This was why the collapse of the towers looked very like a controlled, deliberate demolition. In such controlled demolitions, the usual way to drop a tower vertically is to blow out supports and let the weight of the upper sections do the rest. How much of this was known by the organizers of the attack? Probably they knew very well: the 1993 bomb in the basement of the centre was intended precisely to achieve this effect by blowing the columns out sideways. But down there, the columns were very much stronger: the strength of the columns diminished with height because they carried less and less weight - good engineering practice, in peaceful times - and, as mentioned above, Yamasaki had drawn his many slender columns into fewer, much fatter, elements towards the base, better able to withstand blast.

Even so, the damage to the World Trade Centre caused by the 1993 bomb ran to $525 million. The cost of the whole building programme in 1974 was $800 million. At the time of their destruction, they had recently been purchased by a private developer for $2.2 billion. Clearly, with the enormous amount of collateral damage caused by the collapse of the twin towers - several surrounding buildings also collapsed and others, some huge, will have to be demolished - reinstatement costs will reach a new order of magnitude. A complete city district of Manhattan will have to be rebuilt from scratch.

In view of the almost unimaginable loss of human life in terrifying circumstances, and the doleful prospect of much more havoc to come worldwide as the political repercussions take effect, it might seem insensitive or naïve to consider the architectural implications of the tragedy. Yet this must be done. Even at the height of the Blitz in London in 1941, plans were being laid for the post-war reconstruction.

There are two possible effects. The most obvious and immediate is that skyscraper design will change. The other is that people will stop building skyscrapers. The Twin Towers were leanly designed, engineered precisely for their purpose and no more. In contrast, other towers such as Canary Wharf in London - built in a country that is no stranger to terrorist and wartime bomb attack - is allegedly designed with a fair amount of “structural redundancy”. In other words, it is stronger than is strictly necessary.

Building in such structural redundancy is not especially expensive - the above-ground structure is by no means the costliest element of skyscraper construction. Alternatives to Yamasaki’s exo-skeleton, such as the structural central core, may also come to be preferred. This may well see the end of visionary “skyscraper city” projects such as Norman Foster’s Millennium Tower project in Japan - which owes much to Yamasaki’s pioneering work and also possesses a latticework exo-skeleton.

It is possible to build skyscrapers that could withstand the impact of the biggest airliner yet devised, and the fires that follow. It is possible to devise faster escape systems for skyscrapers. But will people want to? The trend in recent years has been for scattered organizations, private or public, to group themselves into single, landmark buildings, often towers. But even when they are not towers, such buildings are vulnerable to attack. Britain’s spymasters became very visible - and a target for terrorists - when they moved to their admittedly blast-proofed Terry Farrell HQ in London. The Bilbao Guggenheim by Frank Gehry has been a focus for attack by Basque separatists and is seen by some as an example of American cultural imperialism.