Probabilistic considerations and use of WIM data for assessing structural safety effects of permitting 74 ton heavy trucks on Norwegian bridges

Abstract

The Norwegian Public Roads Administration has been asked to provide scientific background on the effect of increasing the maximum allowed traffic load to 74 ton on the structural safety of existing road bridges. This is a challenging task that requires careful attention to the way structural safety is assessed. With this paper, we want to highlight the aspects of importance when addressing this task, and demonstrate the possibilities in utilizing information from weigh-in-motion measurements. The background for the Norwegian traffic load model for assessment of existing bridges is reviewed and probabilistic considerations that are important for evaluating the effect of increased traffic loading are identified. The relation between the legal load limit and the loading situations in different limit states is deliberated, with focus on ultimate limit state and extreme value prediction for long time horizons. One year traffic weight data from a national Norwegian road is used for illustration purposes and the effect of different statistical considerations and assumptions are evaluated. It is indicated that the Norwegian traffic load model for assessment may be considerably less conservative than what can be justified from the measurements, and it is suggested to review the load model further with a probabilistic approach. When making decisions on legal load limits, proper understanding of the impact these limits have on structural safety is crucial.