Self-centering rocking composite frame using double-skin concrete-filled steel tube columns and energy-dissipating fuses in multiple locations

Self-centering rocking frame structures are high-performance seismic-resilient structures that exhibit damage in specified locations and keep the majority of structural elements elastic. Their recentering mechanism efficiently eliminates residual deformations. The present study investigated experimentally the seismic performance of a self-centering rocking composite steel/concrete frame system consisting of a braced frame with double-skin concrete-filled steel tube (CFT) columns made of ultra-high-strength (HS) steel designed to remain elastic to allow a column to rock off its foundation, and a typical all-steel moment-resisting frame (MRF). Both frames were connected at the floor levels using replaceable energy-dissipating fuses made of either conventional steel or low-yield point steel to further enhance deformation capacity. Post-tensioned (PT) steel bars that go through the inner tube of the double-skin CFT columns provide reliable self-recentering forces. An efficient shear-transfer mechanism was designed to effectively allow for uplifting behavior and prevent column base slippage. Equations to predict the base moment-frame rotation relationship of the self-centering rocking frame system were developed. Test results show that the proposed frame system reduces permanent deformations while the uplifting force is controlled by the initial post-tensioned force applied to the steel bars within a 5% margin of error. Furthermore, the proposed system exhibits a flag-shaped hysteresis loop with no residual deformation as long as the PT bars remain elastic.