Moving Load

Moving loads are loads that traverse the structure — typically vehicles on bridges, trains on railway bridges, or rolling cranes on industrial structures. Distinguished from static loads (fixed in position) or even cyclic-but-stationary loads, moving loads cause maximum stresses at different sections at different times as they traverse the structure. Per IRC 6:2017 (road bridges) + IRS Bridge Rules (railway bridges) + IS 875 (general), moving load analysis is essential for bridge design and crane runway beams.

For highway bridges: the IRC vehicle classes (Class A, B, AA, 70R) are positioned at the most adverse location to maximise design forces at each section. For a simply-supported bridge: Class A truck (35 t single load) at midspan gives maximum moment; at quarter-span gives maximum negative moment in continuous bridges. Multiple vehicles in sequence with appropriate spacing per IRC 6 — vehicles placed end-to-end if same class, spaced apart if mixed class. Software (Influence-Line Analysis, Bridge Software like LARS, Bridges) automates the process. Indian routine practice: software computes maximum forces at every section under the worst combination of moving loads.