Study On Seismic Behavior Of Concrete Frame Columns With High-Yield-Strength Rebars

The application of high-yield-strength rebar in concrete structures is favorable to enhancing the development level of reinforcement and has great significance for the sustainable development of China. However, the current researches on seismic behavior and design method of reinforced concrete structural members with high-yield-strength rebars are not sufficient. In this paper, the seismic behavior and designing issues of concrete frame columns employing high-yield-strength longitudinal rebars and stirrups are experimentally investigated and synthetically analyzed. The main research contents and results are as follows.(1)20concrete columns reinforced with500MPa grade longitudinal rebars and500MPa grade or970MPa grade confinement stirrups were tested under the low frequency cyclic loading with various axial compression ratios. The failure mode, force-displacement(deformation) hysteretic curve, seismic energy dissipation, displacement ductility factor, limiting drift ratio, flexural and axial capacity of normal section for the columns are experimentally studied. The effects of the axial compression ratio, strength of concrete, strength of longitudinal rebars, strength of stirrups, forms of stirrup reinforcement and volumetric ratio pv of stirrups on the seismic behavior of the columns are analyzed as well.(2) Based on the experimental data collected from more than300concrete columns, which are tested under the low frequency cyclic loading, the equations to determine the displacement ductility factor and the limiting drift ratio are proposed, which are related to the trail characteristic value of stirrup reinforcement λvt and the trail axial compression ratio nt.(3) Reinforced with HRB335, HRB400, HRB500grade longitudinal rebars respectively, three concrete frames located in the zone of seismic intensity8(0.30g) were designed according to the current relevant codes. Elasto-plastic time-history analysis of these frames was conducted under the condition of5rarely occurred earthquake ground motions. The analysis results indicate that the frame columns reinforced with high-yield-strength rebars make lower demand on the displacement ductility factor than the frame columns with low-yield-strength rebars. As a result, it is more reasonable to estimate the ductility of frame structure (frame column) by employing the elasto-plastic story drift ratio. Furthermore, the application of high-yield-strength rebars proves to be economical.(4) The test results show that the strain of500MPa grade stirrups has exceeded the yield strain when the drift ratio at top of the column approaching2%, and at the same time, the stirrups stress for most of the columns with970MPa grade stirrups has surpassed700MPa. When load-bearing capacity is reduced by15%, the stress of970MPa grade stirrups, both internal and external, could reach1012MPa, which is the actual measured tensile yield strength. Furthermore, the necessity of using continuous overlapping hoops in the concrete frame columns for the seismic design is experimentally verified and135degrees hooks at both ends of the ties should be arranged.(5) For the frame columns reinforced with high-yield-strength longitudinal rebars, new requirements for the minimum characteristic value of stirrup reinforcement are presented in the closely-spaced region at the column ends, by analyzing the equation of displacement ductility factor proposed in this research. When compared with the"Code for Design of Concrete Structures(GB50010-2010)", the average of presented minimum characteristic value of stirrup reinforcement for earthquake-resistant grade I, II and III increases by46%,45%and32%respectively.(6) The design recommendation of high-yield-strength confinement stirrups for the concrete frame columns is proposed, including the scope of applications of high-yield-strength stirrups, assignment of the design value of tensile strength of stirrups fyv, demands of minimum characteristic value of stirrup reinforcement λv, details of stirrup reinforcement, and so on. The recommendation may provide a reference for the application of high-yield-strength stirrups in the concrete frame columns.(7) The checking calculation can prove necessary to take account of the constitutive relationship of confined concrete when calculating the flexural and axial capacity of normal section of concrete columns reinforced with confinement stirrups. The principle and suggestion for assignment of the design value of compressive strength of longitudinal rebars are also discussed.