Strength and Ductility Considerations in Seismic Design

Real strength and ductility characteristics are critical determinants of the seismic behavior of structures. Overstrength, which is defined here as member or structural capacity greater than that assumed in design, may be beneficial or, under certain circumstances, detrimental to seismic behavior. When the yield level under lateral loading of a structure is increased by overstrength, the inelastic seismic demands on the structure are decreased. If relative member overstrengths are not well balanced or tuned, inelastic structural capacity will be reduced. Overstrength for several structural steel systems is estimated and the effect on structural response evaluated.

Inelastic demand and structural capacity can be partially characterized by ductility. Local ductility is the basic measure of member performance and relates to detailing considerations, low cycle fatigue, and a large body of laboratory test data. Global ductility is the basic measure of structural response and offers a convenient design parameter. Overstrength models are implemented to relate local and global ductilities.

Current SEAOC recommendations are evaluated in the context of overstrength and ductility demands. The current approach leads to inconsistent designs with, in some cases, inadequate protection against overload of nonductile elements.

Satisfactory seismic performance requires, in part, selection of sufficient structure strength and ductility capacity. A design approach for single degree of freedom systems employing overstrength concepts developed herein and extensions of previous ductility studies is suggested. Concepts presented are illustrated with analytical studies of structures damaged in the 1985 Mexico earthquake and prototypical structures designed to current SEAOC recommendations.