What is the purpose of using influence lines in bridge structural analysis?

The purpose of using influence lines in bridge structural analysis is to determine the maximum effect of a moving load on a specific point or section of the bridge. An influence line is a diagram that shows how the value of a specific response (like bending moment, shear force, or reaction force) at a particular location changes as a unit load moves across the bridge span. By examining the influence line, engineers can determine where to position a load or series of loads to produce the largest possible value of that response. This is crucial for designing bridge components to withstand the most critical loading scenarios and ensuring structural safety. For example, to find the maximum bending moment at the center of a span, you would position the load where the influence line for bending moment at that point has its highest value.