Considerations of Ndt Quality in Fracture-critical Inspections for Steel Bridges

Non-destructive testing (NDI) techniques are routinely used in bridge inspection. However, detection accuracy, accessibility, cost and consequences of false indication are all factors to consider when selecting the NDI techniques to be used. A probabilistic method is proposed to help select the most suitable NDI technique and an associated optimal schedule for fracture-critical member/detail inspections on a specified steel bridge. The Probability of Detection (POD) curve for each NDI technique which denotes its detection accuracy is employed. In addition, by combining probability calculations based on the use of the POD curve together with numerical Monte Carlo simulations for crack propagation in the fracture-critical detail, a total cost function is formulated that includes the expected cost of inspections, repair and failure resulting from the chosen NDI method and alternative inspection schedules. In summary, the selection of an NDI method with an associated inspection schedule for fracture-critical inspections is formulated as an optimization problem which utilizes appropriate constraints on inspection intervals and on a minimum acceptable (target) structural reliability. One case study for a box girder bridge is presented to demonstrate the proposed probabilistic method for selecting the optimal NDI method and corresponding inspection schedule.