How is fatigue analysis performed for steel bridge structures?

Fatigue analysis involves: (1) Determining stress range spectrum from traffic load models or measured data using rainflow counting, (2) Classifying connection details into fatigue categories (A through E'), (3) Applying Miner's cumulative damage rule: Sum(ni/Ni) <= 1.0, where ni is actual cycles and Ni is allowable cycles from S-N curves, (4) Checking against infinite life threshold below which no fatigue damage occurs. Critical locations include welded connections, cover plate terminations, and stiffener welds. Design life is typically 75-100 years with inspection provisions.