Fatigue Load
Repeated load below yield that causes failure after many cycles. Critical for bridges, cranes, machinery foundations.
Fatigue is the progressive failure of a material under cyclic loading at stresses below the yield strength. Repeated loading causes micro-cracks that grow and eventually cause failure. Per IS 800:2007 + IRC 6:2017, fatigue is critical for: bridges (vehicle traffic causing 10⁶-10⁸ cycles over 50-year design life), cranes, machine foundations, and any structure with repeated significant loading. The fatigue limit (endurance limit) is typically 30-50% of yield strength.
Design approach: (a) Estimate the number of load cycles over the design life. For Indian highway bridges: 50 million Equivalent Standard Axle Load (ESAL) over 50 years. (b) Compute stress range Δσ = σmax - σmin under live load. (c) Compare against fatigue strength Δσf = constant ÷ N^(1/3), where N is number of cycles to failure. For Fe-410 steel: Δσf at 2 million cycles ≈ 230 MPa. (d) Design for required margin (typically 1.5-2.5× safety factor). For RCC bridges: stress range at service load typically 70-120 MPa, well below the fatigue limit. For steel girder bridges: stress range can approach the fatigue limit at critical sections, requiring detailed analysis.
- Highway and railway bridges (IRC 6, IRC 24)
- Crane runway beams (IS 800 + IS 875)
- Machine foundations under repeated dynamic loading
- Wind-loaded tall buildings (cyclic wind action)
- Pre-stressed concrete with fatigue-prone elements