A bridge having overhead cables supporting its roadway is called a suspension bridge.
Modern suspension bridges are light and aesthetically attractive and can span longer distances than any other bridge form. Unfortunately, they are also among the most costly bridges to construct. Although suspension bridges can be made strong enough to support freight trains, they have nearly all been designed for automobile traffic.
Form and mechanics
A suspension bridge carries vertical loads over curved cables in tension. These loads are transmitted to the towers, which take them by vertical compression to the ground, and anchorages, which must resist the cables' inward and sometimes steep pull. The suspension bridge can be seen as an upside-down arch in tension with only the towers in compression. Since the deck is hung in the air, care must ensure that it does not move excessively under loading. The patio, therefore, must be either heavy or stiff or both.
From the early 20th century, deflection theory has been applied in the design of suspension bridges to estimate how the horizontal deck and curved cables work together to carry loads. As spans turn longer and the suspended structure heavier, the needed stiffness of the deck decreases. Hence, deflection theory, as engineers attempted to reduce the ratio of girder depth to span length to attain a lighter, more elegant appearance without compromising safety.
Construction
When bridges needing piers are built over a body of water, foundations are built by sinking caissons into the riverbed and filling them with concrete. Caissons are huge boxes or cylinders that have been constructed from wood, metal, or concrete. As regards suspension bridges, towers are built atop the caissons. The initial suspension-bridge buildings were stone, but now they are either steel or concrete.
