How do skyscrapers resist sinking into the ground due to their mass ?


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How do skyscrapers resist sinking into the ground due to their mass ?
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Foundation of Skyscrapers

First of all, Skyscrapers foundation system is designed to be safe in two things,
1. It should be able to bear the pressure of building without failure.
2. It should not mobilize excessive settlements.

A simple footing of a house looks like,

Sloped foundation

A very simple system, look how the footing area increases over the column area (the central wooden part). Many of these are under every column of your house.

If we increase the height of building, we will have to either increase the total area of footing or go deeper where we can get a good “firm” stratum.

now again, If we increase the area , we get  Raft foundation.

Raft Foundation

If we increase the depth , we get the Pile foundation.

Pile foundation

Now Skyscrapers foundation is either a Raft, Pile or a combined raft pile.
For a great example, lets have a look at Burj Khalifa ,

Burj Khalifa Foundation

Consider the following:

  • If you dug a hole down 36 ft (11 m) the soil you find there is used to having 36 ft worth of soil sit on top of it. i.e. the deeper the foundation the greater weight it can take.
  • If you drive “columns” into the ground, much like a hammer drives a nail into wood. These columns, lets call them piles, can carry weight to a great depth (where the soil is better) AND they can carry even more weight thanks to the friction they have with the soil all around them. Just like pulling a nail out of wood is difficult, but going down not up.
  • Usually tall buildings are built where there is good soil rock! If you can get your foundation to reach rock, you’re golden. Large rocks buried in soil tend to not move.
  • The larger the footprint of the building the less weight per square foot (or square m).

Mass of earth removed from basement

There is also one debate on that most of us  overlook the simple fact that the mass of the building is often less than the mass of the earth removed for the basement of the building. The volume of the building, after all, is mostly occupied by air. The volume of the burden removed from the basement, however, is mostly solid material

But it doesn’t really matter how much earth you remove, or how the weight of the removed earth compares to the weight of the building. What matters is how the remaining earth supports the building. Typically, structures like skyscrapers sit on pile foundations. Pushing a standard wooden pencil into the dirt will give you a good idea of how this type of foundation resists loads. If you push the pencil into the ground, chances are you can’t push hard enough to get it all the way into the ground. As you push it down, a couple of things happen:

1) It displaces the soil around and under it, compacting it and making it more resistant to applied loads,

2) It develops skin friction (which is the friction force on the side of the pencil applied by the fact that soil is present). End bearing (how much of your weight the tip of the pencil supports) is usually small compared to the skin friction (you can test this by sharpening the pencil to a fine point and seeing how much farther you can push it in than an unsharpened one). Now, that’s only a pencil….and if you drive it into normal soil, chances are it would only take 4 pencils to support a 150-lb person. Imagine if you scale that up to where your “pencils” are 36″ concrete piles. You get the idea.

The additional benefit to this type of foundation is that it also resists lateral loads quite well (resistance is usually a function of embedment length). Back to the pencil analogy – if you drive 1/4 of it’s length into the ground, and then try to push the eraser until it falls over, it’s fairly easy. But try that same stunt with it 3/4 of the way into the ground….you won’t be able to do it. That becomes very important when you have 90mph wind gusts acting on 100 stories of building above the foundation.

also read

What are the technical limitations for building skyscrapers today (2016) and how high can we go with today’s technology ?

Udayram Patil

Udayram Patil

Udayram Patil is an enthusiastic graduate civil engineer with an eye for innovative & technical writing.His wish is to combine his knowledge and experience about his field, to deliver the best practical cum accurate information to his audience.
Udayram Patil

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