Practical valuations on the effect of two type uncertainties for optimum design of cable-stayed bridges

Abstract

A procedure for the optimum structural design of cable-stayed bridges is proposed based on minimum expected life-cycle cost (LCC); the procedure is illustrated with the optimum design of a cable-stayed bridge subjected to static and earthquake loads. Reliability analysis of the bridge is performed taking into account the two types of uncertainty in the capacity and loads. The capacity of the bridge is assumed to be determined by its critical members; this is tantamount to the assumption that the capacities and load effects of the structural members are highly correlated. Various designs of a cable-stayed bridge are considered; namely, a standard design, plus several that are weaker as well as several that are stronger than the standard design. For the different alternative designs, the member sections are decreased or increased relative to those of the standard design. The LCC of a particular design is formulated assuming that the cost components (including the maintenance and social costs) are respectively fractions of the initial cost. Reliability of a design associated with the aleatory uncertainties is assessed for each design, and the corresponding expected LCC and safety index are evaluated. The results of the various designs provide the information, safety index vs expected LCC, for determining the design with the minimum expected LCC which can be presented graphically. Because of the epistemic type of uncertainty, the LCC as well as the safety index of the optimum design are random variables; the respective histograms are also generated, from which the various percentile values can be obtained. Especially, the 75% and 90% values of the LCC may be specified to minimize the chance of underestimating the actual LCC of the optimum design; similarly the 75% and 90% values of the safety index may be specified for a conservative design of the cable-stayed bridge.