Why are triangles the most stable shape in construction/engineering?

In: Engineering

It’s because equilateral triangles can take a point load and spread that force/stress/strain evenly across its upright members and its base. Its also the only shape that remains if its corners were hinges.

A quick way to visualize this for yourself is to get 4 popsicle sticks or some other small stick like that – toothpicks would also work (they aren’t necessary if you can picture this mentally).

Take 3 of the sticks and try to touch the ends together to make a triangle. What you will find is that there is only one triangle that can be made and in order to change the shape of that triangle and the angle between any of the two sticks you would have to change the length of one of the sticks.

Now take 4 sticks and touch the ends together to make a quadrilateral. Did you make a square? Or perhaps a rhombus of some sort? What you’ll notice is that using those exact same sticks you can make many different shapes by changing the angles without having to change the length of the sticks.

For this reason when if you have a square structure it is easier for it to change shape than when you have a triangle structure. In other words, it’s easier for the square structure to longer be square than it is for the triangle to no longer be triangle.

There are other shapes such as circles which are very strong as well depending on the circumstances and arches which are often used in architecture to distribute weight more evenly, but for the simplicity of being able to use multiple straight lengths of material a triangle is the easiest to build of the structurally sound shapes.

I’ll start with a square. If a square is made from four boards and every corner is a hinge like your elbow, there is a problem. This shape can easily be squished out of shape. Your square can easily be changed into a parallelagram, A triangle cannot. A triangle is said to be inherently rigid because of this. It’s natural, and so we see it all around us.

Sidenote: triangles can’t be bent. Moving any vertex it will always spin around an axis. That’s really useful in 3d modelling.

Already some great explanations for the triangle, so I’ll just add that triangles are not always the most stable shape. It is mainly the most stable polygon.

Another very stable shape is a sphere. This can be used to distribute a given load quite efficiently. This is the idea behind building a dome, such as The Pantheon in Rome.

Because three points make a plane. A fourth point (square) might sit on the same plane or it might not. If it does not, then the object will wobble.