Cumulative Fatigue Damage (Miner's Rule) & Temperature Stress
IRC 58 checks cumulative fatigue damage: for each axle-load category the applied (wheel-load + temperature curling) flexural stress gives a stress ratio, the fatigue law gives the allowable repetitions, and the damage fractions are summed (Miner's rule) — the total must be ≤ 1 over the design period for both bottom-up (day) and top-down (night) cracking. Temperature differential (curling) stress is added to wheel-load stress, not ignored.
Key Requirements
•For each axle category: stress ratio = (wheel-load stress ± temperature curling stress) / flexural strength
•Allowable repetitions from the stress-ratio fatigue equation; damage = expected / allowable repetitions
•Cumulative fatigue damage Σ(damage) ≤ 1 over the design period (Miner's hypothesis)
•Check both bottom-up cracking (daytime positive temperature differential + edge load) and top-down (nighttime)
•Temperature differential (curling) stress is combined with load stress — not omitted
Cumulative fatigue damage (Miner's rule) using combined load + temperature stress
n_i = expected repetitions of axle category iN_i = allowable repetitions at stress ratio SR_iσ_wheel = wheel-load flexural stressσ_temp = temperature (curling) stressMR = flexural strength (modulus of rupture)
Practical Notes
✓Temperature curling stress is large and is the reason IRC 58 checks both day (bottom-up) and night (top-down) cracking — ignoring it under-designs the slab.
✓Fatigue is dominated by the high-stress-ratio (heavy-axle, high-temperature-differential) cases — they consume almost all the damage budget.
Common Mistakes
⚠Omitting temperature (curling) stress and checking wheel load alone.
⚠Checking only bottom-up cracking, not top-down (night) cracking.
⚠Treating CFD ≤ 1 as a single average case instead of summing over the spectrum.