In the real-world, different types of bridges are chosen based on how they are going to be used, and the type of materials that are available. For example, modern-day bridges are very different from bridges built during the Renaissance era. There are several major bridge designs to choose from, but only a few are able to be reproduced by children in a science classroom.
A truss bridge is made of straight building materials that are connected together at angles where adding a weight or pressure cause the joints and/or connections to compress and push against each other. This type of bridge is particularly strong because it has a dynamic weight-bearing system. The pressure on the joints is lessened as the weight decreases. If the connections or joints are properly made — meaning they are set at the right angle — the bridge can hold significantly more weight than if it was connected by overlapping materials. These bridges are easily built with Popsicle sticks for science projects.
A suspension bridge, also referred to as a rope bridge, is made of two pieces of rope that are placed parallel to one another and attached to opposite sides of an embankment (or piece of wood for a science project). Planks or broad sheets of material strong enough to walk on or hold weight are then attached to the two parallel ropes. This is an easy bridge to make, and its strength depends entirely on the quality of materials used. Using wire cabling rather than hemp rope makes a stronger bridge. This is an easy bridge to replicate in the classroom or home.
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An Arch Bridge
An arch bridge is a flat-topped bridge that has an arch underneath it as the primary form of support. The arch bridge is similar in appearance to many older stone bridges, as this was a good method of constructing a bridge from shorter and smaller pieces of material. This is an easy bridge to build using small rocks or pebbles, and can be a very sturdy bridge. Its strength relies on the quality of the materials and how well it is constructed.
A beam bridge is the simplest bridge design there is. It is merely a single beam that spans a distance between two fixed points. Unlike a truss bridge, a beam bridge does not have materials that connect at angles to absorb the weight that is placed on it. A beam bridge relies on the strength of the materials available and the distance it needs to span. The greater the distance, the sturdier the material must be. This is not as strong a bridge design as the other types, but it is the easiest to build.