The NIST WTC 7 investigation report: A critique, Part 2
By Richard Schulte
Schulte & Associates, Evanston, Ill.
The following report by Richard Schulte is part 2 of a two-part column. Part 1 of this report was published in February’s Fire Protection section of Plumbing Engineer.
“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.