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Structural strength saved Pentagon lives on 9/11

HELICALLY WOUND column reinforcement helped limit the death toll during the terrorist attack on the Pentagon on 11 September 2001, according to the first official report into the disaster.

Energy absorbing helical reinforcement, continuity of beam reinforcement through supports and the insitu concrete outer wall combined to prevent a more extensive collapse of the building, the report says.

The concrete outer wall acted as a transfer girder, creating substantial residual load capacity and alternative load paths, says the report.

During the attack 125 Pentagon personnel and 64 passengers and crew on a hijacked Boeing 757 died when the aircraft flew into the building.

The Pentagon building performance report, published by the American Society of Civil Engineers (ASCE), concludes that fire was the cause of the limited collapse that did occur 20 minutes after impact.

The blaze acted directly on the main steel reinforcement bars of badly damaged columns, weakening them.

The report commends the performance of recently installed anti-blast measures, which helped keep most of the jet fuel and the post impact fireball outside the building.

Some damaged columns suffered dramatic deformations yet still supported the floor above.

The report recommends more research into the residual performance of such extremely deformed columns, and into the energy-absorbing capacity of reinforced concrete elements.

It acknowledges that 'much has been written on means to prevent progressive collapse, but little detailed guidance' has been incorporated into US building codes.

A 'focused effort to accumulate research and practical experience' into structural robustness is also recommended.

British engineers said the report confirms European opinions on the need for robustness and resistance to progressive collapse in structures.

US codes are still trying to absorb the lessons of the 1995 Oklahoma bomb disaster, said ASCE World Trade Center investigation leader Gene Corley.

'This latest report will reinforce the need for change, especially for buildings seen as potential terrorist targets, ' he added.

INFOPLUS www. nceplus. co. uk/wtc www. asce. org

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