Dual-level designs of buildings under seismic loads

Three methods of dual-level seismic design for buildings are proposed based on consideration of reliability against the serviceability and ultimate limit states. The first is a method of calibration of design parameters according to the two target reliability levels within the context of current code procedures. The method is demonstrated by calibration of seismic load factor and drift limit. The second is an extension of the design earthquake approach to allow consideration of two levels of design earthquake. The performance is assured by linear and nonlinear equivalent static (pushover) analyses. It produces designs which come closer to the design objectives of most current codes. The third is an alternative to the current design procedures. It uses uniform hazard response spectra and a pushover analysis-based equivalent single-degree-of-freedom system to explicitly account for variability and uncertainty in the seismic loads and inelastic response behavior of the structures. This avoids the use of the controversial response reduction factor. Uncertainties in the soil amplification and structural system modeling are accounted for by corresponding design factors similar in concept to the LRFD method. It ensures that the structure meets local (interstory) drift and global ductility requirements with specified probabilities.