Impact Load

Impact load is a suddenly-applied load with significant kinetic energy, distinct from a slowly-applied static load. Examples: vehicle wheels on bridges, heavy equipment dropped on industrial floors, rolling crane wheels, lift cars. Per IS 875 Part 2:1987 Cl. 4.6 and IRC 6:2017, impact loads are accounted for by applying a multiplier (typically 1.25-1.50) to the static equivalent. The factor accounts for the dynamic effect — a vehicle bouncing on a road bump exerts more force than its static weight.

For bridges per IRC 6: impact factor varies with span and surface condition. Smaller spans (< 5 m) impact factor up to 1.50; longer spans (> 25 m) impact factor 1.25. The impact factor is applied to live load only, not dead load. For lift wells per IS 875 Part 2: impact factor 1.50 on lift car weight (representing impact during sudden braking). For machine foundations per IS 1893 Part 4: vibratory loads with impact factors specific to machine type. The most-overlooked Indian construction issue: heavy equipment placed on slabs designed only for UDL — impact factor not considered. A 250 kg cabinet dropped from 300 mm produces 1875 N peak vs 2500 N static load — impact much higher.