Though any structure appears still, its parts are always pulling and pushing on each other. Structures remain standing because some parts are being pulled or stretched and other parts are being pushed or squashed. The parts that are being pulled are in tension. The parts that are being squashed are in compression.

Strong structures such as towers, bridges and beams have to be able to cope with pushing (compressive) and pulling (tensile) forces. For example, two bricks side by side can withstand pushing forces, but not pulling forces. A rope can withstand pulling forces but not pushing forces. What shapes can you think of that are strong when you push or pull them?
Bridges are great example of structures that use simple shapes for strength.
Triangles form the strongest structure because all three sides bear the load. Compression pushes down equally on two sides of the triangle, causing the base to be pulled equally in two directions, which creates tension. In a square, two sides of the structure bear the load.


The trianglangular shape is used in the construction of suspension bridges because of its strength.