ABSTRACT
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.
