SUMMARYThe seismic performance of post-tensioned steel connections for moment-resisting frames was examined experimentally and analytically. Cyclic tests were conducted on three full-scale subassemblies, which had two steel beams post-tensioned to a concrete-ÿlled tube (CFT) column with high-strength strands to provide recentring response. Reduced ange plates (RFPs) welded to the column and bolted to the beam ange were used to increase the dissipation of energy. Test results indicated that (1) the proposed buckling-restrained RFP could dissipate energy in axial tension and compression, (2) the subassemblies could reach an interstorey drift of 4% without strength degradation, and (3) buckling of the beam occurred towards an interstorey drift of 5%, causing a loss of the strand force, the recentring response, and the moment capacity. A general-purpose non-linear ÿnite element analysis program (ABAQUS) was used to perform a correlation study. The behaviour of the steel beam under both post-tensioning and exural loadings was compared to the test results and predictions.

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Post-tensioned (PT) self-centering moment frames were developed as an alternative to welded momentresisting frames (MRFs). Lateral deformation of a PT frame opens gaps between beams and columns. The use of a composite slab in welded MRFs limits the opening of gaps at the beam-to-column interfaces but cannot be adopted in PT self-centering frames. In this study, a sliding slab is used to minimize restraints to the expansion of the PT frame. A composite slab is rigidly connected to the beams in a single bay of the PT frame. A sliding device is installed between the floor beams and the beams in other bays, wherever the slab is allowed to slide. Many shaking table tests were conducted on a reduced-scale, two-by-two bay one-story specimen, which comprised one PT frame and two gravitational frames (GFs). The PT frame and GFs were self-centering throughout the tests, responding in phase with only minor differences in peak drifts that were caused by the expansion of the PT frame. When the specimen was excited by the 1999 Chi-Chi earthquake with a peak ground acceleration of 1.87g, the maximum interstory drift was 7.2% and the maximum lateral force was 270 kN, equal to 2.2 times the yield force of the specimen. Buckling of the beam bottom flange was observed near the column face, and the initial post-tensioning force in the columns and beams decreased by 50 and 22%, respectively. However, the specimen remained self-centering and its residual drift was 0.01%. Figure 1. (a) PT connection and (b) flag-shaped hysteretic behavior.of an MRF with PT connections exceeds the performance of an MRF with typical welded connections [12,13].Although the newly developed PT connection provides satisfactory cyclic performance, Kim and Christopoulos [13] and Garlock et al. [14,15] raised the issue of how columns and slabs restrainted PT frame expansion. Gap opening at the beam-to-column interfaces causes an expansion of the PT frame. The column and slab restraints that oppose the frame expansion affect the compression force in the PT beam. Kim and Christopoulos [13] suggested an approximate approach to estimate the restraint, which is appropriate in cases where a more concentrated response occurs at a single floor alone and overly conservative in cases where the structure responds in its fundamental mode. For a PT frame that responds in its fundamental mode, Chou and Chen [16] proposed a method for evaluating the restraint that considered the continuity and boundary conditions of the column. The method requires a structural analysis of the column in a deformed configuration and equations that are derived from the compatibility of deformation of beams, strands, and columns.When gaps open at the beam-to-column interfaces, the concrete slab, if it does not open near the columns, restrains frame expansion, affecting the self-centering [17]. Garlock et al. [14,15] and King [10] suggested that the collector beams or bays transfer the inertial force into the PT frame and accommodate PT frame expansion. Kim and Christopoulos [13] proposed detail...

SUMMARYGaps between beam-to-column interfaces in a post-tensioned (PT) self-centering frame with more than one column are constrained by columns, which causes beam compression force different from the applied PT force. This study proposes an analytical method for evaluating column bending stiffness and beam compression force by modeling column deformation according to gap-openings at all stories. The predicted compression forces in the beams are validated by a cyclic analysis of a three-story PT frame and by cyclic tests of a full-scale, two-bay by first-story PT frame, which represents a substructure of the three-story PT frame. The proposed method shows that compared with the strand tensile force, the beam compression force is increased at the 1st story but is decreased at the 2nd and 3rd stories due to column deformation compatibility. The PT frame tests show that the proposed method reasonably predicts beam compression force and strand force and that the beam compression force is 2 and 60% larger than the strand force with respect to a minor restraint and a pin-supported boundary condition, respectively, at the tops of the columns. Therefore, the earlier method using a pin-supported boundary condition at upper story columns represents an upper bound of the effect and is shown to be overly conservative for cases where a structure responds primarily in its first mode. The proposed method allows for more accurate prediction of the column restraint effects for structures that respond in a pre-determined mode shape which is more typical of low and mid-rise structures.

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