Page 87 - Handbook of Civil Engineering Calculations, Second Edition

1.70 STRUCTURAL STEEL ENGINEERING AND DESIGN

FIGURE 48

Consider that a moving load traverses the bridge floor from right to left and that the
portion of the load carried by the given truss is 1 kip (4.45 kN). This unit load is transmit-
ted to the truss as concentrated loads at two adjacent bottom-chord panel points, the latter
being components of the unit load. Let x denote the instantaneous distance from the right-
hand support to the moving load.
Place the unit load to the right of d, as shown in Fig. 48b, and compute the shear V cd in
panel cd. The truss reactions may be obtained by considering the unit load itself rather
than its panel-point components. Thus: R L x/120; V cd R L x/120, Eq. a.
2. Compute the panel shear with the unit load to the left
of the panel considered
Placing the unit load to the left of c yields V cd R L 1 x/120 1, Eq. b.
3. Determine the panel shear with the unit load within the panel
Place the unit load within panel cd. Determine the panel-point load P c at c, and com-
pute V cd . Thus P c (x 60)/20 x/20 3; V cd R L P c x/120 (x/20 3)
x/24 3, Eq. c.