Constant yield displacement approach for seismic design of structures

Abstract

The existing relations between the vibration period, the strength reduction factor and the ductility demand of stiff fixed-base structures indicate that these structures should not be allowed to yield (i.e. Ry=1). This design approach is based on the argument that the inelastic ductility demand of these structures, if they were allowed to yield, would be very high. This study shows that these ductility demand values are not realistic: they emerge from the constant-period, forced-based design methodology, which leads to unrealistically small yield displacement estimates. Further, this study quantifies the relation between the strength and deformability of structures, based on the argument that the yield displacement of a structure in bending depends mainly on the yield strain of the yielding material and the geometry of the structure and that it does not dependent strongly on the bending strength of that structure. This relation is determined through a statistical analysis of the response of a fixed-base single-degree-of-freedom inelastic structure excited by a large number of recorded ground motions. These motions cover a wide range of ground motion types, magnitudes and distances. Based on this relation, a new seismic design approach is proposed, the Constant-Yield-Displacement-Design (CYDD) approach. This approach is based on the independence of the strength and the yield displacement of a structure. Compared to the existing approaches, it offers: 1) a more realistic calculation of the inelastic displacement ductility demand of structures with a predetermined strength; and 2) a more exact determination of the strength that is required to satisfy certain performance objectives expressed in terms of flexural displacement ductility. The vibration period (i.e. stiffness) of the structure does not play a role in the CYDD approach. --> The existing relations between the vibration period, the strength reduction factor and the ductility demand of stiff fixed-base structures indicate that these structures should not be allowed to yield (i.e. Ry=1). This design approach is based on the argument that the inelastic ductility demand of these structures, if they were allowed to yield, would be very high.This study shows that these ductility demand values are not realistic: they emerge from the constant-period, forced-based design methodology, which leads to unrealistically small yield displacement estimates.Further, this study quantifies the relation between the strength and deformability of structures, based on the argument that the yield displacement of a structure in bending depends mainly on the yield strain of the yielding material and the geometry of the structure and that it does not dependent strongly on the bending strength of that structure. This relation is determined through a statistical analysis of the response of a fixed-base single-degree-of-freedom inelastic structure excited by a large number of recorded ground motions. These motions cover a wide range of ground motion types, magnitudes and distances.Based on this relation, a new seismic design approach is proposed, the Constant-Yield-Displacement-Design (CYDD) approach. This approach is based on the independence of the strength and the yield displacement of a structure. Compared to the existing approaches, it offers: 1) a more realistic calculation of the inelastic displacement ductility demand of structures with a predetermined strength; and 2) a more exact determination of the strength that is required to satisfy certain performance objectives expressed in terms of flexural displacement ductility. The vibration period (i.e. stiffness) of the structure does not play a role in the CYDD approach.