Bridges are all around us. They are in different shapes. How do they hold our weight – railroad trains, cars, or people? Besides, bridges also have to bear environmental forces like wind and water!
Bridges carry loads and stay in precise position. This is made possible by carefully balancing two main kinds of forces: compression (a pushing or squeezing force, acting inward) and tension (a pulling or stretching force, acting outward).
There are five basic types of bridges. Let us explore all of them one by one!
The arch bridge is one of the oldest forms of bridges. It has a semicircular design (an arch) with abutments (the supports on each end). An arch bridge is known as a compression structure. The weight of the central keystone on the top of each arch pushes the surrounding rocks downward and outward. The weight is channelled along the curve of the arch to the abutments. This makes the bridge very rigid and strong. Arch bridges stand on their own for a long time.
A beam bridge is the simplest kind of bridge. Ancient beam bridges were constructed in a simple manner, like placing a log across a stream. Today, bridges are built with the use of rock, stone or mortar.
A beam bridge consists of a horizontal beam supported at each end by piers. Beam bridges work on the principles of compression and tension. A beam must be strong enough to resist twisting and bending under the weight it must support. A beam carries loads by bending. Under load, the beam’s top surface is pushed down (compressed) while the bottom surface is stretched (in tension). However, the farther apart its piers, the weaker the beam becomes. This explains why a beam bridge rarely spans more than 250 feet. If a bridge is needed to cross great distances, a series of beam bridges are joined together.
Suspension bridges get their name from the fact that the roadway is suspended by cables from two tall towers. Smaller cables (suspenders) run vertically from the bridge deck up to the main supporting cables that extend to anchorages on each end. Anchorages are usually bedrock deep underground. On load, the suspenders transfer the compression forces to the towers via the main supporting cables. The load pulls the towers inward while the anchorages pull the towers outward. Suspension bridges can easily span distances as great as 7,000 feet or more.
A cantilever bridge is a bridge built using cantilevers. A cantilever is a horizontal beam supported only on one end. Cantilevers must be firmly anchored on one end to support the weight on the free standing end, which works like a diving board. Since most of the bending happens in the centre of a beam, the pier is usually placed in the centre of the beam. A cantilever bridge balances tension forces above the bridge deck with compression forces below. There’s usually a pair of cantilevers extending from each pier, with a short beam bridge linking them together.
Cable-stayed bridges look similar to suspension bridges, but cable-stayed bridges support the weight differently. Cable-stayed bridges are built when there isn’t room for cables or appropriate bedrock to anchor them into. Straight cable stays extending in one or two planes from the tower or towers are anchored directly into the roadway itself. The balancing two sets of suspension cables on either side of each pier support the weight. The road and tower are in compression while the cable stays are in tension.
Inquiry – Digging Deeper
Before designing a bridge, engineers need to know the important differences among the various kinds of bridges, as well as the bridge purpose and location. In this bridge-building video, some of the important differences among arch, suspension and cable-stayed bridges are explored.