Engineering Wonders of the World
Volume I

106 ENGINEERING WONDERS OF THE WORLD. (Fig. 14-) erected upon a steel arch, the whole being made rigid by diagonal bracing. A suspension bridge (Fig. 18) may be con- sidered an arched bridge inverted, the curved combination of two lattice girders gives a “ double lattice.” Another much-used type of girder is shown in Fig. 15, the vertical members being struts (Fig. 15.) and the diagonals ties. In this design the struts are shorter than in the Warren, and are therefore stiffer for a given section. The remarks that have been made as to the bracing of parallel girders also apply to the “ parabolic.” The curve of the top (or bottom) boom will, of course, be influenced by the nature of the load ; and if the sets of bracings or “ panels ” are few in number, it will be formed of a series of straight lines, after the manner of an inverted queen truss, instead of being a continuous curve. The varying depth of a curved girder has the effect of shortening the end struts and so saving material, and certainly gives an outline more graceful than that of the parallel type. A “bow-string” girder (Fig. 16) with a curved top boom is virtually an arch, the straight bottom boom forming a tie to prevent the ends from spreading, and taking the place of the massive abutments that perform this func- tion in an open arch. If the load could be applied uniformly to the top member, the diagonal bracing might be dispensed with, and the tie could be removed if the curved boom were placed between two immovable abutments. Fig. 17 is an example of a bridge in which a level roadway is carried upon struts (Fig. 16.) member being in tension instead of compres- sion, and tending to draw the end supports or “ anchorages ” together, rather than thrust them apart. In the old form of suspension bridge, with a hanging elastic roadway, the unequal bending action of a rolling load caused undue oscilla- tion. The above example shows a roadway carried on a “ stiffening girder ” to minimize local deflection. In girder bridges variations in (Fig. IS.) length, due to changes of temperature, can be provided for by allowing th© ends to slid© on the abutments ; but in the case of arches and suspension bridges the span is fixed and un- alterable, so that other methods, such, as a hinge admitting of an upward movement in the centre, must be adopted. A cantilever bridge may be likened to a suc- cession of brackets, each arm being composed of a rigid semi-arch or half span of a stiffened suspension bridge (Figs. 19 and 20). It will be noticed that the inshore cantilevers balance those projecting over the water. This plays a most important part in the work of con- struction, as the work may “ grow ” riverwards from the piers, thus avoiding the costly “ false-