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Cities are under pressure to “grow up” instead of “growing out”, which creates a debate on the relative costs of over-crowding vs. urban sprawl. Still, recent reports suggest that the issues associated with growing up are more subtle than many people understand.

First some facts: somebody is starting to build a kilometer-high building. This building is another in a long line of record-setters.  I never knew that a Council on Tall Buildings and Urban Habitat (CTBUH) existed until this week but, apparently, it does and it distinguishes between the merely supertall buildings and the megatall buildings. The meaning of a “high” building also varies from city to city.

But, is a huge building really that surprising?  Are there any real or serious technical constraints on building tall buildings? Or, is it just a matter of finance.  The evidence seems to be the latter.

There is one obvious technical barrier: getting people up and down. Elevators have an amazing history covering several centuries. Otherwise, the CTBUH offers some technical papers (construction, fire safety, earthquake, design, retrofitting) for those who are interested.

Finance is a big issue. Bigger buildings take longer to complete and are more expensive in total. Since few people want to work in a building which is under construction, the costs are likely to be increasing (and likely to be increasing at an increasing rate).

There is some evidence that the decision to build tall buildings is motivated by something other than technology (i.e. cost) and revenue. (This motive also applies to many other activities in big name projects: bragging rights. Every builder wants to attract tenants by their building being certified LEED Neodymium or most innovative or the first of its kind to [fill-in-the-blank] or the biggest and that is why tenants should choose them. Since being the biggest means that somebody else isn’t, the payoff is purely positional and relative. Academic papers have been written studying the game theoretic features of this problem.)

A famous example of truly strategic thinking is evident in the Chrysler Building in New York City. In 1930, two tall buildings were completed at nearly the same time. Both were trying to be the tallest but one builder was more thoughtful. Both builders could see how tall the other building was on any day and could continue building skyward in hopes that the other builder would get tired or run out of money. In this case, the Chrysler Building appeared to concede while the other builder continued building until it was a bit taller. But, the builder of the Chrysler building changed the game by hiding a tower (i.e. it could not be observed daily and the other builder could not react) which would eventually make it taller. Thus, the other building was the tallest for only two months: a failure by one player to anticipate the other’s behaviour led to a very expensive mistake in business strategy.

But, the most insightful analysis might be a response to a question from a five year old: why can’t they build a billion storey building?  The analysis by XKCD is illuminating in many ways.