Physics of Tall Structures

There are a great many things about architecture

that are hidden from the untrained eye. 

- Frank Gehry

What makes a tall structure stable?

I find pretty much all buildings beautiful.

Certain buildings inspire many feelings and emotions in us from old buildings, a reminder of history, or even tall buildings, admiring and applauding how it could possibly stand with such an incredible height. Yeah, I get that emotion when I regularly pass by our CN tower.  

Looking at these comparisons of the tallest
 structures in the world, you can notice how
most have a strong definite wide base
and triangular shapes in their designs.

I think the main component for a strong, sturdy structure would have to be its base. Starting off with a strong base not only assures that the building can increase its height but it also avoids any doubts of the building crumpling down. The base in most tall structures seem to be wide so that they could easily distribute the mass better.

Great Pyramid of Khufu

As well shapes are a big factor for structures and triangles seem to work the best. If you look around, you will notice that most of our buildings are shaped like rectangles and squares. However, if you look closely, you will see that those buildings use triangles. Triangles are the strongest geometric shape in the world. Its shape is very simple: a flat base with two sides that come together at the top to meet at a point. They are rigid, able to stand freely, and able to support their own weight. Triangles are so important in structure design that even many years ago in Ancient Egypt, builders designed huge pyramids with the shape of a triangle in mind. One of the oldest pyramids in Egypt is called the Great Pyramid of Khufu. It was built around 2575 BC. This pyramid is one of the largest surviving pyramids in the world today. I'm guessing future architects examined these ever lasting structures and learned that the triangle shape is the way to go.

Lastly, weight distribution is a huge important factor for tall structures. Would you ever build a building from cardboard at the bottom and the upper ones from brick? Probably not since you'd run into problems quite quickly. Weight distribution from heavy to light helps a building balance and support its own weight.
 

What is the centre of gravity?

The center of gravity is the average

location of the weight of an object

OR

The place where an object's mass

seems to be the most concentrated.

GRAVITY: the magnetic-like force of attraction between any two objects in our universe.

On Earth, gravity seems to be the way that things fall to the floor, but gravity always work two ways. If you drop a pen, it does fall toward the floor—but the floor also jumps up by a microscopic amount to meet it on the way. The force pulling your pen down toward Earth is exactly the same size as the force that pulls Earth up toward the pen.

Different people have different centre of gravity.
For example the baby has a heavier head
than body so its centre of gravity is higher.

Now gravity usually pulls things straight downward, but it can act in other ways too. Suppose you built a really tall brick wall. We can think of gravity acting on it in two different ways. We can see it as a collection of separate bricks, with gravity pulling on each one separately. Or we can think of it as a solid wall with gravity pulling on the whole thing, just as though all its mass were packed into a single point in its center. For a simple brick wall, the center of gravity is right in the middle of the central brick.

If the center of gravity is over to one side (if we've not built the wall straight or if we've built it on sloping ground), the force of gravity acting down will produce a turning effect called a moment. If the moment is small, the mortar between the bricks can resist it and keep the wall upright. But if the moment is too large, the mortar will break apart, the bricks will topple, and the wall will collapse.

Now this doesn't just apply to single walls: it applies to entire buildings. That's why tall buildings need deep foundations (where a significant part of the building is constructed underground to support the part that's above ground). If something tries to push the top the building to one side, the foundations effectively resist and push it back in the opposite direction. In other words, they help to counter the moment that would make a building topple to one side.

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