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Report praises strength of Twin Towers

News special

ENGINEERS PRAISED the performance of the twin World Trade Center towers in the first official report into the terrorist attacks on them on 11 September.

The report says that despite sustaining massive damage, the Twin Towers remained standing long enough for 99% of people to escape from parts of the buildings below the crash impact zones in each.

It says that such an event could not have been considered in the design.

World Trade Center Building performance study: data collections, preliminary observations, and recommendations publishes the findings of an investigation team convened by the American Society of Civil Engineers and the Federal Emergency Management Agency.

Led by consultant CTL's senior vice president Gene Corley, the team was assembled shortly after the attacks to report to the US Federal Emergency Management Agency.

According to the authors, the study was to 'determine probable failure mechanisms and to identify areas of future investigation that could lead to practical measures for improving the damage resistance of buildings against such unforeseen events.'

Running to almost 250 pages, the report provides an overview of the relevant building codes, and a detailed description of the performance of each building in the WTC complex.

Observations and recommendations for future studies are provided.

It says that after the attacks both structures could have remained standing barring severe loading such as a windstorm or earthquake. But the intense fires caused by aviation fuel led to collapse.

The fuel itself - around a third of which burned off in the initial fireball - did not cause collapse alone.

Instead, it ignited contents of the buildings leading to a series of fires with power comparable to that produced by a large commercial power station. The intense heat weakened structural members, leading to collapse.

Key topics identified include:

lNeed for structural redundancy in framing systems lFireproofing must adhere under impact and where members deform lPerformance of structural connections under impact and fire loads needs to be understood to improve design capabilities lBuildings with fire ratings which assume reliance on a sprinkler system must have a reliable water supply, otherwise assumed fire protection is greatly reduced.

lEgress systems must be evaluated for redundancy and robustness in the event of building damage lFire protection ratings and safety factors for structural transfer systems should be linked to the role such systems provide in the stability of the building.

lThe report also describes the collapse of WTC7 - believed to be the first modern steel framed building to have collapsed solely as the result of fire - concluding that much more study is needed to understand why it fell.

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