Risk-based Decision-Making in Bridge Maintenance

The paper focuses on the use of formal decision-making methodologies to balance the competing interests of safety, direct cost, environmental and socio-political factors encountered in managing bridges. Application of the methodologies to the various maintenance options for a major motorway bridge structure is discussed.

It is estimated that there are some 155,000 bridges in public ownership in the UK. They are all subject to periodic inspection and re-assessment (to ensure continued fitness for purpose) where appropriate. For highway bridges the assessment programme is given added impetus with the introduction of the European 40 tonne vehicle.

Current structural assessment procedures provide little guidance about what to do if a bridge fails an assessment. The implications of'failure' is that some sort of remedial action is needed, but the significance of failing an assessment may not be immediately obvious. Bridges fail assessments for a variety of reasons, but it is fairly unusual for signs of distress to be apparent and failure of an assessment does not necessarily imply that structural failure is imminent. The general feeling among bridge engineers is that assessments provide unduly conservative answers because the number of actual collapses of structures identified as approaching collapse is very low. A key question is thus how conservative is the assessment? How can this conservatism (ie. the likelihood of the bridge failing to meet its functional purpose) be quantified? Furthermore how should we account for the various consequences of such a failure? These two aspects (ie. the likelihood of failure and the consequences of it) are the key elements of risk - based methodology for prioritising bridge maintenance programmes.