Study On Moment Redistribution Of Concrete Frame Beam With High Strength Hot Rolled Ribbed Bars

The application of high strength hot rolled steel is an important way to achieve energy conservation and emission reduction in concrete structure engineering,which affords an easier way to save resource and reduce environmental pollution.HRB500steel reinforcement has been accepted as a main reinforcement in"code for design of concrete structures"（GB50010-2010）.At the same time,HRB600 steel reinforcement has also been included in"Steel for the reinforcement of concrete-Part2:Hot rolled ribbed bars"（GB1499.2-2013）.Value of moment redistribution coefficient is based on the test results of continuous beam and frame with HPB235 and HRB335 steel reinforcement in"specification for design of reinforced concrete continuous beams and frames considering redistribution of internal forces"（CECS51-93）.There are new characteristics of moment redistribution of concrete continuous beam with HRB500steel due to the increase of yield strength of steel bars.Firstly,the process becomes longer which is from the tensile concrete turning into plasticity and the extreme tensile fiber of concrete cracking to the steel bar yielding.The moment redistribution of this stage increases.Secondly,since the yield strength of longitudinal steel is improved,the plastic hinge appears to be delayed and the plastic hinge rotation capacity decreases which is unfavorable to moment redistribution.Thirdly,since the yield strength of longitudinal steel is improved,the effect of strain penetration in the anchorage area will increase obviously.There are important theoretical significance and practical value to study the regulation of moment redistribution of reinforced concrete frame with high strength hot rolled bars.Six single layer two-span frames were tested in this paper.Longitudinal reinforcement included HRB500 steel reinforcement.Grade of concrete strength included C40,C50 and C60.Relative depth of compression included 0.1,0.2,0.3 and0.4.The dimension of the cross-section of columns included 250mm×250mm,250mm×350mm,250mm×450mm.The dimension of the cross-section of beams was180mm×300mm.Three-dividing-point symmetrical loading mode was adopted in the experiment.The strain gauges were uniformly arranged in the height of column section and on the outside of the column within 1.5h₀ with 40mm spacing,in order to investigate the development rules of tensile strain of longitudinal tensile reinforcement bars in the height of column section and beam end plastic hinge area during the loading process.Yield curvature（φ_y）is the curvature when tensile steels yield.Ultimate curvature（φ_u）is the curvature when pressure edge concrete reaches the extreme strain.The length of the longitudinal tensile steel strain not lower than the yield strain is the actual plastic hinge length when the beam end control section reaches ultimate bearing capacity state.The area of plastic curvature distribution curve in actual plastic hinge area is equal to the area of the rectangle that height（φ_u-φ_y）.The length of the rectangle is the equivalent plastic hinge length.The moment redistribution of the reinforced concrete frame with high strength hot rolled steel is divided into two stages:the stage before longitudinal reinforcement yields and the stage after longitudinal reinforcement yields.β_I is the moment redistribution coefficient of the stage which is from the concrete entering the tensile plasticity to the longitudinal reinforcement yield.Through the analysis of various affecting factors,it is obvious to know that with the increase ofθ_y,the decrease ofξand the increase of the yield strength of steel bar（f_y）,moment redistribution of the first stage（β_I）increased.Based on experimental results,calculation formulas ofβ_I considering yield strength of reinforcement,relative depth of compression zone andθ_y were proposed.β_II is the moment redistribution coefficient of the stage which is from longitudinal reinforcement yield to the concrete reaching the compression strain.With the increase ofξof the section of support end,the increase of the yield strength of steel bar（f_y）and the decrease of appendix plastic rotation angle,moment redistribution of the second stage(β_II)decreased.Based on experimental results,calculation formulas ofβ_II considering yield strength of reinforcement and plastic rotation angle were proposed.Lastly,the practical formula of bending moment redistribution coefficient was proposed considering relative depth of compression zone（ξ）and the relative anchorage length(l_ah/d)of the tensile bar as independent variables for practical engineering design applications.