World Trade Centre 7 collapse findings revealed

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Calls for engineers to consider fire as a load case were made today, as thermal expansion of long span composite flooring was revealed as the main factor leading the collapse of the World Trade Centre 7 (WTC7).

The fall of the 47-story WTC 7 was primarily due to fires, the National Institute of Standards and Technology (NIST) announced today following an extensive, three-year scientific and technical building and fire safety investigation. According to NIST, this was the first known instance of fire causing the total collapse of a tall building.

WTC7 was located across from the World Trade Centre site and was damaged on 11th September 2001 when the North Tower of the WTC collapsed, following impact from an aircraft.  Fires burnt throughout the afternoon and WTC7 collapsed later that day shortly after five.

According to the report, a key factor leading to the eventual collapse of WTC 7 was thermal expansion of long-span composite floor systems.  The beams expanded relative to the concrete causing shear studs to be snapped, and beams to buckle.  In some cases the expansion of long span primary beams pushed secondary beams, causing them to buckle and lose connection with the columns. 

Eventually, a girder on Floor 13 lost its connection to a critical interior column that provided support for the long floor spans on the east side of the building. The displaced girder and other local fire-induced damage caused Floor 13 to collapse, beginning a cascade of floor failures down to the fifth floor. Many of these floors had already been at least partially weakened by the fires in the vicinity of the critical column. This collapse of floors left the critical column unsupported over nine stories.

"When this critical column buckled due to lack of floor supports, it was the first domino in the chain," said NIST WTC Lead Investigator Shyam Sunder. "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 façade collapsed."

The building had been fitted with sprinklers but due to what had happened earlier that day, the mains water supply had been cut off. Investigators summarized that if the sprinklers had operated, the building would probably still be standing but urged that building codes be strengthened to prevent against structural collapse when sprinklers do not function.

"Structural engineers design for gravity, wind and earthquake loadcases but not fire," said Sunder.  "Architects specify fire proofing from a catalogue, based on building code requirements. What is missing is a connection between the disciplines. No one evaluates the structural response to fire."

The investigation team considered the possibility of other factors playing a role in the collapse of WTC 7, including the possible use of explosives, fires fed by the fuel supply tanks in and under the building, and damage from the falling debris of WTC 1.

The team said that the smallest blast event capable of crippling the critical column would have produced a "sound level of 130 to 140 decibels at a distance of half a mile," yet no noise this loud was reported by witnesses or recorded on videos.

As for fuel fires, the team found that they could not have been sustained long enough, could not have generated sufficient heat to fail a critical column, and/or would have produced "large amounts of visible smoke" from Floors 5 and 6, which was not observed.

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