The NIST WTC 7 investigation report: A critique

By Richard Schulte
Schulte & Associates, Evanston, Ill.

In November 2008, The National Institute of Standards and Technology (NIST) issued the final report on its investigation into the collapse of the WTC 7 building on the afternoon of September 11, 2001. While the reasons why the WTC 1 and WTC 2 buildings, the World Trade Center towers, collapsed on the morning of September 11 were rather obvious - buildings are not designed for the high-speed impact of large commercial aircraft, combined with the effects of the fire that will occur after the impact - the reasons for the collapse of WTC 7 have been the subject of much conjecture. Hence, the NIST report on the collapse of WTC 7 is actually of more interest to those in the fire protection field than the NIST report on the collapse of the WTC towers.

Following are excerpts from NIST's final report on the investigation of the collapse of WTC 7, referred to as NCSTAR 1A:

"At the time of this publication, this effort has resulted in twenty-three changes being adopted into the International Building Code and fifteen introduced into the NFPA 5000 Building Code, NFPA 1 Fire Code and NFPA 101 Life Safety Code. … All of these steps will lead to improved safety and security of buildings, building occupants and emergency responders." (page xxxi)

"This is the final report of the National Institute of Standards and Technology (NIST) investigation into the collapse of WTC 7, conducted under the National Construction Safety Team Act. The report is the result of an extensive, state-of-the-art reconstruction of the events that affected WTC 7 and eventually led to its collapse. Numerous facts and data were obtained then combined with validated computer modeling to produce an account that captures the key features of what actually occurred. However, the reader should keep in mind that the building and the records kept within the building were destroyed, and the remains of all the WTC buildings were disposed of before congressional action and funding was available for this Investigation to begin. As a result, there are some facts that could not be discerned and, thus, there are uncertainties in this accounting. Nonetheless, NIST was able to gather sufficient evidence and documentation to conduct a full investigation upon which to reach firm findings and recommendations." (page xxxv)

"WTC 7 was a 47-story office building located immediately north of the main WTC complex. … On September 11, 2001, WTC 7 endured fires for almost 7 hours, from the time of collapse of the north WTC tower (WTC 1) at 10:28:22 a.m. until 5:20:52 p.m., when it collapsed. This was the first known instance of the total collapse of a tall building primarily due to fires." (page xxxv)

"WTC 7 was unlike the WTC towers in many respects. It was a more typical tall building in the design of its structural systems. It was not struck by an airplane. The fires in WTC 7 were quite different from those in the towers. Since WTC 7 was not doused with thousands of gallons of jet fuel, large areas of any floor were not ignited simultaneously. Instead, the fires in WTC 7 were similar to those that have occurred in several tall buildings where the automatic sprinklers did not function or were not present. These other buildings did not collapse, while WTC 7 succumbed to its fires." (page xxxv)

"The fires in WTC 7 were ignited as a result of the impact of debris from the collapse of WTC 1, which was approximately 110 m (350 ft) to the south. The debris also caused structural damage to the southwest exterior of WTC 7, primarily between Floors 7 and 17. The fires were ignited on at least 10 floors; however, only the fires on Floors 7 through 9 and 11 through 13 grew and lasted until the time of the building collapse. These uncontrolled fires had characteristics similar to those that have occurred previously in tall buildings. Their growth and spread were consistent with ordinary building contents fires. Had a water supply for the automatic sprinkler system been available and had the sprinkler system operated as designed, it is likely that fires in WTC 7 would have been controlled and the collapse prevented. However, the collapse of WTC 7 highlights the importance of designing fire-resistant structures for situations where sprinklers are not present, do not function (e.g., due to disconnected or impaired water supply), or are overwhelmed." (page xxxvi)

"There were no serious injuries or fatalities, because the estimated 4,000 occupants of WTC 7 reacted to the airplane impacts on the two WTC towers and began evacuating before there was significant damage to WTC 7. The occupants were able to use both the elevators and the stairs, which were as yet not damaged, obstructed, or smoke-filled. Evacuation of the building took just over an hour. … The decision not to continue evaluating the building and not to fight the fires was made hours before the building collapsed, so no emergency responders were in or near the building when the collapse occurred." (page xxxvii)

"The stairwells were narrower than those required by the NYCBC, but, combined with the elevators, were adequate for a timely evacuation on September 11, 2001, since the number of building occupants was only about half that expected during normal business hours." (page xxxvii)

"The collapse of WTC 7 could not have been prevented without controlling the fires before most of the combustible building contents were consumed." (page xxxvii)

"The partial or total collapse of a building due to fire is an infrequent event. This is particularly true for buildings with a reliably operating active fire protection system such as an automatic sprinkler system. A properly designed and operating sprinkler system will contain fires while they are small and, in most instances, prevent them from growing and spreading to threaten structural integrity." (page xxxviii)

"The intent of current practice, based upon prescriptive standards and codes, is to achieve life safety, not collapse prevention. However, the key premise of NIST's recommendations is that buildings should not collapse in infrequent (worst-case) fires that may occur when active fire protection systems are rendered ineffective, e.g. when sprinklers do not exist, are not functional, or are overwhelmed by the fire, or where the water supply is impaired." (page xxxviii)

"WTC 7 was an irregular trapezoid, approximately 100 m (329 ft) long on the north face and 75 m (247 ft) long on the south face, 44 m (144 ft) wide, and 186 m (610 ft) tall. The 47-story building contained approximately 200,000 m2 (2 million ft2) of floor area. A typical floor was the size of a football field. The gross floor area was 75 percent of that contained in the Empire State Building." (page 5)

"WTC 7 contained a three-zone system of sprinklers and standpipes." (page 7)

"These fire protection measures addressed the conventional approaches to preserving life safety. However, in the U.S., neither architectural nor structural engineering practice explicitly required (then or currently) an evaluation of the structural system response to heating (fires) as part of the building design." (page 8)

"The combustible fuel load for these open landscaped floors was dominated by workstations. The architectural drawings showed the densities of workstations similar to those on most of the fire floors in the WTC towers. The estimated fuel loads for these floors was about 20 kg/m2 (4 lb/ft2). Simulations of the fires with a higher combusted fuel load (NIST NCSTAR 1 - 9, Chapter 9) resulted in poor agreement with observed fire spread rates." (page 10)

"Shortly before 9:00 a.m. on Tuesday, September 11, 2001, about 4,000 people were at work in WTC 7. This was about half of the roughly 8,000 people who worked there." (page 15)

"At 8:46:30 a.m. EDT, five highjackers flew American Airlines Flight 11 (AA 11), a Boeing 767-200ER aircraft with 11 crew and 76 passengers on board, over the top of WTC 7 and into the north face of WTC 1. Moving at about 710 km/h (440 mph), the nose hit the exterior wall of the tower at the 96th floor. The aircraft cut a gash that was over half of the width of the building and extended from the 93rd to the 99th floor. At 9:02:59 a.m., 16 1/2 minutes after the first impact, five other highjackers flew United Airlines Flight 175, a Boeing 767-200ER with 9 crew and 51 passengers on board into the south face of WTC 2 at about 870 ken/hr (540 mph), about 160 km/hr (100 mph) faster than AA 11. The center of the nose of the plane struck at the 81st floor slab. This entry wound stretched over nine floors, from Floors 77 to 85 (NIST NCSTAR 1)." (page 15)

"The elevators alone could have evacuated the building in about 20 min. The stairwells, although somewhat narrow for the maximum possible 14,000 possible occupants (estimated using the formula in the NYCBC), were more than adequate to evacuate roughly one-third of that number in the building that morning (NIST NCSTAR 1 - 9, Chapter 7)." (page 15)

"Pieces of WTC 1 hit WTC 7, severing six columns on Floors 7 through 17 on the south face and one column on the west face near the southwest corner. The debris also caused structural damage between Floor 44 and the roof." (page 16)

"The emergency responders quickly recognized that WTC 7 had been damaged by the collapse of WTC 1. A number of fire teams entered WTC 7 to examine the damage, locate fires and possibly extinguish them, and search for occupants." (page 18)

"As early as 11:30 a.m., FDNY found that there was no water supplied by the hydrant system to fight fires that were visible. With the collapse of the towers fresh in their minds, there was a concern that WTC 7 too might collapse, risking the lives of additional fire fighters. Within the next two hours, serious discussions were underway regarding the cessation of any efforts to save WTC 7, and the final order to cease was given at about 2:30 p.m. The Con Edison substation was shut down at 4:33 p.m. (NIST NCSTAR 1-9, Chapter 6)." (page 18)

"Most likely, the WTC 7 fires began as a result of burning debris from the collapse of WTC 1 at about 10:28:22 a.m. Soon after that, there were numerous vehicles around WTC 7 that were on fire, presumably ignited by burning debris from the tower. It is likely that nascent fires were also growing within WTC 7 around the same time, although visual evidence of the fires in the building was not available until around noon. From the fire spread patterns, it is likely that the fires began around the western half of the south face." (page 18)

"Fires broke out on at least 10 floors of WTC 7, near the damaged southwest corner of the building (NIST NCSTAR 1 - 9, Chapter 5). They were typically observed as single floor fires, and observations supported a local fire origin on an [a] given floor." (page 18)

"Between 12:10 and 1:00 p.m., there were fires at the southwest corners of the 19th, 22nd, 29th and 30th floors. These fires grew large enough to break glass from nearby windows but did not spread far before dying out. These fires might have also burned along the south sides of the floors, where they would not have been seen, due to limited visibility from smoke obscuration. It is possible that the fires on the 22nd, 29th and 30th floors were limited by automatic sprinklers, whose water [supply] came from the storage tanks on the 47th floor. At any rate, after about 1:00 p.m., there was no evidence of fires on these floors on the east, north, or west faces of the building." (page 19)

"There were no indications of fire on the 9th floor until shortly before 4 p.m., when a small fire appeared on the west side of the north face." (page 20)

"As the fires progressed, some of the structural steel began to heat. According to the generally accepted test standard, ASTM E-119, one of the criteria for establishing the fire resistance rating for a steel column or floor beam is derived from the time at which, during a standard fire exposure, the average column temperature exceeds 538°C (1000°F) or the average floor beam temperature exceeds 593°C (1100°F). These are temperatures at which there is significant loss of steel strength and stiffness. Due to the effectiveness of the SFRM [sprayed fire resistive material], the highest column temperatures in WTC 7 only reached an estimated 300°C (570°F) and only on the east side of the building did the floor beams reach about 600°C (1100°F). The heat from these uncontrolled fires caused thermal expansion of the steel beams on the lower floors of the east side of WTC 7, primarily at or below 400°C (750°F), damaging the floor framing on multiple floors." (page 21)

"The initiating local failure that began the probable WTC 7 collapse sequence was the buckling of Column 79. This buckling arose from a process that occurred at temperatures at or below approximately 400°C (750°F), which are well below the temperatures considered in current practice for determining fire resistance ratings associated with significant loss of steel strength. When steel (or any other metal) is heated, it expands. If thermal expansion in steel beams is resisted by columns or other steel members, forces develop in the structural members that can result in buckling of beams or failures of connections." (page 21)

"Fire-induced thermal expansion of the floor system surrounding Column 79 led to the collapse of Floor 13, which triggered a cascade of failures. In this case, the floor beams on the east side of the building expanded enough that they pushed the girder spanning between Columns 79 and 44 to the west on the 13th floor. … This movement was enough for the girder to walk off of its support at Column 79." (page 22)

"The unsupported girder and other local fire-induced damage caused Floor 13 to collapse, beginning a cascade of floor failures down to the 5th floor (which as noted in Section 1.2.3, was much thicker and stronger). Many of these floors had already been at least partially weakened by the fires in the vicinity of Column 79. This left Column 79 with insufficient lateral support, and as a consequence, the column buckled eastward, becoming the initial local failure for collapse initiation." (page 22)

"As with the WTC towers, much of the information specific to the WTC 7 construction was lost with the destruction of the WTC site. Nonetheless, copious information was obtained from drawings and specifications, reports and available records from The Port Authority, Silverstein Properties, and a number of contractors that worked on the design, construction, or modification of WTC 7. The documents included the erection and fabrication shop drawings of the building, which provided detailed information about the floor and column connections." (page 25)

"Considerable effort was expended to compile evidence and to determine whether intentionally set explosive material might have caused the collapse of WTC 7 (NIST NCSTAR 1- 9, Appendix D). As a minimum, the explosive material would have had to cause sufficient damage to a critical column or truss that it became unable to carry its service load or that a lateral deflection would cause it to buckle." (page 26)

"Preparations for a blast scenario would have been impossible to carry out on any floor in the building without detection. Preparations would have included removal of column enclosures or walls, weld[ing] torches to cut column sections, and placement of wires for detonation. Occupants, support staff, and visitors would have noticed such activities, particularly since they likely would have occurred around more than one column." (page 27)

"Similar to the analyses conducted for the WTC towers, the analyses of WTC 7 continued to advance the current state of the art and tested the limits of computational capabilities. The unprecedented complexity and sophistication of these analyses required the use of various strategies for computational demands while adequately capturing the physics of the problem." (page 28)

"The visual evidence indicated that only fires of significant duration and intensity were on Floors 7 through 9 and 11 through 13." (page 29)

"Based on these analyses and review of numerous interview transcripts, NIST concluded that it was highly unlikely that any fires on the 5th or 6th floors contributed to the collapse of WTC 7." (page 32)

"NIST concluded that the only fires that could have led to structural weakening of WTC 7 were those on the 7th through 9th and 11 through 13th floors." (page 32)

"Once simulation of the global collapse of WTC 7 was underway, there was a great increase in the uncertainty in the progression of the collapse sequence, due to the random nature of the interaction, break up, disintegration, and falling debris. … and the details of the progression of the horizontal failure and final global collapse were increasingly less precise." (page 44)

"While NIST was not able to compile complete documentation of the history of WTC 7, due to loss of records over time and due to the collapses, the investigators were able to acquire information adequate to arrive at and firmly support the findings and recommendations compiled in this chapter and the next [chapter]." (page 47)

"The collapse of WTC 7 represents the first known instance of total collapse of a tall building primarily due to fires. The collapse could not have been prevented without controlling the fires before most of the combustible building contents were consumed." (page 47)

"There was no evidence to suggest that there was damage to the SFRM that was applied to the steel columns, girders, and beams, except in the vicinity of the structural damage from the collapse of WTC 1, which was near the west side of the south face of the building." (page 48)

"Even without the initial structural damage caused by debris impact from the collapse of WTC 1, WTC 7 would have collapsed from fires having the same characteristics as those experienced on September 11, 2001." (page 48)

"Diesel fuel fires did not play a role in the collapse of WTC 7. The worst-case scenarios associated with fires being fed by ruptured fuel lines (a) could not have been sustained long enough, or could not have generated sufficient heat, to raise the temperature of a critical column (i.e., Column 79) to the point of significant loss of strength or stiffness, or (b) would have produced large amounts of visible smoke which would have emanated from exhaust louvers. No such smoke discharge was observed." (page 49)

"Blast events did not play a role in the collapse of WTC 7. Based upon visual and audio evidence and the use of specialized computer modeling to simulate hypothetical blast events, NIST concluded that blast events did not occur, and found no evidence whose explanation required invocation of a blast event. Blast from the smallest charge capable of failing a critical column (i.e., Column 79) would have resulted in a sound level of 130 dB to 140 dB at a distance of at least one-half a mile if unobstructed by surrounding buildings (such as along Greenwich Street or West Broadway). This sound level is consistent with standing next to a jet engine and more than 10 times louder than being in front of the speakers at a rock concert. There were no witness reports of such a loud noise, nor was such a noise heard on audio tracks of video recordings of the WTC 7 collapse." (page 49)

"Due to the focus on rescuing people trapped in the debris field, providing aid to the injured, and the loss of water in the hydrant system, FDNY was not able to consider the possibility of fighting the fires in WTC 7 until approximately 1:00 p.m. At approximately 2:30 p.m., FDNY gave the order to forego firefighting activity and for personnel to withdraw to a safe distance from the building." (page 57)

"The vertical (gravity) load resisting system comprised of core columns and exterior columns, which received gravity loads from the floor framing. Of particular note were the three core columns on the east side of the building (Columns 79, 80, and 81), which supported large span floor areas with approximately 15 m (50 ft) spans on at least one side." (page 58)

"NIST simulations showed that, for the heaviest columns in WTC 7, when properly insulated, it would have taken an exposure of about 7 h [hours] at post-flashover upper layer gas temperatures to raise the steel temperature 600°C (1100°F), the point at which the steel strength would have been reduced by half. A similar calculation indicated that it would have taken about 4 h [hours] to reach this temperature for an insulated lighter column. These times are both far longer than the time over which post-flashover gas temperatures were sustained in the computed WTC 7 fires. For comparison, this steel temperature would have been reached in under one-half hour if the insulation were not applied." (pages 59 and 60)

"It is unlikely that the collapse of WTC 7 would have been prevented had the insulation thickness on the floor beams been increased from 13 mm (1/2 in) to 19 mm (3/4). NIST calculations indicated that the time to reach steel temperatures of 649°C (1200°F) would have increased by about 10 min to 20 min." (page 59)

"The ASTM E119 test does not capture critical behavior of structural systems, e.g., the effect of thermal expansion or sagging of floor beams on girders, connections, and/or columns. The thermal expansion of the WTC 7 floor beams that initiated the probable collapse sequence occurred primarily at temperatures below approximately 400°C (750°F). Thus, to the extent that thermal expansion, rather than loss of structural strength, precipitates an unsafe condition, thermal expansion effects need to be evaluated. The current fire resistance rating system, which does not include thermal expansion effects, is not conservative." (page 59)

"Since the city water main had been compromised as a result of the collapse of the two towers, there was no water supply to control the fires on the 7th through 13th floors. By contrast, the early fires on the 22nd, 29th, and 30th floors may have been limited by the sprinkler system on the upper floors, whose primary water supply was from storage tanks on the 46th floor." (page 59)

"There was no evidence of floor-to-floor fire spread until perhaps just before the WTC 7 collapse. Thus, the fire-rated floors were successful as fire penetration barriers." (page 60)

"In its final report on the collapse of the World Trade Center towers (NIST NCSTAR 1), NIST made 30 recommendations for improving the safety of buildings, occupants and emergency responders. These encompass increased structural integrity, enhanced fire endurance of structures, new methods for fire resistant design of structures, improved active fire protection, improved building evacuation, improved emergency response, improved procedures and practices, and education and training." (page 63)

"The Investigation Team has compiled a list of key factors that enabled ordinary fires to result in an extraordinary outcome. In doing so, the Team recognized that there were additional aspects to be included in the content of some of the 30 earlier recommendations." (page 63)
"Based upon the findings of this Investigation, NIST has identified one new recommendation and has reiterated 12 recommendations from the Investigation of the WTC towers." (page 63)

"The term 'tall building' is used by architects and structural engineers to indicate buildings that are taller than surrounding buildings, slender in their proportions, and/ or require technologies such as wind bracing to carry loads, and are nominally taller than 15 to 20 stories. For fire protection engineers, the term 'high rise building' is used to indicate buildings that are nominally taller than 25 m (75 ft), and external rescue from fires is no longer possible. Both terms apply to WTC 7." (footnote 1, page 63)

"Fire scenarios for structural design based upon single compartment or single floor fires are not appropriate representations of infrequent fire events. Such events have occurred in several tall buildings, resulting in unexpected substantial losses. Instead, historical data suggests that infrequent fires which should be considered in structural design involve: ordinary combustibles and combustible load levels, local fire origin on any floor, no widespread use of accelerants [accelerants], consecutive fire spread from combustible to combustible, fire-induced window breakage providing ventilation for continued fire spread and accelerated fire growth, concurrent fires on multiple floors, and active fire protection systems rendered ineffective. The fires in WTC 7 involved all three of these." (page 64)

"Had contemporaneous standards and practices been available to expressly design WTC 7 for prevention of fire-induced progressive collapse, it would have been sufficiently robust to withstand local failure due to the fires without suffering total collapse." (page 64)

"Engineers should be able to design cost-effective fixes to address areas of concern that are identified by these evaluations. Several existing, emerging or even anticipated capabilities could have helped prevent the collapse of WTC 7. The degree to which these capabilities improve performance remains to be evaluated. Possible options for developing cost-effective fixes include:

• More robust connections and framing systems to better resist the effects of thermal expansion on the structural systems.
• Structural systems expressly designed to prevent progressive collapse. The current model building codes do not require that buildings be designed to prevent progressive collapse.
• Better thermal insulation (i.e., reduced conductivity and/or increased thickness) to limit heating of structural steel and to minimize both thermal expansion and weakening effects. Currently, insulation is used to protect structural strength, but it could also be used to maintain a lower temperature in the steel framing to limit thermal expansion.
• Improved compartmentation in tenant areas to limit the spread of fires.
• Thermally resistant window assemblies which limit breakage, reduce air supply, and retard fire growth." (page 65)

"The effects of restraint of free thermal expansion on the steel framing systems, especially for the long spans on the east side of WTC 7, were not considered in the structural design and led to the initiation of the building collapse." (page 66)

"The floor systems failed in WTC 7 at shorter fire exposure times than specified fire rating (two hours) and at temperatures lower than the endpoint temperature (593°C, 1100°F) because of thermal effects within the structural system, especially thermal expansion, were not considered in setting the endpoint criteria when using ASTM E119 or equivalent testing standard. The structural breakdowns that led to the initiating event and eventual collapse of WTC 7 occurred at temperatures that were hundreds of degrees below the criteria that determine structural fire resistance ratings." (page 67)

"Thermally-induced breakdown of the floor system in WTC 7 was a determining step in causing collapse initiation and progression. Therefore, the floor system should be considered to be an integral part of the primary structural frame." (page 68)

"There is a critical gap in knowledge about how structures perform in real fires, particularly considering: the effects of fire on the entire structural system (including thermal expansion effects at lower temperatures); the interaction between sub-systems, elements and connections; and scaling the fire test results to full-scale structures (especially for structures with long span floor system). The performance standards should be adopted as an alternative method in the model building codes by mandatory reference to, or incorporation of, the latest edition of the standard." (page 69)

"A performance-based assessment of the effects of fire on WTC 7, had it considered all relevant thermal effects (e.g. thermal expansion effects that occur at lower temperatures), would have identified the vulnerability of the building to fire-induced collapse and allowed alternative designs for the structural system." (page 70)

"No water supply was available for the automatic suppression system on the lower 20 stories of WTC 7 once water from the street-level mains was disrupted. This lack of reliability in the source of the primary and secondary water supplies allowed the growth and spread of fires that ultimately resulted in the collapse of the building." (page 70)

"Command posts should be established outside the potential footprint of any building which shows evidence of large multi-floor fires or has serious structural damage. A continual assessment of building stability and safety should be made in such emergencies to guide ongoing operations and enhance emergency responder safety." (page 71)

"An effective command system should be established and operating before a large number of emergency responders and apparatus are dispatched and deployed." (page 71)

"Key decisions (e.g., decisions not to fight the fires in WTC 7 and to turn off the power to the Con Edison substation) were reasonable and would not have changed the outcome on September 11, 2001, but were not made promptly. Under different circumstances (e.g., if WTC 7 had collapsed sooner and fire fighters were still evaluating the building condition), the outcome could have been very different." (page 71)

"The efforts required in locating and acquiring drawings, specifications, tenant layouts, and material certifications, and especially shop fabrication drawings, significantly lengthened the investigation into the collapse of WTC 7." (page 72)

"Following typical practice, none of the design professionals in charge of the WTC 7 project (i.e., architect, structural engineer and fire protection engineer) was assigned the responsibility to explicitly evaluate the fire performance of the structural system. Holistic consideration of thermal and structural factors during the design or review stage could have identified the potential for failure and might have prevented the collapse of the building." (page 72)

"Discerning the fire-structure interactions that led to the collapse of WTC 7 required research professionals with expertise in both disciplines. Assuring the safety of future buildings will require that participants in the design and review processes possess a combined knowledge of fire science, material science, heat transfer, and structural engineering and design." (page 73)
"NIST stretched the state-of-the-art in the computational tools needed to reconstruct a fire-induced building collapse. This enabled identification of the critical processes that led to that collapse. Making these expanded tools and derivative, validated, and simplified modeling approaches usable by practitioners could prevent further disasters." (page 73)

Discussion

With a cursory review of the NIST final report on the investigation into the collapse of the WTC 7, one can't help but be impressed with the depth of the analysis done by NIST. However, after a more in-depth review of the report, flaws in the NIST report begin to become more apparent. In a "nutshell," the NIST report can be best described as advancing a hypothesis on how and why WTC 7 collapsed.

Did the collapse of WTC 7 occur as postulated by NIST, or was there another cause? Unfortunately, it is highly probable that we will really never know for sure. I guess you could say that the investigation into the collapse of WTC 7 is, at best, "indeterminate." In the report, NIST states that the fires that occurred in WTC 7 were fires that typically occur in high rise buildings in the United States, but clearly this is not the case. The report states that the debris from the collapse of WTC 1 (the north tower) ignited multiple fires in WTC 7. If I recall correctly, all of the major fires in high rise buildings in the United States in the past 35 years developed as a result of an ignition at a single point within the building.

The NIST report further indicates that the water supply at the WTC site was impaired as a result of the collapse of the WTC towers and that the New York Fire Department (FDNY) made a decision not to fight the fire either on the interior or the exterior. Once again, I don't recall any major fire in a high rise building where the municipal water supply at the building was impaired to such an extent that no water supply was available for use by firefighters and where the fire department that responded to the fire made no effort to fight the fire.

Based upon the above, it can be concluded that the fires that occurred in WTC 7 were anything but typical fires that have occurred in other high rise buildings in the United States, without even mentioning that the collapse of WTC 7 was the last act of a drama which began with the hijacking of commercial aircraft earlier in the day.

While the NIST report indicates that the water supply at the WTC site was damaged by the collapse of the WTC towers and that the FDNY made the decision not to commit resources to fight the fires in WTC 7, the final report fails to address the obvious question: If the fires in WTC 7 were an isolated event and if the FDNY had deployed to fight the fires as they normally would have on any other day except September 11, would WTC 7 have collapsed? While NIST avoids addressing this question, it is my opinion that this is a key question that needed to be addressed in the report. It is also my opinion that the answer to this question is that WTC 7 would still be standing today if the fires in the building had occurred on any day other than September 11. Hence, it is my conclusion that the collapse of WTC 7 was just another part of the series of events in the disaster referred to as September 11.

While the NIST report outlines the methodology used to develop its hypothesis on the cause of the collapse, the report does not address one critical assumption on which the entire analysis relies - the condition of the fire protection provided for the structural elements of WTC 7 on September 11. While the NIST report on the collapse of WTC 1 and WTC 2 spent considerable time discussing the condition of the structural fire protection provided for the towers on September 11, the report on the WTC 7 collapse only briefly addresses this issue with the following statement:

"There was no evidence to suggest that there was damage to the SFRM [sprayed fire resistive material] that was applied to the steel columns, girders, and beams, except in the vicinity of the structural damage from the collapse of WTC 1, which was near the west side of the south face of the building."

Of course, there is also no evidence that the structural fire protection was properly installed initially and then properly maintained. All of the evidence regarding the application and maintenance of the structural fire protection was destroyed in the collapse and subsequent removal of the construction debris from the site.

My experience as a new construction inspector for the San Jose Fire Department indicates that sprayed-fireproofing materials (SFRM) are often applied to surfaces of structural members that have not been properly prepared. (The listing requirements for sprayed-fireproofing materials indicate that "mill scale" is required to be removed prior to the application of the fireproofing materials.) My inspection experience also indicates that the thickness of the fireproofing materials varies and that it is not uncommon that the thickness applied to portions of structural members is deficient. Of course, once the sprayed-fireproofing materials are applied, the various trades always remove portions of the fire-proofing materials to facilitate the installation of the mechanical, electrical and plumbing systems, as well as to facilitate the construction of partitions. Given this, the NIST report should have included a lengthy discussion on how NIST determined the condition of the sprayed-fireproofing material in WTC 7 immediately prior to the fires.

Given the discussion above, it is possible that the real culprit in the collapse of WTC 7 was not the inadequate consideration of the expansion of long span horizontal members under fire conditions in the structural design, but rather the improper or inadequate application of the sprayed-fireproofing materials or the removal of the sprayed-fireproofing materials by the trades. The NIST report provides a subtle hint at this in the following excerpt:

"For comparison, this steel temperature would have been reached in under one-half hour if the insulation were not applied."

Conclusion

It seems quite obvious that the condition of the sprayed-fireproofing materials providing protection for the structural steel framing members of WTC 7 would have a significant impact on the ability of the building structure to carry the loads imposed on the structure during a fire in the building. The fact that NIST apparently completely ignored the condition of the sprayed-fireproofing materials in its analysis calls into question both NIST's complex analysis and the conclusions drawn from this analysis.

It also seems quite obvious that the FDNY's decision not to fight the fire in WTC 7 had a serious impact on the ability of WTC 7 to remain standing after a seven-hour fire exposure. What NIST fails to explain is why a structural analysis under fire conditions should assume the simultaneous failure of both the sprinkler system and manual fire fighting. Historically, the combined probability of success of sprinkler protection and manual fire fighting is one hundred percent.

Given the historic reliability of the combination of sprinkler protection and manual fire fighting, the need to design the structural fire protection assuming both the failure of the sprinkler system and the failure of manual fire fighting certainly appears to be highly questionable. Just as with other recommendations included in the report, NIST really does not provide any justification for this recommendation. Nor does NIST address the cost issue. One would think that before a government agency would make recommendations with a substantial cost impact that the agency would perform a cost/benefit analysis to determine whether or not the recommendation is an effective use of capital.

Of course, experience tells us that government officials are rarely concerned with cost or even with whether or not policy recommendations are effective. This attitude is what has gotten us into the economic mess we're in right now, and incorporating NIST's recommendations into our model building codes will simply make it more difficult for the construction industry to recover from the economic downturn of 2008. Instead of providing a "bailout" for the building construction industry, why not simply rescind the code provisions for making already "extremely safe" high rise buildings just a little bit "safer"?

Richard Schulte is a 1976 graduate of the fire protection engineering program at the Illinois Institute of Technology. After working in various positions within the fire protection field, he formed Schulte & Associates in 1988. His consulting experience includes work on the Sears Tower and numerous other notable structures. He has also acted as an expert witness in the litigation involving the fire at the New Orleans Distribution Center. He can be contacted by sending e-mail to rschulte@plumbingengineer.com.