All bridges are designed to allow loads to cross obstacles. These obstacles may rivers, valleys or lakes. Generally the loads will either be vehicular traffic, pedestrians or animals. There are four basic types of bridges. These are Beam bridges, Arch bridges, Cantilever bridges and Suspension bridges.
Bridges can twist or bend under severe weather conditions which can have disastrous consequences.
In order to prevent this from happening bridges must be stiff enough to resist this movement and each member from which the bridge is made must be strong enough to withstand the load which is placed upon it.