Fatigue load spectra for a steel girder bridge

The objective is to present an approach for the evaluation of fatigue performance. The proposed method is a combination analytical and experimental approach. The steps include identification of critical components and details, determination of the load distribution factors and stress range, instrumentation of the bridge, measurement of strains under a control vehicle to calibrate the equipment and determine the load distribution factors, measurement of strain under normal traffic, verification of load distribution factors, verification of load range, and evaluation of fatigue performance. Critical components and details are identified on the basis of analysis and past experience. The analysis provides theoretical values of load distribution factors and stress ranges. Measurement results provide a basis for verification of the analytical values. The number of load cycles is determined from the stress record under normal traffic. For a given number of load cycles, the effective stress range is calculated and compared with the critical level. Then the remaining fatigue life is estimated as a percent of the remaining number of cycles to failure. This serves as a basis for the evaluation of fatigue performance. The proposed approach is applied to an existing steel girder bridge. The superstructure consists of four plate girders and transverse floor beams. The bridge is instrumented, and the results of measurements are presented and discussed. For the considered components and details, the measured stress range and estimated number of load cycles are not critical. Some minor cracking may be expected, but in general the bridge may be considered as adequate with regard to fatigue.