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THE DESIGN AND CONSTRUCTION OF THE TOWERS
5.1 BUILDING AND FIRE CODESEdit
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
- Main article: NIST NCSTAR 1 full text:Chapter 6
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.