The industry was this week grappling with the implications for the future design of tall buildings after the horrific terrorist attack on the World Trade Center.
The twin 400 m high towers collapsed after two hijacked aircraft crashed into them on Tuesday.

The disaster directly affected many UK firms, including Bovis Lend Lease, which was working on a fit-out project in one of the towers at the time of the crash.

Since the attack, engineers have been attempting to understand the causes of the catastrophic collapse. Theories have centred on the intense heat created by the crashed planes' burning fuel, the close proximity of the two skyscrapers, and the weight of dampers on top of the towers.

Chris Luebkeman, director of Arup Research and Development, said the disaster could have an influence on future skyscrapers as designers had gradually reduced spare structural capacity in tall buildings to squeeze in more floor space.

He said: "We have optimised and minimised our structural elements. What we have to do is use this to pause and reconsider this direction."

Gordon Masterton, chairman of the structural and building board at the Institution of Civil Engineering, described the towers' destruction as unprecedented.

He said that the industry had to re-examine the risk of aircraft crashing into tall buildings.

He said: "You do not design buildings to be resistant to this kind of impact, except in the event of war. If you did, we would all be living underground."

He said that the combination of the impact of the planes hitting the towers, smashing through the external load-bearing walls, and the intense fires that raged through the interior led to the collapse.

William Frischmann, structural designer of London's former NatWest Tower and its refurbishment after the IRA bomb in 1996, said the proximity of the towers added to the disaster.

Frischmann, senior partner of structural engineer Pell Frischmann, said the second building failed partly as a result of the dynamic forces from the collapsing building next to it.

He said: "In my view, the towers were built too close together. They should have been designed to withstand the accidental crash of an aircraft, especially since they stand so close to an airport."

Another cause could have been a device intended to stabilise the buildings in high winds, according to Arup's Luebkeman.

He said that viscous dampers – huge concrete blocks mounted at the top of both towers – crashed through concrete floor slabs weakened by the impact of the hijacked aircraft.

He said: "There were some huge weights installed in the roof – viscous dampers to reduce sway. After the explosion had weakened the floor slabs, you have this huge continued weight pushing down on them. They could have played a role."

Malaysian skyscraper architect Ken Yeang claimed that no building could have withstood such an impact.

He said: "I don't think any building could have survived that. Once the plane hit, it was gone."

Foster and Partners director Ken Shuttleworth agreed. He said: "You can't stop burning fuel with building systems." He added that buildings are only developed to handle office fires that may cover one or two floors, not up to 20.

Shuttleworth added that a fire of such intensity would inevitably melt the surrounding structure, leaving the floors above to drop on to those below.

Designed with wind loads and earthquakes in mind, the towers were flimsier towards their tops to add to flexibility, as it was anticipated that most of the stress would have been near the foundations. Such immense damage at the top was not anticipated.