Question: how can bridges hold everyone?

Pete Etchells answered on 11 May 2012:

That’s a brilliant question. There are two basic bits to a bridge – the span, which is the bit that use to walk or drive on, and the supports, which are used to hold everything up and deal with the forces that a bridge will be subjected to. There are different types of bridge design, but all of them have the same aim; to make sure that forces (such as compression) don’t all focus in a small area, which would cause the bridge to break. Engineers spend lots of time figuring out where the forces would focus on a bridge depending on how many people are on it, and where they are on it. They then use support structures to spread the effects of these forces out along the whole length of the bridge, which makes the effect of the force in a particular place much weaker, and therefore much less likely to make the bridge break!