Engineering Wonders of the World
Volume I

104 ENGINEERING WONDERS OF THE WORLD. the points o£ support, a vertical “ shearing ” stress, which, unless provided for, would cause carried upon brick piers, form a type of bridge that is much, used for short spans (see Fig. 8). (Fig. 6.) (Fig. S.) fracture somewhat in the manner illustrated by Fig. 6. From what has been said in connection with beams, it is evident that the greatest stress in a girder is nearly always due to the bending moment, and occurs at the point where the bending moment is at its maximum. This is normally at the centre of the span, but shifts to other points with, the load. In any case, to save dead weight and material, a girder itself should be so proportioned that its strength may increase in the same ratio as the bending moment, and render its resisting power con- stant throughout its length. This may be accomplished either—(1) by adding to the sectional area of the flanges, or (2) by in- creasing their distance from the neutral axis towards the centre of the span. An example of the first method is given in Fig. 7, which shows a parallel “ plate ” girder. (Fig. 7.) Its strength is gradually increased towards the centre by augmenting the number of plates in the top and bottom flanges. Incidentally it may be remarked that the vertical plates a, b, c, etc., are “ stiffeners,” whose purpose is to prevent sideways buckling of the thin vertical web connecting the flanges. Two or more girders such as this, placed side by side, with decking between them, and their ends Plate girders are frequently built in a box ” form with two webs. The second type of girder—namely, that in which the stress on the material is rendered uniform throughout the span by varying its distance between flanges and neutral axis—is known as the “ parabolic ” or “ bow-string.” Fig. 9 is an outline of such a girder, with a (Fig. 9.) load evenly distributed. The shape has been arrived at by drawing above each weight a vertical line, the length of which is propor- tional to the bending moment at that point. The curve bounding the upper extremities of the vertical lines is therefore an indication of the depth required to resist bending. Owing to the necessity of providing against shear at the abutments, the actual outline would ap- proach that shown by the dotted lines. The stresses for other conditions of loading may be determined by similar diagrams. With certain reservations, it may be said that, as the out- line so produced represents theoretically the relative depth required at any part of the span, the lines of a well-designed bridge form a “ stress diagram,” and are, to the educated eye, an expression of its fitness to resist the forces acting upon it. Anything in the nature