World Trade Center progressive collapse

The attack and destruction of the NYC World Trade Centers was clearly a political act, and it used a technical approach unique to the construction of the World Trade Center buildings. This article deplores this act of terrorism, but will leave analysis of the politics and techniques of the attacks alone, and focus on the impact of the attack on architecture and cities themselves.

Cities exist where they do for any of a number of reasons, but their site usually owes a large amount to trading and transportation. Traders stopped in New York because of its central location in the colonies and its wonderful port. Consumers, merchants, and manufacturers liked the proximity of each other, and a great city grew. Similarly, office buildings sole purpose is to facilitate human interaction. Office buildings allow easy walking and communications between parts of an organization or business. Transportation for goods, workers, and consumers, is easier when they are located in close proximity. Prior to the acceptance of personal cars, the only way to accomplish this was central locations. The natural result is great cities with lots of transportation, offices, and markets tightly packed together. Getting the required density requires that the city grow upwards.

Very tall buildings are hard to build, not only for the obvious strength and safety reasons, but also because needed services (i.e. elevators, plumbing, wiring, stairs, air-conditioning ducts) require increasing amounts of overhead as the building gets taller. In addition to bigger beams and columns, tall buildings need more elevators, bigger pipes, etc. than short campus-style buildings. As building height increases, these overhead items grow faster more quickly than the usable floor space, making very tall buildings hard to justify economically. Making very tall buildings make business sense requires a significant architectural and engineering effort.

The World Trade Centers existed because of their architecture and the supporting engineering. Unlike other skyscrapers, the exterior walls of the World Trade Centers were load bearing. [See for a brief discussion]. The whole building was a vertical truss, and the interior was column free. Without this design, it is unlikely that the WTC could have been built on that site. The architecture enabled the existence of the building.

As we saw in the painful to watch news footage of September 11, 2001, the load bearing walls were damaged, then the fire heated the steel to the critical temperature, and the building had a progressive collapse. It looked similar to the planned demolitions, with each floor failing and adding its weight along with the load of all floors above it to the floor below. This failure occured because the terrorists hit the building in a nearly perfect location. If they had hit the top floors, the building would not have collapsed. If they had hit lower floors, which have much more strength, it is likely that the buildings would still be standing. The combination was deadly.

Simply telling the engineers to make the building stronger is not a viable answer. Of course they could make it stronger, add redundancy, add heat resistant coatings, or more. But at the cost not only of the material and labor to add the strength, but at the cost of substantially increasing the overhead of the building itself. If the building's internal overhead becomes larger, the economics of the project quickly disappear. There are hard numbers of dollars behind the decisions not to build buildings bigger than the Empire State Building up until the WTC towers, and if the WTC architecture is not feasible or acceptable, then the density it enables will not be possible.

Even if another clever engineering solution is found, will society accept the risks of having this large of a target? No amount of cost effective structure will prevent all possible future attacks. So do we start to accept that there is risk in simply going to work?

The implications for the city are huge. Successful public transportation requires that large numbers of commuters go to the same place at about the same time. Similarly, the density of people is what enables the wonderful shops, markets, theaters and clubs of New York. The lack of density is a direct cause of the decline in quality of life.

The attacks on the World Trade Centers caused a horrible loss of individual human lives today, and there is a significant chance it will cause a significant loss of "life of the city" in the future.