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Comment by eminence32 | Hacker News Reader

> In The Tall Buildings Reference Book, the authors state that under pure gravity loading, a steel-framed building could perhaps reach 10,000 feet (3,000 meters) in height

I'm not a civil engineer, so I have no intuition for these types of things. But I'm certain I wouldn't have guessed that the limit here was 3km! My mind can't even visually picture a thing.

Note that after this quote, the article immediately discusses other constraints, including lateral loading, which dramatically reduce the upper limits. But I'm still boggling over the idea of a 3km tall tower -- it seems like it would just sink right into the earth!

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The key words are "under pure gravity loading" which means if there was no wind, earthquakes or occupants. These forces are more significant the higher the building. I wouldn't be surprised if including wind would halve the limit.

I also think they mean something that could be practically recognised as a building. You can build much higher if you are not restricted by a traditional proportions of a building (for example a pyramid or dome).

As to the building sinking -- it is not a difficult problem to solve. The reason some buildings face problem is that this is just a cost issue -- they guessed they need X amount of effort to provide stable base for the building and they guessed wrong. In practice there exists very simple algorithm to provide stable base -- drill to the rock face and use the underlying rock rather than possibly unstable ground. The reason this is not always used is because it can be very costly.

Fundamentally such a thing has to be possible or else natural occurrences like mountains, waterfalls, cliffs etc. would not be possible since they would collapse on themselves. Steel is stronger than dirt after all.

> "else natural occurrences like mountains would not be possible since they would collapse on themselves"

One way of looking at how mountains can get so high is to think of them as if they already have collapsed on themselves. There's no more collapsing possible, so they can pile up higher. A lot like the Pyramids, they're so big and so old and haven't fallen down because they can't, they're basically a collapsed heap already.

Which is something Isaac Arthur mentions in his YouTube series on space elevators, if you can make the base wider, you can make the top higher, and then you don't need Graphene or Scrith or other future super-strong materials to go high enough (in theory, in practise the base would have to get wider than Earth).

You'd run into two problems: The first is that all materials begins to behave like fluids under enough pressure. The second is that your giant heap of material will have its own gravitation pull. By the time you're talking space elevators, you'll run into very unique engineering challenges.

Good point, that's true. But mountains also have massive bases over which to distribute the load. I assume in the context of a skyscraper, the overall shape is still very much "skyscraperish" (that is, much taller than it is wide)

I don’t think that’s quite accurate. You still need to extend the base to support the top, I imagine a pyramid would be better as well at such heights without futuristic materials.

Steel is stronger than dirt after all.

Rock is also stronger than dirt. In fact, if you like up dirt a km deep, you end up with rock. (Sedimentary rock to be specific.)

I think it could reach that but would have to be pretty wide if I’m not mistaken. That’s the argument behind arcologies, you build this giant building that is basically a mountain and then fill it with trees, shrubs, grasses etc you can create a building that is also a giant forest.

You could build it out of dissimilar materials to optimize the total cost.

Lower levels out of concrete, then going up, steel, then aluminum and finally composite for the spire.

The problem is exponential in nature.

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