A suspension bridge is composed of a deck that is attached to or suspended from cables. Just like the name states, the suspended bridge literally suspends the roadbed from huge cables, which extend form one end of the bridge to the other. The cables are attached to two tall towers and are secured at each end by anchorages. The tower allows the cables to be draped over very long distances. The cable carries the weight on a suspended bridge to the anchorages that are imbedded in solid rock or massive concrete blocks. The cables are spread over a large area in order to evenly distribute the load inside the anchorages to prevent the cables from breaking free.
In the suspension bridge each cable supporting a segment of the roadbed is vertically suspended from the primary drapped cable spanning between main pylon towers. The forces from permanent and moving loads push down onto the roadbed placing it in compression. The cables through tension, then transfer the forces to the towers, which carries the forces, through compression, directly into the earth where they are firmly imbedded. The tension cables running between the two anchorages support the forces. The cables stretch from the weight of the bridge and the traffic that travels from anchor to anchor. In addition to the cables, the anchorages are also under the forces of tension. Because they are firmly imbedded into the earth like the towers, the amount of tension exerted on them is resisted by the counter forces of the dead load. Most suspension bridges also have a supporting truss system underneath the bridge deck to help stiffen the roadbed and to provide a lateral stabilization of the roadbed. This extra support system resists wind and lateral forces and reduces the tendency of the roadbed to ripple and sway.
Suspension bridges come in two different types of designs; the elongated "M" shape and the "A" shaped design called a cable-stayed bridge. The two designs support the load of the roadbed in very different ways. The differences lie in the way the cables are connected to the towers. The cable-stayed bridge attaches all cables that support the roadbed to the tower and they alone carry the weight of the roadbed and the traffic. The series of cables are attached to the roadbed in two basic ways, using a running parallel pattern or a radial pattern. In the parallel pattern, the cables are parallel to one another and attached at different heights along the tower. Each cable carries a segment of the roadbed. In the radial pattern, each cable carries its section of the roadbed and they are attached to the tower at a single point. In the cable stay bridge, all segments of the roadbed must carry a horizontal compressive force to counter balance the equal force from the other side.
