(b) Transmission of load through floor beamsFIGURE 48Consider thai 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 jc 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. 486, and compute the shear Vcd in
panel cd. The truss reactions may be obtained by considering the unit load itself rather
than its panel-point components. Thus: R 1 = x/120; Vcd = RL= #/120, Eq. a.
- 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 Vcd = R 1 - 1 = jt/120 - 1, Eq. b. - Determine the panel shear with the unit load within the panel
Place the unit load within panel cd. Determine the panel-point load Pc at c, and compute
Vcd. Thus Pc = (x- 60)720 = jc/20 - 3; Vcd = R 1 - Pc = Jc/120 - (jc/20 - 3) = -jt/24 + 3, Eq.
c. - Construct a diagram representing the shear associated
with every position of the unit load
Apply the foregoing equations to represent the value of Vcd associated with every position
of the unit load. This diagram, Fig. 48c, is termed an influence line. The pointy at which
this line intersects the base is referred to as the neutral point. - Compute the slope of each segment of the influence line
Line a, dVJdx = 1/120; line b, dVJdx = 1/120; line c, dVJdx = -1/24. Lines a and b are
therefore parallel because they have the same slope.
Truss chordFloor beams Bridge floor(a) Pratt truss6 panels