Study On Bearing Capacity And Seismic Behavior Of Confined Concrete Columns

The restraint of stirrup can effectively improve the bearing capacity and deformability of confined concrete columns.“Code for Design of Concrete Structures”(GB50010-2010)introduces HRB500 hot rolled steel bars.“Steel for the Reinforcement of concrete-Part 2: Hot Rolled Ribbed Steel Bars”(GB1499.2-2018)introduces HRB600 hot rolled steel bars.“Steel Bars Prestressed Concrete”(GB/T5223.3-2005)includes PC steel bars with tensile strength standard values of800,970 MPa and 1270 MPa.Using HRB500,HRB600 hot rolled steel bar and PC steel bar as stirrup is not only a way to promote the application of relatively high-strength steel bars,but also an effective method to ensure the full play of compressive strength of relatively high-strength hot rolled longitudinal reinforcement in confined concrete columns.On the one hand,in a certain range of volumetric ratio,the application of relatively high-strength stirrup can reduce the amount of stirrups and simplify construction.On the other hand,the restraint of relatively high-strength stirrups on confined concrete columns can effectively improve the deformability of confined concrete columns.For confined concrete columns under axial compression or under seismic load,the engineering community concerned two hot issues.One is whether the stirrup yield strength can be fully exerted at peak load and the ultimate load,another one is how to determine the stirrup stress if the stirrup could not yield.With the application of using PC steel bar as stirrup in engineering,the seismic performance and seismic design of concrete columns confined by PC steel bars have gradually become a concern of the engineering community.Therefore,it is of practical engineering value to carry out the bearing capapcity calculation and seismic performance study of concrete columns confined by relatively high-strength stirrup.This paper mainly focuses on the following studies:(1)64 concrete columns confined by PC steel bars were tested under axial compression.The strength grade of concrete included C50,C60,C70 and C80.The volumetric ratio included 0.9%,1.2%,1.6% and 2.0%.The standard tensile strength of stirrup included 800 MPa and 970 MPa.The stirrup configuration included circular spiral and square transverse reinforcement.The test results indicated that the high-strength circular concrete column confined by PC steel bars under axial compression exhibited bulging and shear sliding failure mode.For the columns with determined concrete compressive strength and stirrup tensile strength,the confined concrete columns with lower volumetric ratio exhibited shear sliding failure mode,while the confined concrete columns with higher volumetric ratio exhibited bulging failure mode.Based on the experimental results,the formula of index was established to discriminate the two failure modes.It was found that with the increase in unconfined concrete compressive strength and volumetric raito,the stirrup strain at confined concrete compressive strength decreased.The stirrup strain at confined concrete compressive was larger in circular spiral than that in square transverse reinforcement.A unified formula of stirrup strain at confined concrete compressive strength was established considering unconfined compressive strength,volumetric ratio and stirrup configuration reasonably.The formula of calculating maximum volumetric ratio at which stirrup can yield at the confined concrete compressive strength and the value of maximum volumetric raito was proposed.(2)Based on the experimental results of 115 concrete columns confined by HRB500,HRB600 hot rolled bar and PC steel bar under axial compression,the influence of unconfined concrete compressive strength,volumetric ratio,stirrup yield strength and stirrup configuration on bearing capacity and deformability of concrete columns confined by HRB500,HRB600 hot rolled bar and PC steel bar were observed.The formulas of peak stress and peak strain which comprehensively consider the effect fo unconfined concrete compressive strength,volumetric ratio,stirrup yield strength,stirrup configuration,section size,stirrup spacing and stirrup distance were proposed.The stress-strain relationship of concrete columns confined by HRB500,HRB600 hot rolled bar and PC steel bar was established and the calculating method of axial compressive bearing capapcity of concrete columns confined by high-strength stirrup was proposed.This stress-strain relationship reasonably consider the relatively low lateral deformability of high-strength concrete.This relationship also consider the using HRB500,HRB600 hot rolled steel bars and PC steel bars as stirrup need relatively large lateral deformabili ty if the stirrup yield.Based on 21 experimental results of full-scale confined concrete columns under axial compression,the influence of section form,section size,stirrup spacing,longitudinal reinforcement spacing were reasonably considered by introducing the constraint efficiency coefficient.It concluded that for full-scale confined concrete columns under axial compression,the restrictions of GB50010-2010 and ACI 318-19 on the maximum stirrup spacing in confined concrete columns can be relaxed.The contribution of stirrup to bearing capacity of confined concrete column can be calculated according to the specific stirrup spacing.(3)15 concrete columns confined by 800 MPa steel bars with longitudinal reinforcement of HRB500 were tested subjected to lateral cyclic loading.Shear span ratios included 2,2.5,3,the test axial compression ratio included 0.1,0.2,0.3,and the strength grade of concrete was C50.The longitudinal reinforcement ratio was0.85%,0.95%,1.15%,and the volumetric ratio were 1.1%,1.47%,1.96%.It was found that the strength of stirrup was not fully exerted when the specimens reach the peak load.The stirrup were still not broken when the specimens reach the ultimate displacement(the displacement when the load drops to 85% peak load).The specimens showd excellent bearing capacity and deformability in the whole loading process.