Risk assessment of existing bridge structures

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Abstract

Bridges provide critical links in the transport network, but are vulnerable to damage by human actions, natural hazards and aggressive environmental conditions. To avoid disruption of the network, the adequate performance of the bridges has to be guaranteed by undertaking strengthening, repair or replacement when necessary. To ensure the most efficient use of limited economic resources, it is important that such actions are only undertaken on those bridges which actually need intervention. A bridge-specific analysis has to be undertaken to judge the performance of a bridge with regard to the risk of collapse. Current practice is based on safety considerations only and relies upon deterministic evaluation of a single estimate of the factor of safety, defined as the ratio of the structural resistance of the bridge and the load effects induced. Although some existing assessment codes make allowance for other important parameters such as ductility, redundancy, or consequences of failure, to the author's knowledge there exists no method that combines all these parameters.

In this dissertation a new comprehensive methodology has been developed to evaluate the risk of structural collapse of existing concrete bridges, where risk is defined as the product of the consequences and probability of failure. In the method proposed here, the key parameters influencing the risk of bridge collapse are identified and quantified. The five key risk indicators chosen are: (1) current safety, (2) future safety, (3) warning level (redundancy), (4) condition evaluation and (5) importance (consequences of failure). Standardising curves have been developed to transform the original indicator values of each parameter to a common scale, so that these different indicators can directly be compared quantitatively. A novel graphic representation of the results, referred to as a risk spiderweb, is used to visually highlight the relative significance of each parameter and allows detection of the critical indicators. Finally a single performance measure combining the risk indicators is defined as well as a minimum acceptance criterion. By considering all risk indicators at the same time, the failure to meet acceptance thresholds by one or more of the risk indicators can in some circumstances be compensated for by higher values of the other indicators. This criterion helps to decide whether the performance of the bridge is adequate or whether intervention is necessary.