by There’s a pattern that we frequently see in the development of a new technology. Initially, the practical functionality is limited by the technology itself – what’s built and used is close to the limit of what the technology is physically capable of doing. As the technology develops and its capabilities improve, there’s a divergence between what a technology can physically do and what it can economically do, and you begin to see commercialized versions that have lower performance but are more affordable. Then, as people begin to build within this envelope of economic possibility, capability tends to get further constrained by legal restrictions, especially if the new technology has any (real or perceived) negative externalities. That is Brian Potter opening an essay on why our modern
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Chris Blattman considers the following as important: buildings, construction, Economics, science, Technology, urbanization
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There’s a pattern that we frequently see in the development of a new technology. Initially, the practical functionality is limited by the technology itself – what’s built and used is close to the limit of what the technology is physically capable of doing. As the technology develops and its capabilities improve, there’s a divergence between what a technology can physically do and what it can economically do, and you begin to see commercialized versions that have lower performance but are more affordable. Then, as people begin to build within this envelope of economic possibility, capability tends to get further constrained by legal restrictions, especially if the new technology has any (real or perceived) negative externalities.
That is Brian Potter opening an essay on why our modern buildings are not taller.
For thousands of years, the basic physical and economic limits didn’t vary that much. So tall buildings in ancient times are not so much shorter than buildings built in 1800–something you see from the 1884 diagram above, from Cram’s Unrivaled Family Atlas of the World.
If you like esoteric tidbits of knowledge (as I do) this article is for you. For example, did you know that, when it comes to modern buildings, the main constraint on building height is that the structure cannot move too much side-to-side:
Lateral design controls building height for a few reasons. For one, while gravity loads increase linearly with building height, wind- and earthquake-induced bending moments rise with the square of building height – doubling your building height increases bending moments by a factor of 4. Deflection and lateral sway is even worse – it rises in proportion to the height to the 4th power. Doubling height (while keeping everything else unchanged) increases lateral deflection by a factor of 16.
This creates a challenging design problem. Tall buildings need to be stiff enough to avoid excessive swaying, but stiffness is a function of building mass and geometry more than it is material strength. You can increase the vertical load-bearing capacity of a concrete building simply by using a stronger concrete mix, but this has relatively little impact on building stiffness. Avoiding this motion in supertall buildings often requires things like tuned-mass dampers, and part of the reason for the move towards concrete as a material for use in supertall buildings is its ability to damp lateral motions.
Then there is the tyranny of the elevator:
A tall building requires speedy elevator service to make the upper floors usable, as long elevator waits reduce the rent that can be charged. Like with other conveyance technology, elevator system size is governed by traffic at peak loading times, generally at noon for an office building. As a building gets taller, more and more elevators are needed to service the upper floors, which intrude on the floors below. The result is that a taller building must devote proportionally more and more of its space to elevators. There are ways to squeeze more capacity out of a smaller number of elevator shafts (such as by using double-decker elevators, or transfer floors at higher levels), but they don’t alter the fundamental dynamic. Residential buildings, which have fewer occupants per unit area and lower peak traffic volume, have it somewhat easier here.
Everything in a tall building faces a similar sort of dynamic. Building taller requires more complex mechanical and plumbing systems, due to the higher water pressure and the complexities of handling outside air (which may, for instance, be moving at a high speed). It requires larger and more expensive lateral resisting systems and foundations. While the building is under construction, it takes more time to move workers and materials to the upper floors, resulting in higher construction costs. This all combines to make construction more and more expensive as the building gets taller.
The result is that you see a distinct parabolic shape in the returns on investment for a tall building.
Finally, there are the many peculiar regulations. One quirk I had never heard of:
No modern building is a better illustration than the 5 + 2 podium, a type of building that exists almost entirely due to building code provisions. US building codes allow the use of light-framed wood construction for multifamily apartment buildings so long as the wood portion does not exceed 5 (sometimes 6) storeys in height. And buildings taller than 7 storeys are considered “high-rise” construction, a designation that brings with it many burdensome and expensive code provisions. The result is that multifamily buildings often consist of a “podium” of 1 or 2 storeys of concrete (containing parking, retail space, or amenities), which then has 4 or 5 storeys of wood apartments placed on top of it. Because it’s so economical to build, a large fraction of residential construction in American urban areas is podium construction.
There is a shout-out to some economics, including Ed Glaeser, that suggests some of these legal restrictions make sense, but the regulations are too many, making us all poorer. I am not so persuaded that big cities have too few mega-skyscrapers. But the idea that we should build most buildings taller and more dense fits my basic YIMBY nature.
Potter has a Substack called Construction Physics and here he is on Twitter.