Twin Towers showed 'remarkable' resistance

World Trade Center remains
The towers' survival "astonished" engineers
The World Trade Center showed "remarkable" resistance in the 11 September attacks and its design contributed to saving many lives, a US government report has found.

Wreckage from World Trade Center
The Towers could have survived the impact had fire not broken out

The report, jointly issued by the Federal Emergency Management Agency and the American Society of Civil Engineers, found that the length of time the buildings remained upright after being hit by two passenger planes "astonished most observers, including knowledgeable structural engineers".

It claimed this resistance was testament to its good construction and undoubtedly saved the lives of many within the buildings.

"The fact that the structures were able to sustain this level of damage and remain standing for an extended period of time is remarkable and is the reason that most building occupants were able to evacuate safely," the report said.

Immense heat

Investigators believe the WTC would probably even have survived being hit by two passenger jets on 11 September had fires not weakened the buildings' support structures.

Firefighters and rescue workers help those injured shortly after the attacks
Up to 3,000 people died in the attack on the World Trade Center

The report said engineers now believe that it was a combination of three factors: the force of the aircraft impacts, the heat from burning jet fuel, and the heat from the burning of much of the buildings' contents that caused the towers to collapse.

But most crucially, the team of civil, structural and fire protection engineers found that building fire systems in general are not equipped to deal with simultaneous fires breaking out over an entire building, as happened at the Twin Towers, Reuters news agency reported.

The report found that heat from the many fires ignited in the minutes immediately following the planes hitting the buildings rapidly became equivalent in temperature to "the power produced by a large commercial power generating station".

Designers exonerated

This heat gradually induced additional stresses into the already damaged structural frames while simultaneously softening and weakening them, it said.

The resulting damage then inevitably led to the collapse of both structures.

The findings also exonerated the buildings' designers, stating that many of the structural and fire protection procedures in place in the buildings were "superior" to other constructions.

However it also warned that it may not be "technically feasible" to design a building that could withstand the force of a plane hitting it and, even if possible, the costs would be prohibitive.

Up to 3,000 people are thought to have died in the Twin Towers attack.

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