This chapter reviews concepts related to the uncertainties associated with structures and how the results of load tests can be used to reduce these uncertainties. When an existing bridge is subjected to a load test, it is known that the capacity of the cross section is at least equal to the largest load effect that was successfully resisted. As such, the probability density function of the capacity can be truncated after the load test, and the reliability index can be recalculated. These concepts can be applied to determine the required target load for a proof load test to demonstrate that a structure fulfills a certain reliability index. Whereas the available methods focus on member strength and the evaluation of isolated members, a more appropriate approach for structures would be to consider the complete structure in this reliability-based approach. For this purpose, concepts of systems reliability are introduced. It is also interesting to place load testing decisions within the entire life cycle of a structure. A cost-optimization analysis can be used to determine the optimum time in the life cycle of the structure to carry out a load test.

In the coming years numerous existing traffic bridges in cities are subject to re-evaluation. In this paper a new probabilistic traffic load model for short-span bridges is proposed. As an example, site-specific weigh-in-motion data from Rotterdam, the Netherlands, is included in the model. The method proposed in this paper allows for a very efficient computation and at the same time takes into account the stochastics in axle loads and distances. For a simply supported 6 m single lane bridge the probabilistic load model provides a design load effect that is slightly lower than currently prescribed EN 1991-2 and the Dutch Guidelines for existing structures NEN 8700 and NEN 8701. This indicates that there is possible potential for a small reduction of the traffic load for short-span city bridges compared to the current standards. However, given the questionable quality of recorded measurements, a more extensive measurement campaign using WIM measurements is needed to get an adequate quantification of the traffic load parameters for city traffic conditions.