Study On Moment Redistribution In RC Continuous Beams And Frames With High Strength Hot Rolled Bars As Tensile Longitudinal Reinforcement

High-strength hot rolled steel bar mainly refers to the ones with strength grades of 500 MPa and 600 MPa.Yield strength of high-strength hot rolled steel bar is significantly improved compared with that of 235 MPa,335 MPa and 400 MPa classes steel bars.In the reinforced concrete(RC)continuous beams and frame beams with high-strength hot rolled steel bars as longitudinal tensile reinforcement,the process of moment redistribution before the yielding of critical section becomes longer.Meanwhile,when the ratio of neutral axis depth to effective depth of the critical section is constant,the yield curvature is increased and the formation of plastic hinge is delayed.The process of moment redistribution from the yielding of critical section to the flexure failure becomes shorter.Moreover,the strain penetration effect of high-strength longitudinal tensile reinforcement anchored in beam-column connections is further enhanced,which make the additional fixed-end rotations increase and is beneficial to the moment redistribution in frames.In the light of above,it is of great theoretical significance and practical value to study the moment redistribution in RC continuous beams and frames with high strength hot rolled steel bar as longitudinal reinforcement.The following studies were carried out:(1)For the RC continuous beams and frames with high strength hot rolled steel bar as longitudinal reinforcement,considering that the process of moment redistribution before the yielding of critical section was longer and the process of moment redistribution from the yielding of critical section to the flexure failure becomes shorter,the idea of diving the moment redistribution into two stages was proposed,according to whether the plastic hinges were formed.(2)24 two-span RC continuous beams with HRB500 and HRB600 steel bars as longitudinal reinforcement were tested for moment redistribution.The strength grades of concrete included C40,C50 and C60.The relative depth of compression zone of the critical section was 0.1,0.2,0.3 and 0.4.The widths of the intermediate support refer to 100 mm,150mm,200 mm and 250 mm.From the test results,it can be known that the degrees of moment redistribution before the formation of plastic hinge were between 15.3% to 24.2%,which was accounted for 37.7%-74.0% of the total degrees of moment redistribution.It was indicated that the moment redistribution before the formation of plastic hinge could not be ignored.The degrees of moment redistribution from the yielding of critical section to the flexural failure were between 6.91% to 30.30%,which was accounted for 26.0%-62.3% of the total degrees of moment redistribution.Based on conjugate beam theory,a simulation program of moment redistribution in RC continuous beams was developed.An expanded parametric study of 336 simulating beams was conducted to investigate the variation regular of moment redistribution.The strength grades of longitudinal tensile reinforcement included 400 MPa,500MPa and 600 MPa.The strength grades of concrete was between C20 to C80,the width of the intermediate support was from 100 mm to 400 mm,the relative depth of compression zone of the critical section was between 0.1 to 0.4 and the ratio of span to depth was between 8 to 24.The loading forms included single point load at the middle span,three-point load and uniform load.It was found that with the improvement of yield strength of reinforcement,the degrees of moment redistribution before the formation of plastic hinge were increased and those after the formation of plastic hinge were decreased.With the increase in the width of the intermediate support and the decrease in the ratio of span to depth and the relative depth of compression zone,the degrees of moment redistribution in both of the two stages were increased.A uniform calculation method for the degree of moment redistribution in RC continuous beams was established that considering the influence of the above key parameters.(3)To investigate the influence of the strain penetration of high-strength reinforcement anchored in beam-column connections on the rotation of the critical section,30 beam-column connection RC specimens with HRB500 and HRB600 steel bars as tensile longitudinal reinforcement were tested with static loadings on the free end.The experimental results indicated that due to the strain penetration of high-strength reinforcement,the additional fixed-end rotations at the yielding state(θy,slip)and limit ultimate state(θu,slip)were increased linearly with the increase in the yield strength of tensile longitudinal reinforcement,increased as a power function tendency with the increase in the relative anchorage length of the anchored reinforcement,and decreased as a power function tendency with the increase in the concrete compressive strength.Moreover,with the increase in the relative depth of compression zone,a power function increase tendency of θy,slip and a power function reduction tendency of θu,slip were observed.The calculation formulas of the additional fixed-end rotations at two characteristic states were proposed.(4)12 single layer two-span RC frames with high-strength hot rolled steel bar as longitudinal reinforcement were tested for moment redistribution.In view of the fact that there were two critical sections in the frame beams,namely,the critical section at interior beam-column connection and the critical section at exterior beam-column connection,a principle for calculation of moment redistribution was proposed.The calculation diagram was adjusted based on the orders of the formation of plastic hinges at the corresponding critical section,and then the object of moment redistribution for each critical section was determined.The test results indicated that the degrees of moment redistribution of test frames before and after the formation of plastic hinge were 10.3%-33.4% and 3.4%-30.5%,respectively,which accounted for 48.1%-81.6% and 18.4%-51.9% of total degrees of moment redistribution.The influence of the additional fixed-end rotations due to strain penetration were all taken into consideration in the two stages of moment redistribution.It was found that both the two stages of moment redistribution were increased with the increase in the additional fix-end rotations.A calculation formula for the degree of moment redistribution in RC frames was established that considering the influence of the yield strength of reinforcement,the additional fixed-end rotation and other relevant parameters.(5)The experimental study and theoretical analysis showed that when the other parameters were constant,the degrees of moment redistribution before plastic hinge forming were increased and the degrees of moment redistribution after plastic hinge forming were decreased in the RC beams and frames.Overall,compared with the RC beams and frames with normal strength reinforcement as longitudinal tensile reinforcement,the total degrees of moment redistribution in those with high-strength reinforcement as longitudinal tensile reinforcement were decreased.