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The conspiracy theory

Conspiracy theory junkies will get a major fix from the American Society of Civil Engineers' first report into the World Trade Center disaster:

World Trade Center Building Performance Study. Not from trading unlikely tales about who flew the two Boeing 767s into the twin towers, but from the mystery surrounding the collapse of WTC7.

On 11 September, the moment that was truly unthinkable came seven hours on from the first impacts, when the 47 storey steel framed WTC7 suddenly folded into itself, collapsing like a redundant local authority tower block brought down by demolition professionals.

Buildings are not supposed to fail like that. Even assuming that neither sprinklers nor the efforts of the emergency services stopped the fire spreading, both concrete and steel framed office blocks should burn out to leave empty charred shells, buckled and twisted perhaps, but essentially upright. Total, catastrophic progressive collapse in less than 40 seconds after an unchecked seven hour fire was almost unimaginable.

Much will be made of the identity of the tenants for four of the office floors. What might the Internal Revenue Service, the Central Intelligence Agency, the Secret Service and the Department of Defence have locked away in their storerooms?

Surely something exceptional, given the spectacular failure?

Last week's report confirmed that the towers were eventually brought down by the fires ignited by burning jet fuel after key sections of fire protection were detached by the initial massive impacts. But for anyone seeking an answer to the anomalous collapse of WTC7, the report was disappointing.

Yes, there is some reassurance in that WTC7 was a far from typical structure, sited almost on top of a major electricity substation. The massive trusses spanning the substation created a platform on which a conventional and cost effective office tower could be erected.

Supported on columns fabricated from steel up to 250mm thick, this first seven storey assemblage should have been the toughest section of the building. Yet it was here that one of the major structural elements failed, triggering a progressive collapse.

Exactly which section failed is not the most interesting question.

Cook any steel member long enough and hot enough and it will fail, however massive it may seem. The question that should intrigue structural engineers and conspiracy fantasists alike is what was fuelling this fire.

According to the report, the first seven floors of WTC7 contained few offices, and only the sixth floor was used for storage.

Attention naturally then focused on the fuel tanks for the standby diesel generators, the largest of which were concentrated on the ground floor. But to construct a scenario in which enough fuel found its way to the fifth floor to bring the structure down taxed the imagination of investigators.

They admit their hypothesis is unproven and probably unprovable. But not impossible.

To me, it seems there are two preliminary conclusions to be drawn from WTC7. The first is that any building that can resist an unchecked fire for seven hours is a tribute to its designers.

But the second, and much more important, is that unconventional buildings, such as air rights developments, that sit on complex supporting structures, may be much more likely to catastrophic collapse in extreme circumstances than we have hitherto assumed.

Dave Parker is technical editor of New Civil Engineer.

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