UBC Theses and Dissertations
Seismic performance of buildings with permanent lateral demands Dupuis, Michael Robert Leo
Architectural features and other irregularities in the gravity system which apply permanent lateral demands to the seismic force resisting system are being incorporated in new buildings. There permanent lateral demands raised concerns within the Standing Committee on Earthquake Design due to the perceived potential for a seismic ratcheting effect to occur during seismic loading. Nonlinear, parametric analyses were conducted in OpenSees to investigate the inelastic response of cantilevered and coupled shear wall buildings. The sensitivity of these buildings to permanent lateral demands was investigated across a domain of structural parameters including building height, building strength, and permanent lateral demands. Additional case studies considered the effect of vertical ground motions, subduction ground motions, coupling ratio, seismic demands, and investigated the behaviour of steel-braced frame buildings. The results demonstrate that a seismic ratcheting effect can develop and amplify inelastic deformation demands. The extent of ratcheting increases with the permanent lateral demands and is also highly dependent upon the hysteretic behaviour exhibited by the structural system. Systems with fat hysteresis - such as coupled shear walls and steel braced frames - demonstrate greater ratcheting than systems with flag-shaped hysteresis - such as cantilevered shear walls. An irregularity class is proposed for the National Building Code of Canada which will limit the allowable permanent lateral demands when IEFaSa(0.2) ≥ 0.50. It is suggested to limit permanent lateral demands in coupled shear wall buildings and steel braced frame buildings to 10% of the yield strength required to resist earthquake loads. Cantilevered shear-wall buildings may be subjected to larger permanent lateral demands corresponding to 40% of the yield strength required to resist earthquake loads. The more conservative limit of 10% is recommended for seismic force resisting systems, such as moment frames, which were not considered in this study. Within these limits, it is recommended that an amplification factor of 1.5 - accounting for the increased inelastic deformation demands - be applied to design deformations. The extent of ratcheting in buildings with permanent lateral demands exceeding the proposed limits is large and variable; therefore, the performance of such buildings should be validated with nonlinear dynamic analysis.
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