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Every bridge, no matter the size or length, must withstand and overcome the forces (or stresses) to which it is subjected. Bridges are designed to carry their own weight, or dead load; to carry people and traffic, or live load; and to resist natural forces, such as wind and earthquakes, or environmental load.
The distance between a structure’s main supports is its span, the length normally measured when determining a bridge’s size. A simple span, such as a plank across a stream, rests on two supports, one at each end. A continuous span is supported along its length by piers; the outermost supports are called abutments.
Four types of forces act on bridges, either singly or in combination: tension, compression, shear, and torsion. Tension, which stretches or pulls apart, is the opposite of compression, which squeezes or pushes together. Shear is a sliding force; torsion is a twisting force.
The Verrazano-Narrows Bridge that connects Brooklyn and Staten Island was the world’s longest suspension bridge when it opened in 1964.
Traditional spans utilize one or more of three basic structures, derived from forms found in nature: the beam, from a log fallen across a stream, the arch, from natural rock formations, and the suspension, from a hanging vine. Once constructed of wood, stone, fiber, concrete, or steel, bridges increasingly are made from high-performance composite materials. Smart materials, 3-D modeling software, and hybrid combinations of bridge types are extending structural and aesthetic possibilities in all directions.
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The three types differ in the way they carry their weight. The simplest, the beam bridge, is horizontally self-supporting and exerts a downward force on its piers. The cantilever, a more complex version of the beam bridge, is composed of two anchor arms that extend from opposite sides and are joined by a middle span, which is supported by piers. A cantilever balances the forces of compression and tension. Beam and cantilever bridges can be formed from a truss, a rigid structure usually built from triangular members. Truss spans can be simple or continuous, of the deck type, which is driven over, or the through type, which is driven through.
Arch bridges are characterized by their stability. In an arch, force is carried outward from the crown to the ends of the arch, where abutments exert a compressive force to keep the ends from spreading apart. In the largest bridges, those of the suspension type, the main support members are parallel cables strung over towers that run the length of the bridge and are anchored at either end. The deck is supported by suspenders that are hung from the main cables. The weight of the bridge is supported by the cables, which are in tension, a pull withstood by their anchorages, and the towers, which are in compression. A variation is the cable-stayed bridge, in which the deck is attached directly to the towers by a series of diagonal cables. Cable-stayed bridges generally cover shorter distances than suspended spans and, because anchorages are unnecessary, tend to be less costly and more quickly constructed. The length of the main span largely decides the type of bridge that will be constructed.
As a rule of thumb, box girder designs are used for spans under 500 feet (152 meters), cable-stayed spans for lengths between 500 and 2,000 feet (152 to 610 meters), and suspension bridges for spans that exceed 2,000 feet (610 meters). Movable bridges, although variations on the basic trio, form a family of their own, described by the movement of the deck. To clear a navigation channel, a swing bridge deck pivots about a center point; a bascule bridge deck, or leaf, is raised with counterweights like a drawbridge; and a lift bridge deck is raised vertically, like a huge horizontal elevator. The length of a bridge’s “useful service life,” or the period of time that a bridge is expected to be in operation, has been redefined over the past three decades. Different countries impose different standards for longevity, as do the owners of exceptional bridges. Since 1988, the British Standards Institution has specified a minimum service life of 120 years. In the United States, federally funded bridges conform to American Association of State Highway and Transportation Officials (AASHTO) standards that specify a minimum design life of 75 years, but not a minimum service life. Given international standards and adequate preventive maintenance, it is reasonable to expect new generations of bridges to operate for 100 years or more. As noted in these pages, many owners of landmark North American bridges have imposed century-or-more criteria.
