257 Truss Bridges

25.7.1 Introduction

The structural layout of a truss bridge is shown in Figure 25.43 for a through bridge with the deck located at the level of lower chords. The floor slab, which carries the live load, is supported by the floor system of stringers and cross beams. The load is transmitted to the main trusses at nodal connections, one on each side of the bridge, through the floor system and finally to the bearings. Lateral braces, which also are a truss frame, are attached to the upper and lower chords to resist horizontal forces such as wind and earthquake loads as well as torsional moments. The portal frame at the entrance provides transition of horizontal forces from the upper chords to the substructure.

Truss bridges can take the form of a deck bridge as well as a through bridge. In this case, the concrete slab is mounted on the upper chords and the sway bracing is placed between the vertical members of two main trusses to provide lateral stability.

A truss is composed of upper and lower chords, joined by diagonal and vertical members (web members). This frame action corresponds to beam action in that the upper and lower chords perform like flanges and the diagonal braces behave in much the same way as the web plate. The chords are mainly in charge of bending moment while the web members take the shear force. Trusses are an assembly of bars, not plates, and thus are comparatively easier to erect on site and are often the choice for long bridges.

25.7.2 Type of Truss

Figure 25.44 shows some typical trusses. A Warren truss is the most common and is a frame composed of isosceles triangles, where the web members are either in compression or tension. The web members of a Pratt truss are vertical and diagonal members where the diagonals are inclined toward the center and resist only tension. The Pratt truss is suitable for steel bridges since it is tension that is most effectively resisted. It should be noted, however, that vertical members of Pratt truss are in compression. A Howe truss is similar to the Pratt except that the diagonals are inclined toward the ends, leading to axial compression forces, and the vertical members resist tension. Wooden bridges

Upper lateral bracing Stmt

Portal frame

End post

Figure 25.44 shows some typical trusses. A Warren truss is the most common and is a frame composed of isosceles triangles, where the web members are either in compression or tension. The web members of a Pratt truss are vertical and diagonal members where the diagonals are inclined toward the center and resist only tension. The Pratt truss is suitable for steel bridges since it is tension that is most effectively resisted. It should be noted, however, that vertical members of Pratt truss are in compression. A Howe truss is similar to the Pratt except that the diagonals are inclined toward the ends, leading to axial compression forces, and the vertical members resist tension. Wooden bridges

Upper lateral bracing Stmt

Portal frame

End post

Floor beam Stringer

"" Upper chord

Web (diagonal) Slab

Lower chord Lower lateral bracing

FIGURE 25.43 Truss bridge (Nagai 1994).

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