Seismic design of setback irregular steel structures based on life cycle cost

The seismic design of conventional structures is mainly addressed considering the direct construction cost; the life cycle costs (LCCs) are often neglected. This paper proposes a performance-based framework for optimal seismic design of irregular steel structures; the LCC is involved as an optimization criterion. Two regular 7- and 10-story structures are first designed based on the design earthquake; their geometries are then changed to make them setback irregular having in overall four cases to investigate. Nonlinear analyses are performed to estimate the target displacement for annual exceedance probabilities, different specified acceleration levels, and, accordingly, the extent of the structural damage. The LCCs of the cases studied are calculated to achieve two objectives: an LCC-based optimal design of steel structures, and evaluating the extent of irregularity on the structures’ LCCs. Results indicate that in the regular and irregular 7-story structures, a 40% and a 50% increase in the seismic loads can respectively reduce the LCCs by 31.3% and 34.9%. In the same vein, in the 10-story regular and irregular structures, increasing the seismic loads by 50% can reduce the LCCs by 33.4% and 31.7%, respectively. The results highlight the point that irregular structures, overall, require a higher initial cost than regular structures when the LCC is taken into account as an optimization criterion.