A structure is something that has parts put together in a particular way and is load bearing
Are you at home sitting on a chair looking at your iPad? Or maybe you are at the library sitting at a desk watching on a computer. You might even be in a car or an airplane looking at a smartphone or a tablet. Wherever you are, you are likely sitting on a structure, like a chair, and sitting in a structure, like a building. If you are sitting in a vehicle like a car or an airplane, then you are in a machine with structural components, or parts. So what is a structure anyway?
The simplest definition of a structure is that it is something with parts put together in a particular way. From an engineer's point of view, a structure is something that is 'load bearing'. Let's take a closer look at what each of these two words mean, so we can understand what it means to be 'load bearing'.
Load bearing means to hold up against one or more forces
When you hear the word "load", you might think about carrying a load, like your backpack or a load of laundry. In the case of structures, load is another word for force. A force is a push or a pull on a structure or any other kind of object. "Bearing" might be a word you have never heard before, and it can have a few different definitions (none of them having to do with bears!). In this case, bearing means to hold up or stay strong against something. Putting these two ideas together, a structure is something that can hold up or stay strong against one or more forces, which means it will not break apart, collapse, or otherwise fall down.
Structures support their own weight, which is the force gravity exerts on the structure
Structures serve different purposes, but they are all load bearing. A chair's purpose is to be sat on (and maybe it's the place you put your backpack when you come home from school...). A house provides shelter. A bridge gives you a way to cross an obstacle. Each of these structures does something different, but they all have one load or force in common: their own weight. An object’s weight is the measure of the pull of gravity on that object. Before a chair can support the force of your weight, it has to support its own weight. The same thing is true for the house and the bridge. A structure's own weight is called its 'dead load'. Any other force the object supports (like you or your backpack) is called its 'live load'.
Large structures must also stand up to environmental forces like wind and earthquakes
Some structures have to support more then the force of the weight of people and things that use it. For example, a bridge is designed to support the weight of the vehicles and people crossing it and also to resist the force of the wind blowing against it. It wouldn't be a very useful bridge if it could support a lot of users, but blew over in the first strong wind. Depending on where the bridge is located, it might also be designed to resist the movement of the earth during an earthquake. Earthquakes and wind create environmental forces that act on structures, and they are important considerations for large structures like buildings and bridges.
Engineers help to ensure structures are safe
Engineers help to make sure that buildings and other large structures are safe to use. Engineers also help to ensure the structural safety of other kinds of technology like vehicles and other machines. For example, an engineer would help to make sure that the frame (a structural component) of your car can support the weight of your car, your family, and your stuff. An engineer will also make sure that the frame protects you if you get in an accident.
We rely on the structural safety of not only our buildings and bridges but also the vehicles we use to travel and the other machines that we encounter in our modern world. From chairs to buildings and bridges, we are surrounded by structures. What structures do you see around you?