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NEW YORK CITY—The Commerce Department’s National Institute of Standards and Technology (NIST) today presented its analysis of how the World Trade Center (WTC) towers collapsed after two aircraft were flown into the buildings by terrorists on Sept. 11, 2001. This is the most detailed examination of a building failure ever conducted. “Like most building collapses, these events were the result of a combination of factors,” said Shyam Sunder, lead investigator for the agency’s building and fire safety investigation into the WTC disaster. “While the buildings were able to withstand the initial impact of the aircraft, the resulting fires that spread through the towers weakened support columns and floors that had fireproofing dislodged by the impacts. This eventually led to collapse as the perimeter columns were pulled inward by the sagging floors and buckled.” The probable collapse sequences, which update and finalize hypotheses released by NIST last October, were presented by Sunder at a press briefing in New York City. The specific factors in the collapse sequences relevant to both towers (the sequences vary in detail for WTC 1 and WTC 2) are:
- Each aircraft severed perimeter columns, damaged interior core columns and knocked off fireproofing from steel as the planes penetrated the buildings. The weight carried by the severed columns was distributed to other columns.
- Subsequently, fires began that were initiated by the aircraft’s jet fuel but were fed for the most part by the building contents and the air supply resulting from breached walls and fire-induced window breakage.
- These fires, in combination with the dislodged fireproofing, were responsible for a chain of events in which the building core weakened and began losing its ability to carry loads.
- The floors weakened and sagged from the fires, pulling inward on the perimeter columns.
- Floor sagging and exposure to high temperatures caused the perimeter columns to bow inward and buckle—a process that spread across the faces of the buildings.
- Collapse then ensued.
The sequences are supported by extensive computer modeling and the evidence held by NIST, including photographs and videos, recovered steel, eyewitness accounts and emergency communication records. Additionally, this information was used to document a variety of factors affecting the performance of the buildings, the efforts of emergency responders and the ability of occupants to escape prior to the collapses. In turn, NIST has identified a number of future practices and technologies that potentially could have enhanced building performance and life safety capabilities on 9-11 had they been available for implementation. All are being considered for NIST’s upcoming recommendations. NIST also released drafts of 15 reports from three projects of the investigation: analysis of building and fire codes and practices; occupant behavior, egress and emergency communications; and fire service technologies and guidelines. Recommendations for improvements to building and fire codes, standards and practices derived from these and the other five projects in the investigation will be released for public comment in June, along with the draft of the final investigation report and drafts of 27 reports from the remaining five projects. The NIST WTC investigation’s goal is to recommend improvements in the way people design, construct, maintain and use buildings, especially high-rises. Accompanying this release are selected portions of Sunder’s presentation at today’s press briefing that detail the findings on structural and life safety factors, and the future technologies/practices by which these factors might have been enhanced on 9-11 had they been available. Sunder’s full presentation (including the complete probable collapse sequences for both WTC towers), the text of the 15 reports issued, and all previous WTC investigation findings are available at http://wtc.nist.gov. As a non-regulatory agency of the U.S. Department of Commerce’s Technology Administration, NIST develops and promotes measurement, standards and technology to enhance productivity, facilitate trade and improve the quality of life.
FACT SHEET—Findings on Structural and Life Safety Factors
Factors that Enhanced Building Structural Performance on Sept. 11, 2001
- The unusually dense spacing of perimeter columns, coupled with deep spandrels, that was an inherent part of both the architectural and structural design of the exterior walls, resulted in a robust building that was able to redistribute loads from severed perimeter columns to adjacent intact columns.
- The wind loads used for the World Trade Center (WTC) towers, which governed the design of the perimeter frame-tube system, significantly exceeded the prescriptive requirements of the New York City building code and selected other building codes of the era (Chicago, New York State), including the relevant national model building code (BOCA).
- The robustness of the perimeter frame-tube system and the large dimensional size of the WTC towers helped the buildings withstand the aircraft impact.
- The composite floor system with open-web bar joist elements, framed to provide two-way flat plate action, enabled the floors to redistribute loads without collapse from places of aircraft impact damage to other locations, avoiding larger scale collapse upon impact.
- The hat truss resisted the significant weakening of the core, due to aircraft impact damage and subsequent thermal effects, by redistributing loads from the damaged core columns to adjacent intact columns and, ultimately, by redistributing loads to the perimeter walls from the thermally weakened core columns that lost their ability to support the buildings’ weight.
- As a result of the above factors, the buildings would likely not have collapsed under the combined effects of aircraft impact and the subsequent jet-fuel ignited multi-floor fires, if the fireproofing had not been dislodged or had been only minimally dislodged by aircraft impact. The existing condition of the fireproofing prior to aircraft impact and the fireproofing thickness on the WTC floor system did not play a significant role in initiating collapse on Sept. 11, 2001.
Future Practices and Technologies that Potentially Could Have Improved Building Structural Performance on Sept. 11, 2001 (Requires Analysis)
- Fireproofing not dislodged or only minimally dislodged by aircraft impact.
- Perimeter columns and floor framing with greater mass to enhance thermal and buckling performance.
- Other passive and active fire protection features (e.g., compartmentation to retard spread of building fires; thermally resistant window assemblies to limit air supply and retard the spread of fires; fire-protected and structurally hardened elevators for firefighter access with continuous, redundant water supply for standpipes).
- Steels with improved high-temperature properties (e.g., yield strength and stiffness) and creep behavior.
NOTE: There is far greater knowledge of how fires influence structures in 2005 than was the case in the 1960s. The analysis tools available to calculate the response of structures to fires also are far better now than they were when the WTC towers were built. Factors that Enhanced Life Safety on Sept. 11, 2001
- Since the buildings were occupied by only about one-third of the building’s full capacity of 25,000 occupants, the egress capacity (number and width of exits and stairways) was adequate for those survivors seeking and able to reach and use undamaged exits and stairways.
- Functioning elevators in WTC 2 enabled nearly 3,000 occupants to self-evacuate prior to aircraft impact.
- The greater remoteness of stairwells in the impact areas of WTC 2 enabled one of the stairwells to remain marginally passable after aircraft impact.
- A large number (two-thirds) of surviving occupants participated in a fire drill in the prior 12 months, with almost all of those (93 percent) instructed about the location of the nearest stairwell.
- Upgrades were made to the life safety system components after the 1993 bombing.
- Emergency responders provided evacuation assistance to building occupants.
- As a result of the above factors, approximately 87 percent of the WTC tower occupants, including more than 99 percent below the floors of impact, were able to evacuate successfully.
Future Practices and Technologies that Potentially Could Have Improved Life Safety on Sept. 11, 2001 (Requires Analysis)
- Improved performance to delay or prevent building collapse.
- Improved stairwell integrity via increased remoteness of stairwells and/or enhanced structural integrity of stairwell enclosures.
- Better communications to occupants and among first responders via improved systems and timely information sharing.
- Better command and control for large-scale incident management (e.g., location of command posts and physical assets; interagency coordination).
- Better evacuation training (e.g., practice stairwell evacuation, roof rescue not presently feasible as a standard option, existence of transfer hallways).
- Other life safety features (e.g., fire protected and structurally hardened elevators available for occupant use during emergencies; vibration-protected elevators such as those used in seismic regions; self-evacuation capability for mobility impaired occupants; operational smoke and fire control systems).
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