Cost-optimization based target reliabilities for design of structures exposed to fire

Adequacy of structural fire design in uncommon structures is conceptually ensured through cost-benefit analysis where the future costs are balanced against the benefits of safety investment. Cost-benefit analyses, however, are complicated and computationally challenging, and hence impractical for application to individual projects. To address this issue, design guidance proposes target reliability indices for normal design conditions, but no target reliability indices are defined for structural fire design. We revisit the background of the cost-optimization based approach underlying normal design target reliability indices then we extend this approach for the case of fire design of structures. We also propose a modified objective function for cost-optimization which simplifies the evaluation of target reliability indices and reduces the number of assumptions. The optimum safety level is expressed as a function of a new dimensionless variable named “Damage-to-investment indicator” (). The cost optimization approach is validated for the target reliability indices for normal design condition. The method is then applied for evaluating and the associated optimum reliability indices for fire-exposed structures. Two case studies are presented: (i) a one-way loaded reinforced concrete slab and (ii) a steel column under axial loading. This study thus provides a framework for deriving optimum (target) reliability index for structural fire design which can support the development of rational provisions in codes and standards.

在概念上，通过成本效益分析来确保不常见结构中结构防火设计的充分性，在这种分析中，未来成本与安全投资的效益相平衡。然而，成本效益分析复杂且计算难度大，因此对于单个项目的应用不切实际。为了解决这个问题，设计指南针对正常设计条件提出了目标可靠度指标，但没有为结构防火设计定义目标可靠度指标。我们重新审视了作为正常设计目标可靠度指标基础的基于成本优化方法的背景，然后将这种方法扩展到结构防火设计的情况。我们还提出了一种用于成本优化的修正目标函数，它简化了目标可靠度指标的评估，并减少了假设的数量。最优安全水平表示为一个名为“损害 - 投资指标”（）的新的无量纲变量的函数。成本优化方法针对正常设计条件的目标可靠度指标进行了验证。然后将该方法应用于评估暴露于火灾的结构的 以及相关的最优可靠度指标。给出了两个案例研究：（i）单向受荷的钢筋混凝土板和（ii）轴向受荷的钢柱。因此，本研究为推导结构防火设计的最优（目标）可靠度指标提供了一个框架，该框架可支持规范和标准中合理条款的制定。