Suspension bridges are world-class infrastructures that serve as not only a connecting medium between two locations, but also as an essential landmark with a valuable function. Suspension bridges are massive constructions designed to enable people traverse rivers and valleys. They consist of three main parts: towers, cables, and decks.
Towers are the structural heart of a suspension bridge and provide support for the cable and deck structures. There are several types of towers used in suspension bridges including single-tower, double-tower, and triple-tower designs. The number of towers determines the height of the bridge. For example, a four-panel suspension bridge made up of two pairs of towers would be about half the height of a comparable single-tower bridge. Double- and triple-tower bridges are even lower than this because they require more support than a single tower. Towers can be built from concrete or steel. Concrete towers are usually shorter than their steel counterparts because they are less strong. Steel towers can be very tall if necessary because they have great capacity for strength over length.
Cables are long chains or wires that connect the towers together. They must be extremely strong since they hold the weight of the bridge deck above them. Cables can be made of steel or fiberglass since these materials are both strong and light weight. Fiberglass cables are becoming more common since they are much cheaper than steel ones.
Suspension bridges are so named because the highway is suspended by cables from two towering towers. The two towers bear the majority of the weight. They then direct the compression pressures from the wires into the earth. Suspenders are smaller cables seen on suspension bridges. They help to prevent the bridge from swaying in the wind.
The main types of support for a suspension bridge are anchor rods, which go down into the ground under high tension; counterweights, which provide balance against the load imposed by the traffic crossing the bridge; and tie rods, which connect the girders together and allow them to move relative to one another.
Anchor rods secure the suspension bridge to the ground. They can be made of steel or concrete. If made of concrete, they may also have a protective coating to avoid corrosion. Tie rods connect the girders together and allow them to move relative to one another. They are usually made of steel but some modern bridges use polyurethane ties instead.
Counterweights are objects that weigh more than you would expect for their size. For example, a person weighing 70 kg (154 lb) would need to carry a sack of wheat to make a counterweight for a bridge. Counterweights reduce the amount of steel needed in the structure and therefore reduce its cost. They do this by providing balance where there would otherwise be stress.
A typical suspension bridge is a continuous girder hung by suspension cables that run through the main towers through a specific structure known as a saddle and finish in large anchorages. Tension in the cables and compression in the towers are the primary forces in a suspension bridge. The main components of a suspension bridge that resist these forces are the girders and the roadbed.
The girders are long, thin trusses made out of steel or concrete. They support the weight of the bridge on both sides and connect the center of each side to the center of the next one. Most commonly, there are two girders per side; however, some bridges have more than this or less. The distance between each girder and the one next to it is called the girtdle. If the girdles are equal in length, then the bridge is said to be symmetrical. If one girder is longer than the other, the bridge is said to be asymmetric. Symmetrical bridges are usually stronger because they distribute the load over an equivalent area, while an asymmetric bridge would only carry half its total weight.
The roadbed is the surface of the ground or water beneath the bridge upon which the bridge rests. It must be strong enough to support the weight of the vehicles crossing it plus any load caused by weather conditions.