Civil engineers are currently faced with problems involving all aspects of our aging infrastructure. One area in critical need of such attention is the design, monitoring, and maintenance of bridge piers where local pier scour is a concern. As an example of the pier scour problem, the New York State Thruway bridge that crosses the Schoharie Creek collapsed in April, 1987, killing 10 people. The failure apparently resulted from pier scour. The scour depth at a particular time during the design life of a bridge is a function of the flow-duration characteristics of storms that occurred prior to the time of interest. Current scour models and equations provide a single-valued estimate of the ultimate scour depth for a design discharge; the engineer does not have a time-dependent estimate of the scour depth. In addition, there is a lack of guidance in decision-making regarding the control of scour at existing piers and the design of piers for avoiding failure due to pier scour. The objective of this study was to develop both a conceptual, time-dependent pier scour model and a methodology to assess the risk of bridge failure due to pier scour over the life of the bridge. The time-dependent pier scour model was formulated and calibrated with laboratory data and observations as well as field observations. The time-dependency enables the engineer to estimate the scour depth at any time during the life of the bridge. The risk of failure analysis involves simulating pier scour over some period of time and determining the probability that the bridge will fail at intervals of time during that period. The scour model and risk of failure analysis may be used to determine the effects of scour control (e.g., riprap), pier size and shape, and pier depth on the scour depth and associated risk of failure. The risk-based failure analysis will enable the engineer to base bridge pier designs and scour control on an estimated probability of failure, thus providing for the safer design of bridges.
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