Forces at Work: Suspension Bridges

suspensionbridge PBSBridges are subject to loads. Some loads are known as dead loads. These are loads that are made up of the weight of the structure itself. Another load is known as the live load. This load is made up of the weight of the traffic on the bridge. Another load is the weather load. It is the load that is caused by the wind and other weather-related activity. When a bridge is designed, all of these loads must be taken into account. If they are not all properly balanced, the bridge will not stand.

Depending upon the type of bridge, the loads that are put upon the bridge result in forces that are specific to the design of the bridge. In a suspension bridge, the dead load and live load produce tension on the cables. Tension is a pulling force(<– –>).  In a suspension bridge, the towers exert compression. Compression is the squeezing force (–> <–) that acts against the tension. You can see the compression forces on the towers marked in red. The towers are able to exert this force because they are secure on footings well beneath the soil beneath the water. The tension of the horizontal cables helps to hold the towers in place, too. As the towers push back on the cables, a balance is reached. This balance must be maintained in the face of not only a dead load, but a live load and a weather load, too.

You’ll also notice that the suspension cables that go from shore to shore are anchored at the banks or further back. This results in a finite amount of cable that passes up and over the towers. The shape of the horizontal suspension cables forms a parabola because the cables from those cables to the deck of the bridge are spaced evenly. This uniform placement is important because it ensures an equal distribution of the dead load of the bridge.

Let’s look at this in a less scientific way. Suppose the trees available for the ropes to tie the hammock were not as tall as we’d like, or we did not know how to tie a strong knot. We could make a terrific loop, but not a terrific knot for the purposes of suspending a hammock. We could put the rope over the branch – see how it is not straight and tall – and then secure it some distance away with a stake. This would mean that the rope at either ends of the hammock went up and over a standing structure (in this case, a tree trunk that was not straight up and down) and was supported by a stake driven through the loop at the end of the rope. This is the same idea behind the wires that are secured at an anchorage before going up and over a tower. In this case, there are two simple suspenders, rather than a series of suspenders hanging down from the looping wire, but you get the idea!

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More: Types of Bridges: Suspension More about bridges at PBS

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