Example 364

A designer proposes two alternate life cycle plans (Plan A and Plan B) for a structure, as illustrated in Figure 36.4. Which plan should the client accept?

Answer

There is not enough information available to decide. If the plans are to be evaluated on the average reliability over the lifetime of the structure, the plans are equivalent since blifetime (i.e., 0.5(bmin + bmax)) is the same for both. Plan A initially designs the structure to a higher level of safety, which incurs greater initial costs of construction and allows the structure to deteriorate to a lower level of safety, which increases the likelihood of failure and thus incurs greater failure costs. Plan B, on the other hand, has a narrower band of acceptable reliability and requires more frequent (and probably more costly) maintenance actions throughout the structural life. All estimated costs and the discount rate would be needed to determine the better plan. In addition, the analysis needs to include societal standards to determine a minimum acceptable value of bthreshold. A single value for reliability does not completely define the problem.

36.3 Failure Cost

In most life cycle cost analyses, the failure cost represents the costs incurred by a structural failure multiplied by the probability of failure. What appears to be a straight-forward concept can become quite complex. Costs of failure may be expressed in terms of a point-in-time, annual, or lifetime cost depending on the type of problem. The costs of failure can vary by orders of magnitude over the life of the structure depending on the reliability range the structure is allowed to pass through.

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