Background Essay: Clifton Suspension Bridge
The challenge with all bridge design is to provide a structural system to support the weight of the deck and all other fixed parts (its dead load), as well as any people or vehicles that may cross it (its live load). All bridges stand on feet of some sort, be they piers, towers, or abutments. The main difference between the designs is in how the feet enable upward force from the ground to support the weight of the bridge and its loads.
In suspension bridges, vertical suspension wires, made from steel or another material that is strong in tension, connect the deck to the main cables overhead. The main cables sag between two tall towers in a predetermined way so that each section of cable pulls up on the deck more or less steeply according to its position along the length of the cable. The curve of the main cables, then, is similar to the curve of an arch. The main cables' ends are anchored to immovable concrete blocks or solid rock on either side of the span. Pulled taut by the anchor blocks, the cables rest atop the bridge's towers so that the entire weight of the bridge and its suspended load is supported by these towers. The ground on which the bridge's feet stand pushes up to balance the downward force moving through the towers.
For the Clifton Suspension Bridge that spans Avon Gorge, an early example of a suspension bridge, engineers fashioned lengths of heavy iron bars into a chain to create the bridge's curving overhead cables. Iron is a tension-strong material, but not as strong as steel, the strongest structural material used today. No matter which materials are used, the principles are the same in all suspension bridges: Tension in the cables is transferred to compression in the towers, creating an equilibrium of forces. This enables suspension bridges to span greater distances than other types of bridges with a minimum number of obstacles underneath the structure.