Does capacity design do the job?

Abstract

Current provisions in the New Zealand Loadings code for dynamic amplification of moment and shear force in cantilever wall buildings are critically examined. Based on time-history analyses of six wall structures, from two- to twenty-storeys, it is found that higher mode effects are inadequately represented by either the equivalent lateral force or modal response spectrum design methods. The time-history results indicate that dynamic amplification is dependent on both initial period, and expected displacement ductility level. Two different methods for consideration of higher mode effects in cantilever walls are proposed. The first is based on a simple modification of the modal response spectrum method, while the second is appropriate for single-mode design approaches such as the equivalent lateral force method. Both are found to give excellent representation of expected response. It is shown that providing capacity protection at the design seismic intensity does not ensure against undesirable failure modes at intensities higher than the design level. This has significance for the design of critical facilities, such as hospitals.