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PSEUDO DYNAMIC TESTS OF A FULL-SCALE CFT/BRB COMPOSITE FRAME: DISPLACEMENT BASED SEISMIC DESIGN AND REPONSE EVALUATIONS


K.C.TSAI1 ,Yuan-Tao WENG2,Min-Lang LIN3,Chui-Hsin CHEN4 Juin-Wei LAI4 and Po-Chien HSIAO5

SUMMARY

At time of this writing, a full scale 3-story 3-bay concrete filled steel tube (CFT) composite braced frame has been constructed in NCREE with buckling restrained braces (BRBs) and scheduled to be tested in October 2003. The full scale CFT/BRB frame test is part of an international collaboration between researchers in Taiwan and the United States. Measuring 12 meters tall and 21 meters long, the frame is among the largest frame tests of its type ever conducted. The frame is to be tested using the pseudo-dynamic test procedures applying input ground motions from the 1999 Chi-Chi and 1989 Loma Prieta earthquakes, scaled to represent 50%, 10% and 2% in 50 years seismic hazard levels. This paper describes the displacement-based seismic design procedures adopted in the design of the structural members. A target story drift limit of 0.025 radian for the 2% in 50 years hazard level governs the design strength of the frame .Nonlinear analyses illustrate that the response of individual BRB member can be satisfactorily simulated by using truss elements implemented in two different general purpose nonlinear response analysis programs. Nonlinear dynamic analyses suggest that the peak story drift is likely to reach 0.025 radian after applying the 2/50 design earthquake on the frame specimen. Analytical results also suggest that the arrangement of 4,3 and 3 actuators, each having 980 kN force capacity, might be necessary for the 1st , 2nd and 3rd floor, respectively. The BRBs at the upper two floors are more vulnerable than those in the first floor. The failure of moment connections is not likely to occur after the applying four earthquake load effects. CFT columns hinging at the based are expected, but should not fail at the rotational demand is moderate.
Keywords: concrete filled tube, buckling restrained brace, pseudo-dynamic tests, displacement based seismic design, nonlinear analysis.
INTRODUCTION
Through international collaboration between researchers in Taiwan and the United States, a full-scale 3-story 3-bay RC column and steel beam RCS composite moment frame has been tested in October of 2002 in the structural laboratory of National Center for Research on Earthquake Engineering (NCREE) in Taiwan in October 2002 (Chen et al. 2003). In the year 2003, a full-scale 3-story 3-bay CFT column with the buckling restrained braced composite frame (CFT/BRBF) specimen has been constructed and scheduled to be tested in October in a similar manner. The 3-story prototype structure is designed for a highly seismic location either in Taiwan or United States. The typical bay width of 7m and typical story height of 4m have been found common in Taiwan and US building configuration, it also corresponds well with the 1m spacing of the tie down holes on the strong floor and reaction wall of the lab. The total height of the frame, including the grade beam, is within the strong wall height 15m. The 2150m wide concrete slab is adopted to develop the composite action of the beams. Measuring 12 meters tall and 21 meters long, the specimen is among the largest frame tests of its type ever conducted. The frame will be tested using the pseudo-dynamic test produres applying input ground motions from the 1999 Chi-Chi and 1989 Loma Prieta earthquakes, scaled to represent 50%, 10%, and 2% in 50 years seismic levels. Following the pseudo-dynamic tests, if no brace is fractured, quasi-static loads will be applied to cyclically push the frame to large inter-story drifts up to the failure of the braces, which will provide valuable data to validate possible failure mechanism and analytical models for large deformation response. Being the largest and most realistic composite CFT/BRB frame ever tested in a laboratory, the test provides a unique data set to verify both computer simulation models and seismic performance of CFT/BRB frames. This experiment also provides great opportunities to explore international collaboration and data archiving envisioned for the Networked Earthquake Engineering Simulation (NEES) initiatives or the Internet-based Simulation for Earthquake Engineering (ISEE) (Yang et al. 2003) launched recently in us and Taiwan, respectively. This paper focuses on the displacement-based seismic design procedures adopted in the design of CFT/BRB frame specimen. During the planning stage, extensive nonlinear dynamic analyses were also carried out in order to ensure the possible seismic demands would not exceed the force and displacement limits of the test facility. This paper describes the analytical models and evaluates
The seismic performance observed from the simulation results. Inelastic static and dynamic time history analyses have been conducted using PISA2D (Tsai and Chang 2001) and OpenSees (Open System for Earthquake Engineering Simulation), developed at National Taiwan University and Pacific Earthquake Engineering Research Center (PEER), respectively.