a continuous fire barrier from top of floor to bottom of slab. • There were no requirements in the 1968 NYC Building Code or in The Port Authority guidelines for partitions wholly within tenant spaces. As mentioned in Section 1.2.2, these There were no code requirements nor general practice by which sprayed fireresistive material was to be inspected over the life of the building. The Design and Construction of the Towers NIST NCSTAR 1, WTC Investigation 57 gypsum board walls generally ran from the floor slab to just above the suspended ceiling, although some extended to the slab above when the tenant desired additional sound attenuation. For these partitions to be fire rated, the ceiling would have had to be rated as well but were not required to be so. • Enclosures for vertical shafts, including stairways and transfer corridors, elevator hoistways, and mechanical or utility shafts were required to be of 2 hour fire rated construction. These innovative walls are further described below. There was a conflict regarding the number of partitions within a tenant space. On the one hand, the design of the WTC towers was intended to provide about 30,000 ft2 per floor of nearly uninterrupted space and access to views of the Manhattan panorama. On the other hand, Local Law 5 dictated compartmentation into no more than 7,500 ft2 areas for unsprinklered spaces. These areas could be increased to 15,000 ft2 if protected by 2 hour fire resistive construction and smoke detectors. The compartmentation limit was removed when complete sprinkler protection was provided. Following a 1975 fire, The Port Authority began installing sprinklers at the time a new tenant moved in. By September 11, 2001, the installations had been completed throughout the towers, and, in general, the tenants on the impact floors had few internal partitions except for those surrounding conference rooms and executive offices. Firestopping materials are used to fill gaps in walls and floors through which smoke and flames might pass. Such passage could negate the fire endurance value of the wall or floor. The 1968 NYC Building Code included comprehensive requirements identifying when and where firestopping was required. The 1964 New York State Building Code addressed the issue in less detail, and the Chicago Building Code had no requirements. The National Fire Protection Association (NFPA) Life Safety Code had firestopping requirements for exterior and interior partitions at floor levels, and did allow a trade-off for sprinklered concealed spaces. In the towers, unlike many buildings, the exterior wall was connected with the floors without gaps. 5.3.5 Egress Provisions The primary egress system for the office spaces was the three stairways located in the building core. There were four main requirements for these stairways: number, width (including separate width requirements for the doors), separation of the doors to the stairways, and travel distance to the stairway doors. The number of stairways and the width of the doors resulted from the implementation of the 1968 edition of the NYC Building Code, whose provisions were less restrictive than those in the 1938 edition. The 1968 code eliminated a fire tower (an enclosed staircase accessed through a naturally ventilated vestibule) as a required means of egress and reduced The NYC Building Code used the “units of exit width” method for specifying exit capacity, in which each 22 in. unit of exit width provided the capacity for 60 people. Thus each 44 in. stairwell provided for 120 people and the 56 in. stairwell provided 2½ units, or 150 people, for a total occupant load per floor of 390. Chapter 5 58 NIST NCSTAR 1, WTC Investigation the number of required stairwells from six to three9 and the width of the doors leading to the stairs from 44 in. to 36 in. Of the three staircases, two (designated A and C) were 44 in. wide; stairway B was 56 in. wide. The largest occupant load in the office spaces was 365 people per floor (36,500 ft2 on the largest floor, with 100 ft2 per person). Neither the 1968 NYC Building Code nor any of the contemporaneous codes mandated consideration of the number of building stories in determining the number and widths of the stairwells. For the floors classified in the office use group (all floors except the observation deck and restaurant/meeting spaces), a minimum of two stairwells would have been required to serve the occupants, each equally sized. The three modern building codes considered in this report [International Building Code (IBC) (2000), NYC Building Code (2003), and NFPA 5000 (2003)], as well as the 1968 NYC Building Code, were consistent in this requirement, each regardless of building height. However, the resulting width of these minimum requirements would differ. Two 44 in. stairwells would have satisfied IBC minimum requirements, two 65 in. stairwells would have satisfied NFPA 5000 requirements, and two 78 in. stairwells would have satisfied the 1968 and 2003 NYC Building Code requirements. Alternatively, as was built at WTC 1 and WTC 2, three stairwells of narrower construction, but equivalent or greater total required width, would also satisfy the egress requirements in the modern building codes. The 1968 NYC Building Code contained a requirement that the stairwells be “as far apart as practicable.” Since the stairwells on the impact floors of WTC 1 were substantially closer together than those on the impact floors of WTC 2, it certainly was possible to have designed a greater separation in WTC 1. Local Law 16 (1984) added a quantitative requirement that the separation between exit door openings be at least one-third of the maximum travel distance of the floor. For the WTC towers, this maximum distance was 180 ft, and the smallest separation of stairwell doors was 70 ft. The towers were consistent with this requirement. NFPA 5000 (2003) and IBC (2000) incorporate a requirement that the separation of the stairwells be no less than one-third the overall diagonal length of the building. For the towers this length was 294 ft, and one-third was 98 ft. Thus, the stairwell separations on some floors would have been inconsistent with the later codes (with which the buildings in New York City were not required to comply). At the top of the two towers were floors that were classified as public assembly floors: the Windows on the World restaurant complex in WTC 1 (floors 106 and 107) and the Top of the World observation deck in WTC 2 (floor 107). The design number of occupants on each of these floors was over 1,000. On September 11, 2001, there were about 188 people in the Windows on the World and few in the Top of the World since it was before the opening hour. Thus, had the stairwells remained passable through the impact region, the capacity would have been sufficient for the occupant load observed on that morning. Nonetheless, the egress requirements for assembly occupancy were more stringent than for business occupancy in both the NYC Building Code in 1968 and in 1996, when the Windows on the World re-opened after refurbishment following the 1993 bombing in the basement. NIST found documentation that, in 1996, The Port Authority created areas of refuge consistent with the provisions of the 1968 NYC 9 See discussion of the required number of stairwells later in this section. The Design and Construction of the Towers NIST NCSTAR 1, WTC Investigation 59 Building Code, but NIST was unable to find evidence indicating that the requirements for the number of exits for the evacuation of over 1,000 people from each of these floors had been considered in the design or operation of the buildings. In 1995, the NYC Department of Buildings, however, had reviewed the egress capacity from these floors and apparently concurred that the proposed remodel to these spaces would meet the intent of the NYC Building Code. Subsequently, NIST communications in 2005 with The Port Authority and the NYC Department of Buildings identified a difference of interpretation regarding the number of exits required to serve these floors. The Port Authority stated that a fourth exit was not required since the assembly use space in question constituted less than 20 percent of the area of principal use, with principal use area defined as the entire building. The Department of Buildings stated that the 20 percent rule did not apply to assembly spaces such as restaurants and observation decks that are open to the public, and therefore exit reduction cannot be applied and a fourth exit was required. The Department further clarified that areas of refuge and horizontal exits are not to be credited for required means of egress (unless the spaces are used non-simultaneously) and that for places of assembly, with occupant load in excess of 1,000, the floor shall have a minimum of four independent means of egress (stairs) to street. If the floor were divided into areas of refuge with rated walls, as was the case for the WTC towers, each area is to be considered an independent place of assembly that needs its own access to two means of egress (stairs) without going through another assembly space if they have an occupant load of less than 500 each (or three means of egress if the area of refuge had an occupant load between 500 and 999). Further, since the only means of egress from the roof-top deck was through the space on the observation floor, the Department clarified that occupant load from the deck would need to be added to the occupant load of the observation floor and that the travel distance from the roof deck along the connecting stairs to the required means of egress at the observation floor shall be within the maximum permitted by the NYC Building Code. The Department, however, did not raise the issue of a fourth stairwell in its December 1994 meeting with The Port Authority and when it subsequently concurred with The Port Authority’s proposal to remodel the spaces. Given the low occupancy level on September 11, 2001, NIST found that the issue of egress capacity from these places of assembly, or from elsewhere in the buildings, was not a significant factor on that day. It is conceivable that such a fourth stairwell, depending on its location and the effects of aircraft impact on its functional integrity, could have remained passable, allowing evacuation by an unknown number of additional occupants from above the floors of impact. If the buildings had been filled to their capacity with 20,000 occupants, the required fourth stairway would likely have mitigated the insufficient egress capacity for conducting a full building evacuation within the available time. The elevator system was described in Chapter 1. These were not to be used for emergency evacuation except under the control of the fire department. Roughly 3,000 of the people who were initially at or above the impact floors in WTC 2 and were warned by the attack on WTC 1 survived, however, in large part by taking the elevators downward before the aircraft struck WTC 2. Following the 1993 bombing, The Port Authority instituted the following changes to reduce egress time, in addition to those stairwell improvements mentioned in Section 1.1.2: • Construction of new egress corridors, north (to Church Street and Vesey Street) and south (to Liberty Street) for faster evacuation from the Concourse (mall), and of two escalators from Chapter 5 60 NIST NCSTAR 1, WTC Investigation A Fire Command Desk (Figure 5–1) was located in the lobby of each tower. The computer screen monitored the fire alarms, smoke sensors, sprinkler water flow, elevator lobby smoke detectors, fire signal activation, air handling fans, status of elevators, and troubles with the fire systems. the Concourse (mall), one to the plaza at WTC 5 and one up to WTC 4 and onto Church Street. • Semiannual fire drills in conjunction with the FDNY. • Appointment of Fire Wardens, specially trained and equipped with flashlights, whistles, and identifying hats. Building Communications WTC emergency procedures specified that all building-wide announcements were to be broadcast from the Fire Command Desk (FCD), located in the lobby of each WTC tower (Figure 5–1), using prepared text. A situation requiring evacuation for any reason, including fire or smoke, would have led to the following announcement, enabling a phased evacuation: “Your attention please. We are experiencing a smoke condition in the vicinity of your floor. Building personnel have been dispatched to the scene and the situation is being addressed. However, for precautionary reasons, we are conducting an orderly evacuation of floors _____. Please wait until we announce your floor number over the public address system. Then follow the instructions of your fire safety team. We will continue to keep you advised. We apologize for the inconvenience and we thank you for your cooperation.”10 The announcement to be used when a particular floor required an evacuation was: “Your attention please. It is now time for your floor to be evacuated. In accordance with the directions from your fire safety team, please take the exit stairs nearest to your location. We remind you that communications, emergency lighting and other essential services are in service. We will continue to keep you advised. We apologize for the inconvenience and we thank you for your cooperation.”10 At the discretion of the Fire Safety Director, the information and instructions broadcast to the building occupants could be modified to suit the nature of the emergency. 10 The Port Authority of New York and New Jersey. World Trade Center Emergency Procedures Manual 2001. The Design and Construction of the Towers NIST NCSTAR 1, WTC Investigation 61 5.3.6 Active Fire Protection The provision of fire safety in the WTC towers revolved around a Fire Safety Plan that provided direction for fire emergency response and was organized around a hierarchy of staff trained in its implementation. In charge in each tower was the Fire Safety Director, who oversaw emergency response until the arrival of the Fire Department of the City of New York (FDNY), gathered necessary information, and relayed it to the Fire Chief upon arrival. In an emergency, the Fire Safety Director proceeded to the FCD or the fire scene. He/she had one or more Deputy Fire Safety Directors located at the FCD and at the sky lobbies. The front line was a set of Floor Wardens and Deputy Floor Wardens who were responsible for assessing conditions and assisting the evacuation of occupants on their respective floors. The Floor Wardens had their own communications system. Built into each tower were four resources to mitigate the effects of a fire: an alarm system to alert people to the presence of the fire, an automatic sprinkler system and a standpipe system for controlling the fire by the application of water, and a smoke venting system to improve visibility as people proceeded toward exits. The primary documentation of the design, installation, maintenance, and modification of these systems was stored on the 81st floor of WTC 1 and was lost when that building collapsed. Contractors to the Investigation Team were able to re-create descriptions of the physical systems and their capabilities from limited duplicate information provided by The Port Authority, Silverstein Properties, Inc, and contractors, consultants, and operators involved with the systems. The original fire alarm system used the technology current at the time and was engineered exclusively for the World Trade Center towers. The 1993 bomb explosion in WTC 1 destroyed the communications to the Operations Control Center, and the alarm system was revealed to be vulnerable to a single point of failure. Repair was problematic, since spare parts for the 25-year-old system were unavailable, and the software was no longer supported. The Port Authority immediately commissioned a new state-of-the-art system for WTC 1, WTC 2, WTC 4, WTC 5, and the subterranean levels. This retrofit involved the installation of over 10,000 detectors, pull stations, and monitors; 30,000 notification devices (speakers Figure 5–1. Fire Command Desk in WTC 1, as seen from a mezzanine elevator, looking west. Chapter 5 62 NIST NCSTAR 1, WTC Investigation and strobe lights); 150 miles of conduit; and 1,000 miles of wiring. Redundant Operations Control Centers were located in the basements of both towers. The primary monitoring and control of the fire alarm system was performed at the FCD located in the lobby of each building. The new system included: • Numerous interconnected microprocessors located in each of the four WTC buildings. • Smoke sensors located throughout the tenant spaces, at each elevator landing, in return air ducts, and in electrical and mechanical rooms. • At least one manual fire alarm station installed in each story in the evacuation path. • Emergency voice and alarm speakers for notification and communication in all areas within the buildings, designed to ensure system function in the event 50 percent of the system became inoperable. • Automatic notification of the fire department upon fire alarm activation. • Two-way communications stations at the remote fire panels, at the Floor Warden stations, and at the standpipes. • A two-way telephone system for the firefighters to make announcements. • Emergency voice and alarm communication capability, both under manual control at the FCD. • Strobe lights to provide alarm indications for the hearing impaired. • Water flow indicators for the fire sprinkler system, including indicators for disabled systems. No documentation of the status of the replacement system survived the 2001 attack. However, a 2002 analysis estimated that over 80 percent of the towers had been retrofitted and that about 25 percent of the original system was still in use. Although there were localized carbon dioxide and halon systems within the towers, the Safety Plan predominantly relied on water for containing and suppressing a fire (Figure 5–2). By September 11, 2001, automatic sprinklers had been installed throughout WTC 1 and WTC 2.11 The New York City water distribution system supplied water to the complex from two independent connections located under Liberty Street to the south and Vesey Street to the north. Within each tower were six 5,000 gal water storage tanks, three located on the 110th floor and one each on the 20th, 41st, and 75th floors. These were filled from the domestic water supply in the building. In the event of a fire, the gravity-fed water would flow to as many of the thousands of installed sprinklers as had been activated. The WTC engineering staff would supply additional water upward from the city mains using manually 11 The exceptions to this were the computer rooms (protected with halon and carbon dioxide systems), kitchens (protected with dry chemical and steam smothering systems), mechanical spaces on the 108th through 110th floors, and the electrical rooms throughout the buildings, for which the application of water would have been inappropriate. The Design and Construction of the Towers NIST NCSTAR 1, WTC Investigation 63 started pumps located in the towers; the FDNY could augment the supply using fire department connections and truck-based pumps. While there were redundant vertical supply pipes, there was only a single connection to the array of sprinklers on any given floor. Figure 5–2. Schematic of sprinkler and standpipe systems. The WTC towers were constructed with a manually activated (by Port Authority staff at the direction of FDNY) smoke purge system, use of which was integrated into The Port Authority’s WTC Fire Safety Plan. The system was designed to meet the 1968 NYC Building Code and was functional by September 11, 2001. The non-dedicated system used the existing building ventilation system, in contrast with an alternative dedicated system that would have been used only for smoke management. Each tower was divided into three zones, with the blowers located on the mechanical equipment floors (7, 41, 75, and 108). In the smoke purge mode, the mechanical system was aligned so that an entire zone was vented; there was no provision to vent an individual floor. The smoke from the impact floors in WTC 1 would have been drawn upward to the 108th floor, while the smoke from the impact floors in WTC 2 would have been drawn downward to the 75th floor. The system was designed to clear the zone of smoke after the fire was extinguished, perhaps during post-fire cleanup operations, lest the forced air increase the burning intensity. Chapter 5 64 NIST NCSTAR 1, WTC Investigation 5.4 BUILDING INNOVATIONS 5.4.1 The Need for Innovations Had the towers been built according to conventional design, they would have been heavier and would have had less usable space on each floor. Thus, a resourceful approach was taken in translating The Port Authority’s needs and Yamasaki’s design into practice. The Investigation Team identified six innovations incorporated in the lateral-load-resisting system and the gravity-load-carrying system of the towers. Their roles were discussed in Chapter 1. In addition, there were two innovations in achieving the required fire resistance ratings. The innovative, tiered elevator system was also discussed in Chapter 1. The following sections describe these new technologies. The use of sprayed fire-resistive material is discussed in more detail in Section 5.6. 5.4.2 Framed Tube System WTC 1 and WTC 2 were among the first steel-structure, high-rise buildings built using the framed-tube concept to provide resistance to lateral (wind) loads. The framed-tube system had previously been used in the concrete-framed, 43-story DeWitt-Chestnut and the 38-story Brunswick buildings, both in Chicago and both completed in 1965. In the framed-tube concept, the exterior frame system resists the force of the wind. The exterior columns carry a portion of the building gravity loads, and in the absence of wind, are all in compression, i.e., the loads push down on and shorten the columns. Under the effect of a strong wind alone, columns on the windward side are in tension, i.e., they elongate as the top of the building bends away from the wind. The columns on the leeward side are compressed. The columns on the walls parallel to the wind are half in tension (on the windward side) and half in compression (on the leeward side). The net effect of combined gravity and wind loads is larger compression on the leeward side and reduced compression, or in rare instances even tension, on the windward side. Prior to final design, tests had been performed at the University of Western Ontario to assess the stiffness of the wall panels, which consisted of three columns, each three stories high, and the associated spandrel plates as shown in Figure 1–4. These tests used quarter-scale thermoplastic models of panels planned for the 20th, 47th, and 74th floors. (Recall that the structural members became lighter at the higher floors.) The tests also examined the effect of the spandrel thickness, the width of the box columns, and the presence and thickness of stiffeners. Forces were applied to the models, and the resulting deflections measured. The results of these tests guided the final design of the wall panels and provided support for The Port Authority’s acceptance of the resulting structural design. This included the innovations described in Sections 5.4.3 and 5.4.4. 5.4.3 Deep Spandrel Plates The standard approach to construction of the framed tube would have used spandrel beams or girders to connect the columns. The towers used a band of deep plates as spandrel members to tie the perimeter columns together. The Design and Construction of the Towers NIST NCSTAR 1, WTC Investigation 65 5.4.4 Uniform External Column Geometry In a typical high-rise building, the columns would have been larger near the base of the building and would have become smaller toward the top as they bore less wind and gravity loads. However, the Yamasaki design called for the appearance of tall, uniform columns (Figure 1–2). This was achieved by varying both the strength of the steels and the thickness of the plates that made up the perimeter columns. 5.4.5 Wind Tunnel Test Data to Establish Wind Loads To determine the extreme wind speeds that could be expected at the top of the towers, Worthington, Skilling, Helle & Jackson (WSHJ) collected data on the wind speeds and directions recorded in the New York area over the prior 50 years. From these data, a design wind speed for the buildings was determined for a 50 year wind event, defined as the wind speed, averaged over a 20 min duration at 1,500 ft above the ground. The estimated value was just under 100 mph in all directions. To estimate how the buildings would perform under wind loads, both during construction and upon completion, WSHJ conducted a then unique wind tunnel testing program at Colorado State University (CSU) and the National Physical Laboratory (NPL) in the United Kingdom. In each wind tunnel, a physical model of Lower Manhattan, including the towers, was subjected to steady and turbulent winds consistent with the estimated design wind speeds. The model scale was 1/500 for the CSU tests and 1/400 for the NPL tests. The tower models were thus about 3 ft tall. Separate tests were conducted for the single tower and for the two towers at various spacings, with various values of the tower stiffness and damping, and for various wind directions. The two laboratories obtained similar results. Tests on the twotower models showed that the wind response of each tower was significantly affected by the presence of the other tower. WSHJ also conducted experiments to determine the wind-induced conditions that would be tolerated by the people who would work in and visit the towers. Breaking new ground in human perception testing, the investigators found that surprisingly low building accelerations caused discomfort. The test results led to changes in the building design, including stiffer perimeter columns, and the addition of viscoelastic dampers described in the next section. The dampers were used to reduce the building vibrations due to winds. 5.4.6 Viscoelastic Dampers The tower design included the first application of damping units to supplement the framed-tube in limiting wind-induced oscillations in a tall building. Each tower had about 10,000 dampers. On most truss-framed floors (tenant floors), a damper connected the lower chord of a truss to a perimeter column. A depiction of the units is shown in Figure 5–3. On beam-framed floors (generally the mechanical floors with their heavier loads), a damper connected the lower flange of a wide-flange beam (that spanned between the core and the perimeter wall) to a spandrel. Chapter 5 66 NIST NCSTAR 1, WTC Investigation Figure 5–3. Diagram of floor truss showing viscoelastic damper. Two sets of experiments, conducted by the 3M Company (the manufacturer of the viscoelastic material) and by the Massachusetts Institute of Technology, respectively, examined the damping characteristics of the units. Both studies found that the units provided significant supplemental damping under design conditions. 5.4.7 Long-Span Composite Floor Assemblies The floor system in the towers (as shown in Figure 1–6) was novel in two respects: • The use of open-web, lightweight steel trusses topped with a slab of lightweight concrete • The composite action of the steel and concrete that resulted from the “knuckles” of the truss diagonals extending above the top chord and into the poured concrete Tests conducted in 1964 by Granco Steel Products and Laclede Steel Company (the manufacturer of the trusses for WTC 1 and WTC 2) determined the effectiveness of the knuckles in providing composite action. Another set of tests, performed by Laclede Steel Company, determined that any failure of the knuckles occurred well beyond the design capacity. A third set of tests, performed at Washington University in 1968, confirmed the prior results and indicated that failure was due to crushing of the concrete near the knuckles. 5.4.8 Vertical Shaft Wall Panels While similar to other gypsum shaft wall systems and firewalls, the compartmentation system used in the vertical shafts (e.g., for elevators, stairs, utilities and ventilation) was unique in that it eliminated the need for any framing. The walls consisted of gypsum planks placed into metal channels at the floor and ceiling slabs. The planks were 2 in. thick (2½ in. on floors with 16 ft ceiling heights) and 16 in. wide, with metal tongue and groove channels attached to the long sides that served as wall studs. An assembled wall was The Design and Construction of the Towers NIST NCSTAR 1, WTC Investigation 67 then covered with gypsum wallboard. The planks were likely custom fabricated for this job, as the investigators found no mention of similar products in gypsum industry literature of the time or since. 5.5 STRUCTURAL STEELS 5.5.1 Types and Sources Roughly 200,000 tons of steel were used in the construction of the two WTC towers. The building plans called for an unusually broad array of steel grades and multiple techniques for fabricating the structure from them. The NIST team obtained the information needed to characterize the steels from structural drawings provided by The Port Authority, copies of correspondence during the fabrication stages, steel mill test reports, interviews with fabrication company staff, search of the contemporaneous literature, and measurements of properties at NIST. Sorting through this immense amount of information was made difficult by the large number of fabricators and suppliers, the use of proprietary grades by some of the manufacturers; and the fact that the four fabricators of the impact and fire floor structural elements no longer existed at the time of this Investigation. Fortunately, the potential for confusion had led the building designers to a tracking system whereby the steel fabricators stamped and/or stenciled each structural element with a unique identifying number. The structural engineering drawings included these identifying numbers as well as the yield strengths of the individual steel components. Thus, when NIST found the identifying number on an element such as a perimeter column panel, the particular steel specified for each component of the element was known, as well as the intended location of the steel in the tower. In all, 14 grades of steel were specified in the structural engineering plans, having yield strengths from 36 ksi to 100 ksi. Twelve were actually used, as the fabricators were permitted to substitute 100 ksi steel where yield strengths of 85 ksi and 90 ksi were specified. Table 5–1 indicates the elements for which the various grades were used. The higher yield strength steels were used to limit building weight while providing adequate load-carrying capacity. Table 5–1. Specified steel grades for various applications. Yield Strength (ksi) Application 36 42 45 46 50 55 60 65 70 75 80 100 Perimeter columns 􀀹 􀀹 􀀹 􀀹 􀀹 􀀹 􀀹 􀀹 􀀹 􀀹 􀀹 􀀹 Spandrel plates 􀀹 􀀹 􀀹 􀀹 􀀹 􀀹 􀀹 􀀹 􀀹 􀀹 􀀹 􀀹 Core columns 􀀹 􀀹 (a) (a) Floor trusses 􀀹 􀀹 a. About 1 percent of the wide flange core columns were specified to be of these higher grades. 5.5.2 Properties The Port Authority required a thorough and detailed quality assurance programs to ensure compliance with the specifications for the steel, welds, and bolts. The steel data went beyond the minimum yield strength (the property of greatest importance) to include tensile strength and ductility. The quality assurance program included unannounced inspections and confirming tests. Chapter 5 68 NIST NCSTAR 1, WTC Investigation NIST performed confirmatory tests on samples of the 236 pieces of recovered steel to determine if the steel met the structural specifications. Making a definitive assessment was complicated by overlapping specifications from multiple suppliers, differences between the NIST test procedures and the test procedures that originally qualified the steel, the natural variability of steel properties, and damage to the steel from the collapse of the WTC towers. Nonetheless, the NIST investigators were able to determine the following: • There were 14 grades (strengths) of steel that were specified. However, a total of 32 steels in the impact and fire floors were sufficiently different (grade, supplier, and gage) to require distinct models of mechanical properties. • The steels in the perimeter columns met their intended specifications for chemistry, mechanical properties, yield strengths, and tensile properties. The steels in the core columns generally met their intended specifications for both chemical and mechanical properties. • Roughly 13 percent of the measured strength values for the perimeter and core columns were at or below the specified minimums (Figure 5–4). The strength variation was consistent with the historical variability of steel strength and with the effects from damage during the collapse of the towers. The measured values were within the typical design factor of safety. • The yield strengths of many of the steels in the floor trusses were above 50 ksi, even when they were specified to be 36 ksi. • Tests on a limited number of recovered bolts showed they were much stronger than expected based on reports from the contemporaneous literature. The mechanical properties of steel are reduced at elevated temperatures. Based on measurements and examination of published data, NIST determined that a single representation of the elevated temperature effects on steel mechanical properties could be used for all WTC steels. Separate values were used for the yield and tensile strength reduction factors for bolt steels. The Design and Construction of the Towers NIST NCSTAR 1, WTC Investigation 69 Note: The ratio values less than 1 arose from natural variation in the steel and did not affect the safety of the towers on September 11, 2001. The bars represent maximum and minimum values from multiple measurements. Figure 5–4. Ratio of measured yield strength (Fy) to specified minimum yield strength for steels used in WTC perimeter columns. 5.6 FIRE PROTECTION OF STRUCTURAL STEEL 5.6.1 Thermal Insulation When steel is heated it loses both strength and stiffness. Thus, measures must be taken to protect the steel in a structure from temperature rise (and consequent loss of strength) in case of fire. Bare structural steel components can heat quickly when exposed to a fire of even moderate intensity. Therefore, some sort of thermal protection, or insulation, is necessary. This insulation can be in direct contact with the steel, such as a sprayed fire-resistive material (SFRM), or can be a fire resistant enclosure surrounding a structural element. 5.6.2 Use of Insulation in the WTC Towers The thermal protection of the steel structures in the WTC towers included a combination of SFRM and enclosures of gypsum wallboard. The use of SFRM for floor truss protection was new in high-rise buildings, and the requirements evolved during the construction and life of the towers. By examining documents supplied by The Port Authority, LERA, and the SFRM manufacturers, NIST was able to Chapter 5 70 NIST NCSTAR 1, WTC Investigation document much of the sequence of these changing requirements and arrive at an estimation of the passive protection in place on September 11, 2001. Floor Systems At the time the WTC was designed, the ASTM E 119 test method had been used for nearly 50 years to determine the fire resistance of structural members and assemblies. However, The Port Authority confirmed to the Investigation Team that there was no record of fire endurance testing of the innovative assemblies representing the thermally protected floor system used in the towers. The floor assembly was not tested despite the fact that the Architect of Record and the Structural Engineer of Record stated that the fire rating of this novel floor system could not be determined without testing. Prior to construction, the Architect of Record had used information from (unidentified) manufacturers to recommend a 1 in. thickness of SFRM around the top and bottom chords of the trusses and a 2 in. thickness for the web members of the trusses. This was to achieve the fire endurance requirements for Class 1A construction (Section 5.3.3). In 1969, The Port Authority directed that a ½ in. thick coating of BLAZE-SHIELD Type D (BLAZE-SHIELD D), a mixture of cement and asbestos fibers, be used to insulate the floor trusses. This was to achieve a Class 1A rating, even though the preponderance of evidence suggests that the towers were chosen to be Class 1B, the minimum required by the NYC Building Code. NIST found no evidence of a technical basis for selection of the ½ in. thickness. This coating had been installed as high as the 38th floor of WTC 1 when its use was discontinued due to recognition of adverse health effects from inhalation of asbestos fibers. The spraying then proceeded with BLAZE-SHIELD DC/F, a similar product in which the asbestos was replaced by a glassy mineral fiber and whose insulating value was reported by Underwriters Laboratories, Inc., to be slightly better than that of BLAZE-SHIELD D. On the lower floors, the BLAZE-SHIELD D was encapsulated with a sprayed material that provided a hard coat to mitigate the dispersion of asbestos fibers into the air. In 1994, The Port Authority measured the SFRM thickness on trusses on floors 23 and 24 of WTC 1. In all, average thicknesses were reported for 32 locations, and the overall average thickness was found to be 0.74 in. NIST performed a further evaluation of the SFRM thickness using photographs taken in the 1990s of floor trusses on (non-upgraded) floors 22, 23, and 27 of WTC 1 (Figure 5–5). By measuring dimensions on the photographs, NIST estimated the insulation thicknesses on the diagonal web members of trusses. (The thickness of chord member insulation could not be measured.) The average thickness and standard deviation of web members was 0.6 in. ± 0.3 in. on the main trusses, 0.4 in. ± 0.25 in. on the bridging trusses, and 0.4 in. ± 0.2 in. on the diagonal struts. These numbers indicated that there were areas where the coating thickness was less than the specified 0.5 in. The Design and Construction of the Towers NIST NCSTAR 1, WTC Investigation 71 Note: Enhancement by NIST. Figure 5–5. Irregularity of coating thickness and gaps in coverage on SFRM–coated bridging trusses. In 1995, The Port Authority performed a study to establish requirements for retrofit of sprayed insulation to the floor trusses during major alterations when tenants vacated spaces in the towers. Based on design information for fire ratings of a similar, but not identical, composite floor truss system contained in the Fire Resistance Directory published by Underwriters Laboratories, Inc., the study concluded that a 1½ in. thickness of sprayed mineral fiber material would provide a 2 hour fire rating, consistent with the Class 1B requirements. In 1999, the removal of existing SFRM and the application of new material to this thickness became Port Authority policy for full floors undergoing new construction and renovation. For tenant spaces in which only part of a floor was being modified, the SFRM needed only to be patched to ¾ in. thickness or to match the 1½ in. thickness, if it had previously been upgraded. In the years between 1995 and 2001, thermal protection was upgraded on 18 floors of WTC 1, including those on which the major fires occurred on September 11, 2001, and 13 floors of WTC 2 that did not include the fire floors. The Port Authority reported that the insulation used in the renovations was BLAZE-SHIELD II. In July 2000, an engineering consultant to The Port Authority issued a report on the requirements of the fire resistance of the floor system of the towers. Based on calculations and risk assessment, the consultant concluded that the structural design had sufficient inherent fire performance to ensure that the fire condition was never the critical condition with respect to loading allowances. The report recommended that a 1.3 in. thickness be used for the floor trusses. In December 2000, another condition assessment concluded that the structural insulation in the towers had an adequate 1 hour rating, considering that all floors were now fitted with sprinklers. The report also noted the ongoing Port Authority program to upgrade the fire-resistive material thickness to 1½ in. in order to achieve a 2 hour fire rating. Chapter 5 72 NIST NCSTAR 1, WTC Investigation The Port Authority provided NIST with the records of measurements of SFRM thickness on upgraded floors in both towers. The average thickness and standard deviation on the main trusses was 2.5 in. ± 0.6 in., based on 18 data sets with a total of 256 measurements. NIST analysis of several Port Authority photographs from the 1990s of the upgraded 31st floor of WTC 1 indicated an average thickness and standard deviation on the main trusses of 1.7 in. ± 0.4 in., based on 52 measurements from five web members in two photographs. NIST gave more weight to the measured data, which were taken according to a standard procedure in ASTM E 605, than to the data scaled from photographs, for which there was neither standard procedure nor calibration of the method. Perimeter Columns In 1966, the contractor responsible for insulating the perimeter columns proposed applying a 1 3/16 in. thick coating of BLAZE-SHIELD D to the three external faces (Figure 5–6) to achieve a 4 hour rating, which is a Class 1A rating requirement (1 hour more than Class 1B). NIST found evidence of a technical basis for this decision. In the construction drawings prepared by the exterior cladding contractor, the following SFRM thicknesses were specified: • 7/8 in. of vermiculite plaster on the interior face and 1 3/16 in. of BLAZE-SHIELD D on the other three faces. • ½ in. of vermiculite plaster on the interior surfaces of the spandrels and ½ in. of BLAZE-SHIELD D on the exterior surfaces. Figure 5–6. Thermal insulation for perimeter columns. Vermiculite plaster had a higher thermal conductivity and thereby increased heat migration from the room air to the column steel and, thus, could keep the steel temperature at 70 °F when the temperature was 0 °F outside. The Design and Construction of the Towers NIST NCSTAR 1, WTC Investigation 73 In October 1969, The Port Authority provided the following instructions to the contractor applying the sprayed fire protection, in order to maintain the Class 1-A Fire Rating of the NYC Building Code: • 2 3/16 in. of BLAZE-SHIELD D for columns smaller than 14WF22812 and 1 3/16 in. for columns equal to or greater than 14WF228. • ½ in. covering of BLAZE-SHIELD D for beams, spandrels and bar joists. NIST’s review of available documents has not uncovered the reasons for selecting BLAZE-SHIELD fire-resistive material or the technical basis for specifying ½ in. thickness of SFRM for the floor trusses. As with the trusses, BLAZE-SHIELD DC/F was applied to the perimeter columns above the 38th floor of WTC 1 and all the perimeter columns in WTC 2. Core Columns and Beams Multiple approaches were used to insulate structural elements in the core: • Those core columns located in rentable and public spaces, closets, and mechanical shafts were enclosed in boxes of gypsum wallboard (and thus were inaccessible for inspection). The amount of the gypsum enclosure in contact with the column varied depending on the location of the column within the core. SFRM (BLAZE-SHIELD D and DC/F) was applied on those faces that were not protected by the gypsum enclosure. The thicknesses specified in the construction documents were 1 3/16 in. for the heavier columns and 2 3/16 in. for the lighter columns. • Columns located at the elevator shafts were protected using the same SFRM thicknesses. They were not enclosed and thus were accessible for routine inspections. Inspection of the columns within the elevator shaft spaces in 1993 indicated some loss of SFRM coverage. As a result, new insulation was applied to selected columns within the elevator shaft space. Information provided to NIST indicated that a different SFRM, Monokote Type 2-106, was used. Thickness measurements for columns and beams below the 45th floor indicated average thicknesses of 0.82 in. and 0.97 in., respectively. Information from The Port Authority indicated that the minimum required thickness of the re-applied SFRM was ½ in. for the columns and ¾ in. for the beams. NIST was unable to locate information from which to characterize the insulation of the core columns and beams that were not accessible. Except as noted above, once completed, the core was generally not inspected. NIST was not able to locate any post-collapse core beams or columns with sufficient insulation still attached to make pre-collapse thickness measurements. Summary of SFRM on September 11, 2001 Table 5–2 summarizes the types and thicknesses of the SFRMs used in the towers. According to Port Authority documents, in the upper part of the towers, trusses on floors 92 through 100 and 102 in WTC 1 12 This designation indicates that the column is a 14 in. deep wide flange section and weighs 228 pounds per foot. Chapter 5 74 NIST NCSTAR 1, WTC Investigation had upgraded insulation by September 11, 2001. In WTC 2, truss insulation had been upgraded on floors 77, 78, 85, 88, 89, 92, 96, 97, and 99. Table 5–2. Types and locations of SFRM on fire floors. Thickness (in.) Building Component Material Specifieda Installed Used in Analysisb FLOOR SYSTEM Original Main trusses and diagonal struts BLAZE-SHIELD DC/F 0.5 0.75 0.6 Bridging trusses (one-way zone)c BLAZE-SHIELD DC/F 0.5 0.38d 0.3 Bridging trusses (two-way zone)c BLAZE-SHIELD DC/F 0.5 0.38d 0.6 Upgraded Main trusses BLAZE-SHIELD II 1.5 2.5 2.2 Main truss diagonal struts BLAZE-SHIELD II 1.5 2.5 2.2 Bridging trusses BLAZE-SHIELD II 1.5 2.5 2.2 EXTERIOR WALL PANEL Box columns Exterior face BLAZE-SHIELD DC/F 1 3/16 (e) 1.2 Interior face Vermiculite plaster 7/8 (e) 0.8 Spandrels Exterior face BLAZE-SHIELD DC/F 0.5 (e) 0.5 Interior face Vermiculite plaster 0.5 (e) 0.5 CORE COLUMNS Wide flange columns Light BLAZE-SHIELD DC/F 2 3/16 (e) 2.2 Heavy BLAZE-SHIELD DC/F 1 3/16 (e) 1.2 Box columns Light BLAZE-SHIELD DC/F (f) (e) 2.2(g) Heavy BLAZE-SHIELD DC/F (f) (e) 1.2(g) CORE BEAMS BLAZE-SHIELD DC/F 0.5 (e) 0.5 a. “Specified” means material and thicknesses determined from correspondence among various parties. b. The analysis is described in Chapter 6. c. Not expressly specified. SFRM was required for the areas where the main trusses ran in both directions and, while not required, was also applied in the areas where they ran in one direction only. d. Analysis of photographs indicated that the thickness was approximately one half that on the main trusses. e. Not able to determine. f. Not specified. g. Thickness assumed equal to wide flange columns of comparable weight per foot. The Design and Construction of the Towers NIST NCSTAR 1, WTC Investigation 75 5.7 CONCRETE Two types of concrete were used for the floors of the WTC towers: lightweight concrete in the tenant office areas and normal weight concrete in the core area. Because of differences in composition and weight, the two types of concrete respond differently to elevated temperatures, as shown in Figure 5–7. While their tensile strengths degrade identically, lightweight concrete retains more of its compressive strength at higher temperatures. The degradation of concrete mechanical properties with temperature was included in the structural response analysis of the floor systems. 0 200 400 600 1000 2000 3000 4000 5000 6000 Normal-weight (3000psi) Normal-weight (4000psi) Light-weight (3000psi) Temperature (°C) Compressive Strength (psi) 0 200 400 600 0 100 200 300 400 Normal-weight (3000psi) Normal-weight (4000psi) Light-weight (3000psi) Temperature (°C) Tensile Strength (psi) Figure 5–7. Temperature–dependent concrete properties. 5.8 THE TENANT SPACES 5.8.1 General About 80 percent of the floors had a single tenant. Many of these floors were filled with arrays of modular office cubicles, their low partitions affording sightlines to the windows, with perhaps an occasional perimeter conference room or executive office in the way (Figure 1–11). Trading floors (Figure 1–12) had tables and computers throughout and food service areas to minimize time away from the non-stop transactions. The remaining 20 percent of the floors were each subdivided among as many as 25 tenants. Some of the approximately 25 tenants that occupied two or more contiguous floors installed convenience stairways within their own space. Certain floors were of special interest to the Investigation. These were the floors on which there was structural damage from the aircraft and/or on which extensive fires were observed. These floors, designated as focus floors, and the information NIST obtained regarding them are characterized in Table 5–3. Additional information, obtained from the tenant firms and The Port Authority, is summarized in the remainder of this chapter. Chapter 5 76 NIST NCSTAR 1, WTC Investigation 5.8.2 Walls The plans for the tenant spaces in WTC 1 showed no interior walls whose sole function was to subdivide the floors. There were a number of partitioned offices and conference areas. Although NIST was not able to obtain layout drawings for the fire floors in WTC 2, the verbal descriptions of those floors indicated similarly open space. The types of interior walls were described in Section 5.3.4. 5.8.3 Flooring Truss-supported concrete slabs formed the floors in the office areas of the towers. Some tenants had installed slightly raised (6 in.) floors on top of the slab under which communication cables were run. This was especially true on trading floors. There was a wide range of floor coverings in use. Inlaid wood and marble were used in some reception areas. Most commonly, the expanse of the floor was covered with nylon carpet. 5.8.4 Ceilings There were two different ceiling tile systems originally installed in the towers under Port Authority specification. The framing for each was hung from the bottom of the floor trusses, resulting in an apparent room height of 8.6 ft and an above-ceiling height of about 3.4 ft. The tiles in the tenant spaces were 20 in. square, ¾ in. thick, lay-in pieces on an exposed tee bar grid system. The tiles in the core area were 12 in. square, ¾ in. thick, mounted in a concealed suspension system. Neither system was specified to be fire-rated, and it was estimated that in a fire they might provide only 10 min to 15 min of thermal protection to the trusses before the ceiling frame distorted and the tiles fell. Chemically, the tiles were similar, and their combustible content, flame spread, and smoke production were all quite low. 5.8.5 Furnishings The decorating styles of the tower tenants ranged from simple, modular trading floors to customized office spaces. The most common layout of the focus floors was a continuous open space populated by a large array of workstations or cubicles (Figure 1–11). The number of different types of workstations in the two towers was probably large. However, discussions with office furniture distributors and visits to showrooms indicated that, while there was a broad range of prices and appearances, the cubicles were fundamentally similar to that shown in Figure 5–8. The workstations were typically 8 ft square, bounded on all four sides by privacy panels, with an entrance opening in one side only. Within the area defined by the panels was a Source: Reproduced with permission of The Port Authority of New York and New Jersey. Figure 5–8. A WTC workstation. The Design and Construction of the Towers NIST NCSTAR 1, WTC Investigation 77 self-contained workspace: desktop (almost always a wood product, generally with a laminated finish), file storage, bookshelves, carpeting, chair, etc. Presumably there were a variety of amounts and locations of paper, both exposed on the work surfaces and contained within the file cabinets and bookshelves. The cubicles were grouped in clusters or rows, with up to 215 units on a given floor. NIST estimated the combustible fuel loading on these floors to have been about 4 lb/ft2 (20 kg/m2), or about 60 tons per floor. This was somewhat lower than found in prior surveys of office spaces. The small number of interior walls, and thus the minimal amount of combustible interior finish, and the limited bookshelf space account for much of the differences. While paper in the filing cabinets might have been significant in mass, it did not burn readily due to the limited oxygen available within the drawers. 78 NIST NCSTAR 1, WTC Investigation Chapter 5 Table 5–3. Floors of focus. Building Floor Tenant Damagea Firesb Material Obtainedc General Description of Tenant Layout 92 Carr Futures, empty Y FP (Carr), V 93 Marsh & McLennan (M&M), Fred Alger Mgmt. Y Y FP, F, V M&M occupied the south side. Filled with workstations. Demising walls for the south façade to the edges of the core. Offices along the east side of the south core wall. Stairwell to the 94th floor. 94 Marsh & McLennan Y Y FP, F, V Generally open space filled with workstations. Offices and conference rooms around most of the perimeter. Stairwell to the 93rd floor. 95 Marsh & McLennan Y Y FP, F, V Generally open space filed with workstations. Offices, conferences and work areas in exterior corners. Large walled data center along north and east sides. Two separate stairwells, one to 94th floor, the other to the 96th and 97th floors. 96 Marsh & McLennan Y Y FP, F, V Generally open space filled with workstations. Offices at exterior corners and middle of north and south facades. Some conference rooms on north and south sides of core. Stairwell connection to 95th and 97th floors. 97 Marsh & McLennan Y Y FP, F, V Generally open space filled with workstations. Offices at exterior corners and in the middle of the north façade. Two separate stairwells: one connected to the 95th and 96th floors, the other connected to the 98th, 99th, and 100th floors. 98 Marsh & McLennan Y Y FP, F, V Generally open space filled with workstations. Offices at exterior corners and middle of north and south facades. Some conference rooms on north and south sides of core. Stairwell connected to the 97th, 99th, and 100th floors. 99 Marsh & McLennan Y Y FP, F, V Open space filled with workstations on the east side and east half of the north side. Offices at exterior corners and along south and west sides. Large walled area on west side of north façade. Stairwell connected to the 97th, 98th, and 100th floors. 100 Marsh & McLennan Y FP, F, V Considerable number of workstations, but more individual offices than the other floors. Partitioned offices extended the full length of the west wall and also at other locations along walls and at exterior corners. Stairway connected to the 97th, 98th, and 99th floors. WTC 1 104 Cantor Fitzgerald Y V Trading floor. Tables with many monitors. NIST NCSTAR 1, WTC Investigation 79 The Design and Construction of the Towers Building Floor Tenant Damagea Firesb Material Obtainedc General Description of Tenant Layout 77 Baseline Y Y FP, V Generally open space. Offices along east and west core walls. A few offices in each exterior corner of the floor. 78 Baseline, 1st Commercial Bank Y Y FP,V West side open. Northeast quadrant walled. Offices along south side of east core wall. Offices along east side of south façade. 79 Fuji Bank Y Y V 80 Fuji Bank Y Y FP, V Generally open space filled with workstations. Offices or conference rooms at exterior corners and along south half of west façade. Large vault at southeast corner of core. 81 Fuji Bank Y Y V 82 Fuji Bank Y Y V 83 Chuo Mitsui, IQ Financial Y V Chuo Mitsui had half the area. Wide open space. No information regarding IQ Financial. 84 Eurobrokers Y V Open floor for trading. Tables rather than workstations. Perimeter offices. WTC 2 85 Harris Beach Y FP. V Offices around full perimeter. Offices along east, west and south walls of core. a. Floors on which the exterior photographs indicated direct damage from the aircraft. b. Floors on which the exterior photographs indicated extensive or sustained fires. c. Types of descriptive material obtained: FP, floor plan; F, documentation of furnishings; V, verbal description of interior. Chapter 5 80 NIST NCSTAR 1, WTC Investigation This page intentionally left blank. NIST NCSTAR 1, WTC Investigation 81 Chapter 6 RECONSTRUCTION OF THE COLLAPSES 6.1 APPROACH The following presents an overview of the methods used to reach the accounts in Part I. The details may be found in the companion reports to this document, which are indexed in Appendix C. A substantial effort was directed at establishing the baseline performance of the WTC towers, i.e., estimating the expected performance of the towers under normal design loads and conditions. This enabled meeting the third objective of the Investigation, as listed in the Preface to this report. The baseline performance analysis also helped to estimate the ability of the towers to withstand the unexpected events of September 11, 2001. Establishing the baseline performance of the towers began with the compilation and analysis of the procedures and practices used in the design, construction, operation, and maintenance of the structural, fire protection, and egress systems of the WTC towers. The additional components of the performance analysis were: • The standard fire resistance of the WTC truss-framed floor system, • The quality and properties of the structural steels used in the towers, and • The response of the WTC towers to design gravity and wind loads. The second substantial effort was the simulation of the behavior of each tower on September 11, 2001, providing the basis for meeting the first and second objectives of the Investigation. This entailed four modeling steps: 1. The aircraft impact into the tower, the resulting distribution of jet fuel, and the damage to the structure, partitions, insulation materials, and building contents. 2. The spread of the multi-floor fires. 3. The heating of the structural elements by the fires. 4. The response of the damaged and heated building structure, and the progression of structural component failures leading to the initiation of the collapse of the towers. For such complex structures and complex thermal and structural processes, each of these steps stretched the state of the technology and tested the limits of software tools and computer hardware. For example, the investigators advanced the state-of-the-art in the measurement of construction material properties and in structural finite element modeling. New modeling capability was developed for the mapping of firegenerated environmental temperatures onto the building structural components. For the final analyses, four cases were used, each involving all four of the modeling steps. Case A and Case B were for WTC 1, with Case B generally involving more severe impact and fire conditions than Chapter 6 82 NIST NCSTAR 1, WTC Investigation Case A. For WTC 2, Case D involved more severe impact and fire conditions than Case C. The results of the two cases for each tower provided some understanding of the uncertainties in the predictions. There were substantial uncertainties in the as-built condition of the towers, the interior layout and furnishings, the aircraft impact, the internal damage to the towers (especially the insulation), the redistribution of the combustibles, and the response of the building structural components to the heat from the fires. To increase confidence in the simulation results, NIST used information from an extensive collection of photographs and videos of the disaster, eyewitness accounts from inside and outside the buildings, and laboratory tests involving large fires and the heating of structural components. Further, NIST applied formal statistical methods to identify those parameters that had the greatest effect on the model output. These key inputs were then varied to determine whether the results were reasonably robust. The combined knowledge from all the gathered data and analyses led to the development of a probable collapse sequence for each tower,13 the identification of factors that contributed to the collapses, and a list of factors that could have improved building performance or otherwise mitigated the loss of life. 6.2 DEVELOPMENT OF THE DISASTER TIMELINE Time was the unifying factor in combining photographic and video information, survivor accounts, emergency calls from within the towers, and communications among emergency responders. The visual evidence was the most abundant and the most detailed. The destruction of the WTC towers was the most heavily photographed disaster in history. The terrorist attacks occurred in an area that is the national home base of several news organizations and has several major newspapers. New York City is also a major tourist destination, and visitors often carry cameras to record their visits. Further, the very height that made the towers accessible to the approaching aircraft also made them visible to photographers. As a result there were hundreds of both professional and amateur photographers and videographers present, many equipped with excellent equipment and the knowledge to use it. These people were in the immediate area, as well as at other locations in New York and New Jersey. There was a surprisingly large amount of photographic material shot early, when only WTC 1 was damaged. By the time WTC 2 was struck, the number of cameras and the diversity of locations had increased. Following the collapse of WTC 2, the amount of visual material decreased markedly as people rushed to escape the area and the huge dust clouds generated by the collapse obscured the site. There is a substantial, but less complete, amount of material covering the period from the tower collapses to the collapse of WTC 7 late the same afternoon. 13 The focus of the Investigation was on the sequence of events from the instant of aircraft impact to the initiation of collapse for each tower. For brevity in this report, this sequence is referred to as the “probable collapse sequence,” although it does not actually include the structural behavior of the tower after the conditions for collapse initiation were reached and collapse became inevitable. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 83 There were multiple sources of visual material: • Recordings of newscasts from September 11 and afterward, documentaries, and other coverage provided information and also pointed toward other potential sources of material. • Web sites of the major photographic clearinghouses. • Local print media. • NYPD and FDNY. • Collections of visual material assembled for charitable or historical purposes. • Individuals’ photographs and videos that began appearing on the World Wide Web as early as September 11, 2001. • Responses to public appeals for visual material by the Investigation Team. Investigation staff contacted each of the sources, requested the material, made arrangements for its transfer, and addressed copyright and privacy issues. Emphasis was placed on obtaining material in a form as close as possible to the original in order to maintain as much spatial and timing information as possible: direct digital copies of digital photographs and videos, high resolution digitized copies of film or slide photographs, and direct copies from the original source of analog video. The assembled collection included: • 6,977 segments of video footage, totaling in excess of 300 hours. The media videos included both broadcast material and outtakes. Additionally, NIST received videotapes recorded by more than 20 individuals. • 6,899 photographs from at least 200 photographers. As with the videos, many of the photographs were unpublished. This vast amount of visual material was organized into a searchable database in which each frame was characterized by a set of attributes: photographer (name and location), time of shot/video, copyright status, content (including building, face(s), key events (plane strike, fireballs, collapse), the presence of FDNY or NYPD people or apparatus, and other details, such as falling debris, people, and building damage). The development of a timeline for fire growth and structural changes in the WTC buildings required the assignment of times of known accuracy to each video frame and photograph. Images were timed to a single well-defined event. Due to the large number of different views available, the chosen event was the moment the second plane struck WTC 2, established from the time stamps in the September 11 telecasts. Based on four such video recordings, the time of the second plane impact was established as 9:02:59 a.m. The TV network clocks were quite close to the actual time since they were regularly updated from highly accurate geopositioning satellites or the precise atomic-clockbased timing signals provided by NIST as a public service. Chapter 6 84 NIST NCSTAR 1, WTC Investigation Absolute times were then assigned to all frames of all videos that showed the second plane strike. By matching photographs and other videos to specific events in these initially assigned videos, the time assignments were extended to visual materials that did not include the primary event. Times were also cross-matched using additional characteristics, such as the appearance and locations of smoke and fire plumes, distinct shadows cast on the buildings by these plumes, the occurrence of well-defined events such as a falling object, and even a clock being recorded in an image. By such a process, it was possible to place photographs and videos extending over the entire day on a single timeline. As the time was assigned to a particular photograph or video, the uncertainty in the assignment was also logged into the database. In all, 3,032 of the catalogued photographs and 2,673 of the video clips in the databases were timed with accuracies of ± 3 s or better. This process enabled establishing the times of four major events of September 11, listed in Table 6–1. The building collapse times were defined to be the point in time when the entire building was first observed to start to collapse. Table 6–1. Times for major events on September 11, 2001. Event Time First Aircraft Strike 8:46:30 a.m. Second Aircraft Strike 9:02:59 a.m. Collapse of WTC 2 9:58:59 a.m. Collapse of WTC 1 10:28:22 a.m. There were additional sources of timed information. Phone calls from people within the building to relatives, friends, and 9-1-1 operators conveyed observations of the structural damage and developing hazards. Communications among the emergency responders and from the building fire command centers contributed further information about the areas where the external photographers had no access. 6.3 LEARNING FROM THE VISUAL IMAGES The photographic and video images were rich sources of information on the condition of the buildings following the aircraft impact, the evolution of the fires, and the deterioration of the structure. To enable analysis of this information, a shorthand notation (based on the building design drawings) was used to label the exterior columns and windows of the buildings: • First, the faces of the towers were numbered in a manner identical to those used in the original plans: WTC 1: north: 1 east: 2 south: 3 west: 4 WTC 2: west: 1 north: 2 east: 3 south: 4 • The 59 columns across each tower face were assigned three-digit numbers. Following the floor number, the first digit was that of the face, and the remaining two digits were assigned consecutively from right to left as viewed from outside the building. Thus, the fourth column from the right on the east face of the 81st floor of WTC 1 was labeled 81-204. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 85 • Each of the 58 windows on each floor and tower face was assigned the number of the column to its right as viewed from the outside of the building and was also identified by its floor. Thus the rightmost window on the east face of the 94th floor of WTC 1 was labeled 94-201. As an example of information that was extracted, Figure 6–1 shows an enhanced image of the east face of WTC 2. Figure 6–2 expands a section of interest. The amount of detail available is evident. For instance, large piles of debris are present on the north side of the tower on the 80th and 81st floors, and locations where fires are visible or where missing windows are easily identified. Many details of each frame were important in tracking the evolution of the fires and the damage to the buildings. Note: Enhancements by NIST. Figure 6–1. 9:26:20 a.m. showing the east face of WTC 2. In each photograph and each video frame, each window was also coded to indicate whether the window was still in place or not and the extent to which flames and smoke were visible. Color-coded graphics of the four façades of the two towers were then constructed. Examples of these graphics were shown in Chapters 2 and 3. The results of the visual analysis included: • The locations of the broken windows, providing information on the source of air to feed the fires within. • Observations of the spread of fires. • Documentation of the location of exterior damage from the aircraft impact and subsequent structural changes in the buildings. Chapter 6 86 NIST NCSTAR 1, WTC Investigation Note: Enhancements by NIST. Figure 6–2. Close-up of section of Figure 6–1. • Identification of the presence or absence of significant floor deterioration at the building perimeter. • Observations of certain actions by building occupants, such as breaking windows. The near-continuous observations of the externally visible fires provided input to the computer simulations of fire growth and spread. The discrete observations of changes in the displacement of columns and, to a far lesser degree, floors became validation data for the modeling of the approach to structural collapse of the towers. Table 6–2 lists the most important observations. 6.4 LEARNING FROM THE RECOVERED STEEL 6.4.1 Collection of Recovered Steel NIST had two reasons for obtaining specimens of structural steel from the collapsed towers. The primary objective was characterizing the quality of the steel and determining its properties for use in the structural modeling and analysis of the collapse sequences. The second reason was obtaining information regarding the behavior of the steel in the aircraft impact zone and in areas which had major fires. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 87 Table 6–2. Indications of major structural changes up to collapse initiation. Tower Time (a.m.) Observation 10:18 Smoke suddenly expelled on the north face (floors 92, 94, 95 to 98) and west face (92, 94 to 98). 10:23 Inward bowing of perimeter columns on the east side of the south face from floors 94 to 100; maximum extent: 55 in. ± 6 in. at floor 97. WTC 1 10:28:22 First exterior sign of collapse (downward movement of building exterior). Tilting of the building section above the impact and fire area to due south as the structural collapse initiated. First exterior sign of downward movement of building at floor 98. 9:02:59 Exterior fireball from the east face of floor 82 and from the north face from floors 79 to 82. The deflagration prior to the fireballs may have caused a significant pressure pulse to act on floors above and below. 9:21 Inward bowing of exterior wall columns on most of the east face from floors 78 to 83; maximum extent: 7 in. to 9 in. at floor 80. WTC 2 9:58:59 First exterior sign of collapse (downward movement of building exterior). The northeast corner tilted counterclockwise around the base of floor 82. Column buckling was then seen progressing across the north face and nearly simultaneously on the east face. Tilting of the building section above the impact and fire area to the east and south prior to significant downward movement of the upper building section. The tilt to the south did not increase any further as the upper building section began to fall, but the tilt to the east did increase until dust clouds obscured the view. Within weeks of the destruction of the WTC, contractors of New York City had begun cutting up and removing the debris from the site. Members of the FEMA-sponsored and ASCE-led Building Performance Assessment Team, members of the Structural Engineers Association of New York, and Professor A. Astaneh-Asl of the University of California, Berkeley, CA, with support from the National Science Foundation, had begun work to identify and collect WTC structural steel from various recycling yards where the steel was taken during the clean-up effort. The Port Authority of New York and New Jersey (Port Authority) also collected structural steel elements for future exhibits and memorials. Over a period of about 18 months, 236 pieces of steel were shipped to the NIST campus, starting about six months before NIST launched its Investigation. These samples ranged in size and complexity from a nearly complete three-column, three-floor perimeter assembly to bolts and small fragments. Figures 6–3 through 6–5 show some of the recovered steel pieces. Seven of the pieces were from WTC 5. The remaining 229 samples represented roughly 0.25 percent to 0.5 percent of the 200,000 tons of structural steel used in the construction of the two towers. The collection at NIST included samples of all the steel strength levels specified for the construction of the towers. The locations of all structural steel pieces in WTC 1 and WTC 2 were uniquely identified by stampings (recessed letters and numbers) and/or painted stencils. NIST was successful in finding and deciphering these identification markings on many of the perimeter panel sections and core columns, in many cases using metallurgical characterization to complete missing identifiers. In all, 42 exterior panels were positively identified: 26 from WTC 1 and 16 from WTC 2. Twelve core columns were positively identified: eight from WTC 1 and four from WTC 2. Twenty-three pieces were identified as being parts of trusses, although it was not possible to identify their locations within the buildings. Chapter 6 88 NIST NCSTAR 1, WTC Investigation Source: NIST. Figure 6–3. Examples of a WTC 1 core column (left) and truss material (right). Source: NIST. Figure 6–4. WTC 1 exterior panel hit by the fuselage of the aircraft. Overlaying the locations of the specimens with photographs of the building exteriors following the aircraft impact (for perimeter columns and spandrels) and the extent-of-damage estimates (Section 6.8) (for core columns) enabled the identification of steel pieces near the impact zones. These included five specimens of exterior panels from WTC 1 and two specimens of core columns from each of the towers. 6.4.2 Mechanical and Physical Properties NIST determined the properties of many of the recovered pieces for comparison with the original purchase requirements, comparison with the quality of steel from the WTC construction era, and input to the structural models used in the Investigation. Structural steel literature and producers’ documents were used to establish a statistical basis for the variability expected in steel properties. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 89 The properties of the steel samples tested were consistent with the specifications called for in the steel contracts. In particular, the yield strengths of all samples of the floor trusses were higher than called for in the original specifications. This was in part because the truss steels were supplied as a higher grade than specified. Overall, approximately 87 percent of all perimeter and core column steel tested exceeded the required minimum yield strengths specified in design documents. Test data for the remaining samples were below specifications, but were within the expected variability and did not affect the safety of the towers on September 11, 2001. Furthermore, lower strength values measured by NIST could be expected due to (a) differences in test procedures from those used in the qualifying mill tests and (b) the damaged state of the samples. The values of other steel properties were similar to typical construction steels of the WTC construction era. The limited tests on bolts indicated that their strengths were greater than the specified minimum, and they were stronger than contemporaneous literature would suggest as typical. The tested welds performed as expected. NIST measured the stress-strain behavior at room temperature (for modeling baseline performance), high temperature strength (for modeling structural response to fire), and at high strain rates (for modeling the aircraft impact). Based on data from published sources, NIST estimated the thermal properties of the steels (specific heat, thermal conductivity, and coefficient of thermal expansion) and creep behavior for use in the structural modeling of the towers’ response to fire. 6.4.3 Damage Analysis NIST performed extensive analyses of the recovered steel specimens to determine their damage characteristics, failure modes, and (for those near the fire zones) fire-related degradation. In some cases, assessment of enhanced photographic and video images of the towers enabled distinguishing between damage that occurred prior to the collapse and damage that occurred as a result of the collapse. Because the only visual evidence was from the outside of the buildings, this differentiation was only possible for the perimeter panel sections. The observations of fracture and failure behavior, confirmed by an Investigation contractor, were also used to guide the modeling of the towers’ performance during impact and subsequent fires and to evaluate the model output. Source: NIST. Figure 6–5. WTC 1 exterior panel hit by the nose of the aircraft. Chapter 6 90 NIST NCSTAR 1, WTC Investigation For two of the five exterior panels from the impact zone of WTC 1, the general shape and appearance of the recovered pieces matched photographs taken just before the building collapse. Thus, NIST was able to attribute the observed damage to the aircraft impact. NIST also made determinations regarding the connections between structural steel elements: • There was no evidence to indicate that the joining method, weld materials, or welding procedures were inadequate. Fractures of the columns in areas away from a welded joint were the result of stretching and thinning. Perimeter columns hit by the plane tended to fracture along heat-affected zones adjacent to welds. • The failure mode of spandrel connections varied. At or above the impact zone, bolt hole tearout was more common. Below the impact zone, it was more common for the spandrels to be ripped from the panels. There was no evidence that fire exposure changed these failure modes. • The exterior column splices at the mechanical floors, which were welded in addition to being bolted, generally did not fail. The column splices at the other floors generally failed by bolt fracture. • The perimeter truss connectors (or seats) below the impact zone in WTC 1 were predominantly bent down or torn off completely. Above the impact zone, the seats were as likely to be bent upward as downward. Core seats could not be categorized since their asbuilt locations could not be determined. • Failure of core columns was a result of both splice connection failures and fracture of the columns themselves. Examination of photographs showed that 16 of the exterior panels recovered from WTC 1 were exposed to fire prior to the building collapse. None of the nine recovered panels from within the fire floors of WTC 2 were directly exposed to fire. NIST used two methods to estimate the maximum temperatures that the steel members had reached: • Observations of paint cracking due to thermal expansion. Of the more than 170 areas examined on 16 perimeter column panels, only three columns had evidence that the steel reached temperatures above 250 °C: east face, floor 98, inner web; east face, floor 92, inner web; and north face, floor 98, floor truss connector. Only two core column specimens had sufficient paint remaining to make such an analysis, and their temperatures did not reach 250 °C. NIST did not generalize these results, since the examined columns represented only 3 percent of the perimeter columns and 1 percent of the core columns from the fire floors. • Observations of the microstructure of the steel. High temperature excursions, such as due to a fire, can alter the basic structure of the steel and its mechanical properties. Using metallographic analysis, NIST determined that there was no evidence that any of the samples had reached temperatures above 600 ºC. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 91 These results were for a very small fraction of the steel in the impact and fire zones. Nonetheless, these analyses indicated some zones within WTC 1 where the computer simulations should not, and did not, predict highly elevated steel temperatures. 6.5 INFORMATION GAINED FROM OTHER WTC FIRES There had been numerous fires in the towers prior to September 11, 2001. From these, NIST learned what size fire WTC 1 and WTC 2 had withstood and how the tower occupants and the responders functioned in emergencies. While The Port Authority’s records of prior fires were lost in the collapses, FDNY provided reports on 342 fires that had occurred between 1970 and 2001. Most of these fires were small, and occupants extinguished many of them before FDNY arrival. Fortyseven of these fires activated one to three sprinklers and/or required a standpipe hose for suppression. Only two of the fires required the evacuation of hundreds of people. There were no injuries or loss of life in any of these fires, and the interruptions to operations within the towers were local. A major fire occurred in WTC 1 on February 13, 1975, before the installation of the sprinkler system. A furniture fire started in an executive office in the north end of an 11th floor office suite in the southeast corner of the building. The fire spread south and west along corridors and entered a file room. The fire flashed over, broke seven windows, and spread to adjacent offices north and south. The air conditioning system turned on, pulling air into the return air ducts. Telephone cables in the vertical shafts were ignited, destroying the fire-retarded wood paneling on the closet doors. The fire emerged on the 12th and 13th floors, but there was little nearby that was combustible. The fire also extended vertically from the 9th to the 19th floors within the telephone closet. Eventually the fire was confined to 9,000 ft2 of one floor, about one-fourth of the total floor area. The trusses and columns in this area had been sprayed with BLAZE-SHIELD D insulation to a specified ½ in. thickness. Four trusses were slightly distorted, but the structure was not threatened. Only one major fire incident resulted in a whole-building evacuation. At 12:18 p.m. on February 26, 1993, terrorists exploded a bomb in the second basement underground parking garage in the WTC complex. The blast immediately killed six people and caused an estimated $300 million in damage. An intense fire followed and, although the flames were confined to the subterranean levels, the smoke spread into four of the seven buildings in the WTC complex. Most of the estimated 150,000 occupants evacuated the buildings, including approximately 40,000 from the affected towers. In all, 1,042 people were injured in the incident, including 15 who received blast-related injuries. The evacuation of the towers took over 4 hours. The incident response involved more than 700 firefighters (approximately 45 percent of FDNY’s on-duty personnel at the time). In addition, there was a fire on the 104th floor of WTC 1 on September 11, 2001, that apparently did not contribute to the eventual collapse, yet was quite severe. At 10:01 a.m., flames were first observed on the west face, and by 10:07 a.m., intense flames were emanating from several windows in the southern third of that face. The fire raged until the building collapsed at 10:28 a.m. Thus, the tower structure was able to withstand a sizable fire for about 20 min, presumably with the ceiling tile system heavily damaged and the truss system exposed to the flames. The 104th floor was well above the aircraft impact zone, so there should have been little damage to the sprayed fire-resistive material, which was the same (Table 5–3) as Chapter 6 92 NIST NCSTAR 1, WTC Investigation on the floors where the fires led to the onset of the collapse. The photographic evidence showed no signs of column bowing or a floor collapse. 6.6 THE BUILDING STRUCTURAL MODELS 6.6.1 Computer Simulation Software Structural modeling of each tower was required in order to: • Establish the capability of the building, as designed, to support the gravity loads and to resist wind forces; • Simulate the effects of the aircraft impacts; and • Reconstruct the mechanics of the aircraft impact damage, fire-induced heating, and the progression of local failures that led to the building collapse. The varied demands made different models necessary, and different software packages were used for each of these three functions. The reason for the choice in each case is presented in the next three sections of the report. 6.6.2 The Reference Models Under contract to NIST, Leslie E. Robertson Associates (LERA) constructed a global reference model of each tower using the SAP2000, version 8, software. SAP2000 is a software package for performing finite element calculations for the analysis and design of building structures. These global, three-dimensional models encompassed the 110 stories above grade and the six subterranean levels. The models included primary structural components in the towers, resulting in tens of thousands of computational elements. The data for these elements came from the original structural drawing books for the towers. These had been updated through the completion of the buildings and also included most of the subsequent, significant alterations by both tenants and The Port Authority. LERA also developed reference models of a truss-framed floor, typical of those in the tenant spaces of the impact and fire regions of the buildings, and of a beam-framed floor, typical of the mechanical floors. LERA’s work was reviewed by independent experts in light of the firm’s earlier involvement in the WTC design. It was that earlier work, in fact, that made LERA the only source that had the detailed knowledge of the design, construction, and intended behavior of the towers over their entire 38-year life span. The accuracy of the four models was checked in two ways: • The two global models were checked by Skidmore, Owings & Merrill (SOM), also under contract to NIST, and by NIST staff. This entailed ensuring consistency of the models with the design documents, and testing the models, for example, to ensure that the response of the models to gravity and wind loads was as intended and that the calculated stresses and deformations under these loads were reasonable. • The global model of WTC 1 was used to calculate the natural vibration periods of the tower. These values were then compared to measurements from the tower on eight dates of winds Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 93 ranging from 11.5 mph to 41 mph blowing from at least four different directions. As shown in Table 6–3, the N-S and E-W values agreed within 5 percent and the torsion values agreed within 6 percent, both within the combined uncertainty in the measurements and calculations. • SOM and NIST staff also checked the two floor models for accuracy. These reviews involved comparison with simple hand calculations of estimated deflections and member stresses for a simply supported composite truss and beam under gravity loading. For the composite truss sections, the steel stress results were within 4 percent of those calculated by SAP2000 for the long-span truss and within 3 percent for the short-span truss. Deflections for the beams and trusses matched hand calculations to within 5 percent to 15 percent. These differences were within the combined uncertainty of the methods. Table 6–3. Measured and calculated natural vibration periods (s) for WTC 1. Direction of Motion N-S E-W Torsion Average of Measured Data 11.4 10.6 4.9 Original Predicted Values 11.9 10.4 – Reference Global Model Predictions 11.4 10.7 5.2 The few discrepancies between the developed models and the original design documents, as well as the areas identified by NIST and SOM as needing modification, were corrected by LERA and approved by NIST. The models then served as references for more detailed models for aircraft impact damage analysis and for thermal-structural response and collapse initiation analysis. NIST also used these global reference models to establish the baseline performance of the towers under gravity and wind loads. The two key performance measures calculated were the demand-to-capacity ratio (DCR) and the drift. • Demand is defined as the combined effects of the dead, live, and wind loads imposed on a structural component, e.g., a column. Capacity is the permissible strength for that component. Normal design aims at ensuring that DCR values for all components be 1.0 or lower. A value of DCR greater than 1.0 does not imply failure since designs inherently include a margin of safety. • Drift is the extent of sway of the building under a lateral wind. Excessive deflection can cause cracking of partitions and cladding, and, in severe cases, building instability that could affect safety. Using SAP2000, NIST found that, under original WTC design loads, a small fraction of the structural components had DCR values greater than 1.0. (Most DCR values of that small fraction were less than 1.4, with a few as high as 1.6.) For the perimeter columns, DCR values greater than 1.0 were mainly near the corners, on floors near the hat truss, and below the 9th floor. For the core columns, these members were on the 600 line between floors 80 and 106 and at core perimeter columns 901 and 908 for much of their height. (See Figure 1–5 for the column numbers.) One possible explanation to the cause of DCRs in excess of 1.0 may lie in the computer-based structural analysis and software techniques employed for this Chapter 6 94 NIST NCSTAR 1, WTC Investigation baseline performance study in comparison with the relatively rudimentary computational tools used in the original design nearly 40 years ago. As part of its wind analysis, NIST calculated the drift at the top of the towers to be about 5 ft in a nearly 100 mph wind—the wind load used in the original design. Common practice was, and is, to design for substantially smaller deflections; but drift was not, and still is not, a design factor prescribed in building codes. The estimation of wind-induced loads on the towers emerged as a problem. Two sets of wind tunnel tests and analyses were conducted in 2002 by independent laboratories as part of insurance litigation unrelated to the NIST Investigation. The estimated loads differed by as much as 40 percent. NIST analysis found that the two studies used different approaches in their estimations. This difference highlighted limitations in the current state of practice in wind engineering for tall buildings and the need for standards in the field of wind tunnel testing and wind effects estimation. 6.6.3 Building Structural Models for Aircraft Impact Analysis Ideally, the Investigation would have used the reference global models of the towers as the “targets” for the aircraft. However, this was not possible. The impact simulations required inclusion of both a far higher level of detail of the building components and also the highly nonlinear behavior of the tower and aircraft materials, and the larger model size could not be accommodated by the SAP2000 program. There were also physical phenomena for which algorithms were not available in this software. Another finite element package, LS-DYNA, satisfied these requirements and was used for the impact simulations. Early in the effort, it became clear to both NIST and to ARA, Inc., the NIST contractor that performed the aircraft impact simulations, that the model had to “fit” on a state-of-the-art computer cluster and to run within weeks rather than months. To minimize the model size while keeping sufficient fidelity in the impact zone to capture the building deformations and damage distributions, various tower components were depicted with different meshes (different levels of refinement). For example, tower components in the path of the impact and debris field were represented with a fine mesh (higher resolution) to capture the local impact damage and failure, while components outside the impact zone were depicted more coarsely, simply to capture their structural stiffness and inertial properties. The model of WTC 1 included floors 92 through 100; the model of WTC 2 extended from floor 77 through floor 85. The combined tower and aircraft model of more than two million elements, at time steps of just under a microsecond, took approximately two weeks of computer time on a 12-noded computer cluster to capture the needed details of the fraction of a second it took for the aircraft and its fragments to come to rest inside the building. The structural models, partially shown in Figures 6–6 through 6–9, included: • Core columns and spliced column connections; • Floor slabs and beams within the core; • Exterior columns and spandrels, including the bolted connections between the exterior panels in the refined mesh areas; and • Tenant space floors, composed of the combined floor slab, metal decking, and steel trusses. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 95 They also included representations of the interior partitions and workstations. The live load mass was distributed between the partitions and cubicle workstations. Figure 6–6. Structural model of the 96th floor of WTC 1. Figure 6–7. Model of the 96th floor of WTC 1, including interior contents and partitions. Chapter 6 96 NIST NCSTAR 1, WTC Investigation Figure 6–8. Multi-floor global model of WTC 1, viewed from the north. Figure 6–9. Multi-floor global model of WTC 2, viewed from the south. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 97 Within these models, it was critical that the structural and furnishing materials behaved correctly when impacted by the aircraft or debris. For each grade of steel, the stress-strain behavior and the yield strength were represented using data from tests conducted at NIST. The weakening and failure of the concrete floor slabs were simulated using material models embedded in LS-DYNA. The primary influence of the nonstructural components on the impact behavior was their inertial contribution. Values for the resistance to rupture of gypsum panels and the fracture of the wood products in the workstations were obtained from published studies. In order to complete the global models of the two towers, models of sections of the buildings were developed. As shown in Section 6.8.1, these submodels enabled efficient identification of the principal features of the interaction of the buildings with specific aircraft components. 6.6.4 Building Structural Models for Structural Response to Impact Damage and Fire and Collapse Initiation Analysis The structural response and collapse analysis of the towers was conducted in three phases by NIST and Simpson Gumpertz & Heger, Inc. (SGH), under contract to NIST. The first phase included component and detailed subsystem models of the floor and exterior wall panels. The objectives of Phase 1 were to gain understanding into the response of the structure under stress and elevated temperatures, identify dominant modes of failure, and develop reductions in modeling complexity that could be applied in Phase 2. The second phase analyzed major subsystem models (the core framing, a single exterior wall, and full tenant floors) to provide insight into their behavior within the WTC global system. The third phase was the analysis of global models of WTC 1 and WTC 2 that took advantage of the knowledge gained from the more detailed and subsystem models. A separate global analysis of each tower helped determine the relative roles of impact damage and fires with respect to structural stability and sequential failures of components and subsystems and was used to determine the probable collapse initiation sequence. Phase 1: Component and Detailed Subsystem Analyses Floor Subsystem Analysis The floors played an important role in the structural response of the WTC towers to the aircraft impact and ensuing fires. Prior to the development of a floor subsystem model, three component analyses were conducted, as follows: • Truss seats. Figure 6–10 shows how an exterior seat connection was represented in the finite element structural model. The component analysis determined that failure could occur at the bolted connection between the bearing angle and the seat angle, and the truss could slip off the seat. Truss seat connection failure from vertical loads was found to be unlikely, since the needed increase in vertical load was unreasonable for temperatures near 600 °C to 700 °C. • Knuckles. The “knuckle” was formed by the extension of the truss diagonals into the concrete slab and provided for composite action of the steel truss and concrete slab. A model was developed to predict the knuckle performance when the truss and concrete slab acted compositely. Chapter 6 98 NIST NCSTAR 1, WTC Investigation Figure 6–10. Finite element model of an exterior truss seat. • Single composite truss and concrete slab section. A floor section was modeled to investigate failure modes and sequences of failures under combined gravity and thermal loads. The floor section was heated to 700 °C (with a linear thermal gradient through the slab thickness from 700 °C to 300 °C at the top surface of the slab) over a period of 30 min. Initially the thermal expansion of the floor pushed the columns outward, but with increased temperatures, the floor sagged and the columns were pulled inward. Knuckle failure was found to occur mainly at the ends of the trusses and had little effect on the deflection of the floor system. Figure 6–11 shows that the diagonals at the core (right) end of the truss buckled and caused an increase in the floor system deflection, ultimately reaching approximately 42 in. Two possible failure modes were identified for the floor-truss section: sagging of the floor and loss of truss seat support. Figure 6–11. Vertical displacement at 700 oC. Stand-off Plates Seat angle 5/8 in. Diameter bolt Truss top chord Gusset plate Strut Bearing angle MN MX -42.11 -37.357 -32.603 -27.849 -23.095 -18.342 -13.588 -8.834 -4.081 .673211 Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 99 A finite element model of the full 96th floor of WTC 1 was translated from the SAP2000 reference models into ANSYS 8.1 for detailed structural evaluation (Figure 6–12)14. The two models generated comparable predictions of the behavior under dead or gravity loads. Figure 6–12. ANSYS model of 96th floor of WTC 1. The model was used to evaluate structural response under dead and live loads and elevated temperatures, identify failure modes and associated temperatures and times to failure, and identify reductions in modeling complexity for global models and analyses. The structural response included thermal expansion of steel and concrete members, temperature-dependent properties of steel and concrete that affected material stiffness and strength, and bowing or buckling of structural members. The deformation and failure modes identified were floor sagging between truss supports, floor sagging resulting from failure of a seat at either end of the truss, and failure of the floor subsystem truss supports. Exterior Wall Subsystem The exterior walls played an important role in each tower’s reaction to the aircraft impact and the ensuing fires. Photographic and video evidence showed inward bowing of large sections of the exterior walls of both towers just prior to the time of collapse. A finite element model of a wall section was developed in ANSYS for evaluation of structural response under dead and live loads and elevated structural temperatures, determination of loads that would have caused buckling, and identification of reductions of modeling complexity for global models and analyses. The modeled unit consisted of seven full column/spandrel panels (described in Section 1.2.2) and portions of four other panels. The model was validated against the reference model developed by LERA (Section 6.6.2) by comparing the stiffness for a variety of loading conditions. 14 ANSYS allowed including the temperature-varying properties of the structural materials, a necessary feature not available in SAP 2000. Chapter 6 100 NIST NCSTAR 1, WTC Investigation The model was subjected to several gravity loads and heating conditions, several combinations of disconnected floors, and pull-in from sagging floors until the point of instability. In one case, the simulation assumed three disconnected floors, and the top of the wall subsystem was subjected to “pushdown” analysis, i.e., an increasing force to provide a measure of remaining capacity in the wall section. The model captured possible failure modes due large lateral deformations, column buckling from loss of support at floor truss seats and diagonal straps, failure of column splice bolts, and failure of spandrel splice bolts or tearing of spandrel or splice plates at bolt holes. The model also showed: • Large deformations and buckling of the spandrels could be expected at high temperatures, but they did not significantly affect the stability of the exterior columns and generally did not need to be precisely modeled in the tower models. • Partial separations of the spandrel splices could be expected at elevated temperatures, but they also did not significantly affect the stability of the exterior columns. • Exterior column splices could be expected to fail at elevated temperatures and needed to be accurately modeled. • Plastic buckling of columns, with an ensuing rapid reduction of load, was to be expected at extremely high loads and at low temperatures. • The sagging of trusses resulted in approximately 14 kip of inward pull per truss seat on the attached perimeter column. Phase 2: Major Subsystem Analyses Building on these results, ANSYS models were constructed of each of the three major structural subsystems (core framing, a single exterior wall, and full composite floors) for each of the towers. The models were subjected to the impact damage and elevated temperatures from the fire dynamics and thermal analyses to be described later in this chapter. Core Framing The two tower models included the core columns, the floor beams, and the concrete slabs from the impact and fire zones to the highest floor below the hat truss structure: from the 89th floor to the 106th floor for WTC 1 and from the 73rd floor to the 106th floor for WTC 2. Within these floors, aircraft-damaged structural components were removed. Below the lowest floors, springs were used to represent the stiffness of the columns. In the models, the properties of the steel varied with temperature, as described in Section 5.5.2. This allowed for realistic structural changes to occur, such as thermal expansion, buckling, and creep. The forces applied to the models included gravity loads applied at each floor, post-impact column forces applied at the top of the model at the 106th floor, and temperature histories applied at 10 min intervals with linear ramping between time intervals. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 101 Under these conditions, the investigators first determined the stability of the core under impact conditions and then its response under thermal loads: • In WTC 1, the core was stable under Case A (base case) impact damage, but the model could not reach a stable solution under Case B (more severe) impact damage. • The WTC 1 core became unstable under Case A impact damage and Case B thermal loads as it leaned to the northwest (due to insulation dislodged from the northwest corner column); the core model was restrained in horizontal directions at floors above the impact zone half way through the thermal loads. • The WTC 2 core was stabilized for Case C (base case) by providing horizontal restraint at all floors representing the restraint provided by the perimeter wall to resist leaning to the southeast. A converged, stable solution was not found for Case D (more severe) impact damage. • The WTC 2 stabilized core model for Case C impact damage was subjected to Case D thermal loads. Following each simulation, a pushdown analysis was performed to determine the core’s reserve capacity. The analysis results showed that: • The WTC 1 isolated core structure was most weakened from thermal effects at the center of the south side of the core. (Smaller displacements occurred in the global model due to the constraints of the hat truss and floors.) • The WTC 2 isolated core was most weakened from thermal effects at the southeast corner and along the east side of the core. (Larger displacements occurred in the global model as the isolated core model had lateral restraints imposed that were somewhat stiffer than in the global model.) Composite Floor The composite floor model was used to determine the response of a full floor to Case A and B thermal loads for WTC 1 floors and Case C and D thermal loads for WTC 2 floors. It included: • A reduced complexity truss model, validated against the single truss model results. • Primary and bridging trusses, deck support angles, spandrels, core floor beams, and a concrete floor slab. • Fire-generated local temperature histories applied at 10 min intervals with linear ramping between time intervals. • Temperature-dependent concrete and steel properties, except for creep behavior. Chapter 6 102 NIST NCSTAR 1, WTC Investigation • Restraint provided by exterior and core columns, which extended one floor above and below the modeled floor. The potential for large deflections and buckling of individual structural members and the floor system were included. The results showed that: • At lower elevated temperatures (approximately 100 °C to 400 °C), the floors thermally expanded and displaced the exterior columns outward by a few inches; horizontal displacement of the core columns was insignificant. None of the floors buckled as they thermally expanded, even with the exterior columns restrained so that no horizontal movement was allowed at the floors above and below the heated floor, which maximized column resistance to floor expansion. Even with this level of column restraint, the exterior columns did not develop a sufficient reaction force (push inward to resist the expansion outward) to buckle any of the floors. • At higher elevated temperatures (above 400 °C), the floors began to sag as the floors’ stiffness and strength were reduced with increasing temperature, and the difference in thermal expansion between the trusses and the concrete slab became larger. As the floor sagging increased, the outward displacement of the exterior columns was overcome, and the floors exerted an inward pull force on the exterior columns. • Floor sagging was caused primarily by either buckling of truss web diagonals or disconnection of truss seats at the exterior wall or the core perimeter. Except for the truss seat failures near the southeast corner of the core in WTC 2 following the aircraft impact, web buckling or truss seat failure was caused primarily by elevated temperatures of the structural components. • Analysis results from the detailed truss model found that the floors began to exert inward pull forces when floor sagging exceeded approximately 25 in. for the 60 ft floor span. • Sagging at the floor edge was due to loss of vertical support at the truss seats. The loss of vertical support was caused in most cases by the reduction in vertical shear capacity of the truss seats due to elevated steel temperatures. • Case B impact damage and thermal loads for WTC 1 floors resulted in floor sagging on the south side of the tower over floors that reasonably matched the location of inward bowing observed on the south face. Case A impact damage and thermal loads did not result in sagging on the south side of the floors. • Cases C and D impact damage and thermal loads for WTC 2 both resulted in floor sagging on the east side of the tower over floors that reasonably matched the location of inward bowing observed on the east face. However, Case D provided a better match. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 103 Exterior Wall Exterior wall models were developed for the south face of WTC 1 (floors 89 to 106) and the east face of WTC 2 (floors 73 to 90). These sections were selected based on photographic evidence of column bowing. Many of the simulation conditions were similar to those for the isolated core modeling: removal of aircraft-damaged structural components, representation of lower floors by springs, temperature-varying steel properties, gravity loads applied at each floor, post-impact column forces applied at the 106th floor, and temperature histories applied at 10 min intervals with linear ramping between time intervals. The analysis results showed that: • Inward pull forces were required to produce inward bowing consistent with the displacements measured from photographs. The inward pull was caused by sagging of the floors. Heating of the inside faces of the exterior columns also contributed to inward bowing. • Exterior wall sections bowed outward in a pushdown analysis when several consecutive floors were disconnected, the interior face of the columns was heated, and column gravity loads increased (e.g., due to load redistribution from the core and hat truss). At lower temperatures, thermal expansion of the inside face was insufficient to result in inward bowing of the entire exterior column. At higher temperatures, outward bowing resulted from the combined effects of reduced steel strength on the heated inside face, which shortened first under column gravity loads, and the lack of lateral restraint from the floors. • The observed inward bowing of the exterior wall indicated that most of the floor connections must have been intact to cause the observed bowing. • The extent of floor sagging observed at each floor was greater than that predicted by the full floor models. The estimates of the extent of sagging at each floor was governed by the combined effects of insulation damage and fire; insulation damage estimates were limited to areas subject to direct debris impact. Other sources of floor and insulation damage from the aircraft impact and fires (e.g., insulation damage due to shock and subsequent vibrations as a result of aircraft impact or concrete slab cracking and spalling as a result of thermal effects) were not included in the floor models. • Case B impact damage and thermal loads for the WTC 1 south wall, combined with pull-in forces from floor sagging, resulted in an inward bowing of the south face that reasonably matched the observed bowing. The lack of floor sagging for the Case A impact damage and thermal loads resulted in no inward bowing for the south face. • Cases D impact damage and thermal loads for the WTC 2 east wall, combined with pull-in forces from floor sagging, resulted in an inward bowing of the east face that reasonably matched the observed bowing. Chapter 6 104 NIST NCSTAR 1, WTC Investigation Phase 3: Global Modeling The global models were used for the two final simulations and provided complete analysis of results and insight into the subsystem interactions leading to the probable collapse sequence. Based upon the results of the major subsystem analyses, impact damage and thermal loads for Cases B and D were used for WTC 1 and WTC 2, respectively. The models extended from floor 91 for WTC 1 and floor 77 for WTC 2 to the roof level in both towers. Although the renditions of the structural components had been reduced in complexity while maintaining essential nonlinear behaviors, based on the findings from the component and subsystem modeling, the global models included many of the features of the subsystem models: • Removal of aircraft-damaged structural components. • Application of gravity loads following removal of aircraft damaged components and prior to thermal loading. • Temperature-dependent concrete and steel properties. • Creep strains for column components. • Representation of lower floors by springs. • Local temperature histories applied at 10 min intervals with linear ramping between time intervals. There were several adjustments to the models based on the findings from the subsystem modeling: • Removal of thermal expansion from the spandrels and equivalent slabs in the tenant area to avoid local buckling that affected convergence but had little influence on global collapse initiation. • Representing the WTC 2 structure above the 86th floor as a single “super-element” to reduce model complexity. The floors above the impact zone had only exhibited linear behavior in the previous analyses. This modification assumed linear behavior of the hat truss, which was checked as part of the review of analysis results. • Representation of the lower part of the tower (starting several floors below the impact damage) as a super-element. This prevented the use of construction sequence in applying gravity loads to the model (where loads are applied in stages to simulate the construction of the building). The lack of construction sequence increased the forces on the exterior columns slightly, and decreased those on the core columns slightly. The inclusions of creep for column components was necessary for the accuracy of the models, but its addition also greatly increased the computation time. As a result, the simulations of WTC 1 took 22 days and those of WTC 2 took 14 days on a high-end computer workstation. The results of these simulations are presented in Section 6.14. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 105 6.7 THE AIRCRAFT STRUCTURAL MODEL Due to their similarity, the two Boeing 767-200ER aircraft were represented by a single, finite element model, two views of which are shown in Figure 6–13. The model consisted of about 800,000 elements. The typical element dimensions were between 1 in. and 2 in. for small components, such as spar or rib flanges, and 3 in. to 4 in. for large parts such as the wing or fuselage skin. Structural data on which to base the model were collected from the open literature, electronic surface models and CAD drawings, an inspection of a 767-300ER, Pratt and Whitney Engine Reference Manuals, American Airlines and United Airlines, and the Boeing Company website. Figure 6–13. Finite element model of the Boeing 767-200ER. Chapter 6 106 NIST NCSTAR 1, WTC Investigation More detailed models of subsections of the aircraft were constructed for the component level analyses described below. Special emphasis was placed on modeling the aircraft engines, due to their potential to produce significant damage to the tower components. The element dimensions were generally between 1 in. and 2 in., although even smaller dimensions were required to capture some details of the engine geometry. The various components of the resulting engine model are shown in Figure 6–14. Fuel was distributed in the wing as shown in Figure 6–15 based on a detailed analysis of the fuel distribution at the time of impact. Figure 6–14. Pratt & Whitney PW4000 turbofan engine model. Figure 6–15. Boeing 767-200ER showing the jet fuel distribution at time of impact. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 107 6.8 AIRCRAFT IMPACT MODELING 6.8.1 Component Level Analyses Prior to conducting the full simulations of the aircraft impacting the towers, a series of smaller scale simulations was performed to develop understanding of how the aircraft and tower components fragmented and to develop the simulation techniques required for the final computations. These simulations began with finely meshed models of key components of the tower and aircraft structures and progressed to relatively coarsely meshed representations that could be used in the global models. Examples of these component-level analyses included impact of a segment of an aircraft wing with an exterior column, impact of an aircraft engine with exterior wall panels, and impact of a fuel-filled wing segment with exterior wall panels. Figure 6–16 shows two frames from the last of these analyses, with the wing segment entering from the left, being fragmented as it penetrates the exterior columns, and spraying jet fuel downstream. t = 0.0 s t = 0.04 s Figure 6–16. Calculated impact on an exterior wall by a fuel-laden wing section. The Investigation Team gained valuable knowledge from these component impact analyses, for example: • Moving at 500 mph, an engine broke any exterior column it hit. If the engine missed the floor slab, the majority of the engine core remained intact and had enough residual momentum to sever a core column upon direct impact. • The impact of the inner half of an empty wing significantly damaged exterior columns but did not result in their complete failure. Impact of the same wing section, but filled with fuel, did result in failure of the exterior columns. Chapter 6 108 NIST NCSTAR 1, WTC Investigation 6.8.2 Subassembly Impact Analyses Next, a series of simulations were performed for intermediate-sized sections of a tower. These subassembly analyses investigated different modeling techniques and associated model sizes, run times, numerical stability, and impact response. Six simulations were performed of an aircraft engine impacting a subassembly that included structural components from the impact zone on the north face of WTC 1, exterior panels, truss floor structures, core framing, and interior contents (workstations). One response of the structure to the engine impact is shown in Figure 6–17. Figure 6–17. Response of a tower subassembly model to engine impact. Typical knowledge gained from these simulations were: • The mass of the concrete floor slab and nonstructural contents had a greater effect on the engine deceleration and subsequent damage than did the concrete strength. • Variation of the failure criteria of the welds in the exterior columns did not result in any noticeable difference in the damage pattern or the energy absorbed by the exterior panels. 6.8.3 Aircraft Impact Conditions From the NIST photographic and video collection, the speed and orientation of the aircraft (Table 6–4) were estimated at the time of impact. The geometry of the wings, different in flight from that at rest, was estimated from the impact pattern in the photographs and the damage documented on the exterior panels by NIST. United Airlines and American Airlines provided information on the contents of the aircraft, the mass of jet fuel, and the location of the fuel within the wing tanks. Table 6–4. Summary of aircraft impact conditions. Condition AA 11 (WTC 1) UAL 175 (WTC 2) Impact Speed (mph) 443 ± 30 542 ± 24 Vertical Approach Angle 10.6° ± 3° below horizontal (heading downward) 6° ± 2° below horizontal (heading downward) Lateral Approach Angle 180.3° ± 4° clockwise from Plan Northa 13° ± 2° clockwise from Plan Northa Roll Angle (left wing downward) 25° ± 2° 38° ± 2° a. Plan North is approximately 29 degrees clockwise from True North. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 109 6.8.4 Global Impact Analysis From the component and subassembly simulations, it became apparent that each computation of the full tower and aircraft would take weeks. Furthermore, the magnitude and location of damage to the tower structure were sensitive to a large number of initial conditions, to assumptions in the representation of the collision physics, and to any approximations in the numerical methods used to solve the physics equations. Thus, it was necessary to choose a manageable list of the factors that most influenced the outcome of a simulation. Careful screening was conducted at the component and subassembly levels, leading to identification of the following prime factors: • Impact speed, • Vertical approach angle of the aircraft, • Lateral approach angle of the aircraft, • Total aircraft weight, • Aircraft materials failure strain, • Tower materials failure strain, and • Building contents weight and strength. Guided by these results and several preliminary global simulations, two global simulations were selected for inclusion in the four-step simulation of the response of each tower, as described in Section 6.1. The conditions for these four runs are shown in Table 6–5. The computers simulate the aircraft flying into the tower, calculated the fragments that were formed from both the aircraft and the building itself, and then followed the fragments. The jet fuel, atomized upon impact into about 60,000 “blobs” averaging one pound, dispersed within and outside the building. Each simulation continued until the debris motion had reduced to a level that was not expected to produce any significant further impact damage. Table 6–5. Input parameters for global impact analyses. WTC 1 WTC 2 Analysis Parameters Case A Case B Case C Case D Impact Speed 443 mph 472 mph 542 mph 570 mph Vertical Approach Flight Parameters Angle 10.6° 7.6° 6.0° 5.0° Lateral Approach Angle 180.0° 180.0° 13.0° 13.0° Weight 100 % 105 % 100 % 105 % Aircraft Parameters Failure Strain 100 % 125 % 100 % 115 % Failure Strain 100 % 80 % 100 % 90 % Live Load Weighta Tower Parameters 25 % 20 % 25 % 20 % Contents Strength 100 % 100 % 100 % 80 % a. Live load weight expressed as a percentage of the design live load. Chapter 6 110 NIST NCSTAR 1, WTC Investigation These simulations each took about 2 weeks on a 12-node computer cluster. Figure 6–18 shows six frames from the animation of one such simulation. (a) Time=0.00 s (b) Time=0.10 s (c) Time=0.20 s Figure 6–18. Side view of simulated aircraft impact into WTC 1, Case B. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 111 (d) Time=0.30 s (e) Time=0.40 s (f) Time=0.50 s Figure 6–18. Side view of simulated aircraft impact into WTC 1, Case B (Cont.) Chapter 6 112 NIST NCSTAR 1, WTC Investigation 6.9 AIRCRAFT IMPACT DAMAGE ESTIMATES 6.9.1 Structural and Contents Damage Each of the four global simulations generated information about the state of the structural components following the impact of the aircraft. The four degrees of column damage are defined as follows and shown graphically in Figure 6–19. The unstrained areas are blue and the highly strained areas are red. • Lightly damaged column: column impacted, but without significant structural deformation; • Moderately damaged column: visible local distortion, but no deformation of the column centerline; • Heavily damaged column: Permanent deflection of the column centerline; and • Failed column: Column severed. (a) Light (b) Moderate (c) Heavy (d) Severed Figure 6–19. Column damage levels. Figure 6–20 shows the calculated damage to a floor slab. Figure 6–21 shows the response of the furnishings and the jet fuel to the impact. Figures 6–22 through 6–25 show the combined damage for all floors for the four global simulations. The latter proved useful in visualizing the extent of aircraft impact in one graphic image. Figure 6–20. Case B damage to the slab of floor 96 of WTC 1. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 113 (a) Pre-impact configuration (b) Calculated impact response (c) Calculated impact response (fuel removed) Figure 6–21. Case B simulation of response of contents of 96th floor of WTC 1. Chapter 6 114 NIST NCSTAR 1, WTC Investigation Figure 6–22. Combined structural damage to the floors and columns of WTC 1, Case A. Figure 6–23. Combined structural damage to the floors and columns of WTC 1, Case B. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 115 Figure 6–24. Combined structural damage to the floors and columns of WTC 2, Case C. Figure 6–25. Combined structural damage to the floors and columns of WTC 2, Case D. Chapter 6 116 NIST NCSTAR 1, WTC Investigation 6.9.2 Validity of Impact Simulations Assessment of the aircraft impact simulations of exterior damage to the towers involved comparing the predicted perimeter wall damage near the impact zone with post-impact photographs of the walls. Figure 6–26 shows a photograph of the north face of WTC 1 after impact and the results of the Case A simulation. The calculated silhouettes capture both the position and shape of the actual damage. Figures 6–27 and 6–28 depict more detailed comparisons between the observed and calculated damage. The aircraft hole is shown in white. The colored dots characterize the mode in which the steel or connection failed (e.g., severed bolt, ripped weld) and the magnitude of the deformation of the steel: • Green: proper match of failure mode and magnitude • Yellow: proper match in the failure mode, but not the magnitude • Red: neither the failure mode nor the magnitude matched • Black: the observed damage was obscured by smoke, fire, or other factors The predominance of green dots and the scarcity of red dots indicate that the overall agreement with the observed damage was very good. The agreement for Cases B and D was slightly lower. Assessment of the accuracy of the predictions of damage inside the buildings was more difficult, as NIST could not locate any interior photographs near the impact zones. Three comparisons were made: • The Case A simulation for WTC 1 predicted that the walls of all three stairwells would have been collapsed. This agreed with the observations of the building occupants. The Case A simulation for WTC 2 showed that the walls of stairwell B would have been damaged, but that Stairwell A would have been unaffected. Stairwell C was not included in the WTC 2 model, but is adjacent to where damage occurred. The building occupants reported that Stairwells B and C were impassable; Stairwell A was damaged but passable. • The two simulations of WTC 2 showed accumulations of furnishings and debris in the northeast corner of the 80th and 81st floors. These piles were observed in photographs and videos. • Two pieces of landing gear penetrated WTC 1 and landed to the south of the tower. The Case B prediction showed landing gear penetrating the building core, but stopping before reaching the south exterior wall. For WTC 2, a landing gear fragment and the starboard engine penetrated the building and landed to the south. The Case D prediction correctly showed the main landing gear emerging from the northeast corner of WTC 2. However, Case D showed that engine not quite penetrating the building. Minor modifications to the model (all within the uncertainty of the input data) would have resulted in the engine passing through the north exterior wall of the tower. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 117 (a) Observed Damage = (b) Calculated damage Figure 6–26. Observed and Case A calculated damage to the north face of WTC 1. Chapter 6 118 NIST NCSTAR 1, WTC Investigation Figure 6–27. Schematic of observed damage (top) and Case A calculated damage (lower) to the north face of WTC 1. Figure 6–28. Schematic of observed damage (above) and Case C calculated damage (right) to the south face of WTC 2. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 119 Not all of the observables were closely matched by the simulations due to the uncertainties in exact impact conditions, the imperfect knowledge of the interior tower contents, the chaotic behavior of the aircraft breakup and subsequent debris motion, and the limitations of the models. In general, however, the results of the simulations matched these observables sufficiently well that the Investigation Team could rely on the predicted trends. Simulations of the damage to the core columns had been performed previously by staff of Weidlinger Associates, Inc. (WAI) and the Massachusetts Institute of Technology (MIT). Each developed a range of numbers of failed and damaged columns, as did NIST. The range of the MIT results straddled the NIST results. WAI’s analysis resulted in more failed and damaged columns, with WTC 2 being unstable immediately following impact. 6.9.3 Damage to Thermal Insulation The dislodgement of thermal insulation from structural members could have occurred as a result of (a) direct impact by debris and (b) inertial forces due to vibration of structural members as a result of the aircraft impact. The debris from the aircraft impact included the fragments that were formed from both the aircraft (including the contents and fuel) and the building (structural members, walls, and furnishings). In interpreting the output of the aircraft impact simulations, NIST assumed that the debris impact dislodged insulation if the debris force was strong enough to break a gypsum board partition immediately in front of the structural component. Experiments at NIST confirmed that an array of 0.3 in. diameter pellets traveling at approximately 350 mph stripped the insulation from steel bars like those used in the WTC trusses. Determining the adherence of SFRM outside the debris zones was more difficult. There was photographic evidence that some fraction of the SFRM was dislodged from perimeter columns not directly impacted by debris. NIST developed a simple model to estimate the range of accelerations that might dislodge the SFRM from the structural steel components. As the SFRM in the towers was being upgraded with BLAZESHIELD II in the 1990s, The Port Authority had measured the insulation bond strength (force required to pull the insulation from the steel). The model used these data as input to some basic physics equations. The resulting ranges of accelerations depended on the geometry of the coated steel component and the SFRM thickness, density and bond strength. For a flat surface (as on the surface of a column), the range was from 20g to 530g, where g is the gravitational acceleration. For an encased bar (such as used in the WTC trusses), the range was from 40g to 730g. NIST estimated accelerations from the aircraft impacts of approximately 100g. In determining the extent of insulation damage in each tower, NIST only assumed damage where dislodgement criteria could be established and supported through observations or analysis. Thus, NIST made the conservative assumption that insulation was removed only where direct debris impact occurred and did not include the possibility of insulation damage or dislodgement from structural vibration. This assumption produced a lower bound on the bared steel surface area, thereby making it more difficult to heat the steel to the point of failure. Chapter 6 120 NIST NCSTAR 1, WTC Investigation An intact ceiling tile system could have provided the floor trusses with approximately 10 min to 15 min of thermal protection from ceiling air temperatures near 1,000 °C. These temperatures would quickly heat steel without thermal insulation to temperatures for reduction of the strength of structural steels. 6.9.4 Damage to Ceiling System The aircraft impact modeling did not include the ceiling tile systems. To estimate whether the tiles would survive the aircraft impact, the University at Buffalo, under contract to NIST, conducted tests of WTC-like ceiling tile systems using their shake table (Figure 6–29) and impulses related to those induced by the aircraft impact on the towers. The data indicated that accelerations of approximately 5g would most likely result in substantial displacement of ceiling tiles. Given the estimated impact accelerations of approximately 100g, NIST assumed that the ceiling tiles in the impact and fire zones were fully dislodged. This was consistent with the multiple reports of severely damaged ceilings (Chapter 7). Source: NIST Figure 6–29. Ceiling tile system mounted on the shaking table. 6.9.5 Damage to Interior Walls and Furnishings As shown in Figure 6–18, the aircraft impact simulations explicitly included the fracture of walls in the debris path and the “bulldozing” of furnishings. Damage to the impacted furnishings was not modeled. Walls and furnishings outside the debris paths were undamaged in the simulations. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 121 6.10 THERMAL ENVIRONMENT MODELING 6.10.1 Need for Simulation Following the impact of the aircraft, the jet-fuel-ignited fires created the sustained and elevated temperatures that heated the remaining building structure to the point of collapse initiation. The photographic evidence provided some information regarding the locations and spreading of the fires. However, the cameras could only see the periphery of the building interior. The steep viewing angles of nearly all of the photographs and videos further limited the depth of the building interior for which fire information could be obtained. NIST could not locate any photographic evidence regarding the fire exposure of the building core or the floor assemblies. The simulations of the fires were the second computational step in the identification of the probable sequences leading to the collapse of the towers. The required output of these simulations was a set of three-dimensional, time varying renditions of the thermal and radiative environment to which the structural members in the towers were subjected from the time of aircraft impact until the onset of building collapse. The rigor of the Investigation placed certain requirements on the computational tool (model) used to generate these renditions: • Resolution of the varying thermal environment across key dimensions, e.g., the truss space; • Representation of the complex combustibles; • Computation of flame spread across the large expanses of the WTC floors; and • Confidence in the accuracy of the predictions. 6.10.2 Modeling Approach The time frame of the Investigation and the above requirements led to the use of the Fire Dynamics Simulator (FDS). Under development at NIST since 1978, FDS was first publicly released in February 2000 and had been used worldwide on a wide variety of applications, ranging from sprinkler activation to residential and industrial fire reconstructions. However, it had never before been applied to spreading fires in a building with such large floor areas. Figure 6–30 shows how FDS represented the eight modeled floors (92 through 99) of the undamaged WTC 1. A similar rendition was prepared for floors 78 through 83 of WTC 2. The layout of each floor was developed from architectural drawings and from the information described in Section 5.8. There was a wide range of confidence in the accuracy of these floor plans, varying from high (for the floors occupied by Marsh & McLennan in WTC 1, for which recent and detailed plans were obtained) to low (for most of the space in WTC 2 occupied by Fuji Bank, for which floor plans were not available). The effects of the aircraft impact were derived from the simulations described in Section 6.8. The portions of walls and floors that were “broken” in those simulations were simply removed from the FDS models of the towers. The furnishings outside the aircraft-damaged regions were assumed to be unmoved and undamaged. The treatment of furnishings within the impact zone is discussed later in this section. Chapter 6 122 NIST NCSTAR 1, WTC Investigation FDS represented the spaces in which the fires and their effluent were to be modeled as a grid of rectangular cells. These grids included the walls, floors, ceilings, and any other obstructions to the movement of air and fire. In the final simulations, the grid size was 0.5 m x 0.5 m x 0.4 m high (1.6 ft x 1.6 ft x 1.3 ft.). Each floor contained about 125,000 grid cells, and the nature of each cell was updated every 10 ms (100 times every second). The computations were performed using parallel processing, in which the fires on each floor were simulated on a different computer. At the end of each 10 ms update, the processors exchanged information and proceeded to the computations for the next time interval. Each simulation of 105 min of fires for WTC 1 took about a week on eight Xeon computers with a combined 16 GB of memory. The simulations for WTC 2, with fewer floors and 60 min of real time fires, took about half the time. Figure 6–30. Eight floor model of WTC 1 prior to aircraft impact. The fires were started by ignition of the jet fuel, whose distribution was provided by the aircraft impact simulations. The radiant energy from these short-lived fires heated the nearby combustibles, creating flammable vapors. When these mixed with air in the right proportion within a grid cell, FDS burned the mixture. This generated more energy, which heated the combustibles further, and continued the burning. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 123 The floors of the tower on which the dominant burning occurred were characterized by large clusters of office workstations (Figure 1–11). NIST determined their combustion behavior from a series of single-workstation fire tests (Figure 6–31). In these highly instrumented tests, the effects of workstation type, the presence of jet fuel, and the presence of fallen inert material (such as pieces of ceiling tiles or gypsum board walls) on the burning surfaces were all assessed. While FDS properly captured the gross behavior of these fires, the state of modeling the combustion of real furnishings was still primitive. Thus, the results of this test series were used to refine the combustion module in FDS. The accuracy of FDS predictions was then assessed using two different types of fire tests. In each case, the model predictions were generated prior to conducting the test. The first series provided a measure of the ability of FDS to predict the thermal environment generated by a steady state fire. A spray burner generating 1.9 MW or 3.4 MW of power was ignited in a 23 ft by 11.8 ft by 12.5 ft high compartment. The temperatures near the ceiling approached 900 °C. FDS predicted: • Room temperature increases near the ceiling to within 4 percent. • Gas velocities at the air inlet to the compartment (and thus the air drawn into the compartment by the fire) within the uncertainty in the experimental measurements. • The leaning of the fire plume due to the asymmetry of the objects within the compartment. The extent of the leaning was underestimated. • Radiant heat flux near the ceiling to within 10 percent, within the uncertainty of the experimental measurements. The second series was a preamble to the modeling of the actual WTC fires. Arrays of three WTC workstations were burned in a 35.5 ft by 23 ft by 11 ft high compartment (Figure 6–32). The tests examined the effects of the type of workstation, the presence of jet fuel, and the presence of fallen inert material on the burning surfaces. In one of the tests, the workstations were rubblized (Figure 6–33). Figure 6–34 depicts the intensity of the test fires. Figure 6–35 shows the measured and predicted heat release rate data from one of the tests in which there was no jet fuel nor inert material present. Source: NIST. Figure 6–31. Fire test of a single workstation. The large fires discussed in this report are characterized by heat release rate, or burning intensity, (in MW), by total energy released (in GJ), and by the heat flux, or radiant intensity (in kW/m2). Chapter 6 124 NIST NCSTAR 1, WTC Investigation Figure 6–32. Interior view of a three-workstation fire test. Source: NIST. Figure 6–33. Rubblized workstations. Source: NIST. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 125 Source: NIST Figure 6–34. Three-workstation fire test, 2 min after the start. Figure 6–35. Measured and predicted heat release rate from the burning of three office workstations. The differences in the fire behavior under the different experimental conditions were profound in these roughly hour-long tests. The jet fuel greatly accelerated the fire growth. Only about 60 percent of the Chapter 6 126 NIST NCSTAR 1, WTC Investigation combustible mass of the rubblized workstations was consumed. The near-ceiling temperatures varied between 800 °C and 1,100 °C. Nonetheless, FDS successfully replicated: • The general shape and magnitude of the time-dependent heat release rate. • The time at which one half of the combustion energy was released to within 3 min. • The value of the heat release rate at this time to within 9 percent. • The duration of the fires to within 6 min. • The peak near-ceiling temperature rise to within 10 percent. All these predictions were within the combined uncertainty in the model input data and the experimental measurements. Combined, these results led to the assessment that the uncertainty in the thermal environment predictions of the WTC fires would be dominated less by the FDS errors and more by the unknowns in such factors as the distribution of the combustibles, ventilation, and building damage. 6.10.3 The Four Cases Four fire scenarios (Case A and Case B for WTC 1 and Case C and Case D for WTC 2) were superimposed on the four cases of aircraft-driven damage of the same names (Section 6.9). A number of preliminary simulations had been performed to gain insight into the factors having the most influence on the severity of the fires. The most influential was the mass of combustibles per unit of floor area (fuel load); second was the extent of core wall damage, which affected the air supply for the fires. The aforementioned workstation fire tests had also indicated that the damage condition of the furnishings also played a key role. The scenario variables and their values are shown in Table 6–6. Table 6–6. Values of WTC fire simulation variables. WTC 1 WTC 2 Variable Case A Case B Case C Case D Tenant combustible fuel loada 4 lb/ft2 5 lb/ft2 4 lb/ft2 5 lb/ft2 Distribution of disturbed combustibles Even Weighted toward the core Heavily concentrated in the northeast corner Moderately concentrated in the northeast corner Condition of combustibles Undamaged except in impact zone Displaced furniture rubblized All rubblized Undamaged except in impact zone Representation of impacted core wallsb Fully removed Soffit remained Fully removed Soffit remained a. In addition, approximately 27,000 lb of solid combustibles from the aircraft were distributed along the debris path. b. In Cases A and C, the walls impacted by the debris field were fully removed. This enabled rapid venting of the upper layer into the core shafts and reduced the burning rate of combustibles in the tenant spaces. In Cases B and D, a more severe representation of the damage was to leave a 4 ft gypsum wallboard soffit that would maintain a hot upper layer on each fire floor. This produced a fire of longer duration near the core columns and the attached floor membranes. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 127 FDS contained no algorithm for breaking windows from the heat of the fires. Thus, during each simulation, windows were removed at times when photographs indicated they were first missing. Damage to the ventilation shafts was derived from the aircraft impact simulations. For undamaged floors, all the openings to the core area were assumed to total about 50 ft2 in area. 6.10.4 Characterization of the Fires For each of the four scenarios, FDS was used to generate a time-dependent gas temperature and radiation environment on each of the floors. The results of the FDS simulations of the perimeter fire were compared with the fire duration and spread rate as seen in the photographs and videos. For ease of visualization, contour plots of the room gas temperature 1.3 ft below the ceiling slab (in the “upper layer” of the compartment) were superimposed on profiles of the photographed fire activity. An example is shown in Figure 6–36. The stripes surrounding the image represent a summary of the visual observations of the windows, with the black stripes denoting broken windows, the orange stripes denoting external flaming, and the yellow stripes denoting fires that were seen inside the building. Fires deeper than a few meters inside the building could not be seen because of the smoke obscuration and the steep viewing angle of nearly all the photographs. Figure 6–36. Upper layer temperatures on the 94th floor of WTC 1, 15 min after impact. Chapter 6 128 NIST NCSTAR 1, WTC Investigation Given the uncertainties in some of the floor plans, the damage to the internal walls, and movement of the office furnishings, the intent of the simulations was to capture the magnitudes of the fires and the broad features of their locations and movement; and they did so. The following sections summarize the simulated behavior of the fires (which was used in the following stages of the disaster reconstruction) and their correlation with the analysis of the photographic evidence. WTC 1 Much of the fire activity was initially in the vicinity of the impact area in the north part of the building. As a result of the orientation of the impacting aircraft and its fuel tanks, the early fires on the 92nd through 94th floors tended toward the east side of the north face, while the early fires on the 97th through 99th floors tended toward the west side of the north face. The fires on all the floors spread along the east and west sides and were concentrated in the south part of the building at the time of collapse, as depicted in Figure 6–37. Figure 6–37. Direction of simulated fire movement on floors 94 and 97 of WTC 1. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 129 The fire simulation results for Case A and Case B were similar, indicating only a modest sensitivity to the fuel load and the degree of aircraft-generated damage. This was because, in general, the size and movement of the fires in WTC 1 were limited by the supply of air from the exterior windows. Since the window breakage pattern was not changed in Case B, the additional and re-distributed combustibles within the building did not contribute to a larger fire. The added fuel did slow the spread slightly because the fires were sustained longer in any given location. Although there was generally reasonable agreement between the simulated and observed fire spread rates, there were instances where the fires burned too quickly and too near the windows. This resulted from an artifact of the model: the combustible vapors burned immediately upon mixing with the incoming oxygen. Simulations performed with doubled fuel loads slowed the fire spread well below the observed rates. Combined with the above results, this suggested that the estimated overall combustible load of 4 lb/ft2 was reasonable. The simulations showed high temperatures in some of the elevator shafts. The late fire observed on the west face of the 104th floor may have started from fuel gases in the core shafts that had accumulated over the course of the first hour of fires below. The presence of fire in the shafts on the 99th floor provided some support for this hypothesis, but no simulations were performed for floors higher than the 99th. The predictions of maximum temperatures (e.g., red zones in Figure 6–37) were consistent with those in the three-workstation fire tests. The use of an “average” gas temperature was not a satisfactory means of assessing the thermal environment on floors this large and would also have led to large errors in the subsequent thermal and structural analyses. The heat transferred to the structural components was largely by means of thermal radiation, whose intensity is proportional to the fourth power of the gas temperature. At any given location, the duration of temperatures near 1,000 °C was about 15 min to 20 min. The rest of the time, the calculated temperatures were near 500 °C or below. To put this in perspective, the radiative intensity onto a truss surrounded by smoke-laden gases at 1,000 °C was approximately 7 times the value for gases at 500 °C. WTC 2 Simulating the fires in WTC 2 posed challenges in addition to those encountered in simulating the fires in WTC 1. The aircraft, hitting the tower to the east of center, splintered much of the furnishings on the east side of the building and plowed them toward the northeast corner. Neither the impact study nor the validation experiments performed at NIST could be completely relied upon to predict the final distribution, condition, and burning behavior of the demolished furnishings. In addition, only the layouts of the 78th and 80th floors were available to the Investigation; the other floors were only roughly described by former occupants. As a result of these unknowns, the uncertainty in these calculations was distinctly greater than in those for WTC 1. To help mitigate gross differences between the simulations and the observables, NIST made floor-specific adjustments, based on the results of preliminary computations. In particular, the fuel load and volatility on the 80th floor were reduced, and the fuel load on the 81st and 82nd floors was increased. Chapter 6 130 NIST NCSTAR 1, WTC Investigation In contrast with WTC 1, in WTC 2 there was less movement of the fires. The major burning occurred along the east side, with some spread to the north. There was no significant burning on the west side of the tower. Also unlike WTC 1, changing the combustible load in WTC 2 had a noticeable effect on the outcome of the simulations. Because so many windows on the impact floors in WTC 2 were broken out by the aircraft debris and the ensuing fireballs, there was an adequate supply of air for the fires. Thus, the burning rate of the fires was determined by the fuel supply. In the Case D simulation, the office furnishings and aircraft debris were spread out over a wider area, and the furnishings away from the impact area were undamaged. Both of these factors enabled a higher burning rate for the combustibles. In general, the Case D simulations more closely approximated the observations in the photographs and videos, although there was still some prediction of burning too close to the perimeter, especially on the east side of the 78th, 79th, 81st and 83rd floors. The burning in the northeast corner of the 81st and 82nd floors was more intense in Case D than in Case C. The fire in the east side of the 79th floor burned more intensely and reached the south face sooner. Nothing in the simulations explained the absence of fires in the “cold spot,” the 10-window expanse toward the east of the north face of the 80th, 81st, and 82nd floors. 6.10.5 Global Heat Release Rates Much of the information needed to simulate the fires came from laboratory-scale tests. While some of these involved enclosures several meters in dimension and fires that reached heat release rates of 10 MW and 12 GJ in total heat output, they were still far smaller than the fires that burned in the WTC towers. Figure 6–38 shows the heat release rates from the FDS simulations of the WTC fires. The peak plateau heat release rates were about 2 GW for WTC 1 and 1 GW for WTC 2. Integrating the areas under these curves produced total heat outputs from the simulated fires of about 8,000 GJ from WTC 1 and 3,000 GJ from WTC 2. Time (min) 0 20 40 60 80 100 Heat Release Rate (GW) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 WTC 1, Case A WTC 2, Case C WTC 1, Case B WTC 2, Case D Figure 6–38. Predicted heat release rates for fires in WTC 1 and WTC 2. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 131 6.11 DATA TRANSFER The following data from FDS were compiled for use as boundary conditions for the finite-element calculation of the structural temperatures: • The upper and lower layer gas temperatures, time-averaged over 100 s and spatially averaged over 3 ft. The upper layer gas temperatures were taken 1.3 ft (one grid cell) below the ceiling. The lower layer temperatures were taken 1.3 ft above the floor. • The depth of the smoke layer. • The absorption coefficient of the smoke layer 1.3 ft below the ceiling. 6.12 THERMAL MAPPING 6.12.1 Approach Simulating the effect of a fire on the structural integrity of a building required a means for transferring the heat generated by the fire to the surfaces of the insulation on structural members and then conducting the heat through those members. In the Investigation, this meant mapping the time- and space-varying gas temperatures and radiation field generated by FDS onto and throughout the (insulated) columns, trusses and other elements that made up the tower structure. This process was made difficult for these large, geometrically complex buildings by the wide disparity in length and time scales that had to be accounted for in the simulations. FDS generated thermal maps with dimensional resolution of the order of a meter and temperatures fluctuating on a time scale of milliseconds. The finite element models for thermal analysis resolved length of the order of ½ in. on a time scale of seconds. Devising a computation scheme to accommodate the finest of these scales, while simulating the largest of these scales, presented a software challenge in order to avoid unacceptably long computation times. 6.12.2 The Fire-Structure Interface NIST developed a computational scheme to overcome this difficulty, the Fire Structure Interface (FSI). These computations began with the structural models of each WTC tower as described in Section 6.6.4, damaged by the aircraft as described in Section 6.8.4 and exposed to fire-generated heat, as described in Section 6.10.4. For a particular tower and damage scenario, FSI “bathed” each small section of each structural member in an air environment that had been generated by FDS. For efficiency of computation, two simplifications were made: • The fluctuating environment was averaged over 30 s intervals, and The transfer of radiant energy from a hot mass to a cool mass is proportional to the absolute temperature (Kelvin) to the fourth power. Thus, the contribution of the hot upper layer dominates the overall radiative heat transfer. Convective heat transfer is linearly proportional to the difference in temperature between the hot gas and the cool solid. Chapter 6 132 NIST NCSTAR 1, WTC Investigation • The local environment was represented by a hot, soot-laden upper layer and a cooler, relatively clear lower layer. FSI then calculated the radiative and convective heat transfer to each of these small sections using conventional physics. Finally, the temperature data were read into the ANSYS 8.0 finite element program, which applied the temperature distribution to the structural elements. 6.12.3 Thermal Insulation Properties Equivalent Uniform Thickness of SFRM Preliminary simulations with FSI explored the extent to which bare steel structural elements would heat more rapidly than the same elements would if they were well insulated. In one such calculation experiment, one of the largest columns in the tower structure was immersed in a furnace at 1,100 °C. Uninsulated, it took just 13 min for the steel surface temperatures to reach 600 °C, in the range where substantial loss of strength occurs. When insulated with 1 1/8 in. of SFRM, the same column had not reached that temperature in 10 hours. This established that the fires in WTC 1 and WTC 2 would not be able to significantly weaken the insulated core or perimeter columns within the 102 min and 56 min, respectively, after impact and prior to collapse. Thus, it was important to know whether the insulation was present or removed and much less important to know the exact thickness of the SFRM. It was likely that the thinner steel bars and angles in the floor trusses would be more sensitive to the condition of the insulation. If the insulation were present, but too thin or imperfectly applied, these components might have been heated to failure in times on the order of an hour. NIST performed additional simulations to probe the effect of gaps in the truss insulation and of variations in the thickness, similar to those observed in real SFRM application (Figure 5–6). It was evident that incorporation of these small-scale variations into the description of the structural members would have lengthened the FSI computations to an extreme. Furthermore, there was insufficient information to determine how the thickness varied over the length of the structural members. NIST combined the measured variations in the SFRM thickness (as described in Section 5.6.2) with simulations of the heat transfer through the uneven material. This led to the identification of a uniform thickness that provided the same insulation value as did the measured coatings. These values, shown in Table 5–3, were used in the thermal calculations. They were found to be greater than the specified thicknesses but slightly smaller than the average measured thicknesses, as they should be. SFRM Thermophysical Properties When the Investigation began, there were few published data on the insulating properties of SFRMs, especially at elevated temperatures. It was expected, and soon confirmed, that the fires could generate temperatures up to 1,100 °C. Therefore, NIST contracted for measurement of the key SFRM thermophysical properties that, along with coating thickness, determine the insulating effect of the coatings. These properties included thermal conductivity, specific heat capacity, and density. These were measured for each SFRM at temperatures up to 1,200 °C. Since there were no ASTM test methods developed specifically for characterizing the thermophysical properties of SFRMs as a function of temperature, ASTM test methods developed for other materials were used. Samples were prepared by the Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 133 manufacturers of the fire-resistive material, which included BLAZE-SHIELD DC/F and BLAZE-SHIELD II. • The thermal conductivity measurements were performed according to ASTM C 1113, Standard Test Method for Thermal Conductivity of Refractories by Hot Wire (Platinum Resistance Thermometer Technique). The room temperature values were in general agreement with the manufacturer’s published values for both materials. The thermal conductivities increased with temperature. • Specific heat capacity was measured in accordance with ASTM E 1269, Standard Test Method for Determining Specific Heat Capacity by Differential Scanning Calorimetry (DSC). By including DSC measurement of a NIST Reference Material (sapphire), the measured SFRM quantities were directly traceable to NIST standards. • The densities of the SFRMs were calculated from measurements of changes in the mass and dimensions of samples as their temperatures were increased. The length-change measurements were performed according to ASTM E 228, Standard Test Method for Linear Thermal Expansion of Solid Materials. The mass loss measurements were performed according to ASTM E 1131, Standard Test Method for Compositional Analysis by Thermogravimetry. It was not known which type(s) of gypsum wallboard were used to enclose the core columns. Therefore, the thermophysical properties of four types of gypsum panels were examined. • Thermal conductivity was measured using the heated probe technique described in ASTM D 5334, Standard Test Method for Determination of Thermal Conductivity of Soil and Soft Rock by Thermal Needle Probe Procedure. In general, the thermal conductivity initially decreased as the temperature increased to 200 °C and then increased with increasing temperature above 300 °C. • Specific heat capacities of the cores of the four gypsum panel samples were measured using a differential scanning calorimeter at NIST according to ASTM E 1269, Standard Test Method for Determining Specific Heat Capacity by Differential Scanning Calorimetry. The four panels had nearly identical specific heat capacities as a function of temperature. • The variation of density with temperature was determined from the change in volume of the gypsum material and the mass loss. The linear expansion was determined using a dilatometer and the mass loss from thermogravimetric analysis. All four materials showed the same trend as a function of temperature. 6.12.4 FSI Uncertainty Assessment As was done for FDS, it was necessary to establish the quality of FSI’s predictions of temperature profiles within insulated and bare structural steel components. This was accomplished using data from a series of six tests in which assorted steel members were exposed to controlled fires of varying heat release rate and radiative intensity. The steel members, depicted in Figures 6–39 through 6–41, were either bare or coated with sprayed BLAZE-SHIELD DC/F in two thicknesses. The fibrous insulation was applied by an Chapter 6 134 NIST NCSTAR 1, WTC Investigation experienced applicator, who took considerable care to apply an even coating of the specified thickness. As such, the insulated test subjects represent a best case in terms of thickness and uniformity. Figure 6–42 shows some of the coated components. Figure 6–39. Simple bar dimensions (in.). Figure 6–40. Tubular column dimensions (in.). 132 10 14 1/4 1/4 1 118 Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 135 Figure 6–41. Truss Dimensions (in.). Source: NIST. Figure 6–42. SFRM-coated steel components prior to a test. 31 181 1 3 3 1/4 1/4 2.5 2.5 1/4 1/4 Web Bar Top Chord Angles Bottom Chord Angles Chapter 6 136 NIST NCSTAR 1, WTC Investigation Table 6–7 shows the dimensions and variability of the insulation for the two successful tests involving coated steel. The thickness measurements were taken at numerous locations along the perimeter and length of each specimen using a pin thickness gauge specifically designed for this type of insulation. Table 6–7. Summary of insulation on steel components. Applied Thickness (in.) Test Item Specified Thickness (in.) Mean Std. Deviation 5 Bar 0.75 0.91 0.22 Column 1.50 1.61 0.12 Truss A 0.75 1.06 0.28 Truss B 1.50 1.59 0.32 6 Bar 0.75 1.00 0.18 Column 0.75 0.84 0.14 Truss A 0.75 1.02 0.27 Truss B 0.75 1.01 0.27 Temperatures were recorded at multiple locations on the surfaces of the steel, the insulation, and the compartment. As an example, Figure 6–43 shows the finite element representation of the coated truss. Figure 6–43. Finite element representation of the insulated steel truss (blue), the SFRM (violet), and the ceiling (red). Figure 6–44 compares the measured and predicted temperatures on the steel surface of the top chord of a bare truss. Figure 6–45 is the analogous plot of the measured and predicted temperatures on the steel surface of the top chord of a truss insulated with 3/4 in. of BLAZE-SHIELD DC/F. Similar curves were generated for each of the steel pieces, bare and insulated. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 137 Figure 6–44. Comparison of numerical simulations with measurements for the steel surface temperature at four locations on the top chord of a bare truss. Figure 6–45. Comparison of numerical simulations with measurements for the temperature of the steel surface at four locations on the top chord of an insulated truss. Chapter 6 138 NIST NCSTAR 1, WTC Investigation Examination of the graphs for the insulated steel pieces indicated the following: • FSI captured the shape of the temperature rise at the steel surfaces and the significant decrease in the rate of temperature rise when the SFRM was present. • The times to the peak temperature (or a near-plateau) were predicted to within about a minute in all cases. • There was no consistent pattern of overprediction or underprediction of the surface temperatures. • On the average, the numerical predictions of the steel surface temperature were within 7 percent of the experimental measurements for bare steel elements and within 17 percent for the insulated steel elements. The former was within the combined uncertainty in the temperature measurements and the heat release rate in the fire model. The increase in the latter was attributed to model sensitivity to the SFRM coating thickness and thermal conductivity. In general, the FSI added little to the overall uncertainty in the simulation of the temperatures at the outer surfaces of bare steel elements and, more importantly, at the SFRM-steel interface. An additional, important outcome of the experiments was the demonstration of the insulating effect of even 3/4 in. of SFRM. Trusses, made of relatively thin steel, were far more susceptible to heating than the perimeter and core columns. As shown in Figure 2–10, in 15 min, a bare truss reached a temperature at which significant loss of strength was imminent. An identical, but insulated truss had not reached that temperature in 50 min. 6.12.5 The Four Cases FSI imposed the thermal environment from each of the four FDS fire scenarios (Cases A and B for WTC 1 and Cases C and D for WTC 2) on the four damaged structures from the aircraft simulations, which carried the same case letters. The FSI output files carried the same case letters as the input files. The FSI calculations were performed at time steps ranging from 1 ms to 50 ms. Use of the resulting data set for structural analysis would have required a prohibitive amount of computation time. Thus, for each case, the instantaneous temperature and temperature gradient for each grid volume was provided at 10 min intervals after aircraft impact. For WTC 1, there were 10 such intervals, ending at 6,000 s; for WTC 2 there were 6 intervals, ending at 3,600 s. Comparison of these coarsely timed output files with files at 1 min resolution showed any differences to be within the combined uncertainty. Each floor in the FSI simulation provided thermal information for the floor assembly above. Thus, there was not sufficient information for FSI to model the lowest floor in the FDS simulations. For WTC 1, the global thermal response generated by FSI included floors 93 through 99; for WTC 2, the included floors were 79 through 83. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 139 For ease of visualization, two graphic representations were developed. Figure 6–46 shows an example of the temperature map for the 96th floor of WTC 1. Severed columns and broken floor segments are not shown. Figure 6–47 shows a similar map for the 81st floor of WTC 2. Figure 6–46. Temperatures ( C) on the columns and trusses of the 96th floor of WTC 1 at 6,000 s after aircraft impact, Case B. Figure 6–47. Temperatures ( C) on the columns and trusses of the 81st floor of WTC 2 at 3,000 s after aircraft impact, Case D. A third visualization tool was animation of the evolving temperatures of the columns. Frames from an example, again of the 96th floor of WTC 1, Case A, are shown in Figure 6–48. The size of the square representing a column represents its yield strength. Columns may have been heated when the fire was nearby and then cooled after the local combustibles were consumed. 140 NIST NCSTAR 1, WTC Investigation Chapter 6 (a) Time = 1000 s (b) Time = 2000 s (c) Time = 3000 s (d) Time = 4000 s (e) Time = 5000 s (f) Time = 6000 s Figure 6–48. Frames from animation of the thermal response of columns on the 96th floor of WTC 1, Case A. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 141 6.12.6 Characterization of the Thermal Profiles Tables 6–8 and 6–9 summarize the regions of the floors in which the structural steel reached temperatures at which their yield strengths would have been significantly diminished. Instances of brief heating of one or two columns early in the fires were not included. Even in the vicinity of the fires, the columns and trusses for which the insulation was intact did not heat to temperatures where significant loss of strength occurred. Unlike the simulations of the aircraft impact and the fires, there was no evidence, photographic or other, for direct comparison with the FSI results. Table 6–8. Regions in WTC 1 in which temperatures of structural steel exceeded 600 C. Trusses Perimeter Floor Columns Core Columns Number Case A Case B Case A Case B Case A Case B 93 – – – – – – 94 – – – – N, S NE, S 95 N N, S – – S NW, S 96 N N, S – S S W, S 97 N, S N, S – S N W, S 98 N N, S – – – – 99 – – – – – – Key: N, north; NE, northeast; NW, northwest; S, south; W, west. Table 6–9. Regions in WTC 2 in which temperatures of structural steel exceeded 600 C. Floor Trusses Perimeter Columns Core Columns Number Case C Case D Case C Case D Case C Case D 79 – – – – – – 80 – – – – – – 81 NE NE NE NE – NE 82 E E E E E E 83 E E – E – E Key: E, east; NE, northeast. 6.13 MEASUREMENT OF THE FIRE RESISTANCE OF THE FLOOR SYSTEM As described in Section 5.4.7, the composite floor system, composed of open-web, lightweight steel trusses topped with a slab of lightweight concrete, was an innovative feature. As further noted in Section 5.6.2, the approach to achieving the specified fire resistance for these floors was the use of a SFRM. Documents indicated that the fire performance of the composite floor system of the WTC towers was an issue of concern to the building owners and designers. However, NIST found no evidence regarding the technical basis for the selection of insulation material for the floor trusses or for the insulation thickness to achieve a 2 hour rating. Further, NIST has found no evidence that fire resistance tests of the WTC floor system were conducted. Chapter 6 142 NIST NCSTAR 1, WTC Investigation Most of the possible building collapse sequences included some contribution from the floors, ranging from their ability to transfer load to their initiating the collapse by their failure. Thus, it became central to the Investigation to obtain data regarding the limits of the insulated floors in withstanding the heat from the fires. The standard test for determining the fire endurance of floor assemblies is ASTM E 119, “Standard Test Methods for Fire Tests of Building Construction and Materials.” The conduct of the test is described in Section 1.2.2 under “Fire Protection Systems.” Accordingly, NIST contracted with Underwriters Laboratories, Inc. to conduct tests to obtain information on the fire endurance of trusses like those in the WTC towers. The objective was to understand the effects of three factors: • Scale of the test. There were no established facilities capable of testing the 60 ft lengths of the long spans that were used in the towers, but there is a history of testing reduced-scale assemblies and scaling them to practical dimensions. In the Investigation’s tests, the fullscale test specimens were 35 ft long, equal to the shorter span between the core and the perimeter of the WTC towers. Their construction replicated, as closely as possible, the original short-span floors. The reduced-scale specimens were half that length and height. All assemblies were 14 ft wide. The simulation of a “maximum load condition,” as required by ASTM E 119, involved placing a combination of concrete blocks and containers filled with water on the top surface of the floor. The load on the shorter truss was double that of the longer truss to achieve the same state of stress in both trusses. Traditionally, relatively smallscale assemblies have been tested and results have been scaled to practical floor system spans. • SFRM thickness. The Port Authority originally specified BLAZE-SHIELD D as the SFRM, applied to a ½ in. covering. The average measured thicknesses were found to be approximately 0.75 in. These two thicknesses of BLAZE-SHIELD D were used in the Investigation tests. • Test restraint conditions. In 1971, well after the design of the towers was completed, the ASTM E 119 Standard began differentiating between thermally restrained and unrestrained floor assemblies. An unrestrained assembly is free to expand thermally and to rotate at its supports; a restrained assembly is not. It is customary in the United States to conduct standard fire tests of floor assemblies in the restrained condition. The current standard describes a means to establish unrestrained ratings for floor assemblies from restrained test results. In practice, a floor assembly such as that used in the WTC towers is neither restrained nor unrestrained but is likely somewhere in between. Testing under both restraint conditions, then, is thought to bound performance under the standard fire exposure. In addition, it provided a comparison of unrestrained ratings developed from both restrained and unrestrained test conditions. The test plan included four tests, which varied the three factors: Test 1: 35 ft floor, ¾ in. insulation, restrained Test 2: 35 ft floor, ¾ in. insulation, unrestrained Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 143 Test 3: 17 ft floor, ¾ in. insulation, restrained Test 4: 17 ft floor, ½ in. insulation, restrained The results of the four tests are summarized as follows: • All four test assemblies were able to withstand standard fire conditions for between ¾ hour and 2 hours without exceeding the limits prescribed by ASTM E 119. • All four test specimens sustained the maximum design load for approximately 2 hours without collapsing. • The restrained full-scale floor system obtained a fire resistance rating of 1½ hours, while the unrestrained floor system achieved a 2 hour rating. Past experience with the ASTM E 119 test method led investigators to expect the unrestrained floor assembly to receive a lower rating than the restrained assembly. • For assemblies with a ¾ in. SFRM thickness, the 17 ft assembly’s fire rating was 2 hours; the 35 ft assembly’s rating was 1½ hours. This result raised the question of whether or not a fire rating of a 17 ft floor assembly is scalable to the longer spans in the WTC towers. • The specimen in Test 4, with a fire rating of ¾ hour, would not have met the 2 hour requirement of the NYC Building Code. The Investigation Team was cautious about using these results directly in the formulation of collapse hypotheses. In addition to the scaling issues raised by the test results, the fires in the towers on September 11, and the resulting exposure of the floor systems, were substantially different from the conditions in the test furnaces. Nonetheless, the results established that this type of assembly was capable of sustaining a large gravity load, without collapsing, for a substantial period of time relative to the duration of the fires in any given location on September 11. 6.14 COLLAPSE ANALYSIS OF THE TOWERS 6.14.1 Approach to Determining the Probable Collapse Sequences At the core of NIST’s reconstruction of the events of September 11, 2001, were the archive of photographic and video evidence, the observations of people who were on the scene, the assembled documents describing the towers and the aircraft, and Investigation-generated experimental data on the properties of construction and furnishing materials and the behavior of the fires. Information from all of these sources fed the computer simulations of the towers, the aircraft impacts, the ensuing fires and their heating of the structural elements, and the structural changes that led to the collapse of the towers. To the extent that the input information was complete and accurate, the output of the simulations would have provided definitive responses to the first three objectives of the Investigation. However, the available information, as extensive as it was, was neither complete nor of assured precision. As a result, the Investigation Team took steps to ensure that the conclusions of the effort were credible explanations for how the buildings collapsed and the extent to which the casualties occurred. Chapter 6 144 NIST NCSTAR 1, WTC Investigation One principal step was the determination of those variables that most affected the outcome of the various computer simulations. Sensitivity studies and examination of components and subsystems were carried out for the modeling of the aircraft impact, the fires, and the structural response to impact damage and fires. For each of the most influential variables, a central or middle value and reasonable high and low values were identified. Further computations refined the selection of these values. The computations also were improved to include physical processes that could play a significant role in the structural degradation of the towers. The Investigation Team then defined three cases for each building by combining the middle, less severe, and more severe values of the influential variables. Upon a preliminary examination of the middle cases, it became clear that the towers would likely remain standing. The less severe cases were discarded after the aircraft impact results were compared to observed events. The middle cases (which became Case A for WTC 1 and Case C for WTC 2) were discarded after the structural response analysis of major subsystems were compared to observed events. The more severe case (which became Case B for WTC 1 and Case D for WTC 2) was used for the global analysis of each tower. Complete sets of simulations were then performed for Cases B and D. To the extent that the simulations deviated from the photographic evidence or eyewitness reports, the investigators adjusted the input, but only within the range of physical reality. Thus, for instance, the observed window breakage was an input to the fire simulations and the pulling forces on the perimeter columns by the sagging floors were adjusted within the range of values derived from the subsystem computations. The results were a simulation of the structural deterioration of each tower from the time of aircraft impact to the time at which the building became unstable, i.e., was poised for collapse. Cases B and D accomplished this in a manner that was consistent with the principal observables and the governing physics. 6.14.2 Results of Global Analysis of WTC 1 After the aircraft impact, gravity loads that were previously carried by severed columns were redistributed to other columns. The north wall lost about 7 percent of its loads after impact. Most of the load was transferred by the hat truss, and the rest was redistributed to the adjacent exterior walls by spandrels. Due to the impact damage and the tilting of the building to the north after impact, the south wall also lost gravity load, and about 7 percent was transferred by the hat truss. As a result, the east and west walls and the core gained the redistributed loads through the hat truss. Structural steel and concrete expand when heated. In the early stages of the fire, temperatures of structural members in the core rose, and the resulting thermal expansion of the core columns was greater than the thermal expansion of the (cooler) exterior walls. The floors also thermally expanded in the early stages of the fires. About 20 min after the aircraft impact, the difference in the thermal expansion between the core and exterior walls, which was resisted by the hat truss, caused the core columns’ loads to increase. As floor temperatures increased, the floors sagged and began to pull inward on the exterior wall. As the fires continued to heat areas of the core that were without insulation, the columns weakened and shortened and began to transfer their loads to the exterior walls through the hat truss until the south wall started to bow inward due to the inward pull of the sagging floors. At about 100 min, approximately 20 percent of the core loads had been transferred by the hat truss to the exterior walls due to weakening of Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 145 the core, the loads on the north and south walls had each increased by about 10 percent, and those on the east and west walls had about a 25 percent increase. The increased loads on the east and west walls were due to their relatively higher stiffness compared to the impact damaged north wall and bowed south walls. The inward bowing of the south wall caused failure of exterior column splices and spandrels, and these columns became unstable. The instability spread horizontally across the entire south face. The south wall, now unable to bear its gravity loads, redistributed these loads to the thermally weakened core through the hat truss and to the east and west walls through the spandrels. The building section above the impact zone began tilting to the south as the columns on the east and west walls rapidly became unable to carry the increased loads. This further increased the gravity loads on the core columns. The gravity loads could no longer be redistributed, nor could the remaining core and perimeter columns support the gravity loads from the floors above. Once the upper building section began to move downwards, the weakened structure in the impact and fire zone was not able to absorb the tremendous energy of the falling building section and global collapse ensued. 6.14.3 Results of Global Analysis of WTC 2 Before aircraft impact, the load distribution across the exterior walls and core was symmetric with respect to the centerline of each exterior wall. After aircraft impact, the exterior column loads on the south side of the east and west walls and on the east side of south wall increased. This was due to the leaning of the building core towards the southeast. After aircraft impact, the core carried 6 percent less load. The north wall load reduced by 6 percent and the east face load increased by 24 percent. The south and west walls carried 2 percent to 3 percent more load. In contrast to the fires in WTC 1, which generally progressed from the north side of the building to the south side over approximately 1 hour, the fires in WTC 2 were located on the east side of the core and floors from the time of impact until the building collapsed, with the fires spreading somewhat from south to north. With insulation dislodged over much of the same area, the structural temperatures became elevated in the core, floors, and exterior walls at similar times. During the early stages of the fires, columns with dislodged insulation elongated due to thermal expansion. As the structural temperatures continued to rise, the columns thermally weakened and consequently shortened. Thermal expansion of the floors also occurred early in the fires, but as floor temperatures increased, the floors sagged and began to pull inward on the exterior columns. The south exterior wall displaced downward following the aircraft impact, but did not displace further until the east wall became unstable 43 min later. The inward bowing of the east wall, due to the inward pull of the sagging floors, caused failure of exterior column splices and spandrels and resulted in the east wall columns becoming unstable. The instability progressed horizontally across the entire east face. The east wall, now unable to bear its gravity loads, redistributed them to the thermally weakened core through the hat truss and to the east and west walls through the spandrels. The building section above the impact zone began tilting to the east and south as column instability progressed rapidly from the east wall along the adjacent north and south walls, and increased the gravity load on the weakened east core columns. The gravity loads could no longer be redistributed, nor could the remaining core and perimeter columns support the gravity loads from the floors above. As with WTC 1, once the upper building section began to move downwards, the weakened structure in the impact Chapter 6 146 NIST NCSTAR 1, WTC Investigation and fire zone was not able to absorb the tremendous energy of the falling building section and global collapse ensued. 6.14.4 Events Following Collapse Initiation Failure of the south wall in WTC 1 and east wall in WTC 2 caused the portion of the building above to tilt in the direction of the failed wall. The tilting was accompanied by a downward movement. The story immediately below the stories in which the columns failed was not able to arrest this initial movement as evidenced by videos from several vantage points. The structure below the level of collapse initiation offered minimal resistance to the falling building mass at and above the impact zone. The potential energy released by the downward movement of the large building mass far exceeded the capacity of the intact structure below to absorb that through energy of deformation. Since the stories below the level of collapse initiation provided little resistance to the tremendous energy released by the falling building mass, the building section above came down essentially in free fall, as seen in videos. As the stories below sequentially failed, the falling mass increased, further increasing the demand on the floors below, which were unable to arrest the moving mass. The falling mass of the building compressed the air ahead of it, much like the action of a piston, forcing material, such as smoke and debris, out the windows as seen in several videos. NIST found no corroborating evidence for alternative hypotheses suggesting that the WTC towers were brought down by controlled demolition using explosives planted prior to September 11, 2001. NIST also did not find any evidence that missiles were fired at or hit the towers. Instead, photographs and videos from several angles clearly show that the collapse initiated at the fire and impact floors and that the collapse progressed from the initiating floors downward, until the dust clouds obscured the view. 6.14.5 Structural Response of the WTC Towers to Fire without Impact or Thermal Insulation Damage To complete the assessment of the relative roles of aircraft impact and ensuing fires, NIST examined whether an intense, but conventional, fire, occurring without the aircraft impact, could have led to the collapse of a WTC tower, were the tower in the same condition as it was on September 10, 2001. NIST used the observations, information, and analyses developed during the Investigation to enable the formulation of probable limits to the damage from such a fire. Since a complete analysis beyond the actual collapse times of the towers was not conducted, the findings in this section represent NIST’s best technical judgment based on the available observations, information, and analyses: • Ignition on a single floor by a small bomb or other explosion. If arson were involved, there might have been multiple small fires ignited on a few floors. • Air supply determined by the building ventilation system. • Moderate fire growth rate. In the case of arson, several gallons of an accelerant might have been applied to the building combustibles, igniting the equivalent of several workstations. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 147 • Water supply to the sprinklers and standpipes maliciously compromised. • Intact structural insulation and interior walls. The four cases described in this chapter represented fires that were far more severe than this: • About 10,000 gallons of jet fuel were sprayed into multiple stories, quickly and simultaneously igniting hundreds of workstations and other combustibles. • The aircraft and subsequent fireballs created large open areas in the building exterior through which air could flow to support the fires. • The impact and debris removed the insulation from a large number of structural elements that were then subjected to the heat from the fires. Additional findings from the Investigation showed that: • Both the results of the multiple workstation experiments and the simulations of the WTC fires showed that the combustibles in a given location, if undisturbed by the aircraft impact, would have been almost fully burned out in about 20 min. • In the simulations of Cases A through D, none of the columns and trusses for which thermal insulation was intact reached temperatures at which significant loss of strength occurred. Thermal analyses showed that steel temperatures in areas where the insulation remained intact rarely exceeded 400 °C in WTC 1 and 500 °C in WTC 2. • In WTC 1, if fires had been allowed to continue past the time of building collapse, complete burnout would likely have occurred within a short time since the fires had already traversed around the entire floor and most of the combustibles would already have been consumed (see Figure 6–38). During the extended period from collapse to burnout, the steel temperatures would likely not have increased very much. The installed insulation in the fire-affected floors of this building had been upgraded to an average thickness of 2.5 in. • In a fire simulation of WTC 2, that was extended for 2 hours beyond Case D and with all windows broken during this period, the temperatures in the truss steel on the west side of the building (where the insulation was undamaged) increased for about 40 min before falling off rapidly as the combustibles were consumed. Results for a typical floor (floor 81) showed that temperatures of 700 °C to 760 °C were reached over approximately 15 percent of the west floor area for less than 10 min. Approximately 60 percent of the floor steel had temperatures between 600 °C and 700 °C for about 15 min. Approximately 70 percent of the floor steel had temperatures that exceeded 500 °C for about 45 min. At these temperatures, the floors would be expected to sag and then recover a portion of the sag as the steel began to cool. Based on results for Cases C and D, the temperatures of the insulated exterior and core columns would not have increased to the point where significant loss of strength or stiffness would occur during these additional 2 hours. With intact, cool core columns, any inward bowing of the west exterior wall that might occur would be readily supported by the adjacent exterior walls and core columns. Chapter 6 148 NIST NCSTAR 1, WTC Investigation • Both WTC 1 and WTC 2 were stable after the aircraft impact, standing for 102 min and 56 min, respectively. The global analyses with structural impact damage showed that both towers had considerable reserve capacity. This was confirmed by analysis of the post-impact vibration of WTC 2, the more severely damaged building, where the damaged tower oscillated with a peak amplitude that was between 30 percent and 40 percent of the sway under hurricane force winds for which the towers were designed and at periods nearly equal to the first two translation and torsion mode periods calculated for the undamaged structure. • Computer simulations, supported by the results of large-scale fire tests and furnace testing of floor subsystems, showed that insulated structural steel, when coated with the average installed insulation thickness of ¾ in., would not have reached high temperatures (i.e., greater than 650 °C) from nearby fires for a longer time than the burnout time of the combustibles (approximately 20 min for 4 lb/ft2 of combusted material). Simulations also showed that variations in thickness resulting from normal application, even with occasional gaps in coverage, would not have changed this result. • Inward bowing of the exterior walls in both WTC 1 and WTC 2 was observed only on the face with the long-span floor system. In WTC 1, this was found to be the case even though equally extensive fires were observed on all faces. In WTC 2, fires were not observed on the long-span west face and were less intense on the short-span faces than on the east face. • Inward bowing was a necessary but not sufficient condition to initiate collapse. In both WTC 1 and WTC 2, significant weakening of the core due to aircraft impact damage and thermal effects was also necessary to initiate building collapse. • The tower structures had significant capacity to redistribute loads (a) from bowed walls to adjacent exterior walls with short-span floors via the arch action of spandrels, and (b) between the core and exterior walls via the hat truss and, to a lesser extent, the floors. In evaluating how the undamaged towers would have performed in an intense, conventional fire, NIST considered the following factors individually and in combination: • The temperatures that would be reached in structural steel components with intact insulation. • The extent of the area over which high temperatures (e.g., greater than 600 °C where significant thermal weakening of the steel occurs) would be reached at any given time. • The duration over which the high temperatures would be sustained concurrently in any given area. • The length of the floor span (long or short) where high temperatures would be reached. • The number of floors with areas where high temperatures would be sustained concurrently in the long-span direction. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 149 • The potential for inward bowing of exterior walls (i.e., magnitude and extent of bowing over the width of the face and the number of floors involved) due to thermally induced floor sagging of long-span floors and associated inward pull forces. • The capacity of the structure to redistribute loads (e.g., via the spandrels, hat truss, and floors) if the thermal conditions were sufficiently intense to cause inward bowing of the exterior walls. In addition, NIST considered the following known facts: • WTC 1 did not collapse during the major fire in 1975, which engulfed a large area (about one-fourth of the floor area or 9,000 ft2) on the southeast quadrant of the 11th floor. At the time, office spaces in the towers were not sprinklered. The fire caused minimal damage to the floor system with the ½ in. specified insulation thickness applied on the trusses (four trusses were slightly distorted), and at no time was the load-carrying capacity compromised for the floor system or the structure as a whole. • Four standard fire resistance tests of floor assemblies like those in the WTC towers conducted as part of this Investigation showed that (a) it took about 90 min of sustained heating in the furnace for temperatures to exceed 600 °C on steel truss members with either ½ in. or ¾ in. insulation thickness, and (b) in no case was the load-carrying capacity compromised by heating of the floor system for 2 hours at furnace temperatures, with applied loads exceeding those on September 11 by a factor of two. From these findings, factors, and observed performance, NIST concluded: • In the absence of structural and insulation damage, a conventional fire substantially similar to or less intense than the fires encountered on September 11, 2001, likely would not have led to the collapse of a WTC tower. • The condition of the insulation prior to aircraft impact, which was found to be mostly intact, and the insulation thickness on the WTC floor system contributed to, but did not play a governing role, in initiating collapse of the towers. • The towers likely would not have collapsed under the combined effects of aircraft impact and the subsequent multi-floor fires encountered on September 11 if the thermal insulation had not been widely dislodged or had been only minimally dislodged by aircraft impact. These findings apply to fires that are substantially similar to or less intense than those encountered on September 11, 2001. They do not apply to a standard fire or an assumed fire exposure which has (a) uniform high temperatures over an entire floor or most of a floor (note that the WTC floors were extremely large) and concurrently over multiple floors, (b) high temperatures that are sustained indefinitely or for long periods of time (greater than about 20 min at any location), and (c) combusted fire loads that are significantly greater than those considered in the analyses. They also do not apply if the capacity of the undamaged structure to redistribute loads via the spandrels, hat truss, and floors is not accounted for adequately in a full 3-dimensional simulation model of the structure. Chapter 6 150 NIST NCSTAR 1, WTC Investigation 6.14.6 Probable WTC 1 Collapse Sequence Aircraft Impact Damage • The aircraft impact severed a number of exterior columns on the north wall from the 93rd to the 98th floors, and the wall section above the impact zone moved downward. • After breaching the building’s perimeter, the aircraft continued to penetrate into the building, severing floor framing and core columns at the north side of the core. Core columns were also damaged toward the center of the core. Insulation was damaged from the impact area to the south perimeter wall, primarily through the middle one-third to one-half of the core width. Finally, the aircraft debris removed a single exterior panel at the center of the south wall between the 94th and 96th floors. • The impact damage to the exterior walls and to the core resulted in redistribution of severed column loads, mostly to the columns adjacent to the impact zones. The hat truss resisted the downward movement of the north wall. • Loads on the damaged core columns were redistributed mostly to adjacent intact core columns and to a lesser extent to the north perimeter columns through the core floor systems and the hat truss. • As a result of the aircraft impact damage, the north and south walls each carried about 7 percent less gravity load after impact, and the east and west walls each carried about 7 percent more load. The core carried about 1 percent more gravity load after impact. Thermal Weakening of the Structure • Under the high temperatures and stresses in the core area, the remaining core columns with damaged insulation were thermally weakened and shortened, causing the columns on the floors above to move downward. The hat truss resisted the core column shortening and redistributed loads to the perimeter walls. The north and south walls’ loads increased by about 10 percent, and the east and west walls’ loads increased by about 25 percent, while the core’s loads decreased by about 20 percent. • The long-span sections of the 95th to 99th floors on the south side weakened with increasing temperatures and began to sag. Early on, the floors on the north side had sagged and then contracted as the fires moved to the south and the floors cooled. As the fires intensified on the south side, the floors there sagged, and the floor connections weakened. About 20 percent of the connections on the south side of the 97th and 98th floors failed. • The sagging floors with intact floor connections pulled inward on the south perimeter columns, causing them to bow inward. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 151 Collapse Initiation • The bowed south wall columns buckled and were unable to carry the gravity loads. Those loads shifted to the adjacent columns via the spandrels, but those columns quickly became overloaded as well. In rapid sequence, this instability spread all the way to the east and west walls. • The section of the building above the impact zone (near the 98th floor), acting as a rigid block, tilted at least 8 degrees to the south. • The downward movement of this structural block was more than the damaged structure could resist, and global collapse began. 6.14.7 Probable WTC 2 Collapse Sequence Aircraft Impact Damage • The aircraft impact severed a number of exterior columns on the south wall from the 78th floor to the 84th floor, and the wall section above the impact zone moved downward. • After breaching the building’s perimeter, the aircraft continued to penetrate into the building, severing floor framing and core columns at the southeast corner of the core. Insulation was damaged from the impact area through the east half of the core to the north and east perimeter walls. The floor truss seat connections over about one-fourth to one-half of the east side of the core were severed on the 80th and 81st floors and over about one-third of the east perimeter wall on the 83rd floor. The debris severed four columns near the east corner of the north wall between the 80th and 82nd floors. • The impact damage to the perimeter walls and to the core resulted in redistribution of severed column loads, mostly to the columns adjacent to the impact zones. The impact damage to the core columns resulted in redistribution of severed column loads, mainly to other intact core columns and the east exterior wall. The hat truss resisted the downward movement of the south wall. • As a result of the aircraft impact damage, the core carried about 6 percent less gravity load. The north wall carried about 10 percent less, the east face carried about 24 percent more, and the west and south faces carried about 3 percent and two percent more, respectively. • The core was then leaning slightly toward the south and east perimeter walls. The perimeter walls restrained the tendency of the core to lean via the hat truss and the intact floors. Thermal Weakening of the Structure • Under the high temperatures and stresses in the core area, the remaining core columns with damaged insulation were thermally weakened and shortened, causing the columns on the floors above to move downward. Chapter 6 152 NIST NCSTAR 1, WTC Investigation • At this point, the east wall carried about 5 percent more of the gravity loads, and the core carried about 2 percent less. The other three walls carried between 0 percent and 3 percent less. • The long-span floors on the east side of the 79th to 83rd floors weakened with increasing temperatures and began to sag. About one-third of the remaining floor connections to the east perimeter wall on the 83rd floor failed. • Those sagging floors whose seats were still intact pulled inward on the east perimeter columns, causing them to bow inward. The inward bowing increased with time. Collapse Initiation • As in WTC 1, the bowed columns buckled and became unable to carry the gravity loads. Those loads shifted to the adjacent columns via the spandrels, but those columns quickly became overloaded. In rapid sequence, this instability spread all across the east wall. • Loads were transferred from the failing east wall to the weakened core through the hat truss and to the north and south walls through the spandrels. The instability of the east face spread rapidly along the north and south walls. • The building section above the impact zone (near the 82nd floor) tilted 7 degrees to 8 degrees to the east and 3 degrees to 4 degrees to the south prior to significant downward movement of the upper building section. The tilt to the south did not increase any further as the upper building section began to fall, but the tilt to the east was seen to increase to 20 degrees until dust clouds obscured the view. • The downward movement of this structural block was more than the damaged structure could resist, and the global collapse began. 6.14.8 Accuracy of the Probable Collapse Sequences Independent assessment of the validity of the key steps in the collapse of the towers was a challenging task. Some of the photographic information had been used to direct the simulations. For example, the timing of the appearance of broken windows was an input to the fire growth modeling. However, there were significant observables that were usable as corroborating evidence, as shown in Tables 6–10 and 6–11. Some of these were used to establish the quality of the individual simulations of the aircraft impact and the fire growth, as described in Sections 6.9 and 6.10. While the agreement between observations and simulation was not exact, the differences were within the uncertainties in the input information. The generally successful comparisons lent credibility to the overall reconstruction of the disaster. There remained a small, but important number of observations against which the structural collapse sequences could be judged. The comparisons are for Cases B and D impact damage and temperature histories, for which the better agreement was obtained. Reconstruction of the Collapses NIST NCSTAR 1, WTC Investigation 153 Table 6–10. Comparison of global structural model predictions and observations for WTC 1, Case B. Observation Simulation Following the aircraft impact, the tower still stood. The tower remained upright with significant reserve capacity. The south perimeter wall was first observed to have bowed inward at 10:23 a.m. The bowing appeared over nearly the entire south face of the 94th to 100th floors. The maximum bowing was 55 in. on the 97th floor. (The central area in available images was obscured by smoke.) The inward bowing of the south wall at 10:28 a.m. It extended from the 94th to the 100th floor, with a maximum of about 43 in. As the structural collapse began, the building section above the impact and fire zone tilted at least 8 degrees to the south with no discernable east or west component in the tilt. Dust clouds obscured the view as the building section began to fall downward. The south side bowed and weakened. The analysis stopped as the initiation of global instability was imminent. The time to collapse initiation was 102 min from the aircraft impact. There was significant weakening of the south wall and the core columns. Instability was imminent at 100 min. Table 6–11. Comparison of global structural model predictions and observations for WTC 2, Case D. Observation Simulation Following the aircraft impact, the tower still stood. The tower remained upright with significant reserve capacity. The east perimeter wall was first observed to have bowed inward approximately 10 in. at floor 80 at 9:21 a.m. The bowing extended across most of the east face between the 78th and 83rd floors. The inward bowing of the east wall had a maximum value of about 9.5 in. at 9:23 a.m. The bowing extended from the 78th floor to the 83rd floor. The building section above the impact and fire area tilted to the east and south as the structural collapse initiated. The angle was approximately 3 degrees to 4 degrees to the south and 7 degrees to 8 degrees to the east prior to significant downward movement of the upper building section. The tilt to the south did not increase as the upper building section began to fall, but the tilt to the east rose to approximately 25 degrees before dust clouds obscured the view. At point of instability, there was tilting to the south and east. The time to collapse initiation was 56 min after the aircraft impact. The analysis predicted global instability after 43 min. The agreement between the observations and the simulations is reasonably good, supporting the validity of the probable collapse sequences. The exact times to collapse initiation were sensitive to the factors that controlled the inward bowing of the exterior columns. The sequence of events leading to collapse initiation was not sensitive to these factors. Chapter 6 154 NIST NCSTAR 1, WTC Investigation 6.14.9 Factors that Affected Building Performance on September 11, 2001 • The unusually dense spacing of perimeter columns, coupled with deep spandrels, resulted in a robust building that was able to fragment the aircraft upon impact and redistribute loads from severed perimeter columns to adjacent, intact columns. • The wind loads used for the WTC towers, which governed the design of the framed-tube system, significantly exceeded the requirements of the building codes of the era and were consistent with the independent NIST estimates that were based on current state-of-the-art considerations. • The robustness of the perimeter framed-tube system and the large lateral dimension of the towers helped the buildings withstand the impact of the aircraft. • The composite floor system enabled the floors to redistribute loads from places of aircraft impact damage to other locations, avoiding larger scale collapse upon impact. • The hat truss resisted the significant weakening of the core by redistributing loads form the damaged columns to intact columns. As a result of these factors, the buildings would likely not have collapsed under the combined effects of the aircraft damage and subsequent fires if the insulation had not been widely dislodged. The thickness and the condition of the insulation prior to aircraft impact did not play a governing role in the initiation of building collapse. NIST NCSTAR 1, WTC Investigation 155 Chapter 7 RECONSTRUCTION OF HUMAN ACTIVITY 7.1 BUILDING OCCUPANTS 7.1.1 Background While much attention has properly focused on the nearly three thousand people who lost their lives at the World Trade Center (WTC) site that day, the circumstances and efforts that led to the successful evacuation of five times that many people from the WTC towers also have been given attention. Understanding why the loss of life was high or low was one of the four objectives of the Investigation. Success in clearing a building in an emergency can be characterized by two quantities: the time people need to evacuate and the time available to them to do so. For the WTC towers, the times available for escape were set by the collapse of the buildings. Neither the building occupants nor the emergency responders knew those times in advance. Moreover, the times were also three or four times shorter than the time needed to clear the tenant spaces of WTC 1 following the 1993 bombing. The investigators examined the design of the buildings, the behavior of the people, and the evacuation process in detail to ascertain the factors that figured prominently in the time needed for evacuation. In analyzing these factors, NIST recognized that there were inherent uncertainties in constructing a valid portrayal of human behavior on that day. These included limitations in the recollections of the people, the need to derive findings from a statistical sampling of the building population, the lack of information from the decedents on the factors that prevented their escape, and the limited knowledge of the damage to the interior of the towers. NIST carefully considered these uncertainties in developing its findings and is confident in those findings and related recommendations. 7.1.2 The Building Egress System Examination of drawings, memoranda, and calculations showed that the standard emergency evacuation procedures required using the three stairwells. The elevators were not to be used, and the doors to the roof were to be kept locked. Under most circumstances, a local evacuation would be ordered. The people on the floors near the threat would move to three floors below the incident. Under more severe circumstances, a full building evacuation would be ordered, requiring all occupants to leave the building by way of the stairwells. As noted in Section 1.2.2, the locations of the stairwells differed at various heights in the buildings. This, combined with the aircraft impacting different floors in the two towers, the different aircraft impact location relative to the center of the building, and the different orientation of the core (Section 1.2.2), led to different damage to the stairwells. As shown in Figure 7–1, a frame from a simulation from a NIST contractor, Applied Research Associates (Section 6.9), the stairwell separation in this region of WTC 1 was the smallest in the building—clustered together well within the building core—and American Airlines Flight 11 destroyed all three stairwells from the 92nd floor upward. By contrast, the separation of the stairwells in the impacted region of WTC 2 was the largest in the building, i.e., they were located Chapter 7 156 NIST NCSTAR 1, WTC Investigation along different boundaries of the building core. United Airlines Flight 175 destroyed Stairwells B and C, but not Stairwell A (Figure 7–2). Figure 7–1. Simulated impact damage to 95th floor of WTC 1, including stairwells, 0.7 s after impact. Figure 7–2. Simulated impact damage to WTC 2 on floor 78, 0.62 s after impact. Reconstruction of Human Activity NIST NCSTAR 1, WTC Investigation 157 7.1.3 The Evacuation—Data Sources To document the egress from the two towers as completely as possible, NIST: • Contracted with the National Fire Protection Association and the National Research Council of Canada to index a collection of over 700 previously published interviews with WTC survivors. • Listened to and analyzed 9-1-1 emergency phone calls made during the morning of September 11. • Analyzed transcripts of emergency communication among building personnel and emergency responders. • Examined complaints filed with the Occupational Safety and Health Administration by surviving occupants and families of victims regarding emergency preparedness and evacuation system performance. In addition NIST, in conjunction with NuStats, Partners, LLP as a NIST contractor, conducted an extensive set of interviews with survivors of the disaster and family members of occupants of the buildings. First, telephone interviews were conducted with 803 survivors, randomly selected from the list of approximately 100,000 people who had badges to enter the towers on that morning. The results enabled a scientific projection of the population and distribution of occupants in WTC 1 and WTC 2, as well as exploration of factors that affected evacuation. Second, 225 face-to-face interviews, averaging 2 hours each, gathered detailed, first-hand accounts and observations of the activities and events inside the buildings on the morning of September 11. These people included occupants near the floors of impact, witnesses to fireballs, mobility-impaired occupants, floor wardens, building personnel with emergency response responsibilities, family members who spoke to an occupant after 8:46 a.m., and occupants from regions of the building not addressed by other groups. Third, six complementary focus groups, a total of 28 people, were convened, consisting of: 1. Occupants located near the floors of impact, to explore the extent of the building damage and how the damage influenced the evacuation process. 2. Floor wardens, to explore the implementation of the floor warden procedures and the effect those actions had on the evacuation of the occupants on a floor and the evacuation of the floor warden. 3. Mobility-impaired occupants, to explore the effect of a disability on the evacuation of the occupant and any other individuals who may have assisted or otherwise been affected by the evacuee. 4. Persons with building responsibilities, to capture the unique perspective of custodians, security, maintenance, or other building staff. Chapter 7 158 NIST NCSTAR 1, WTC Investigation 5. Randomly selected evacuees in WTC 1, to explore further the variables that best explained evacuation delay and normalized stairwell evacuation time, including environmental cues, floor, and activities. 6. Randomly selected evacuees in WTC 2, for the same purpose as the preceding group. The following sections describe the key findings from this large data set. 7.1.4 Occupant Demographics The following were estimated from statistical analysis of the telephone interview data: • There were 17,400 ± 1,180 occupants inside WTC 1 and WTC 2 at 8:46 a.m. Of these, 8,900 ± 750 were inside WTC 1 and 8,540 ± 920 were inside WTC 2. • Men outnumbered women roughly two to one. • The mean and median ages were both about 45, with the distribution ranging from the early 20s to the late 80s. • The mean length of employment at the WTC was almost six years, but the median was only two years tenure within WTC 1 and three years within WTC 2. • Sixteen percent of the evacuees were present during the 1993 bombing, although many others knew of the evacuation. • Two-thirds had participated in at least one fire drill in the 12 months prior to the 2001 disaster. Eighteen percent did not recall whether they had participated or not; 18 percent reported that they had not. New York City law prohibited requiring full evacuation using the stairs during fire drills. • Six percent reported having a limitation that constrained their ability to escape. (This extrapolated to roughly 1,000 of the WTC 1 and WTC 2 survivors.) The most common of these limitations, in decreasing order, were recent injury, chronic illness, and use of medications. Estimates based on the layouts of the tenant spaces indicated that approximately 20,000 people worked in each tower. Relatively few visitors would have been present at 8:46 a.m. Thus, the towers were between one-third and one-half full at the time of the attack. 7.1.5 Evacuation of WTC 1 The number of survivors evacuated from WTC 1 was large, given the severity of the building damage and the unexpectedly short available time. Of those who were below the impact floors when the aircraft struck, 99 percent survived. About 84 percent of all the occupants of the tower at the time survived. The aircraft impact damage left no exit path for those who were above the 91st floor. It is not known how many of those could have been saved had the building not collapsed. While it is possible that a delayed Reconstruction of Human Activity NIST NCSTAR 1, WTC Investigation 159 or avoided collapse could have improved the outcome, it would have taken many hours for the FDNY to reach the 92nd floor and higher and then to conduct rescue and fire suppression activity there. The general pattern of the evacuation was described in Chapter 2. The following are specific facts derived from the interviews: • The median time to initiate evacuation was 3 min for occupants from the ground floor to floor 76, and 5 min for occupants near the impact region (floors 77 through 91). The factors that best explained the evacuation initiation delays were the floor the respondent was on when WTC 1 was attacked, whether the occupant encountered smoke, damage or fire, and whether he or she sought additional information about what was happening. • Occupants throughout the building observed various types of impact indicators throughout the building, including wall, partition, and ceiling damage and fire and smoke conditions. The filled-in squares in Figure 7–3 indicate the floors on which the different observations were reported. • Damage to critical communications hardware likely prevented announcement transmission, and thus occupants did not hear announcements to evacuate, despite repeated attempts from the lobby fire command station. • Evacuation rates reached a maximum in approximately 5 min, and remained roughly constant until the collapse of WTC 2, when the rate in WTC 1 slowed to about 20 percent of the maximum. • The maximum downward travel rate was just over one floor per minute, slower than the slowest speed measured for non-emergency evacuations. This was in part because: − Occupants encountered smoke and/or damage during evacuation. − Occupants were often unprepared for the physical challenge of full building evacuation. − Occupants were not prepared to encounter transfer hallways during the descent. − Mobility-impaired occupants were not universally identified or prepared for full building evacuation. − Occupants interrupted their evacuation. • The mobility-impaired occupants did not evacuate as evenly as the general population. − Those who were ambulatory generally walked down the stairs with one hand on each handrail, taking one step at a time. They were typically accompanied by another occupant or an emergency responder. Combined, they blocked others behind them from moving more rapidly. − On the 12th floor, FDNY personnel found 40 to 60 people, some of whom were mobility impaired. The emergency responders were assisting about 20 of these mobility-impaired Chapter 7 160 NIST NCSTAR 1, WTC Investigation people down the stairs just prior to the collapse of the building. It is unknown how many of this group survived. − Some mobility-impaired occupants requiring assistance to evacuate were left by coworkers, thereby imposing on strangers for assistance. 7.1.6 Evacuation of WTC 2 The evacuation from WTC 2 was markedly different from that from WTC 1. Over 90 percent of the occupants had started to self-evacuate before the second aircraft struck, and three-quarters of those from above the 78th floor had descended below the impact region prior to the second attack. (Nearly 3,000 occupants were able to survive due to self-evacuation and the use of the still-functioning elevators.) As a result, 91 percent of all the occupants survived. Eleven people from below the impact floors perished, about 0.1 percent. Eighteen people in or above the impact zone when the plane struck are known to have found the one passable stairway and escaped. It is not known how many others from the impact floors or above found their way to the passable stairway and did not make it out or how many could have been saved had the building not collapsed. A delayed or avoided collapse could have provided the additional time for more people to learn about and use the passable stairway. The general pattern of the evacuation was described in Chapter 3. The following are specific facts derived from the interviews: • The median time to initiate evacuation was 6 min, somewhat longer than in WTC 1. • As in WTC 1, occupants observed various types of impact indicators throughout the building, including wall, partition, and ceiling damage and fire and smoke conditions (Figure 7–4). • Building announcements were cited by many as a constraint to their evacuation, principally due to the 9:00 a.m. announcement instructing occupants to return to their work spaces. Crowdedness in the stairways, lack of instructions and information, as well as injured or disabled evacuees in the stairwells were the most frequently reported obstacles to evacuation. • Evacuation rates from WTC 2 showed three distinct phases: (1) Before WTC 2 was attacked, occupants used elevators, as well as stairs, to evacuate, resulting in approximately 40 percent of the eventual survivors leaving the building during that 16 min window. (2) After WTC 2 was attacked and the elevators were no longer operational, the evacuation rate slowed down to a steady rate equivalent to the rate observed in WTC 1, which also had only stairs available to occupants. (3) About 20 min prior to building collapse, the rate in WTC 2 slowed to approximately 20 percent of the stairwell-only evacuation rate. NIST NCSTAR 1, WTC Investigation 161 Reconstruction of Human Activity Smoke Sprinklers / water Fatally injured people Power outage tiles –– 110 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? –– 100 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? –– 90 –– 80 Smoke Sprinklers / water Fatally injured people Power outage fuel Collapsed –– 70 –– 60 –– 50 Smoke Sprinklers / water Fatally injured people Power outage Jet fuel Fallen ceiling tiles Fire alarms Collapsed walls Extreme heat Fire Fireballs –– 40 –– 30 –– 20 ? ? ? ? ? ? ? ? ? ? ? –– 10 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Jet fuel Fallen ceiling Fire alarms Collapsed walls Extreme heat Fire Fireballs Jet Fallen ceiling tiles Fire alarms walls Extreme heat Fire Fireballs Figure 7–3. Observations of building damage after initial awareness but before beginning evacuation in WTC 1. 162 NIST NCSTAR 1, WTC Investigation Chapter 7 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Smoke Sprinklers / water Fatally injured people Power outage Jet fuel Fallen ceiling tiles Fire alarms Collapsed walls Extreme heat Fire Fireballs –– 70 ? ? ? ? ? ? ? ? ? ? ? –– 60 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? –– 50 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Smoke Sprinklers / water Fatally injured people Power outage Jet fuel Fallen ceiling tiles Fire alarms Collapsed walls Extreme heat Fire Fireballs –– 40 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? –– 30 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? –– 20 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? –– 10 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Smoke Sprinklers / water Fatally injured people Power outage Jet fuel Fallen ceiling tiles Fire alarms Collapsed walls Extreme heat Fire Fireballs ? ? ? ? ? ? ? ? ? ? ? –– 110 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? –– 100 ? ? ? ? ? ? ? ? ? ? ? –– 90 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? –– 80 ? ? ? ? ? ? ? ? ? ? ? Figure 7–4. Observations of building damage from tenant spaces in WTC 2. Reconstruction of Human Activity NIST NCSTAR 1, WTC Investigation 163 7.2 EMERGENCY RESPONDERS 7.2.1 Data Gathered The attack on the World Trade Center produced a massive response from the emergency services within New York City. As a result, copious information was produced concerning the attack and the emergency response. Although some key information was lost when the buildings collapsed, an extensive amount was obtained from three organizations that contributed to the emergency response: The Port Authority of New York and New Jersey (Port Authority), FDNY, and the New York City Police Department (NYPD). There also was a significant amount of information available through the various media services. Some of the items were transferred to NIST; the Investigation Team examined others at locations in the New York City area. The data fell into four categories. Documentary Data This included procedures for conducting operations at the WTC, records generated during the WTC operations, and records generated following the event. The last group of documents included detailed investigative reports of the FDNY and NYPD operations by McKinsey and Company, documents of investigative first-person interviews, and lists of decedents. Electronic Data These were recordings of radio and telephone communications. Some were already in digital format; those on tape were digitized and/or transcribed. Some recordings required sound enhancement to improve comprehension. First-Person Interviews In October 2003, NIST entered into a three-party agreement between NIST, New York City (NYC), and the National Commission on Terrorist Acts Upon the United States (the 9/11 Commission). The agreement provided procedures under which NIST and the 9/11 Commission would interview a maximum of 125 NYC emergency responders, 100 from FDNY and 25 from NYPD. In December 2003, NIST officially requested and the Port Authority agreed to interviews with 15 Port Authority personnel, including emergency responders, safety, security, and management personnel. In addition to the interviews conducted under the agreements described above, NIST interviewed eight people who contacted NIST directly and volunteered. The first-person interviews were conducted beginning in October 2003 and were completed in December 2004. The organizations and the number of interviews conducted were: • FDNY (68 interviews): Senior management and officers, mid-level officers, company officers, firefighters, emergency medical personnel, and dispatchers • NYPD (25 interviews): Senior management and officers, mid-level officers, Emergency Service Unit personnel, aviation personnel, and dispatchers Chapter 7 164 NIST NCSTAR 1, WTC Investigation • PANYNJ/PAPD (15 interviews): Senior management personnel, facility safety personnel, building security personnel, facility communications personnel, building vertical transportation personnel, senior PAPD officers, mid-level PAPD officers, and line PAPD officers • Other (8 interviews): A building security guard, dispatcher, firefighters, WTC building engineer, and a fire safety director Each interview generally took from 1 hour to 4 hours to complete, depending on the person’s job and the complexity of their involvement in emergency operations. An interview included a self-narrative regarding the emergency responder’s experience at the WTC and follow-up questions by staff from NIST and the 9/11 Commission. Visual Data These still photographs and video footage became part of the collection described in Section 6.3. 7.2.2 Operation Changes following the WTC 1 Bombing on February 26, 1993 This unprecedented act had provided insight into the complex nature of responding to a large incident at the WTC towers. As a result, numerous issues were raised concerning the WTC buildings in relationship to the emergency response. A multiagency study identified issues of security, occupant safety, and emergency responder operations and safety. The following changes made by The Port Authority and the FDNY had a direct impact on emergency responder operations on September 11, 2001. The Port Authority • Improved egress from the towers at the Concourse Level. • Made improvements to the stairwells: battery operated emergency lighting, photoluminescent floor strips indicating the path to be followed, and explicit signs on each doorway to indicate where it led. • Established a PAPD Command Center inside of WTC 5. • Installed Fire Command Desks in the lobbies of WTC 1 and WTC 2. • Installed in WTC 5 a radio repeater that operated on the FDNY city-wide high-rise frequency. (The radio repeater’s function was to receive FDNY radio communication on a specified radio frequency, amplify the signal power, and retransmit the radio communications on another specified radio frequency that the FDNY radios could receive. This could enhance communications in buildings made of steel and reinforced concrete that pose challenges to radio-frequency communication.) The antenna was located on the top of WTC 5 and was directed at WTC 1 and WTC 2 (Figure 7–5). On September 11, 2001, the controls for operating the repeater were located at the Fire Command Desks in the tower lobbies. Reconstruction of Human Activity NIST NCSTAR 1, WTC Investigation 165 • Upgraded the elevator intercom system to be monitored at the lobby Fire Command Desks. • Constructed an Operations Control Center on the B1 level of WTC 2 with the capability to monitor all HVAC systems and elevators. • Installed a decentralized fire alarm system, with three separate data risers to transponders located every three floors, redundant control panels and electronics, and multiple control station announcement capability. • Conducted fire drills in conjunction with FDNY. FDNY • Published a new Incident Command System manual in May 1997. • Purchased eighty 800 MHz radios for use by deputy fire commissioners, each staff chief, and the Field Communications Unit. Twenty of the radios were to be distributed by the Field Communications unit at an incident, if needed. • Issued Port Authority radios to those FDNY companies located near the WTC that often responded to the WTC, allowing them to communicate with the building’s Deputy Fire Safety Directors and with PAPD. In addition, The Port Authority and New York City signed two agreements applying to the fire safety of Port Authority facilities located in New York City. The first agreement was for the implementation of fire safety recommendations that would be made by FDNY after they had inspected Port Authority facilities located in New York City. The second recognized the agreement that FDNY could conduct fire safety inspections of Port Authority properties in New York City. It provided guidelines for FDNY to communicate needed corrective actions to The Port Authority, and it assured that new or modified fire safety systems were to be in compliance with local codes and regulations. It also required a third party review of the systems by a New York State licensed architect or engineer. Figure 7–5. Location of the radio repeater. Source: Original artwork by Marco Crupi. Enhanced by NIST. Chapter 7 166 NIST NCSTAR 1, WTC Investigation 3 26 35 66 121 171 214 0 0 6 23 30 74 103 0 50 100 150 200 250 8:46 8:48 8:50 9:00 9:15 9:59 10:29 Time Number of Units Dispatched Signal Arrival 10-84 7.2.3 Responder Organization The emergency response to the attack was immediate. Within 3 min of the aircraft impact on WTC 1, PAPD was providing information on the attack to the police desk, FDNY had dispatched 26 units to the scene, and NYPD had called a department mobilization that included dispatching aviation units to the WTC for visual assessment. Within 10 min, PAPD had called a chemical mobilization; NYPD had dispatched five Emergency Service Unit (ESU) teams and had two aviation units at the scene providing observations. Within 30 min, 121 FDNY units had been dispatched to the scene and 30 units had signaled their arrival at the scene by pushing the “10-84” button on the vehicle’s communications console (Figure 7–6). FDNY Under New York City policy, since this was identified as a fire incident, FDNY was to be in control of the site. By 8:50, FDNY was operating from the Fire Command Desk in the lobby of WTC 1. Within minutes, the Incident Command Post was moved outside to West Street. The FDNY also maintained the lobby Command Post inside WTC 1 and established one in WTC 2. Additional command posts were established in the lobby of the Marriott Hotel (WTC 3) and on the corner of West and Liberty Streets. Some of the first FDNY personnel on the scene had actually seen the aircraft hit the building and knew that the upper floors were badly damaged, including the building safety systems. They also saw the victims burned by the fireball that came into the building lobby. Upon meeting with Port Authority personnel and the previous WTC 1 Deputy Fire Safety Director, who had recently trained the new Fire Safety Director, to learn more about building conditions, FDNY personnel quickly made judgments related to building conditions and emergency response operations that were, in retrospect, highly accurate, for example: • There were large fires burning on multiple floors at and above the impact zone. • Smoke, fire, and structural damage in the buildings prevented many building occupants from evacuating floors above the impact zones. • Many of the people trapped above the impact zones were already dead or would likely die before emergency responders could reach them. • Localized collapses within and above the impact zones were possible due to the structural damage and fires. Figure 7–6. Timing of FDNY unit arrivals. Reconstruction of Human Activity NIST NCSTAR 1, WTC Investigation 167 • The elevators, some with people trapped inside, were generally not working and/or were not safe for use during the WTC operations. • Firefighters would have to gain access to the injured and trapped occupants by climbing the stairs and carrying the equipment needed up the stairs. • It would take hours to accumulate sufficient people and equipment to access the impact zones. • The sprinkler and standpipe systems were compromised at the impact zone and firefighting would not be an option until a reliable water supply was established and equipment was carried up. • Jet fuel had flowed into the elevator shafts and into other parts of the buildings and presented a danger to building occupants and emergency responder personnel. Those in command decided that the response strategy was to enable the evacuation of those below the impact and fire zones. However, those directing initial operations inside the buildings followed an additional strategy: get sufficient people and equipment upstairs to cut a path through the fire and debris to rescue occupants above the fires. The strategy of company-level personnel, who were trained to fight fires and perceived this as a conventional, large high-rise fire, was to get to the fire floors and extinguish the fires. Overlaying this trinity of operational strategies was the fact that this was the largest emergency response in FDNY history, with roughly 1,000 firefighters on the scene. Even the singularly large response to the 1993 bombing involved about 700 emergency personnel. A typical two-alarm fire might have involved about 100 personnel. Thus, keeping track of what all these people were doing, where there were located, where they were going, and what they would do when they got there was a task without precedent. The principal tools for this were three 18 in. x 28 in. magnetic boards known as Fire Command Boards (Figure 7–7). They were located in the lobbies of WTC 1 and WTC 2 and at the Incident command Post on West Street. On each Board, magnetic identifiers of different colors identified engines, ladder and tower ladders, battalions, special units, and sectors. Unit numbers were written on the identifiers with marking pens. These Boards became overwhelmed after about 30 min due to the large number of people and units arriving at the scene. Some emergency personnel that arrived at the site did not report to the Command Posts or were not logged in on the Command Board. A formal analysis of arrivals and missions of the various units was compromised by the loss of the Boards in the collapse of the towers; there were no backup records. NYPD The roles of the NYPD were to establish traffic control and perimeter security at the site, provide security for the command posts, and conduct evacuation and rescue operations within the towers. Their aviation units supplied observation capability and assessed the potential for roof rescue. The primary mobilization point for the NYPD Special Operations Division (SOD) that sent Emergency Service Unit (ESU) rescue teams into the WTC was at the corner of Church and Vesey Streets at the Chapter 7 168 NIST NCSTAR 1, WTC Investigation Figure 7–7. Fire Command Board located in the lobby of WTC 1. northeast corner of the WTC tract. The post was managed by a SOD detective who had just gone off of duty and was still at his office when the attack occurred. He dispatched six ESU teams, each consisting of about five people. Records for each team were written on paper attached to a clipboard. A second SOD mobilization point was established at the corner of West and Vesey Streets at the northwest corner of the WTC tract. The armed NYPD officers and ESU teams provided security for the FDNY Incident Command Post. Since there were few NYPD units and since they typically arrived with all members, keeping track of the units was less problematic than for the FDNY. However, with the collapse of WTC 2, all written records were lost as the high winds and debris blew through the mobilization points. Since NYPD had only about 50 personnel operating in or near the towers, the managers of the mobilization points were able to easily reconstruct the lost data on their personnel. Although The Port Authority had not endorsed a plan for roof rescue from the towers, it appeared to be one of the few options available for occupants trapped above the fires. NYPD helicopters reached the scene by 8:52 to assess the possibility of roof rescue. They were unable to land on the roof due to heavy smoke conditions. During the first hour, FDNY did not consider the option of roof rescue. When the aircraft struck WTC 2, it was clear that this was criminal activity, and the decision regarding roof top operations became the responsibility of NYPD. The NYPD First Deputy Commissioner ordered that no roof rescues were to be attempted, and at 9:43 a.m., this directive was passed to all units. Roof rescue was not intended to be an option, and The Port Authority reported that it never advised tenants to evacuate upward. The Port Authority’s standard full-building occupant evacuation procedures and drills required the use of stairways to exit at the bottom of the WTC towers. The standard procedures were to keep the doors to the roof locked. Roof access required use of an electronic swipe card to get through the first two doors and a security officer watching a closed-circuit camera on the 22nd floor of WTC 1 to open the third door via a buzzer. (The 1968 NYC Building Code required access to roofs like these, most likely to provide FDNY access. The 2003 code does not intend roof access to be used for evacuation and has no prohibition on locking this access.) Reconstruction of Human Activity NIST NCSTAR 1, WTC Investigation 169 The NYPD and FDNY did not consider roof rescue a viable strategy for general evacuation. First, the NYPD and FDNY policies for roof operations were focused mainly on providing emergency responders with access into the building above the fire floors for firefighting, conventional rescue, and comforting occupants. Roof rescue was considered a measure of last resort to be used, for example, to assist occupants with medical emergencies. Second, although on September 11, an NYPD aviation unit was early on the scene to consider the possibility, smoke and heat conditions at the top of the towers prevented the conduct of safe roof operations, despite repeated attempts. Even if it had been possible for a helicopter to gain access to the roof, only a very small fraction of the large number of people trapped above the impact zone could have been rescued before the towers collapsed. Nonetheless, perhaps as an indication of the dire situation in the top floors of the towers, at least two decedents tried to get to the roof and found the roof access locked in both the WTC towers. Personnel at the WTC 1 Security Control Center on the 22nd floor attempted to electronically release the doors to the roof, but were unsuccessful due to damage to the computerized control system. PAPD The roles of the PAPD were to establish security at the WTC and to conduct evacuation operations. PAPD officers were performing their normal law enforcement duties at the WTC site when the attack on WTC 1 occurred. Several additional PAPD teams were dispatched from various locations from around the city and from Jersey City, with some arriving before the collapse of WTC 2 and reporting to PAPD personnel at the WTC 1 lobby Fire Command Desk. There were dozens of PAPD officers on site and on orders to report to the site. With the collapse of WTC 2, the PAPD Police Desk (in WTC 5) and the Command Center were evacuated. Many of the emergency response records were lost initially, but were recovered some days later. Interdepartmental Interactions The coordination of communications and operations between the responding authorities at the WTC site was a challenge for all emergency responders working that morning. The short time duration between the initial attack and the collapse of the towers, coupled with the large number of responders and their staggered arrivals, compounded the difficulty of establishing a unified operation. FDNY (and the Emergency Medical Services), NYPD, PAPD, The Port Authority, and OEM were attempting to work together. These efforts were stymied by a lack of existing protocols that clearly defined authorities and responsibilities, communications systems problems, and multiple major attacks and threats. Although there was merit to having the FDNY and NYPD Command Posts separated, there was no uniform means for communicating between the two Command Posts at the time when WTC 2 collapsed. FDNY and NYPD were primarily operating as independent organizations based on their operational responsibilities. 7.2.4 Responder Access Fighting fires in the upper levels of tall buildings is not the same as fighting fires in buildings that are less than 100 ft high. In the case of the WTC towers, the people needing assistance were mostly many stories above the ground, and climbing tens of flights of stairs was the only way upward for the emergency Chapter 7 170 NIST NCSTAR 1, WTC Investigation responders. In the time available, they were not able to get very far. For example, emergency responders wearing police uniforms, not wearing Self-contained Breathing Apparatus (SCBA), and not carrying extra equipment, were able to climb the stairs at a rate of approximately 1.4 min per floor while climbing to floors in the 40s inside of WTC 1. The climbing rate for firefighters wearing protective clothing and SCBA and carrying extra firefighting and rescue equipment was about 2 min per floor. The downward flow of evacuees, especially those who had physical disabilities or were obese, also slowed the responders’ progress, especially in the 44 in. wide stairwells. The flow of the evacuees caused teams of emergency responders to become separated, further disrupting team operations. Neither the number of responders who entered the towers nor the floors they reached are known, due to the incompleteness of the Command Boards and their eventual destruction. From radio communications and first-person interviews, it appears that there were responders as high as floors in the 50s in WTC 1 and the 78th floor in WTC 2. 7.2.5 Communications There were multiple equipment systems for command-to-field communications, for responders to communicate among themselves, and for contact to and from building occupants: • Landline telephone system (including access to the 9-1-1 system), • Emergency announcement systems within WTC 1 and WTC 2, • Cellular systems (including access to the 9-1-1 system), • Warden phones (tower stair landings to Command Post), • Firefighter phones, called standpipe phones, in the WTC towers, and • FDNY handie-talkies, with booster support from a repeaters on WTC 5 and a Battalion car repeater located inside WTC 2. Within WTC 1, the system used to make the emergency announcements was disabled by the first aircraft impact, communications to the elevators in the upper third of the buildings were lost, the Warden phones did not work, and attempts to use the landline phones to contact people upstairs were unsuccessful due to the failure of some phones in the building. Little is known about the function of the internal communications inside WTC 2 after the aircraft struck the building. This is because all of the key emergency responders working inside WTC 2 died when the building collapsed. However, interviews with some occupants who evacuated from the building and interviews with emergency responders who communicated with counterparts inside WTC 2 indicated the following: some of the building’s public address systems were working, some of the elevator phone systems were working, and some of the landline telephones were working. It is not known if the Warden Phone system was fully operational or if the standpipe phones were operational. Emergency responder communications inside WTC 2 primarily depended on radio and face-to-face communications. Reconstruction of Human Activity NIST NCSTAR 1, WTC Investigation 171 The collapse of WTC 2 caused the cellular phone system in Lower Manhattan to fail. However, there were still landlines working in the city blocks adjacent to the WTC site, and calls were still emanating from inside WTC 1. All of the radio systems analyzed were working well just before the attack on the WTC. However, PAPD, FDNY, and NYPD were aware that radio communications had not fared well in high-rise buildings, including WTC 1 following the 1993 bombing. The vast amount of metal and steel-reinforced concrete in high-rise buildings was known to attenuate and block radio signals, especially the low output power emergency responder handie-talkies. This was again a problem on September 11, 2001, when all three agencies encountered difficulties with their hand-held units. Thus, there was a heavy burden placed on the FDNY repeater to boost the weak signals to a discernable level. The repeater was functional during operations at the WTC; apparently the antenna was not damaged by debris from the aircraft impacts. However, within WTC 1, the system did not function correctly. The cause of this malfunction could not be determined since the unit was destroyed in the collapse of WTC 1. Repeater recording communications suggest that it was used within WTC 2. The radio recordings showed that communications readability using the repeater channel was generally good to excellent. Where readability levels were poor, it was generally caused by multiple people attempting to communicate over the radio at one time. The heavy traffic continued until the repeater failed with the collapse of WTC 2. Had communications using the repeater been adequate in WTC 1, there would have been opposing effects on the quality of operations and life safety. On the positive side, the emergency personnel in the tower would have been in at least some contact with the Command Posts. However, two serious counterpoints would have occurred. First, if the responders in both towers were using the same repeater at the same time, the traffic would have been heavier, and more of the calls would have been indecipherable. Second, a firefighter in either tower would have had difficulty discerning which communications related to operations in his tower. Given the inadequate markings within the towers and the unfamiliarity of some emergency responders with the site, there was already a high degree of confusion as to which tower a responder was in. The poor radio communications at the WTC had a serious impact on the FDNY Command Post’s attempts to maintain command and control in general. All emergency responders struggled with the high volume and low quality of radio communications traffic at the WTC, described as “radio gridlock.” NIST estimates that one-third to one-half of the emergency responder radio communications were undecipherable or incomplete. The poor communications had a critical effect on the conveyance of evacuation instructions. As early as 8:48, there was an order to WTC personnel to clear WTC 1. At 8:59 a.m., a senior PAPD officer called for the evacuation of the two towers. At 9:01 a.m., this was extended to the entire complex. This was before the second aircraft struck. At 9:04 a.m., WTC Operations told people to evacuate an unidentified building. At 10:06 a.m., an NYPD aviation unit reported that it wouldn’t be much longer before WTC 1 would come down. Some survivors reported not having received any of these messages. It is not known how many others did not, nor whether their locations were such that they could have made it out of the buildings in time. Chapter 7 172 NIST NCSTAR 1, WTC Investigation 7.2.6 The Overall Response It was difficult to quantify the responders’ degree of success. There were multiple reports of FDNY, NYPD, and PAPD efforts making the difference between death and survival. There were reports of assistance where the survival of the occupants was not determined. There were reports of firefighters quenching small fires on the lower floors of the towers and at the impact point in WTC 2. However, it would have been impossible for them to have had any significant effect on the fires that eventually led to the collapse of the structures. 7.3 FACTORS THAT CONTRIBUTED TO ENHANCED LIFE SAFETY 7.3.1 Aggregate Factors • Reduced number of people in the buildings at the times of aircraft impact. • Functioning elevators in WTC 2 for the 16 min prior to 9:02:59 a.m. • Remoteness of Stairwell A from the impact zone and debris field. • Participation of two-thirds of surviving occupants in recent fire drills. • Upgrades to the life safety system components after the 1993 bombing. • Evacuation assistance provided by emergency responders to evacuees. 7.3.2 Individual Factors • Location below the floor of impact. • Shortness of delay in starting to evacuate.