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.

中文翻译：

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两种不确定性对斜拉桥优化设计影响的实用评估

根据最小预期生命周期成本（LCC），提出了斜拉桥最佳结构设计的程序。通过斜拉桥在静态和地震荷载作用下的最佳设计来说明该过程。考虑到容量和负载的两种不确定性，对桥梁进行可靠性分析。假定桥梁的能力由其关键成员确定；这等于假设结构构件的承载力和载荷效应高度相关。考虑了斜拉桥的各种设计。即标准设计，再加上比标准设计要弱的几个以及比标准设计要强的几个。对于不同的替代设计，相对于标准设计而言，成员部分减少或增加。假定成本构成部分（包括维护和社会成本）分别是初始成本的一部分，则可以制定特定设计的LCC。针对每个设计评估与不确定性相关的设计的可靠性，并评估相应的预期LCC和安全指数。各种设计的结果提供了有关安全指数与预期LCC的信息，用于确定具有最小预期LCC的设计，该信息可以通过图形显示。由于不确定性的认知类型，LCC以及最佳设计的安全性指标都是随机变量。还生成相应的直方图，从中可以获得各种百分位值。特别，可以指定LCC的75％和90％的值，以最大程度地降低低估最佳设计的实际LCC的机会；同样，对于斜拉桥的保守设计，可以指定安全指数的75％和90％。