Average Weight of Metal in Pounds per Lineal Foot of Entire Bridge 200 400 600 800 Length of Main Opening in Feet FIG. 55ccc. Double-track-railway, Riveted, Cantilever Bridges, Type A-Metal in Trusses and Total Metal in Bridge. Average Weight of Metal in Pounds per Lineal Foot of Entire Bridge 200 400 600 800 1000 1200 1400 1600 1800 Length of Main Opening in Feet FIG. 55bbb. Double-track-railway, Cantilever Bridges, Type A-Metal in Floor System, Laterals, and on Piers. Average Weight of Metal in Pounds per Lineal Foot of Entire Bridge 200 400 600 20000 Length of Main Opening in Feet FIG. 55ccc. Double-track-railway, Riveted, Cantilever Bridges, Type A-Metal in Trusses and Total Metal in Bridge. are then calculated, and their weights are determined from Fig. 55ff. The weights of the girder bracing, of the longitudinal and transverse bracings of the towers, and of the transverse bracing of the bents will be about the same as in the case of a single-track-railway trestle, provided that " minimum thickness of metal be employed. If the use of 516" metal be permitted, the weights should be reduced twenty (20) per cent. The weights of the columns can be found by the formula, in which C = = CR E weight of columns for a single-track electric railway trestle, weight of columns for single-track steam railway trestle, and ratio of the live plus impact loads for the electric railway trestle to the live plus impact loads for the steam railway trestle, as above defined. In case it be desired to apply the diagrams to a double-track electric railway trestle, it will be necessary first to figure the weights of metal for a single-track electric railway structure as just indicated, and then increase the weights of girders, of girder bracing, of transverse bracing in bents and towers, and of columns as previously explained for double-track railway trestles. The weight of the longitudinal bracing of the towers will be about the same as that for the single-track steam railway structure. For electric railway trestles on curves, the weights found in the above manner are to be increased two per cent for each degree of curvature, as in the case of steam railway trestles. The weights of electric railway trestles obtained as above are, of course, approximate only. CANTILEVER BRIDGES Cantilever bridges may be divided into four general types, as shown in Fig. 55aaa. Type A consists consecutively of an anchor arm, a cantilever arm, a suspended span, a cantilever arm, and an anchor arm. This is the most commonly used of the four. Type B consists consecutively of an anchor arm, a cantilever arm, a suspended span, a cantilever arm, a central anchor span, a cantilever arm, a suspended span, a cantilever arm, and an anchor arm. Type C consists consecutively of a suspended span, a cantilever arm, an anchor span, a cantilever arm, and a suspended span, each of the two suspended spans being hung at one end to a cantilever arm and supported by a pier at the other. Type D consists consecutively of a suspended span, a cantilever arm, |