There are four major types of bridges built today; the Beam, the Arch, the Suspension (or Cable) and the Cantilever. The type of bridge chosen depends on several factors. A bridge for a 5,000-foot span across an ocean bay to allow huge ships come and go, or a 700-foot span across a deep canyon gorge, or a 120-foot bridge across a freeway require different approaches. The strength and weaknesses of each type of bridge is taken into consideration while choosing the one that spans the landscape.
One of the oldest types of bridge in existence is the Arch bridge. They date back to the pre-Roman era and have been extensively in use because of their natural strength. An Arch bridge can be visualised as two parts of a whole, the curved bottom across the landscape and a flat span. The weight on an arch bridge is transferred outward along the curve of the arch to the supports at each end. As a consequence, the supports, called abutments keep the ends of the bridge from spreading out.
How to Build an Arch bridge
Cut a strip of plexiglass, about two inches wide by one foot long. Gently bend the strip so that it makes a curvature in the form of an arch. Next, build the abutments as a stack of books or with a stack of wooden blocks strong enough to hold the arch in place and be able to handle a small load. Use a light paperweight or press down gently on the plexiglass. Do you notice that the weight keeps the arch in place and prevents it from spreading apart?
Historically, most Arch bridges were built with stone. Due to the stone construction, there was less flexibility in using this bridge to build longer spans. In modern times, steel and pre-stressed concrete make for better handling of weight and compression and longer bridge spans.
A beam bridge is the most basic type of bridge that can be constructed. It consists of a horizontal beam that is supported on the ends. Any weight that is on the bridge is transferred from the horizontal beam to the vertically supported ends. A beam bridge is also called a girder bridge, and the supported ends are often known as piers.
The beam itself is built from materials that are strong, yet flexible, so that it is able to support its own weight as well as the weight on the bridge. When a weight pushes down on the horizontal beam, the beam stretches at the bottom and compresses on the top. This is commonly called tension and compression.
How to build a beam bridge: Use a flexible material like plexiglass and use it as a beam by supporting the two ends with wooden blocks. Press down on the centre. Did you notice that the top squeezes in compression and the bottom expands?
Suspension bridges are majestic due to the span distances they cover. They usually appear light and strong, and can run longer than any other kind of bridge. As the name suggests, a suspension bridge suspends the causeway from huge main cables, which extend from one end of the bridge to the other. These cables rest on high towers and are secured at each end by anchorages.
The towers help the main cables to be extended over long distances, and hence derives the majestic look. In the case of these bridges, the weight of the bridge is carried by the cables to the anchorages. The anchorages in turn are embedded in either solid rock or heavy concrete blocks. Inside these anchorages, cables evenly distribute the load and prevent them from breaking free.
Suspension bridges are slightly different from cable-stayed bridge in the way the supports work and the two types should not be confused with each other.
How to build a simple Suspension bridge
Tie two loops of a strong piece of string to two wooden blocks which are similar. Create a third loop so that it hangs loosely between the two blocks. When the centre is pressed down, the bridge collapses.
Next, stand two wooden blocks about a foot apart. Put a heavy weight on one end of string to secure it. Then pass the string over each wooden block while allowing some of the string to hang loosely. Place a second heavy weight on the other end of the string. Press again on the centre of the string. Now, the anchorages or the heavy weights, help to stabilise the bridge.
A cantilever bridge is another major type of bridge where two horizontal structures support a third horizontal structure on which a load is carried. The two horizontal structures must be anchored, and this must be done well. Cantilever bridges are a modified form of beam bridge, where the support instead of being placed at the end, is placed somewhere in the middle of the span. A cantilever is a structure that is unsupported at one end but supported at the other, very similar to diving boards in a swimming pool. This configuration made longer spans possible and wider clearance beneath.
How to Build a Cantilever Bridge
Stack a set of wooden blocks. Cut a piece of cardboard 6" long and 1" wide. Secure the piece of cardboard to the middle to the stack using glue. Repeat the steps to create a second set ensuring they are identical in all respects. Place the two supports side by side with a 4" space between the two.
Cut a third piece of cardboard with similar dimensions as the other two. Glue one end of this piece to one of the support and the other end to the end of the second support such that the third piece of cardboard is now supported at its ends. You have successfully created a cantilever bridge.