Because of the huge scale of the failure, the World Trade Center has now entered into history. It is estimated that 1.4 million m 2of space have been destroyed. The nature of the progressive collapse that occurred there is shocking. But there is a genuine weakness here in almost all buildings, even though failure of this sort is almost uniquely dangerous in its extent in high-rise structures.
The direct cause of the collapse at the World Trade Center was not the impact of the aircraft.
Most large, multi-storey buildings are well able to accept dynamic loads - even from large aircraft - without this necessarily causing an immediate, general collapse. The trigger for catastrophe was the eventual failure of the columns that remained around the burning aircraft. But the ultimate cause was the crude effect of the weight of the building itself: debris crashing through each floor in turn; the load of this debris increasing as each floor was breached. While the columns in the building, collectively, are designed to resist the weight of everything above them, the floors are not. It would be possible to provide specially strengthened floors, although this is not likely to be an economic option.
The way forward still seems to be in preventing progressive collapse from developing but, if it does, limiting its extent on plan. It is a fact that the columns remained standing after the initial impact, for 60 and 100 minutes respectively. This was in spite of supporting weights and fire loads immeasurably higher than those assumed.
Even so, these times would be sufficient in most lower-rise buildings to allow everyone to get out. It is regrettable that the emergency services were unable to seek advice from a structural engineer, who would, undoubtedly, have warned of the dangers of progressive collapse, overriding the temptation to carry on the rescue.
In principle, maximum safety is obtained by increasing the number of columns, spreading them over the plan area as far as possible and increasing their energyabsorption, ductility, strength and fire resistance.
Usually, this increases the chances of floor areas being left standing after damage is sustained, and allows for better redistribution of loads. The theoretical argument for increasing column provision is good, as columns only occupy about four per cent of the volume of each floor.
Attention also has to focus on the types of columns used. Almost all the codes of practice about building design relate to static loads, but dynamic loads are different. A column type with an exceptionally good combination of properties is the concrete-filled steel tube. This really is more than the sum of its parts and gives dogged resistance under dynamic and fire loading.
It is quite possible that the eventual collapse of the World Trade Center towers could have been delayed further by forms of construction specially adapted to the control of damage; this may have allowed more people to escape. Certainly, it seems important to limit the risk of large-scale damage to buildings but, as usual, the issue will have to centre on costs and all the imponderables of the public's attitude to personal risk. As elsewhere, some risks will always have to be accepted.