Bridges come in four main types. One of those types is the arch. You may have noticed arched bridges before – certainly now that you’re aware of them you’ll see them often. Have you ever wondered how it is that an arched bridge stays up?
As with any other type of bridge, it’s all about the forces at work. For a bridge to be successful, those forces must be in balance under a variety of conditions. The bridge must be able to:
- support its own weight. In the case of an arched bridge, that means that the weight of the stone or rocks is an important consideration in the functionality of the bridge.
- support the weight of the “live load-” the people and vehicles moving over the bridge.
- withstand the force of the water moving past it in low water conditions and flood times.
An arch bridge meets these conditions:
- The semicircle formed by the arch distributes the force known as compression across the top of the semicircle.
- The inside of the arch is subject to the force known as tension. In an arch bridge, there is not a lot of tension – unless the arch is spread out; the semicircle is large.
- The weight is held by the abutments – the “legs” of the arch.
When an arched bridge is constructed properly, it can stand for years. The proof of this? Many bridges originally constructed by the Romans are still not only standing, but in use today.