APPLICATION OF LOAD SPECTRA TO BRIDGE RATING

Important safety decisions are made each time a bridge is evaluated. Field inspections have concentrated on estimating deterioration and dimensions of load-carrying members. How to measure and use a load spectrum at the site is described. Information on truck loads, dynamic impact, and girder distribution can provide additional data for rating bridges. Five sites in Ohio are reported. In addition, almost 100 other instrumented bridges have been studied by a similar weigh-in-motion operation, which uses existing bridges to provide equivalent static weights of passing vehicles. Weight data are unbiased because the field operation is undetected by drivers. The measured bridge load spectra can replace conservative AASHTO rating recommendations for impact and girder distribution factors. In order to enhance this application a reliability or probabilistic approach incorporates the measured site load spectra in evaluating the bridge safety. Loading is modeled by random variables including truck weight, traffic volume (affecting multiple presence), axle spacings and loads, impact, girder distribution, and measured stresses. A load simulation forecasts the maximum response for periods corresponding to inspection intervals. The calculation incorporates uncertainties and provides a reliability measure for comparing bridge safety. Examples include ultimate strength and fatigue-limit states. Strategies are described for using the load spectra and the reliability model to develop load factors for rating, schedules for inspection intervals, posting control, and redundancy evaluation.