Influence Line
Graph of a response at one point as a unit load moves across the structure
An influence line (ILD) is a diagram showing how a chosen response — reaction, shear, bending moment or deflection at a specific point — varies as a unit load travels across the structure. Unlike a bending-moment diagram (which shows a response everywhere for fixed loads), an ILD shows one response at one section for a load anywhere, making it the fundamental tool for analysing structures under moving loads.
Its decisive use is in bridge and crane-gantry design: positioning the IRC 6 standard vehicle/train of wheel loads (or the crane wheels) along the ILD to find the most adverse position that maximises a given moment/shear/reaction at a section. The maximum effect is obtained by placing loads at the peak ordinates (and using the area under the ILD for distributed loads). Influence lines underpin live-load envelopes for bridge girders, gantry girders (IS 800), continuous beams and trusses, and are constructed by the Müller-Breslau principle for indeterminate structures.
- Bridge girder + deck moving-load analysis (IRC 6)
- Crane gantry-girder design (IS 800)
- Live-load envelopes for continuous beams + trusses
- Determining most-adverse vehicle/wheel positions
- Maximum reaction/shear/moment from travelling loads