Tsai et al. 2008

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A full scale three story, three bay CFT buckling restrained braced (BRB) frame (designed using displacement based seismic design) was constructed and tested under pseudo dynamic loading. As the third of a three part paper, this report summarizes the analytical studies performed along with the experiment as well as evaluates the performance of the frame.

Experimental Study, Results and Discussions

The frame had a total height of 43 ft. and a length of 69 ft. Square CFTs were used for the two exterior columns and circular CFTs for the two center columns. BRBs were installed in the center bay only and the composite beams had a 7 ft. concrete slab. All of the steel used was A572 GR50 with nominal yield strength of 50 ksi. The concrete for all of the CFT columns had a compressive strength of 5 ksi. Three types of moment connections were used in the exterior beam-column connections, specifically through beam, external diaphragm, and bolted end plate connections. Several types of BRBs were used in the frame, including single-core, double-cored, and all metal.

The earthquake records chosen for the pseudo dynamic loading were the 1999 Chi-Chi and 1989 Loma Prieta earthquakes scaled to simulate seismic hazard levels of 50%, 10%, and 2% in 50 years. No fracturing was observed in the braces after the application of the first phase of records, so the frame was loaded cyclically until failure of the braces. After this sequence of loading the damaged components were repaired and a second phase of tests continued. The repairs included a modification of the gusset plates at the brace connections as well as new BRBs.

Free vibration tests were conducted after each pseudo dynamic test to measure the changes in stiffness and damping. Only minor changes were observed.

Analytical Study

Both OpenSees and PISA3D were used to model the frame. The PISA3D model of the frame used two-surface plastic strain hardening truss elements for the BRBs, bi-linear beam-column elements for the beam members, and three-parameter degrading beam-column elements for the columns. The OpenSees model used flexibility-based nonlinear beam-column fiber elements for all of the CFT columns and steel beams. The BRBs were modeled with the truss element and the Menegotto-Pinto steel material with isotropic and kinematic strain hardening.

Several displacement and base shear time histories from the experiment and analytical models were presented, with reasonable agreement. The authors noted that the analytical models performed well since much of the inelasticity and energy absorption occurred in the braces which can be more easily modeled. They also mention that the brace ends undergo significant rotation and that further research on BRB end rotation is needed.

References

  • Lin, M.-L., Weng. Y.-T., Tsai, K.-C., Hsiao, P.-C., Chen, C.-H., and Lai, J.-W. (2004). “Pseudo-Dynamic Test of a Full-Scale CFT/BRB Frame: Part 3 – Analysis and Performance Evaluation,” Proceedings of the 13th World Conference on Earthquake Engineering (13WCEE), Vancouver, B.C., Canada, 1-6 August 2004.
  • Tsai, K.-C., Hsiao, P.-C., Wang, K.-J., Weng, Y.-T., Lin, M.-L., Lin, K.-C., Chen, C.-H., Lai, J.-W., and Lin, S.-L. (2008). “Pseudo-Dynamic Tests of a Full-Scale CFT/BRB Frame - Part I: Specimen Design, Experiment and Analysis.” Earthquake Engineering & Structural Dynamics, 37(7), 1081–1098. doi:10.1002/eqe.804
  • Tsai, K.-C., and Hsiao, P.-C. (2008). “Pseudo-dynamic test of a full-scale CFT/BRB frame - Part II: Seismic performance of buckling-restrained braces and connections.” Earthquake Engineering & Structural Dynamics, 37(7), 1099–1115. doi:10.1002/eqe.803