Load Capacity Evaluation of Existing Bridges

This project developed a proposed guideline for the evaluation of existing bridges. It is primarily intended to replace the present provisions in the AASHTO Manual for Maintenance Inspection of Bridges pertaining to the bridge posting calculations. The proposed procedures for all steel and prestressed concrete spans eliminate the existing inventory and operating stress levels for posting and provide rating levels which depend on the available information and level of effort expended by the rating engineer. A load and resistance factor format similar to the present AASHTO load factor method is proposed for the strength checking equations. The safety margins are calibrated herein by structural reliability procedures similar to those used for developing design codes throughout the world. The report reviews the load information available from bridge tests and numerous weigh-in-motion (WIM) studies. Statistical data are accumulated on overweight trucks, vehicle configuration, multiple presence, distribution of load to stringers and dynamic impact. These data are assembled in a prediction model for extreme load effect. Variables include truck volume, presence of heavy trucks, deck smoothness, method of girder distribution analysis and quality of inspection and maintenance. In a similar manner, statistical data were assembled on component strength for steel and prestressed members. Corrosion data were also studied to estimate the impact on future loss of strength and the effects of inspection and maintenance in assessing short term strength changes. Statistical descriptions of load and strength were meshed together in a reliability formulation to assess the safety index corresponding to the safety margins. The criterium for selecting load and resistance factors is to achieve consistent target reliability for all situations. To simplify calculations the existing AASHTO rating vehicles are utilized to determine nominal member load effects.