The 2003 draft of a new AUSTROADS highway bridge design specification includes a radically upgraded section on fatigue. The upward trend in gross vehicle mass (GVM) and individual axle mass, combined with calculation of fatigue loading as a function of span (base length of influence line) has resulted in a fatigue loading which is quite severe on short spans compared with previous specifications. Detailed current truck axle weight configurations from many weigh- in-motion sites were passed over bridges of various spans with typical influence lines to determine the stress history, from which fatigue loading was determined by Rainflow analysis. These compared well with the draft specification, and were used to adjust the loading downwards, especially for routes which were not interstate or urban freeways.
The reductions in fatigue loading through calibration with weigh-in-motion data were made possible by using the two slopes of the S-N curve, where stress cycles with amplitude below the constant amplitude fatigue limit (which account for most stress cycles) cause less damage per cycle than if one slope is used.
Short span steel bridges will be “governed” by fatigue criteria in their design, especially if there is welding in critical areas. More sophisticated grillage analysis will replace load distribution factors, and design efficiencies such as changes in the spacing of beams will be required.