Reliability of design of seismically isolated buildings

M. Constantinou

This presentation investigates the reliability of design provisions for seismically isolated buildings. It is shown that buildings designed by the minimum criteria of currently applicable design provisions may have unacceptable collapse risk, which may result from either collapse of the superstructure due to excessive inelastic action or from collapse of the isolators. It is demonstrated that an acceptable collapse risk is achieved when the displacement capacity of the isolators is increased, the strength of the superstructure is made consistent with the isolator displacement capacity, and isolator stiffening behavior or isolation moat walls are utilized. It is shown that buildings performed best when designed to have essentially elastic behavior in the design earthquake (R or q factor equal to unity) with isolators having stiffening behavior at a displacement equal to 1.5 times the demand in the maximum considered earthquake, or equivalently having moat walls installed at a distance of 1.5 times the demand in the maximum considered earthquake. When additional design criteria in terms of story drift limits and floor acceleration spectra are implemented, the probability in the lifetime of isolated buildings to develop damage in the structural and nonstructural systems, and to their contents, is minimized leading to substantially improved performance by comparison to non-isolated buildings.