Bridge evaluation using proof load testing

The objective of the presented research is to develop an efficient proof load testing procedure for existing bridges. Many deteriorated structures are found unacceptable by traditional analysis. However, by engineering judgment, some of them seem to have adequate strength to carry normal highway traffic. Proof load testing can reveal the hidden safety reserve. Proof load level required for meaningful tests is approximately twice the legal load. In the State of Michigan, the legal 11-axle truck can weigh over 70 tons. Therefore, it is difficult to find a vehicle heavy enough to be used as proof load in bridge tests. In this study, military tanks were used. Each M-60 tank weighs about 55 tons over the length of about 4.5 m. Three bridges were selected for proof load tests. LVDT's were used to measure deflection. Strain transducers were also installed to determine the stress level in critical locations. For the selected bridges, tanks were positioned within a lane. They were gradually moved towards the center of the span to increase the load effect (midspan moment). After each move, the measurements were taken. Increased stress level, or any non-linearity in response, were considered as indications of inadequate strength. For all the considered bridges, stress levels were unexpectedly low. This can be justified by unintended composite action, effect of non-structural components such as parapets, and a more uniform distribution of load. It was found that in many cases, the support conditions are different than what was assumed in the design (partially fixed supports). All structural response observed followed a linear behavior.