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.