IRC 58 designs against the realistic axle-load spectrum — the distribution of single, tandem and tridem axle loads from an axle-load survey — and the cumulative number of repetitions over the design period (traffic volume × growth rate × design life × lane/directional distribution). Overloading (heavy-tail axles) disproportionately consumes fatigue life, so the spectrum, not an average, governs.
Key Requirements
•Use an axle-load spectrum (single/tandem/tridem categories) from an axle-load survey, not a single equivalent axle
•Cumulative repetitions = initial commercial traffic × growth factor over the design period × lane/directional distribution factors
•Design period typically long (e.g. ~30 years for rigid pavements) — confirm per the current IRC 58
•Account for overloading explicitly — heavy axles consume fatigue life disproportionately (4th-power-like effect)
•Wheel load + tyre pressure define the load footprint for stress computation
Formulas
C = A · D · L · [(1+r)^n − 1] / r
Cumulative commercial-vehicle repetitions over the design period (per IRC 58 traffic computation)
C = cumulative repetitionsA = initial daily commercial trafficD = directional distributionL = lane distribution factorr = annual growth raten = design period (years)
Practical Notes
✓Overloaded axles are the dominant fatigue consumer — designing to the legal axle limit while the road carries overloaded trucks under-designs the pavement; use the surveyed spectrum.
✓Growth rate and design period compound: a small growth-rate error over 30 years swings cumulative repetitions substantially.
Common Mistakes
⚠Using a single equivalent axle instead of the axle-load spectrum.
⚠Designing to legal axle loads while the corridor actually runs overloaded trucks.