The design of such structures requires that the most critical positions of the loads are

identified for various functions, e.g. the support reactions, axial loads in trusses, shear

force or bending moment in beams. Consider a vehicle moving along a simply supported

span as shown in Figure 9.2:

Figure 9.2

The position of the vehicle required to determine the maximum value of a function/design

load effect must be identified for design purposes. This can be achieved by the use of

influence lines. An influence line is a graph of the variation of a function e.g. the support

reaction, shear force, bending moment etc. in a beam or the axial load in a pin-jointed

trussed frame, at a given position in a structure as a unit load traverses the structure. It is

important to recognise that, unlike shear and bending moment diagrams, influence lines

indicate the variation of a function at a specific point in a structure.