Bridges

Bridges

Bridges are structures that span over some sort of obstacle such as a body of water, a trench, or a valley in order to provide transportation over said obstacle. Certain bridges must be able to withstand certain conditions such as the terrain or weather in which the bridge is anchored. For instance, in San Francisco, bridges must be able to withstand the frequent earthquakes that occur as the city is placed on a major fault line.

The Golden Gate Bridge in San Francisco

There are many different types of bridges that have spanned throughout history such as the beam, truss, arch, suspension, and many others.

Beam Bridge

The beam bridge is constructed of horizontal beams which are supported by structures at each end. These components support the downward weight of the bridge and any traffic traveling upon it. In supporting weight though, the bridge endures compressional and tensional stress. To try and spread out the forces, sometimes a longer beam is used. Sometimes the pressure is too much though and a truss may need to be used.

These are both examples of beam bridges.

Truss Bridges

A truss bridge is a bridge whos lattice-like superstructure that bears the load of the force. The support truss adds rigidity to the structure, greatly increasing its ability to dissipate the compression and tension caused by weight on the bridge. Once the beam begins to compress, the force is spread through the truss. 

Arch Bridge

The arch bridge has been used for over 2,000 years. Its elegant structure is composed mainly of two abutments, or structures at the end of the bridge where the superstructure mainly rests, at the end of each side of the arch that take the main amount of force. The curve of the arch and abutments greatly dissipates the amount of tension on the underside of the bridge. 

The second picture shows how the tension dissipates throughout the bridge.

Suspension Bridge

Suspension bridges suspend the roadway in between two towers held by cables. The towers support much of the weight as weight pushes down on the roadway, is transferred to the cables and then to the towers. The towers then dissipate the force directly into the earth. The cables however, receive most of the bridges' tensional force. The cables are attatched to anchorages, or large slabs of rock or concrete in which the bridge is anchored to, and the tensional force travels through the cables, to the anchorages, and then is dissipated into the ground.

Bridges come in all shapes and sizes, but they all basically do the same job: support weight over a certain obstacle. In order to do that job, they must find a way to dissipate the forces so that they do not buckle and break.