Development of Minimum Life-Cycle cost Design Criteria

9510243 Wen环境荷载下民用基础设施系统的设计程序已逐渐从主要基于判断/经验的半概率性转变为概率性。然而，以优化长期风险和利益为基础的程序尚未制定。这种程序在评价和修理/改造现有设施时特别需要，这些设施的危害暴露时间短，但其故障仍可能造成严重后果。该国大量的民用基础设施系统正在老化，许多需要更换。维修/改造问题将变得更加尖锐。基于寿命周期成本的合理设计准则需要考虑载荷和阻力不确定性、危害暴露时间、故障后果和运行维护成本以及成本/损失随时间的折扣率等因素的影响。该项目的目标是：(1)发展确定基于生命周期的设计标准的方法；(2)将其应用于常规规范和标准未涵盖的新设施和改造设施。该问题被表述为相对于设计变量（即设施的设计负载和阻力或相应的LRFD格式的负载和阻力因素）的预期总生命周期成本最小化的问题。所要求的研究任务是：(1)将设计制定为考虑荷载和阻力不确定性的约束最小化问题，包括随机荷载发生时间、强度和相关性、失效后果成本以及随时间的贴现成本/损失；(2)开发求解的优化算法；(3)研究载荷和电阻的不确定性以及电阻随时间劣化的影响；(4)建立成本函数模型并检验设计的灵敏度；(5)确定设计载荷。这项研究将影响新概念和设计方法的发展，对下一代规范和标准具有重要意义。研究结果还将对制定维修/改造指南产生更直接的好处。＊＊＊

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. ***