9510243 Wen Design procedures for civil infrastructure systems under environmental loadings have gradually changed from largely judgments/experience-based, semi- probabilistic, to probabilistic. Procedures based on optimization of long-term risk and benefit, however, have not been developed. Such a procedure is specially needed in evaluation and repair/retrofit of existing facilities whose hazard exposure time is short but whose failure may still cause serious consequences. A large number of the nation's civil infrastructure systems are aging and many are in need of replacement. The repair/retrofit problem will become more acute. Rational life-cycle cost based design criteria are needed in which effects of loading and resistance uncertainty, hazard exposure time, costs of consequence of failure and operation and maintenance, and discount of cost/loss over time are all taken into consideration. The objectives of this project are: (l) development of methodology for determination of design criteria based on life-cycle and (2) application to new and retrofitted facilities not covered by regular codes an standards. The problem is formulated as that of minimization of expected total life-cycle cost with respect to the design variables, i.e., design loads and resistance of the facility or corresponding load and resistance factors in a LRFD format. The required research tasks are to: (1) formulate the design as a constrained minimization problem considering uncertainties in the load and resistance including random load occurrence time, intensity, and correlation, costs of failure consequences, and discounting cost/loss over time; (2) develop of optimization algorithm for solution; (3) study the effect of load and resistance uncertainty and resistance deterioration over time; (4) model cost functions and examine sensitivity of design and (5) determine the design loads. The research will impact the development of new concepts and metho dologies of design that have important implications in the next generation codes and standards. The results will also have more immediate benefit in development of guidelines for repair/retrofit. ***
