Proportioning: Missing Step in Model Building Code Seismic Design

Abstract

Over the past 30 years, many structural designs have included explicit consideration of major seismic events. The vast majority of these structures were designed so that significant inelastic deformations would be required to successfully sustain the demands of the target earthquake. The ability to deform in a ductile fashion remains a fundamental premise of the seismic design provisions included in each of the model building codes. To this end, model building code seismic design provisions go to great lengths to ensure that certain components (typically, vertical elements such as shear walls or frames) of a structure's lateral load resisting system have substantial ductility. Unfortunately, none of the model codes require that a lateral load resisting system be proportioned so that the painstakingly detailed ductile elements would ever yield during an earthquake. In fact, some current model code provisions actually promote proportions that preclude the intended yielding of the ductile elements. Examples of problematic code provisions, inadequate but “acceptable” designs, and some related earthquake damage case studies are discussed.