Seismic behavior of moment resisting frames with high-strength square CFT columns

The seismic behavior of the low-rise (six-story) and medium-rise (twelve-story) moment resisting frames consisted with high-strength square concrete filled steel tube (CFT) columns (CFT MRFs) were investigated analytically. The fiber-based finite element implemented in the nonlinear structural analysis program DRAIN-2DX and capable of modeling the behavior of structural members while accounting for the effects of inelastic local buckling is used to model CFT beam-columns and steel beams of the CFT MRFs. The spring element implemented in DRAIN-2DX and capable of modeling the inelastic response of the Double Split-Tee (DST) connections is used to model DST connections of the CFT MRFs.; Six-story and twelve-story buildings are designed as office buildings located in the Los Angeles area to investigate the seismic behaviors of CFT MRFs with high-strength square CFT columns. The low-rise and medium-rise CFT MRFs are designed to meet all the requirements of the current seismic design provisions. The seismic behavior of the six-story and twelve-story CFT MRFs was analytically investigated by performing static pushover analysis following FEMA 356 procedures and adaptive pushover analysis. In addition, the CFT MRFs were subjected to three sets of SAC ground motion records for Los Angeles, including 2%, 10% and 50% probability of exceedance in 50 years, to investigate the inelastic dynamic behavior of the CFT MRFs.; The results of the analytical investigations demonstrate that the CFT MRFs designed to meet all the requirements of the current seismic design provisions have good ductility and can meet the global and local seismic performance requirements of FEMA 350 and 368. The analytical results indicate that incorporating the effects of inelastic local buckling on member behavior has a small influence on the overall seismic response of the CFT MRFs when the roof drift angles are smaller than 1.5%. When the roof drift angle is larger than 1.5%, local buckling of steel beams controls the post-yield behavior of the CFT MRFs, local buckling of the CFTs has a smaller influence than the steel beams. In addition, the pushover analysis provides a good index to distinguish the influence zones of the higher mode and second-order P-Delta effects. When the roof displacements of the time-history analysis are larger than the design drift, Deltad, the second-order P-Delta effects on the seismic performance of the structure should not be neglected.; Based on the analytical results, the current seismic design provisions could be used to design moment resisting frames consisted with high-strength CFT columns (□y > 50 ksi, f 'c > 10 ksi), as long as the width-to-thickness (b/t) ratio of the CFT is less than or equal to 32. Moreover, the upper bound limitation on fundamental period, Tmax set by IBC-2000 for the purposes of strength check, provides a good index to distinguish relatively rigid and flexible CFT MRFs. The flexible ( 1 < T1/Tmax < 1.15) distributed CFT MRF, which has smaller CFT sections than the low rigid (0.9 < T1/T max < 1) distributed CFT MRF, is recommended to use in the distributed frame system.