Bridge Trusses. The main structural elements of a typical bridge

truss are shown in Fig. 3–4. Here it is seen that a load on the deck is first

transmitted to stringers, then to floor beams, and finally to the joints of the

two supporting side trusses.The top and bottom cords of these side trusses

are connected by top and bottom lateral bracing, which serves to resist the

lateral forces caused by wind and the sidesway caused by moving vehicles

on the bridge. Additional stability is provided by the portal and sway

bracing. As in the case of many long-span trusses, a roller is provided at

one end of a bridge truss to allow for thermal expansion.

A few of the typical forms of bridge trusses currently used for single

spans are shown in Fig. 3–5. In particular, the Pratt, Howe, and Warren

trusses are normally used for spans up to 200 ft (61 m) in length. The

most common form is the Warren truss with verticals, Fig. 3–5c. For

larger spans, a truss with a polygonal upper cord, such as the Parker truss,

Fig. 3–5d, is used for some savings in material. The Warren truss with

verticals can also be fabricated in this manner for spans up to 300 ft (91 m).

The greatest economy of material is obtained if the diagonals have a

slope between 45° and 60° with the horizontal. If this rule is maintained,

then for spans greater than 300 ft (91 m), the depth of the truss must

increase and consequently the panels will get longer. This results in a

heavy deck system and, to keep the weight of the deck within tolerable

limits, subdivided trusses have been developed.Typical examples include

the Baltimore and subdivided Warren trusses, Figs. 3–5e and 3–5f. Finally,

the K-truss shown in Fig. 3–5g can also be used in place of a subdivided

truss, since it accomplishes the same purpose.

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