The Viaduct Works' Handbook

THE VIADUCT WORKS’ HANDBOOK. IRON BRIDGES. Parallel iron girder bridges of the class shown in woodcuts, Figs. 1, 2, 3, for ordinary cases, are the most convenient in form for moderate spans, whether of triangular, as Fig. 1, latticed, as Fig. 2, or plain plate, as Fig. 3, and are either with floor on top of the main girders, and surmounted by parapet railing, or with floor at bottom as in Fig. 4, or at any convenient height wherein the main girders may serve as parapets. It is generally found that the most economical depth of which girders may be constructed, as regards the quantity of material used in them, is about a twelfth of their span, although special circumstances make it necessary to deviate slightly from this. When height beneath a bridge is an object desired, with the minimum depth from bottom of girders to surface of the road carried by them, the arrangement shown by Fig. 4 is used; in other cases it is most economical to place the floor on top of the main girders.