# Page:1902 Encyclopædia Britannica - Volume 26 - AUS-CHI.pdf/418

BRIDGES

376

Wj is at C. Then the reaction at B and shear at 0 is 'Rnjl. Next let the loads advance a distance a so that W2 comes to C. Then the shear at C is R(n + a)/7-Wj, plus any reaction d at B, due to any additional B load which has come on the girder during t the movement. The

shear will therefore be
• increased by bringing

4 W2 to C, if Ra/Z + ^Wj F’S- 6. an(l ^ is generally small and negligible. This result is modified if the action of the load near the section is distributed to the bracing intersections by rail and cross girders. In Fig. 7 the action of W is distributed to A and B by the flooring. Then the loads at A and B are W(_p - x)!p and ~Wx/p. Now let 0 (Fig. 8) be the section at which the greatest shear is required, and let the loads advance from the left till Wj is at C. If R is the resultant of the loads then on the girder, the reaction at B and shear at C is Rn/l. But the shear may he greater when W2 is at C. In that case the shear at C becomes R(rc + a)/7 +^-Wj, if a>p, and R(?i + a.)/7 ■-d— W^t/p, if a<p. If we neglect d, then the shear increases by moving W2 to C, if Ra/l>Wi in the first case, and if ~Ra/l>~W1a/p in the second case. For the greatest bending moment due to a travelling live load, let a load of w per foot run advance from the left abutment (Fig. 9), and let its centre be at x from the left abutment. The reaction at B is '2wx2/1 and the bending moment at any section C, at m from the left abutment, is 2wx2(l-m)/l, which increases as x increases till the span is covered. Hence, for uniform travelling R C 