Design Examples of Steel and Steel-Concrete Composite Bridges - Finite Element Analysis and Design of Steel and Steel-Concrete Composite Bridges - page 277

q vk = -0.83 kN/m

Figure 4.54 Negative distributed live loads acting on main trusses.

can be now used to determine the forces in the members of main trusses
using the influence line method as shown in the coming sections.

4.3.3.2 Calculation of Force in the Upper Chord Member U 5
To determine the force in the upper chord truss member U 5 (see Figure 4.55 )
using the influence line method, we can follow the simple procedures of put-
ting a unit concentrated moving load at midspan (point a), and using the sec-
tioning method, we take a section s - s , as shown in Figure 4.55 , and then take
the moment at point a to calculate the force in the member due to the applied
unit load. After that, we can put the previously calculated dead and live loads
acting on a main truss in the longitudinal direction. The total force in the
member will be the summation of the concentrated loads multiplied by
the companion vertical coordinate in the influence line diagram and the
summation of the distributed loads multiplied by the companion areas in
the diagram. Hence, the forces due to the dead and live loads can be calcu-
lated as follows:

F D : L : U ðÞ¼ 0 : 5 60 2 78 : 1 ¼ 4686 kN

s

U 5 J 6 U 6

7.5

B

a

s

A

30 m 30 m

g vk = 78.1 kN/m

375 kN 375 kN

q vk = 43.8 kN/m

1.2 m

-

1.92

30 × 30/(60 × 7.5) = 2

Figure 4.55 Determination of the compressive force in upper chord member U 5 using
the influence line method.