Bridges

The longest bridges in the world are all suspension bridges. Some examples are the Humber Bridge in Hull, England, with a length of 4,626 ft (1,410 m), the Verrazano-Narrows Bridge in lower New York Bay (4,620 ft [1,298 m]); the Golden Gate Bridge over the entrance to San Francisco Bay (4,200 ft [1,280 m]); and the Mackinac Straits Bridge connecting the Upper and Lower Peninsulas of Michigan (3,800 ft [1,158 m]).

In a suspension bridge, the dead and live loads are carried by thick wire cables that run across the top of at least two towers and are anchored to the shorelines within heavy abutments. In some cases, the bridge deck is supported directly by suspendors from the cables, while in other cases, the suspendors are attached to a truss, on top of which the deck is laid. In either case, the dead and light load of the bridge are transmitted to the cables which, in turn, exert stress on the abutments. That stress is counteracted by attaching the abutments to bedrock.

The towers in a suspension bridge typically rest on massive foundations sunk deep into the river bed or sea bed beneath the bridge itself. The wire cables that carry the weight of the bridge and its traffic are made of parallel strands of steel wire woven together to make a single cable. Such cables typically range in diameter from about 15 in (38 cm) to as much as 36 in (91 cm). Smaller cables can be ordered from a factory, while thicker cables may have to be assembled on the construction site itself.

An interesting hybrid of the cantilever and suspension bridge is known as the cable-stayed bridge. The 1,200 ft (366 m) Sunshine Skyway across the entrance to Tampa Bay in Florida is one of the most beautiful examples of the cable-stayed bridge. In a cable-stayed bridge, the deck is cantilevered outward in both directions from a central tower. The deck is then attached to the tower by a series of cables, similar to those in a suspension bridge. Often, a cable-stayed bridge will make use of two towers. In that case, the cantilevered sections extending towards each other in the middle of the bridge can be joined together, producing an unusually long central span. The advantage of the cable-stayed bridge is that support for dead and live loads come from three distinct places: the towers, the cables, and the abutments to which the bridge is attached at each end.