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Wooden Bridges

united, girders, piers, truss, figs and placed

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Chtssilication.—Considered in respect of their construction, wooden bridges may be classified as: (I) Plain girder bridges (pi. 39, figs. 1-3), in which the roadway of planks lies crosswise upon a number of girders placed parallel to one another. (2) Dovetailed and dowelled girder bridges, in which the girders are formed of a number of pieces placed one above the other and united by screw-bolts and dovetailed joints, or of cut timbers separated by dowels of hard wood and firmly united by means of screw bolts. (3) Strut bridges (figs. 5, io), in which the girders are stiffened and strengthened by struts placed below them and consisting- of two stanchions and a straining--beam stiffened against the bridge supports. (4) Truss-frame bridges (fig. 4), or, properly, bridges with struts placed above the roadway, in which the roadway is suspended from the truss-frame by the vertical suspension-rods. The thrust of the oblique pieces, or braces, is taken by the girders into which they are mortised, so that the supporting points receive it as vertical pressure. In its simplest form the truss-frame has no straining-beain, but consists merely of the two oblique pieces or braces, united to a king-post at the centre of the girder front which the latter is suspended. (5) Lattice bridges, which are preferred for spans of considerable length, are truss bridges which differ materially in the arrangement and functions of their members. Town's lattice truss Col. 39, figs. S, 9; pi. .12, jig . 3) is formed of timbers crossing one another at right angles and united at the point of intersection by wooden tree-nails and having hori zontal string-pieces at top and bottom, from which the floor and roof of the structure may be supported. This form of structure WaS at one time popular in the United States, where wooden truss bridges are largely used, but has been abandoned in favor of improved forms. In the Howe truss (pl. 39, Jigs. 6, 7; /5/. 42, fig. I) vertical suspension-rods are introduced, and the main as well as the counter braces bear against cast-iron angle plates and sockets extending through the chord-pieces. By reason of the

considerable depth of these girder constructions, the wooden lattice bridges are notable for their stiffness. (6) Arch bridges, whose arched main girders consist of various modifications of timber structures of arched form, accord ing to the width of span. In the United States lattice-work arches are in vogue, and according as the roadway is carried under or over the arches they are distinguished as truss arch and strut arch. Combinations of the arch with the lattice-beam are not infrequent.

The Roadway proper in the case of wooden railway-bridges (pi. 39, figs. 6, 7) is formed of cross-ties laid at right angles to the axis of the bridge and extending from one side-girder to the other. The rails are laid either directly upon these cross-ties or, as in the illustrations, upon longitudinal sleepers placed upon the latter. Upon road bridges (figs. 8, 9) it is cus tomary to lay a heavy flooring of planks upon the cross-ties, and upon this to place a lighter flooring—wood-pavement, asphalt, etc.—for the road surface. To give the bridge-structure sufficient transverse strength to enable it to withstand the force of the wind, it is necessary to provide it with special cross-braces and wind-stanchions.

Piers ana' of wooden-pile piers and trestles are shown in Figures r, 2, 6, and 7. The construction of these supports will vary according to their height and as to whether they stand upon the land or in the water. Where the height of the structure is moderate, the first form is employed; where the height is considerable, trestle-work is used, formed, generally, of four-sided or polygonal pyramids supported upon low stone piers. To protect from damage by ice, wooden guards are often built around the piers, though a better plan is to construct special ice-breakers in front of the piers.

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