Tension, compression, and torsion are three different ways that structures respond to the forces, pushes and pulls, exerted on them
Structures are load bearing, which means they can support or stay strong against all of the forces acting on them. A force is a push or a pull on one object by another object. When you think about pushing or pulling something, you might think about moving it. For example you might open a door by either pushing or pulling on it. But not all objects move when you push or pull on them. If you push on a wall, you are definitely exerting a force, but unlike the door, it isn't going to move. Structures still experience the force being exerted, but since their job is to stay put, they have other ways of reacting to forces besides moving. Tension, compression and torsion are three different ways that structures respond to the pushes and pulls exerted on them.
When a structure is being pulled, it is under tension
When a structure is being pulled, it is under tension. For example when you sit down on a swing, the force of your weight pulls down on the swing, creating tension in the swing. The chains on the swing act like a rubber band and stretch. Unlike a rubber band, you can't actually see the chain stretch because the amount of the stretch is incredibly small. Imagine instead that the chain links are made of rubber bands instead of metal. With rubber bands, you would definitely be able to see the stretch. But since the purpose of a swing is to allow you to swing, not to bounce or sink straight to the ground, it has to be built with something that stretches just a tiny amount under tension.
When you get off the swing, the chains return to their normal length. If they didn't, the swing would eventually sink to the ground, and it wouldn't be able to continue being a swing. The swing chains act just like a rubber band that returns to its original size once you stop pulling on it. This ability to stretch and then return back to its original size is a behavior known as elasticity.
When a structure is being pushed, it is under compression
When you sit on a chair, the force of your weight pushes down on the chair, putting the chair under compression. Much as the chains on the swing got a little bit longer when your weight pulled down on them, the chair gets a little bit shorter when you push down on it. When you stand up, the chair goes back to its normal height. Just like the chains on the swing, the chair has elastic behavior.
When a structure is being twisted, it is under torsion
When a structure is being twisted, it is under torsion. If instead of swinging back and forth, you twist your swing up instead, then the swing is under torsion. You know what happens once you lift your feet off the ground: the chair spins right back to its resting position. Not only is the twisted swing a fun ride, it also demonstrates how a structure is able to return to its normal position after being twisted.
All structures, no matter what size, or what they do, are either under compression or tension, or both forces at the same time.
We used small structures like swings and chairs to demonstrate how structures respond to different pushes and pulls through tension, compression, and torsion. Larger structures like buildings and bridges react to forces these exact same ways. The cables on a suspension bridge experience tension as they help support the weight of the bridge and its users. The roof of your house experiences compression if there is a lot of snow on top of it. A skyscraper can experience torsion if it twists slightly in the wind. Structures can also experience a combination of these reactions. For example a twisted swing is experiencing both torsion from the twist and tension as you are still pulling down on the chains. Tension, compression and torsion describe three ways structures respond to the different forces acting on them.