Seismic Performance On The Buckling-restrained Braces And Buckling-restrained Braced Frames With Low-yield-point Steel

Buckling-restrained braces(BRBs) can solve the buckling problem of common braces under cyclic loading during earthquake and have stable bearing capacity. Low-yield-point(LYP) steel has low initial yield strength along with high ultimate-to-yield strength ratio and good ductility. BRBs using LYP steel as the inner core could yield before the major structural members and dissipate earthquake energy through large plastic deformation. As a result, BRBs with LYP steel could protect the main structure, which is just like the fuse. Structural system of buckling-restrained braced frames(BRBFs) with LYP steel is a type of the efficient earthquake-resisting structural systems. Based on the behavior of LYP steel, seismic performance of BRBs and BRBFs with LYP steel was systematically investigated and evaluated in this dissertation. Meanwhile design suggestions were proposed. The following six sections were contained in this dissertation,(1) Behavior of 20 specimens made of LYP steel was tested under monotonic and cyclic loading. Combined hardening parameters were calibrated and the values were verified. This work provided basic data for calculating and analyzing of LYP steel(Chapter 2).(2) Hysteretic tests of BRBs with LYP steel were completed. The data in Chapter 2 were adopted in the numerical simulation of test specimens in ABAQUS. Through results of 86 BRB numerical examples, design suggestions were proposed for the key geometric parameters of BRBs(Chapter 3).(3) By analyzing the test results and numerical simulation results of BRBs in Chapter 3, characteristics of their load–displacement curves were summarized. Based on these, the equivalent model for BRBs was proposed. Multi-scale braced frame model with high precision finite element for BRBs and simplified braced frame model with equivalent model for BRBs were compared through static and dynamic analysis. The simplified braced frame model was verified by comparing the simulating results with a shaking table test(Chapter 4).(4) 408 series of time history analysis were carried out including 9 BRBFs with LYP steel, 18 commonly braced frames and 18 BRBFs with conventional mild steel and 6 bare frames. The performances of different structures were compared throughmember states, story shear forces and story drifts. By incremental dynamic analysis of BRBFs with LYP steel, the working mechanism of dual seismic system was investigated(Chapter 5).(5) 3024 series of time history analysis for 51 structures of BRBFs with LYP steel and 6 bare frames were carried out. Different performance levels were defined based on story drift angles and member damage index. Seismic performance evaluation and seismic fragility analysis were conducted for BRBFs with LYP steel. Structures with braces of different stiffness and forms were compared as well as structures with different stories. Curves of story drift angles and member damage index with earthquake intensity were provided to predict the performance level under given earthquake intensity(Part of Chapter 6).(6) Design method for BRBs with LYP steel was proposed taking stiffness degradation of LYP steel into consideration. Values of overstrength factor were suggested for BRBs with LYP steel. The minimum brace stiffness was listed according to different performance levels for different structural types. So that practical suggestions were provided for the performance-based design method of this kind of structural system(Part of Chapter 6).