Load Rating a Prestressed Concrete Double T-Beam Bridge without Plans by Field Testing

Bridges with no design plans are an issue in New Mexico because of the many that exist throughout the state. Conventional load rating techniques cannot be used because these bridges have limited or no design documentation. This lack of information has created uncertainties regarding the load-carrying capacity of these structures. Only a few states have formal procedures on how these particular bridges should be load rated. A project was conducted for the New Mexico Department of Transportation to develop a procedure for load rating bridges without plans, including prestressed concrete bridges. In accordance with the AASHTO Manual for Bridge Evaluation, a prestressed concrete double T-beam bridge was evaluated with advanced analyses and experimental methods (including load testing and nondestructive material evaluation techniques). A four-step load rating procedure was implemented that included estimating the prestressing steel by Magnel diagrams, verifying the estimate with a rebar scanner, testing the bridge at both diagnostic and proof loads based on strain measurements, and using the proof test results to rate the bridge. Rating factors and posting loads were determined for AASHTO and New Mexico legal loads. Because of the poor condition of the shear keys (some of which were broken), it is shown that the load distribution between beams was adversely affected and the bridge should be load posted.