Stress Cycles for Fatigue Design of Steel Bridges

Analytical methods are used to develop design values for the number of stress cycles caused by the passage of a truck across various types of spans of steel highway bridges. Vibration stresses and close truck spacings are shown to have little effect on such design values under normal conditions. For the main longitudinal members in simple‐span bridges, the design value is 1.0 for spans above 40 ft (12 m) and 2.0 for shorter spans. For the main longitudinal members near interior supports of continuous‐span bridges, the design value is 1.5 for spans above 40 ft and 2.0 for shorter spans. For the main longitudinal members elsewhere in continuous‐span bridges, the value is 1.0 for spans above 40 ft and 2.0 for shorter spans. For transverse members, the value is 1.0 if the spacing between members exceeds 20 ft (6 m) and 2.0 for smaller spacings.