Fatigue cracks have been reported at out-of-plane welded gussets in main girders of existing steel girder highway bridges on severe traffic routes. In this paper, a simplified method to estimate live load stress of main girders for fatigue design by using load distribution factor (LDF) was discussed. First, structural parameter versus fatigue live load stress relations were investigated based on 22 design data sets computed by grillage analyses of 14 girder bridges designed by the past design code with various structural parameters. Regression analyses based on the data sets were conducted, and simplified equations which provide approximate estimates for the analysis values were extracted.
