Minimum life-cycle cost structural design against natural hazards

A methodology is developed for the determination of design criteria based on minimum expected life-cycle cost. Feasibility of application of the methodology to design for earthquake and wind load is also shown. A formulation is adopted and extended for multiple loads. The emphasis is on proper modeling of the uncertainty of loads and load effects in the structural lifetime and treatment of the lifetime costs. Discounting of cost over time is also considered. Parametric studies for these parameters and numerical analysis are carried out. The methodology is then applied to the design of a 9-story office building under seismic loads in Los Angeles. Multiple structural limit states and twelve structures designed. The seismic hazard and evaluation of structural response are based on USGS data, FEMA 273 provisions and the equivalent SDOF methods. A nonlinear inelastic push-over analysis is carried out, the inelastic response spectra method and a structural capacity uncertainty correction factors are used to evaluate the structural limit state probability. The initial cost is calculated and limit state cost functions are developed. The optimal design obtained by minimizing total expected life-cycle cost using a numerical procedure. A sensitivity analysis of the optimal design to change in design life, death and injury cost, structural capacity uncertainty and discount rate is carried out. The application is then extended to Seattle, Charleston and Boston. For wind design, optimal structural strength and optimal envelope strength are considered separately. The response of structural frame to the wind load is calculated using the provisions in ASCE 7-98. For envelope failure, twelve glass types according to thickness are considered. Probability of glass failure by the wind pressure and windborne missiles is calculated. The optimal design for wind load is then obtained, analysis of sensitivity of optimal design is carried out. The application is then extended to design for both earthquake and wind loads. Analysis of sensitivity is also carried out. The life-cycle cost based design method presented here is useful tool for decision on design load intensity. The results have important implications in the development of future codes and design guide lines.