What is the practical limit on tall buildings?

Prompted by this story.

Saudi Arabia to begin building world’s tallest building.

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.

Base jumpers all around the world are salivating.

All super-tall towers have gone past the practical already. They are statuary, monuments to their builders. Parts of them may be useful, but not in an economic sense.

So, what might be the practical, economically useful limit, given current means and presuming a high density area with high land price?

Seems to me that there ought to be a formulae for it. Land cost plus building cost over rental value times predicted life: I made that up - does anyone have a sensible one?

Are you going to factor in prestige? Since that’s the major driving force for most of these projects.

Look at the tall buildings that are not obviously done as monuments. They top out at around 1000 feet. I’d use that as a rule of thumb maximum.

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.

Elevators would have to be replaced with something like vertical rail cars, that would use “up only” and “down only” shafts and shunt between them at top and bottom.

Like the arcologies from Simcity.

Frank Lloyd Wright Mile High Tower

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.

Pumping water that high could be challenging.

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 they ever make a space elevator, would it be unreasonable to assume they could make some sort of building around it? Would be expensive, but could it be done?

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.

Actually, the backbone system might better be thought of as a vertical subway, with stations every 20 floors or so. Local elevators would serve each 20-floor block.