Suspension bridges with cables made of high-strength, zinc-coated, steel wires are suitable for the longest of spans. Such bridges usually become economical for spans in excess of 1000 ft, depending on specific site constraints.

Nevertheless, many suspension bridges with spans as short as 300 or 400 ft have been built, to take advantage of their esthetic features. The basic economic characteristic of suspension bridges, resulting from use of high strength materials in tension, is lightness, due to relatively low dead load.

But this, in turn, carries with it the structural penalty of flexibility, which can lead to large deflections under live load and susceptibility to vibrations. As a result, suspension bridges are more suitable for highway bridges than for the more heavily loaded railroad bridges.

Nevertheless, for either highway or railroad bridges, care must be taken in design to provide resistance to wind- or seismic-induced oscillations, such as those that caused collapse of the first Tacoma Narrows Bridge in 1940.

A pure suspension bridge is one without supplementary stay cables and in which the main cables are anchored externally to anchorages on the ground. The main components of a suspension bridge are illustrated in Fig. 15.8.

Most suspension bridges are stiffened; that is, as shown in Fig. 15.8, they utilize horizontal stiffening trusses or girders. Their function is to equalize deflections due to concentrated live loads and distribute these loads to one or more main cables.

The stiffer these trusses or girders are, relative to the stiffness of the cables, the better this function is achieved. (Cables derive stiffness not only from their crosssectional dimensions but also from their shape between supports, which depends on both cable tension and loading.)

For heavy, very long suspension spans, live-load deflections may be small enough that stiffening trusses would not be needed. When such members are omitted, the structure is an unstiffened suspension bridge.

Thus, if the ratio of live load to dead load were, say, 1:4, the\ midspan deflection would be of the order of 1⁄100 of the sag, or 1/1,000 of the span, and the use of stiffening trusses would ordinarily be unnecessary. (For the George Washington Bridge as initially constructed, the ratio of live load to dead load was approximately 1:6. Therefore, it did not need a stiffening truss.)

FIGURE 15.8 Main components of a suspension bridge.

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