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.
Where used
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
Acceptance / threshold
Per IS 800:2007 Cl. 5.5 + IRC 6:2017 + IRC 24:2010: fatigue check for cyclic loading with adequate margin (1.5-2.5×) over endurance limit; stress range < Δσf at design number of cycles.
Site example
Site reality: a Hyderabad steel-truss bridge had fatigue-check failure at one diagonal member due to over-stressing during construction. The issue was caught in monthly bridge inspection (year 4) when minor cracks at the rivet connection were detected. Replacement of the diagonal cost ₹4.8 lakh; without inspection, fatigue failure could have caused collapse within 5-10 years.
Frequently asked
What is fatigue in structural engineering?
Fatigue is the progressive failure of a material under repeated cyclic loading at stresses below the yield strength. Critical for bridges, cranes, machine foundations subject to millions of load cycles. Per IS 800 + IRC 6: fatigue check mandatory for cyclic-loaded structures.
What is the fatigue limit of steel?
Fatigue limit (endurance limit) is the stress below which steel can withstand infinite cycles without failure. For Fe-410 steel: ~150 MPa. For Fe-500 steel: ~180 MPa. Practical design uses S-N curves with appropriate safety factor (1.5-2.5×) to account for variability in stress range and cycle count.
How is fatigue checked in bridges?
Per IRC 6:2017 + IS 800: estimate ESAL over design life (50 million for major highway over 50 years); compute stress range at critical sections under live load; compare with fatigue strength S-N curve from Annex G. Design for required margin (1.5-2.5×) over endurance limit.