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Fatigue of existing steel bridges

Reliability and risk analyses for decision-making

Time: Mon 2023-08-28 13.00

Location: Kollegiesalen, Brinellvägen 8, Stockholm

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Language: English

Subject area: Civil and Architectural Engineering, Structural Engineering and Bridges

Doctoral student: Ruoqi Wang , Bro- och stålbyggnad

Opponent: Associate Professor Boulent Imam, University of Surrey

Supervisor: Associate Professor John Leander, Bro- och stålbyggnad; Professor Raid Karoumi, Bro- och stålbyggnad

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Fatigue is one of the primary reasons that limit the service life of steel bridges. Many bridges in Sweden and other countries have progressively approached their anticipated service lives. Since funding and resources are limited, simultaneously replacement or maintenance of bridges is impractical. The effort should be put into extending the service life as far as possible and making optimal decisions on when and which interventions to implement.

This doctoral thesis aims to develop a risk-based framework supporting rational decisions to extend the service life of existing steel bridges, allowing bridges to maintain a desired safety level with minimised expected costs. It requires (i) an accurate fatigue life prediction considering different levels of uncertainties and (ii) an evaluation of interventions and consequences for an optimal decision.

Considering part (i), the reliability analysis is first performed on a single critical detail by comparing the detail's time-dependent reliability to a prerequisite safety level. The feasibility of advanced simulation techniques has been evaluated. Then, the spatial correlation between stochastic variables of several similar details has been considered. With the assumption of equivalent correlation, the system reliability can be estimated by an efficient modelling approach. A system perspective allows a better understanding of the condition of the entire bridge, since bridges are structures with typically high redundancy. A probabilistic finite element analysis has been performed to investigate the impact of local fatigue damage on the entire bridge.

Decisions based on reliability analysis can differ from those based on risk analysis, since they have different objective criteria. Decisions made according to the probability of failure or reliability are not necessarily economic. A utility-based risk analysis is motivated for part (ii). Several interventions and potential consequences are evaluated in different scenarios. The expected costs to extend the service life of bridges are minimised by choosing the most optimal intervention. The conditional probability and system modelling are investigated in parametric studies, revealing if and when the optimal decision will alter. An optimised inspection planning is also motivated to make each inspection more effective than conventional inspections at constant intervals. The number of inspections is reduced, and the inspection intervals are prolonged using an optimised strategy and an inspection method with high detection probability.