444. Hence one of these frames is almost as strong as a beam supported at the points c and d, and therefore, from the principles of [Art. 388], its strength is three times as great as that of an unsupported beam.
445. The two frames placed side by side and carrying a roadway form an admirable bridge, quite independent of any external support, except that given by the piers upon which the extremities of the frames rest. It would be proper to connect the frames together by means of braces, which are not, however, shown in the figure. The model is represented as carrying a uniform load in contradistinction to [Fig. 58], where the weight is applied at a single point.
446. With eight stone ranged along it, the bridge of [Fig. 62] did not indicate an appreciable deflection.
LECTURE XIV.
THE MECHANICS OF A BRIDGE.
Introduction.—The Girder.—The Tubular Bridge.—The Suspension Bridge.
INTRODUCTION.
447. Perhaps it may be thought that the structures we have been lately considering are not those which are most universally used, and that the bridges which are generally referred to as monuments of engineering skill are of quite a different construction. Every one is familiar with the arch, and most of us have seen suspension bridges and the celebrated Menai tube. We must therefore allude further to some of these structures, and this we propose to do in the present lecture. It will only be possible to take a very slight survey of an extensive subject to which elaborate treatises have been devoted.
We shall first give a brief account of the use of iron in the arts of construction. We shall then explain simply the principle of the tubular bridge, and also of the suspension bridge. The more complex forms are beyond our scope.
THE GIRDER.
448. A horizontal beam supported at each end, and perhaps at intermediate points, and designed to support a heavy load is called a girder. Those rods upon which we have performed experiments, the results of which have been given in [Table XXIV]., are small girders; but the term is generally understood to relate to structures of iron: the greatest girders for railway bridges are made of bars or plates of iron riveted together.