Cities keep leapfrogging over each other to have the prestige of the world’s tallest but surely there has to be a limit somewhere. So what is the practical height beyond which it just isn’t feasible to go? Or is it literally a case of the sky’s the limit?
The limiting factor in building supertall skyscrapers is providing enough space for elevators to move people around. We could conceivably construct a building five kilometers tall, but the logistics of transporting people between the top and bottom floors in any reasonable timeframe would be a significant challenge.
I’d be skeptical about a 5 km building. Wind loads grow very fast with height, and at 5 km the gravity loads are already a significant fraction of the ultimate stress of structural steel or modern concrete. That’s just taking into account self weight of the structure, let alone office space and mechanical systems.
I remember Bill Baker (lead structural engineer for the Burj Khalifa) giving a talk at my school and suggesting something around a mile is doable. Maybe a bit more.
So, the structural limiting factors are quite a bit above what’s been built so far. I’m not sure where we are with respect to elevator limits. The economic limits will depends where in the world the tower is built. There aren’t a lot of super tall building built in the US compared to the Middle East and Asia. That’s not because we can’t do it, most of the design firms are American, it’s that it doesn’t make financial sense to go much about 1,000 ft in most cases.
In recent trips to vegas, they’ve worked around this by having some elevators skip floors. The lobby has 4 elevators, one goes 1-10 and 20-30, another goes 11-19 and 31-40, another goes 41-60, etc. In really tall buildings, I can see how you need to catch two elevators. So, first you catch the one that goes to 40, then transfer to a different one that goes to 60, etc.
To me, the real hurdle is physics. Each floor you add to the top increases the weight required for the bottom floor to support. But again, the workaround for this is simply making a pyramid shaped building where each successive floor weighs less than the preceding floor, until the top floor weighs maybe 1/100th the weight of the first floor. But, most of these “tallest buildings in the world” cheat by putting a huge antenna on the top.
A building made of steel has a certain limit to its height before it becomes unstable. I do not remember what that limit is though.
Carbon fiber could build much taller skyscrapers.
Another issue is the elevators. People will have to use multiple elevators, and it will take several minutes to get to where you want to go.
If they built a skyscraper that was totally self sufficient (the way a medium size city is self sufficient), then I could see something a mile or so taller. If on one floor you had your apartment, on another you had your office building, on another you had your grocery store/department store, on another you had a recreational area, on another you had the businesses you like to frequent in your leisure time, etc then I could see building huge buildings. The building would become like a city, a lot of people would only leave a few times a month since everything they need was inside.
This idea keeps appearing, and in fact the Chinese are doing some that are on this style. They never work. You can’t get thousands or tens of thousands of residents who want the same couple of stores or restaurants or leisure. Your offices will always wind up needing to draw from a different population of workers. You will never have the right sized and aged population for schools. A collection of people that size are going to be too varied to make self-sufficiency work. You’ll end up with huge numbers of failed businesses leading to empty and uneconomic space that kills the monetary assumptions you made going in.
Oh, and self-sufficiency means many times more trips in the elevators every day, so either you have to increase elevator space hugely or tell people they’ll just have long waits for every daily activity.
You use a series of air-gapped tanks every 30 floors or so to pressurize the floors below. That’s not so hard. The practical realities of adequate elevator coverage make such gigantic buildings less feasible. The only way they can work as self-sufficient communities is if you built a whole city of mega-skyscrapers, all linked to each other by horizontal skyways at various standard levels. Then the elevator problems are somewhat mitigated.
If you can build a space elevator at all, it’s possible; but I’m not sure what the point would be. From a structural point of view you would be hanging dead weight on the end of the tether that would have to be supported by the rest of the structure.