Impact Factors for Fatigue Design

Theoretical and experimental information is presented on impact factors for the fatigue design of steel highway bridges. Theoretical studies suggest that the impact factors for individual trucks in actual traffic should generally be less than 0.25, but that much higher factors would occur occasionally due to unusual conditions, such as a bump at a critical location. Measurement of impact factors in actual steel bridges confirms these theoretical conclusions. Impact factors approaching 100 percent have been reported in studies aimed at determining peak impact factors for nonfatigue design. However, impact factors determined for individual trucks in traffic, or for test trucks, are generally much lower. This suggests that the impact factors given by the present American Association of State Highway and Transportation Officials (AASHTO) formula are representative of actual bridge loading and, therefore, are generally appropriate for fatigue design, which should be based on typical, rather than extreme, conditions.