Proof Load Testing of Highway Bridges

It is estimated that there are more than half a million highway bridges in the United States. Many of these bridges have deteriorated considerably because of age and inadequate maintenance and inspections, whereas the average daily truck traffic and truck loads have increased. Careful evaluation is required to make rational economic and safety decisions concerning the rehabilitation of each bridge. Proof load testing is an efficient tool that can be used to evaluate the performance of existing bridges. The objective of the study was to ensure the safety of two highway bridges located in Michigan by proof load testing. For the first bridge the design details were not available, and the second bridge had heavy corrosion in the lower flanges of its steel girders. Usually, the proof load level is about twice the maximum allowable load. Allowable loads in Michigan are considerably higher than those in other states. Therefore, military tanks on flatbed trailers were used to achieve the high target proof load levels. These tanks were provided by the Michigan National Guard. Load was increased in several steps, and the strains and deflections in the girders were measured. No sign of distress was observed during the tests. The capacity of each bridge was found to be significantly higher than the analytical capacity.