Optimal Structural Design under Seismic Risk Using Engineering and Economic Performance Objectives

In structural design, a wide spectrum of requirements, expectations, and concerns needs to be properly addressed. Engineering design criteria must be considered together with societal requirements and client preferences, with the latter often relating to economic objectives. Most of these design objectives are affected by the uncertainties surrounding a design. Therefore, realistic design frameworks must be able to handle multiple performance objectives and incorporate uncertainties from numerous sources into the process. In this work, a multi-criteria based design framework for optimal structural design under seismic risk is explored. The emphasis is on reliability-based performance objectives and their interaction with economic objectives. In the probabilistic response analysis, seismic loading uncertainties as well as modeling uncertainties are incorporated. A two-stage design procedure is explored. In the first stage, using preference aggregation, reliability-based objectives are combined with others. In the second stage, a socio-economics based approach is used where societal preferences are treated through reliability-based engineering performance measures, but emphasis is given to economic objectives. A rational net asset value formulation including losses from uncertain future earthquakes is used to assess the economic performance of the structure specified by a design. An assembly-based vulnerability analysis is incorporated into the loss estimation. In the implementation of the two-stage procedure, a structure is first designed to satisfy two engineering performance objectives, that is, two limit-states with associated reliabilities: a serviceability performance level chosen from a set of possibilities and a life-safety performance requirement that would be code-specified in practice. In the second stage, the design is assessed based on the expected net asset value of the structure, which, in most cases, reduces to considering its expected life-cycle costs over a specified lifetime. The intention is to provide an economic basis for the owner of the structure to be able to make a trade-off between different designs, for example, different structural configurations satisfying the same engineering performance requirements. Currently, a pure socio-economics based approach to structural design is considered to be impractical and therefore, the proposed two-stage procedure is recommended for use. The procedure is illustrated by designing a simple steel frame using alternative structural systems in the first stage, and comparing the economic consequences in the second stage. The results clearly indicate the importance of considering losses from uncertain future earthquakes while making design decisions.