Cyclic Responses of Double-Tube Unbonded Brace Elements and Connections

• Main Authors: Chung-Hsin Weng, 翁崇興
• Other Authors: K.C.Tsai
• Format: Others
• Language: zh-TW
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/78408223195938500017

碩士 === 國立臺灣大學 === 土木工程學研究所 === 90 === In recent years, a number of researches have confirmed that the buckling restrained braced frame (BRBF) is an effective system for severe seismic application. Buckling restrained bracing (BRB) members can be conveniently made from several kinds of structural steel shape encased in steel tube and confined by infill concrete. When the brace is subjected to compressions, an unbond material placed between the core bracing and the concrete infill is required in order to reduce the friction while restrain the bracing from buckling. In order to reduce the length and the number of bolts in the brace-to-gusset connection, the double-tee double-tube (DTDT) unbonded braces have been developed and extensively tested in National Taiwan University in the past few years. The objectives of this research include: 1) further investigating the effectiveness of various types of unbond material, 2) developing guidelines for the design of the restrain tubes as well the tie elements connecting the two tubes, 3) constructing the constant cyclic axial strain versus fatigues cycle or strain energy relationships for the A36 unbonded braces, and 4) providing guidelines for the design of bolted joint for the brace-to-gusset connections. In this research, a total of ten unbonded braces employing single tube and flat core steel plate details were fabricated and tested cyclically in order to investigate the effectiveness of various kind of unbond material. It is confirmed that the 2-mm thick silicon rubber sheet is most effective in minimizing the variations of peak brace compressive and tensile responses. Test results also suggest that a strain hardening factor of at 1.5 and 1.4 should be considered for the analysis and design of the unbonded braces employing A36 and A572 Grade 50 steels, respectively. Additional ten double-plate double-tube (DPDT) unbonded braces using A36 material for the core element, 2-mm thick silicon rubber sheet for the unbond material were fabricated and tested by applying first cyclically increasing then constant fatiguing strains. Test results also confirm that strain hardening factor can be as high as 1.8 when the steel core peak tensile strain reaches 0.02. Tests confirmed that the proposed strength and stiffness requirements developed for the tie elements between the two tubes can be conveniently applied in the design and construction of the double tubed unbonded braces subjected to large inelastic strain reversals. Test results also confirm that the constant cyclic axial strain versus fatigues cycle or strain energy relationships can be satisfactorily predicted by the log-log linear empirical relationships developed from regression analysis of the fatigue test data. Finally, six DPDT brace to gusset connection specimens incorporating two different levels of roughness at the contacting surface were tested. A set of cyclically increasing forces and displacements was applied to find out the slip load of the bolted joints. Then, two additional sets of similar cyclically increasing forces and displacements but reduced in magnitude subsequently were applied to confirm the cyclically degrading of the slip capacity. This research presents the responses of the bolts and the stiffeners at the brace-to-gusset joints and concludes with the design recommendations for the brace end connections.