Diagnostic Load Testing for Bridge Load Rating

Diagnostic load testing uses field-measured data for bridge load rating without requiring certain information or the assumptions needed for analytical rating, and it can help arrive at more-reliable load ratings. It usually consists of three steps: preparation, execution, and analysis of the results. It is critical to estimate the probability of success in the preparation step before the execution step. A case of such a test for a steel highway bridge is presented. The bridge’s fascia beams were given a low rating because their cover plates were removed. Diagnostic load testing was used to obtain more reliable load ratings for the fascia beams. Under the test loads covering service loads, the support-fixity of the bridge was confirmed. It was identified as reserve strength and was not taken into account in the original analytical rating. Thus, it was concluded that no strengthening is needed for the fascia beams, and their rating factor was recommended to be increased from 0.7 to 1.4 on the basis of the measured load effects.