Self-centering Moment-resisting Frames Research Articles

Seismic behavior of post-tensioned column base for steel self-centering moment resisting frame

This paper presents experimental and analytical investigation of cyclic response of a post-tensioned column (PT) base connection. The PT column base connection is designed to eliminate structural damage at column bases in self-centering moment resisting frames (SC-MRFs) under seismic loading; the softening behavior at the connection is provided by gap opening and elongation of PT bars rather than yielding in the column. Additional shear resistance is provided by bolted keeper plates; additional energy dissipation is provided by buckling restrained steel (BRS) plates. To investigate the cyclic behavior of the PT column base connection, a series of PT column base connection subassemblies were subjected to axial load and cyclic lateral displacements. Test parameters included initial post-tension force, initial axial force in column (constant or varying), column size and loading history. Limit states investigated for the PT column base connections included PT bar yielding and fracture of BRS plates. The test results demonstrated that properly designed PT column base connections were able to undergo lateral displacement up to 4% interstory drift while the columns and grade beams remained elastic. Also, the BRS plates showed good energy dissipation capacity by yielding in tension and compression without fracture. Structural damage at column bases was limited to the replaceable fuse BRS plates. In addition, analytical models were developed to predict the moment–rotation relationship of the PT column base connection and showed good correlation with the experimental data.

Seismic Performance of a Large-Scale Steel Self-Centering Moment-Resisting Frame: MCE Hybrid Simulations and Quasi-Static Pushover Tests

This paper presents an experimental study of a 0.6-scale 2-bay 4-story steel self-centering moment-resisting frame (SC-MRF) test structure under maximum considered earthquake (MCE) ground motions. A SC-MRF uses high-strength posttensioning (PT) strands to precompress the beams to the columns and to close the gaps between the beam flanges and column flanges that occur at the beam-column interface under earthquake loading, returning the frame to its initial position (i.e., the frame is self-centering). In this study, a beam web friction device is included in each beam-column connection to dissipate energy under seismic loading. The SC-MRF design objectives are to be without structural damage, creating the potential for immediate occupancy performance under the design basis earthquake, and to suffer only modest damage, leading to collapse prevention (CP) performance under the MCE. The CP performance is achieved by avoiding beam web buckling and PT strand yielding and fracture. A special fuse that prevents PT strands from yielding is described. Experimental results from MCE-level earthquake hybrid simulations and quasi-static pushover tests on the SC-MRF test structure are presented. The experimental results show that the SC-MRF did not collapse under the MCE, and that the fuse is a viable alternative to protect PT strands from yielding.

Reliability-based evaluation of design and performance of steel self-centering moment frames

Recently developed steel self-centering moment-resisting frames (SC-MRFs) have been analytically and experimentally validated as having the potential to eliminate structural damage under a design basis earthquake and restore their original vertical position following a major earthquake. Using Monte Carlo simulation, we subjected three nonlinear models of prototype SC-MRFs to thousands of synthetic ground motions, and recorded peak demand responses such as story drift and beam-column relative rotation. We used this data to examine the sensitivity of SC-MRF behavior to structural properties and geometry, seeking to generate recommendations to improve the existing design procedure. A reliability-based methodology was used to assess the likelihood of reaching the limit state of post-tensioned strand yielding. This study proposes modifications to the existing design procedure and illustrates a reliability-based methodology for developing improved seismic design recommendations.

Steel self-centering moment frames with collector beam floor diaphragms

Following a major earthquake, self-centering moment resisting frames (SC-MRFs) are capable of returning to their original plumb position and sustaining little structural damage. Under earthquake loading, these frames are characterized by a gap that develops between one of the beam flanges and the column face at the connection. This gap causes the frame to expand and therefore imposes special design requirements on the floor diaphragm and on the floor system (e.g. filler beams and slab) that are connected to it. Through nonlinear analyses of several floor diaphragm designs, this paper examines the influence that the floor diaphragm stiffness, strength, and configuration have on the seismic response of SC-MRFs. The floor diaphragm is represented by collector beams that transfer the inertia forces from the building floor to the SC-MRF. This work also presents several equations that are used to predict the effect that the floor diaphragm has on the axial force and moments that develop in the SC-MRF beams. This effect is shown to not be negligible. Finally, practical considerations related to the construction of SC-MRFs that use collector beams as the floor diaphragm are discussed.