Bridges 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.