SEISMIC BEHAVIOR OF SELF-CENTERING POST-TENSIONED ECCENTRICALLY BRACED STEEL FRAME STRUCTURES WITH STAINLESS STEEL FUSE MEMBERS

Abstract

Eccentrically braced frame (EBF) is a highly effective seismic-resistant structural system when properly designed. However, permanent drift of EBFs after design basis earthquakes may affect the structure’s continuous occupancy and seismic retrofit can be very costly. To address the resilience issues such as uninterrupted use even after strong earthquakes, self-centering behavior is incorporated into EBF through post-tensioning to develop the self-centering eccentrically braced frame (SC-EBF) system. Post-tensioned connections are formed at the interfaces between rocking link beam and EBF beams using post-tensioned (PT) tendons. Gaps are allowed to open at these post-tensioned connections in strong earthquakes; and can be firmly closed under the re-centering force of the PT tendons. The columns, beams, braces, and the rocking link beam are designed to behave elastic under the design basis earthquake. Therefore, no residual drift is expected to occur in the SC-EBF system and damage is concentrated to replaceable fuse members under the design basis earthquake. The proposed SC-EBF system provides a competitive design option in high seismic hazard regions with its self-centering behavior, stiffness and strength comparable to conventional EBF structural systems, and tunable energy dissipation capacity provided by conveniently replaceable fuse devices. The link beam which is welded or bolted to the beams of the conventional EBF system is replaced by the rocking link beam. Self-centering behavior is observed in all the investigated SC-EBF structures in this study. A nonlinear finite element analysis based parametric study is conducted on the SC-EBF systems with K-type or D-type EBF configurations. Effects of two key design parameters are evaluated: PT tendon area and the rocking link beam length. To cope with the large link rotation demand, replaceable fuse devices in the SC-EBF systems are made of highly ductile AISI 316L stainless steel and are designed to utilize the large deformation in the SC-EBF systems for energy dissipation during strong earthquakes. It is observed that the ductility of the SC-EBF with short rocking link beam is generally lower than that of the SC-EBF with longer rocking link beam if the same PT tendon length is used for both cases. To take advantage of the findings from both the SC-EBF system with short rocking link beam and the SC-EBF with long rocking link beam, a modified design of the SC-EBF with two parallel short rocking link beams is proposed. Nonlinear finite element analysis was conducted on this modified SC-EBF system under cyclic loading, showing that this type of SC-EBF system exhibits improved self-centering performance, strength, ductility, and energy dissipation behaviors. The fuse members used for this type of SC-EBF system is redesigned and shows simple configuration, ease of fabrication and installation.