Computer modeling confirms cause of 9/11 building collapse

Using sophisticated computer modeling to reproduce the Sept. 11, 2001, collapse of World Trade Center Building 7, the National Institute of Standards and Technology concluded that it was fire and not explosives or impact from debris that caused the 47-story building to fail.

NIST announced the findings of the three year study Thursday [http://wtc.nist.gov/], and recommended changes in national building codes to take into account the affects of thermal expansion on structural members.

'Our study found that the fires in WTC 7, which were uncontrolled but otherwise similar to fires experienced in other tall buildings, caused an extraordinary event,' said Shyam Sunder, lead investigator in the NIST World Trade Center study. 'Heating of floor beams and girders caused a critical support column to fail, initiating a fire-induced progressive collapse that brought the building down.'

Building 7 was the third building to collapse in the World Trade Center attacks. The two iconic towers collapsed shortly after being hit by hijacked airliners that morning. Building 7 was set ablaze by debris from the attacks and collapsed several hours later. NIST began investigating the attacks in August 2002 and released its findings on the collapse of towers 1 and 2 three years later. Investigators spent the last three years investigating Building 7.

'The investigation was an extensive, state-of-the-art reconstruction of the events that affected WTC 7 and eventually led to its collapse,' NIST said. 'Numerous facts and data were obtained, then combined with validated computer modeling that is believed to be close to what actually occurred. A single computer simulation of the structural response to fires took about eight months to complete on powerful computing workstations and clusters.'

The investigation was somewhat hampered by the fact that steel samples from the building were not available for examination and testing. Debris from the attacks was removed as quickly as possible from the sites so that emergency responders could work in the area. Once removed from the scene, steel from Building 7 could not be clearly identified. Unlike pieces from the two primary towers, which were painted red and contained distinguishing marks, remains of Building 7 contained nothing to distinguish them.

This meant that there was much less physical evidence to examine from Building 7. 'Nonetheless, the NIST investigation of WTC 7 is based on a huge amount of data,' investigators wrote. 'These data come from extensive research, interviews, and studies of the building, including audio and video recordings of the collapse. Rigorous, state-of-the-art computer methods were designed to study and model the building's collapse. These validated computer models produced a collapse sequence that was confirmed by observations of what actually occurred.'

Temperatures that caused the collapse of Building 7 were 'hundreds of degrees below those typically considered in current practice for fire resistance ratings,' investigators found. Sprinklers would likely have prevented the collapse had water to the building not be cut by the attacks.

The impact of debris from the collapse of WTC 1 ignited fires on at least 10 floors of Building 7, which burned out of control. Eventually a girder on the 13th floor lost its connection to a critical interior column that provided support for long floor spans on the east side of the building. The floor collapsed, beginning a cascade of floor failures down to the 5th floor. This collapse left the critical column unsupported, causing the building failure.

'When this critical column buckled due to lack of floor supports, it was the first domino in the chain,' Sunder said. 'What followed in rapid succession was a progression of structural failures. Failure first occurred all the way to the roof line'involving all three interior columns on the most eastern side of the building. Then, progressing from east to west across WTC 7, all of the columns in the core of the building failed. Finally, the entire fa'ade collapsed.'

The report included 12 recommendations for building codes originally made during the investigation of towers 1 and 2, and added a 13th regarding thermal expansion.

'While the partial or total collapse of a tall building due to fires is a rare event, we strongly urge building owners, operators and designers to evaluate buildings to ensure the adequate fire performance of the structural system,' the investigators wrote. 'Of particular concern are the effects of thermal expansion in buildings with one or more of the following features: long-span floor systems, connections not designed for thermal effects, asymmetric floor framing and/or composite floor systems.'

NIST is accepting comments on the draft report and recommendations until noon EDT, Sept. 15. Comments should be e-mailed to wtc@nist.gov, faxed to 301-869-6275, or mailed to WTC Technical Information Repository, Attn: Stephen Cauffman, NIST, 100 Bureau Dr., Stop 8611, Gaithersburg, Md. 20899-8610.