Bridge Overstress Criteria: Fid-Bau Portal

Bridge Overstress Criteria

M. Ghosn / C. G. Schilling / F. Moses / G. Runco

This report presents a reliability-based procedure to determine the optimal allowable load on highway bridges considering both static and dynamic effects. A truck weight (bridge) formula was developed to provide acceptable levels of safety for bridges designed according to the 15th edition of the AASHTO specifications. Using the safety index as a measure of safety, the truck weight formula was developed to produce a safety index value of 2.5. Twelve bridges of different materials types, span lengths and configurations were analyzed for truck loads corresponding to the proposed truck weight formula. The results of the rating evaluation of these bridges showed large variations between the rating values for Load Factor Design (LFD) and Working Stress Design (WSD) procedures and inventory or operating stresses. Application of the higher truck weight limits to a large sample of bridges from the National Bridge Inventory indicated an increase in the number of deficient bridges if the inventory rating stress is used in the evaluation procedure. However, very few of the existing bridges would be considered deficient if the operating ratings are used. A fatigue analysis determined the relative fatigue damage caused by various new truck types and traffic scenarios that might result from changes in truck regulations. The fatigue calculations for actual bridges suggest that many existing bridges would not be affected by the possible truck regulation changes. Even for bridges with fatigue stresses above the fatigue limit, the reduced fatigue lives with the new regulations may still be sufficient for practical requirements.