Why is a Triangle a Strong Shape?

You may have heard people say that a triangle is a strong shape. But why is this?

Why Triangles are a Strong Shape

When engineers build structures, they want to make sure that the structure can bear weight. In other words, they do not want the structure to fall down when a force is applied to it. For example, bridges must be able to hold up the materials that make the bridge, as well as all of the traffic travelling across it. One of the shapes that can bear weight very well is the triangle. But what makes triangles so good at this?

Let’s look at how a triangle transfers a force. 

When a force (the load) is applied to one of the corners of a triangle, it is distributed down each side. The two sides of the triangle are squeezed. Another word for this squeezing is compression. The third side of the triangle is pulled, or stretched sideways. Another word for this stretching is tension.

How Triangles are Used in Bridges

You often see triangles used to create bridges. Bridges combine multiple triangles. They apply compression and tension in different places.

Triangles can be used to make trusses. Trusses are used in many structures, such as roofs, bridges, and buildings. Trusses combine horizontal beams and diagonal beams to form triangles. Bridges that use trusses are called truss bridges.

There are several different types of trusses used in bridge design. The type of truss depends on how the horizontal and diagonal beams are arranged. There are four main styles of trusses used to make bridges.

Warren truss 

This uses diagonal beams that form equilateral triangles. These triangles have three sides of the same length. 

Howe Truss

This uses diagonal beams that slope upwards towards the centre of the bridge until they meet in an upright isosceles triangle. This type of triangle has two sides of the same length, and a third side of a different length.

Pratt Truss

This uses diagonal beams that slope downward towards the centre of the bridge until they meet in an upside down isosceles triangle. 

K Truss

This is a more complex type of truss that is made from back-to-back triangles that form K shapes.

Did you know? 

The longest bridge in Canada is the Confederation Bridge, which is 12.9 km long. It connects New Brunswick and Prince Edward Island.

How Triangles are Used in Roofs

A type of truss commonly used to support roofs is called a King Post truss. A king post truss has a horizontal beam called the tie beam and a vertical beam called the king post (hence its name). There are two posts that join the top of the king post with the tie beam to create triangle shapes. These posts are called principal rafters. Within these triangles, diagonal struts are added to connect the principal rafters to the king post. These struts make the roof even stronger.

Image - Labels

(1) King post

(2) Tie beam

(3) Principal rafters

(4) Struts

These king post trusses are then connected together with more beams to form a strong and stable structure that can support a roof.

Image - Labels

(1) Ridge beam

(2) Purlins

(3) Common rafters

This is an example of a "double roof" with principal rafters and common rafters.

Triangles are extremely important shapes when it comes to engineering. From bridges to houses, and even airplanes -- using triangles in structures will always be important!