Study On The Flexural Behavior Of The Part Negative Bending Moment Rebars At The End Of Reinforced Concrete Frame Beam After Moving To The Side Floor Of The Beam

In multi-high-rise buildings,frame structures are widely used in structures because of their advantages of improving space utilization rate and flexible functional layout.As the most important superstructure of the frame structure,the frame beam not only transmits the load from floor slab and support beam to the perpendicular bearing member,but also connects with the perpendicular bearing member such as frame column and shear wall to form a joint,to resist together the two-way seismic forces acting on the structure,which plays an important role in ensuring the lateral resistance and seismic performance of the structure.In recent years,the frequent occurrence of earthquakes has seriously threatened the life and property safety of the domestic people,and put forward more strict requirements for the bearing capacity of the members in the structure.In order to meet the requirements of the seismic performance of the joint,the designers arranged two or three rows of longitudinal bars are arranged at the end of the beam,which results in the dense arrangement of the rebars at the at the beam-column joint,so it is difficult to ensure the pouring quality of the concrete in the joint core area,which has a negative effect on the holistic behavior of the structure.In view of the construction quality problems to be solved in the traditional structural design,it is a new idea of some negative bending moment rebars of the beam end are arranged on the floor slab to reduce the number of reinforcement layers.Therefore,through experimental study,numerical simulation and theoretical analysis,this paper deeply studies the influence of this new reinforcement arrangement on the bending performance of reinforced concrete beams,which provides a reference for engineering application.The main work and findings are as follows:(1)Taking the distribution width of negative bending moment rebars at the end of the beam moving to the flange,the ratio of negative bending moment rebars at the end of the beam moving to the flange,and the spacing of transverse distributed rebars as the variation parameters,the monotonic static loading tests were carried out on 9 new reinforcement arrangement beams and 1 traditional reinforcement arrangement beam,to obtain the crack distribution,failure mode,load-mid span deflection curves,bearing capacity eigenvalue,ductility and strain of rebars in pure bending section of mid span.The results show that,compared with the traditional rebars arrangement beam,the beams that some negative bending moment rebars of the beam end are arranged on the floor slab have more cracks and more uniform distribution,more regular transverse penetration cracks at the bottom of the slabs,greater pre-yield stiffness,higher flexural capacity and better ductility,it is shown that this rebar arrangement method can improve the bending performance of frame beam end.(2)By studying the influence of different parameters on the bending performance of the new rebar arrangement beam,the results show that the distribution width and proportion of the beam end negative moment rebars in the flange have obvious influence on the bending capacity.With the increase of distribution width,the shear lag effect between rebars is more obvious,the ultimate bearing capacity increases first and then decreases,and the ductility coefficient increases gradually,with the increase of the proportion of negative moment rebars arrange in the flange,the ultimate bearing capacity is increased first and then decreased,the ductility coefficient decreased gradually.(3)The ABAQUS finite element software was used to model and analyze of the new rebar arrangement test beam,and the failure mode,load-mid span deflection curve and ultimate bearing capacity of the simulated beam are in good agreement with the test values.The accuracy of the model is proved,based on this,the influence of the distribution width of negative bending moment rebars moved to the flange on the bending performance with the new rebar arrangement beam is refined,At the same time,expanded and analyzed the changing parameters such as flange plate thickness,shear span ratio and span-depth ratio,to obtain the influence rules of each parameter on its bending performance.The results show that reducing the shear span ratio and increasing the distribution width of negative bending moment rebars moved to the flange can effectively improve the flexural bearing capacity of the new rebar arrangement beam.However,when the distribution width of the negative moment rebars reaches 6 times flange plate thickness,the tensile shear lag effect is more obvious between the negative moment rebars,if the distributed width of the negative rebars continue to increase,the negative rebars far away from the beam rib unable to yield,the further improvement of the flexural bearing capacity will be limited.(4)Analyze the distribution of the strain along the width direction of the beam end negative moment rebars in the flange.Combined with the test data,the relation formula of the influence of the distribution width and the ratio of the beam end negative moment rebars moving to the flange on the stress of negative bars in the flange is proposed,appropriately modify the formula for calculating the bending capacity of reinforced concrete beams in "code for design of reinforced concrete structures",derive a calculation method for the bending capacity of the frame beams for the new reinforcement arrangement.