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Thermal turmoil - why WTC7 collapsed

Investigators confirmed that thermal expansion of structural steel beams caused the progressive collapse of World Trade Center 7 following the 9/11 terrorist attacks. Jessica Rowson reports.

Seven years ago next week two terrorist-hijacked passenger jets flew into the twin towers of New York’s World Trade Center (WTC), causing their collapse and the death of 2,752 people. Shortly after their destruction, a third structure – WTC7 – also collapsed. Its demise has been the subject of much speculation among engineers and numerous conspiracy theories – including one that the Central Intelligence Agency (CIA) demolished the buildings using explosives.

Officials investigating the WTC7 collapse until now have only said that it was due to fire, offering little in terms of technical explanation. This week’s report from the National Institute of Standards and Technology (NIST) sets out exactly why the fire that engulfed WTC7 was able to do something no fire had done before – fell a tall building. The explanation is expected to influence building codes across the world.

WTC7 was built in 1987. It was a 47-storey steel framed structure with composite floors. In plan the building was an asymmetric parallelogram. Long span beams of about 15m ran between external columns and the central grid of internal columns (see diagram).

The external columns were connected by secondary beams, in turn connected to the central grid by a series of long span primary beams. Columns in the internal grid were also connected by secondary beams. Primary beams running between the internal and external frame supported the composite floor slab.

The asymmetric parallelogram shape of the building footprint meant that internal columns at the corner of the grid supported large floor areas. This was particularly apparent in the north east corner of the building.

The floor slabs were made up of between 110mm and 200mm thick concrete poured into steel decking that was generally 76mm deep. They were supported at each end by long beams which spanned between larger secondary beams running between the columns.In the north east corner of the building an area of the central grid was arranged asymmetrically and a critical column, known as column 79, was supporting the largest floor area.

The NIST report says that when the twin towers collapsed, debris from the north tower severed seven of WTC7’s exterior columns and started fires inside the building.
Sprinkler systems failed to operate as the water mains had been cut when the WTC twin towers collapsed earlier that day. The fires in WTC7 continued to burn until the building collapsed shortly after 5pm, seven hours after the second of the twin towers came down.

As fires burned unchecked in WTC7 the primary floors began to expand in the heat, pushing against the secondary beams until they lost their connection with the internal columns.
The report says that eventually a girder on the 13th floor lost its connection to column 79. The displaced girder and other local fire-induced damage caused the floor 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 close to the critical column.

Collapse of these floors left column 79 without lateral support over nine stories so it buckled.
The failure of column 79 continued the progressive collapse. hen column 79 buckled, load was redistributed to neighbouring columns 80 and 81, causing them to buckle too. All floor connections to these three columns failed, causing floors on the east side to fall, leaving just a hollow shell.

The failure then proceeded to the west. Falling debris hit a transfer truss, causing it to fail, followed by columns 76, 77 and 78. Each north-south line of three columns then buckled in succession from east to west, due to loss of lateral support and load redistribution.
Exterior columns buckled between the 7th and 14th floors and the entire building above the buckled column region then moved downwards as a single unit.

“When column 79 buckled due to lack of floor supports, it was the first domino in the chain,” says 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 twin towers lost their fire protection as a result of the impact of the hijacked aircraft. This exposed their structural steel to high temperatures causing it to lose strength, and led to the towers’ eventual collapse. This was not the case in WTC7 where fire protection remained intact and prevented the beams from losing strength. Instead, thermally induced expansion began the collapse sequence. The collapse of WTC7 occurred in just 8.2 seconds, but its repercussions for designers may yet echo from some time.

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