The intensity of compressive stress on the rib at the springing when the bridge is fully loaded is The details of the bridge are sufficiently illustrated in Plates V., Va, and Vb. The calculations for wind pressure present no difficulty; the sections adopted for the wind braces are shown on the Plate Va. Abutments. The abutments are composed of sandstone ashlar masonry, set in cement mortar, with wing walls and pilasters; inequalities in the rock face may be filled with cement concrete. The bedstones or skewbacks should be made of granite or other suitable stone. Erection of the Ribs. --The ribs may be riveted on a staging, so that the longitudinal girders may have, when fixed in position, an even fall of 3 inches from the central hinge to the end girder when the temperature in the sun is 70° Fahr. Should the ribs be erected on a warmer or colder day, the necessary allowance for temperature must be made in wedging up the end hinges. The foregoing method of calculation may be used for either a plate web or lattice web; if the web is designed with a single system of triangulation as in Fig. 360, the stresses may be obtained graphically as shown in Fig. 361, or by passing a section through any three members of the rib and taking moments in the usual way. Suspension Bridges consisting of Steel Wire Cables stiffened near the Towers by means of Inclined Stays. This is not a satisfactory construction as ordinarily carried out, as at best the stays leave the centre portion of the span unsupported, and the lengths of the inclined stays cannot be adjusted so that they will act efficiently under variations of temperature; the only useful purpose they serve is to check the vibrations and oscillations produced by the rolling load. Suspension Bridges consisting of Steel Cables with a Stiffening Girder. The steel cables may be combined with a horizontal stiffening girder, in which case the cables must sustain the whole live and dead load, the girder merely acting when the bridge is partially loaded, and the effect is to distribute the load over the cables. The cables themselves tend to adapt themselves to the form of the equilibrium curve for the load they sustain. If the load is equally distributed along the horizontal, the curve of equilibrium may be proved to be a parabola. Ordinary suspension bridges approximate closely to this curve, and it is assumed that the curve is a parabola in the following calculations. Since the steel cables carry the whole of the load, and the stiffening girders merely distribute the load without relieving the stress in the cables-in fact, adding to it by their own weight-this form of construction is by no means as satisfactory as the suspended rib proposed by Fidler, in which there is no increase in the flange areas of the ribs for stiffening the bridge when it is partially loaded; the form secures rigidity, and therefore requires no stiffening. The use of the stiffening girder for cable suspension bridges has been very general, and is still in use; steel-wire ropes may be obtained at a moderate cost which will bear a tensile stress of 100 tons per square inch of sectional area, and the cost of these bridges is certainly not great where the site is suitable. The equation of maximum bending moments in the stiffening girder hinged at the centre, where the quantities have the same meaning as in the arched rib (Fig. 370), is, if the cables are parabolic and the girder rigid For a uniformly distributed load over the whole bridge, there is no stress on the girder; for a load distributed uniformly over one-half of the bridge, the maximum flange stress in the stiffening girder is + "1l2 16d The stresses in the flanges alternate between tension and an equal compression, so that a low value must be taken for the working stress. The stiffening girder is generally hinged at the centre to avoid temperature stresses, and it should be secured at one end by bolting it to one of the piers or abutments, so as to fix it horizontally, and it should be allowed to expand freely over the other supports. The hinge at the centre renders the calculations an easy matter, but if the girder were made strong enough to withstand the temperature stresses without a hinge, and securely fixed to |