Seismic design of steel moment-resisting frame structures using multi objective optimization

Design of seismic-resistant civil structural systems necessitates a balanced minimization of two general conflicting objective functions: the short-term construction investment and the long-term seismic risk. Many of the existing seismic design optimization procedures use single objectives of either the traditional minimum material usage (weight or cost) or the recent minimum expected life-cycle cost, while imposing constraints from relevant code specifications as well as additional seismic performance requirements. The resulting single optimized structural design may not always perform satisfactorily in terms of other important but competing merit objectives; the designer's individual risk-taking preference is not explicitly integrated into the design process. This paper presents a practical and general framework for design optimization of code-compliant seismic-resistant structures. Multiple objective functions, which reflect material usage, initial construction expenses, degree of design complexity, seismic structural performance, and lifetime seismic damage cost, respectively, are treated simultaneously as well as separately in the optimization process. Member-sizing problems of unbraced, regular planar, steel moment-resisting frames are selected as application examples. The resulting wide spread of optimized trade-off design alternatives enables structural engineers to actively select a cost-effective design solution that compromises conflicting merit objectives in a preferred manner.