Inelastic seismic response and ductility evaluation of steel frames with fully, partially restrained and composite connections

Abstract

The damage suffered by steel structures during recent strong earthquakes forced the profession to reevaluate issues related to the seismic design of steel structures. The evaluation of the maximum inelastic deformation of a structure subjected to a strong motion earthquake is a critical part of this process. A ductility parameter can also be used to calculate the maximum inelastic deformation of a structure. It is pointed that there is no unanimity on the definition of ductility, although it is constantly used in the profession. In this research several definitions of story ductility for MDOF systems are studied and the most appropriate one is identified. Definitions for local and global ductility are proposed. The presence of PR and composite connections on the structural response is also addressed in this study. Conventional analysis and design of steel frames structures is based on the assumptions that beam-to-column connection are either fully restrained (FR) or perfectly pinned (PP) connections. However, almost all steel connections used in practice are essentially partially restrained (PR) connections with different rigidities. The effect of PR and composite connections on the nonlinear seismic response of steel frames is evaluated. For this purpose, first the structural responses in terms of maximum interstory displacements and maximum top lateral displacements of three steel frames are calculated considering all of the frame connections to be of FR-type. Then the structural responses are evaluated for the frames with PR connections and finally for the frames with composite connections. Responses are compared for the three different cases. The recommendations to consider the effect of the vertical component on the structural response of two major seismic design guidelines for buildings are also studied. The first one is the National Earthquake Hazard Reduction Program (NEHRP) Recommended Provisions for Seismic Regulations for New Building (1994) and the second one is the Mexico City Seismic Code. Specifically, the effect of the vertical component on the structural responses of steel frames is evaluated first analytically and then according to the NEHRP Provisions and the Mexican Code. Finally, the analytical results are compared with the codes' recommendations.