Friday, February 25, 2011

Mehaffy on Skyscrapers

Over at New Urban Network, Michael Mehaffy has posted an in-depth account of the drawbacks of tall buildings, echoing some of the points made here previously but with much greater depth, detail and scholarly support.  Most interesting to me is the suggestion of a density optimum, a point at which density and livability are maximized while minimizing construction costs and other negative externalities of tall buildings:
"[R]esearch shows that the benefits of density are not linear, but taper off as density increases. In other words, there is an optimum density, above which the negative effects of density start to increase over the positive ones. That "sweet spot" seems to be in the neighborhood of about 50 people per acre. And many cities around the world achieve this density without tall buildings, and while creating a very appealing, livable environment (e.g., Paris and London, as well as the aforementioned parts of New York, Vancouver et al.)."
Now, since people per acre is an unreliable predictor of form, and because the negative characteristics of tall buildings Mehaffy describes are mostly attributable to their form, rather than their population density (if any, in the case of office buildings), it would be interesting to see numbers couched in terms of floor area per acre, ground coverage and building heights rather than population density.   One of the studies cited in the article mentions a six-story "sweet spot," and the House of Commons report includes illustrations of a few hypothetical configurations, but this is an area that deserves further exploration.

4 comments:

  1. I understand your general point that skyscrapers are overrated, but I'm wondering how anyone can justify this report when you consider the huge amount of over-6-story buildings that have been built by profit-seeking entrepreneurs. Do you think they just made a mistake? Surely plenty of people profit from skyscrapers – what exactly is the basis for saying that the "negative effects" of density kick in after six stories? What does "negative effects" even mean? Surely there are many people who believe that negative effects kick in once you become more dense than your average American suburb – what is it about this opinion that makes you think it's more justifiable than the average American NIMBY's?

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  2. Stephen: the most obvious harm is obstruction of light and views, which detrimentally affects not only neighboring landowners but also the public, who must now walk on a darkened street. Since the USA nowhere has a cause of action for "ancient lights," like Britain does, and doesn't generally consider the blocking of light to be a public or private nuisance, the builder has no disincentive to hoarding the sunlight for his own parcel by building very tall.

    I suppose one could argue that a by-right zoning law for unlimited height underprices air rights by allowing the builder to avoid paying for the harms caused to others (while retaining all the benefits). A zoning law like New York's of 1916 could be seen as a response by the public to this problem.

    The six-story cutoff I don't think has any significance except as a way to show how high densities can be attained at low heights (assuming that if density is held constant, we'd prefer to have it delivered in low-rise rather than high-rise form).

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  3. The low limits on height actually do have some basis. Christopher Alexander for example suggested a maximum of four stories, which is the approximate limit at which someone can properly observe/police/be engaged with the activities on the street below. Much above four stories you can't talk out the window to someone on the street without yelling real loud, and it gets more difficult to recognize faces and tell what's going on.

    Four stories does seem a bit low to my mind however, as the most important factor is making sure that people on the lower floors of a building can better engage those on the street, like by having operable windows, good sight lines, etc. A more tenable limit is six or seven stories, one advocated by James Howard Kunstler for instance. This is the limit for walkups, which is basically why Paris' height limit is what it is for all practical purposes. If an elevator isn't needed, then that saves a lot on construction and operational costs, since you still need the same amount of stairs for fire egress reasons anyway. Also, buildings of that size can be more simply framed and structured, not requiring a lot of reinforced concrete or complicated steel superstructure.

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