BRIDGES.
eassificatio;z.—The term Bridge is applied to certain forms of construc tion the object of which is to connect two points separated from each other by water, a roadway, or a ravine in such a manner that a free or open space shall be left beneath the new avenue of communication. According to the kind of traffic for which the structure is destined, road-bridges, railway bridges, and viaducts may be distinguished. The open space under the bridge-structure may serve, according to circumstances, for the passage of foot- or roadways, or of watercourses, small or great, or of a railway; or, as in the case of viaducts, its object may be principally the saying of material. Where the two lines of communication are not at right angles to each other, the bridge which crosses one of them must be placed obliquely to the face of its abutments, and is termed an askew bridge; when the inter secting ways of communication are at right angles, the bridge is a r4,Elit bridge.
Bridges may be either fixed or movable in certain parts of their struc ture. The latter constitute the so-called " drawbridges," designed origi nally as the means of communication across the moat surrounding the castles and strongholds of the medixyal period (p/. 4o, fig. 4), and later to pro vide an open channel for navigation where bridges were built across navi gable streams at so low an elevation as to interfere with the free passage of vessels beneath them. According to the mode of operating these mov able parts, several inodifications—such as lift, swing, rolling, and pivot bridges—may be distinguished. In this category, likewise, may be placed the pontoon-bridges, and flying-bridges or ferries.
Every bridge consists of four essential parts: (1) the roadway; (2) the supporting strncture (girders, trusses, arches, chains, cables, etc.); (3) the bridge-supports (piers, abutments); and (4) the foundations.
Pridg:,es may be classified according to the material of which they are built or according to the form of their superstructure. The first classifica tion may be divided into wooden, stone, iron (or steel), and combination bridges, the latter being composite structures of wood and iron. In the
system of construction of the superstructure, bridges may be generally classified as girder bridges (fi/. 39, figs. 1, 2, 3, 6; pl. 43, figs. 1, 3, 8, to 13, 17), in which the stress clue to the load is transmitted to the supporting points as vertical pressure only; framed-girder or truss bridges (pt. 39,figs. 4b, 5, to which class the arch bridges of iron, pi. 43, fig. 2o, as well as stone belong), in which the stress due to the load is transmitted to the sup porting points also as a horizontal thrust; and suspension bridges (fil. 41, figs. 1-3), in which the supporting structure must sustain, in addition to the vertical pressure due to the load, an inward pull tending to drag the chains or wire cables from their anchorages; or, to put the case somewhat differently, in girder bridges the material of the girders, as it yields beneath the stress due to the load, suffers compression in one part and extension in another; in arch bridges the material of the arch suffers compression; and in suspension bridges the material of the structure carrying the load (chains, cables) suffers extension. Combinations of the foregoing systems are not in frequent.
The superstructure may be formed of a sing-le span, in which case two points of support only will be needed, and these may consist of the two banks of the stream, or of masonry (pl. 39, fig. I), or of piles (fig. 6); or it may be formed of a number of spans, in which case the intermediate sup ports are of wooden (figs. 2, 3, 6) or iron trestle-work, or of stone (p/. 43, figs. 8, 2o) or iron piers or columns. The latter are either hollow cast-iron columns of one or more tubes sunk into the foundation and bound together, or of wrought- or cast-iron pillars or columns, usually formed each of a number of columns of small diameter firmly united with one another by bracing and supported upon low stone piers.