The Mechanical Effects Of Strength Joints Between Column And Beam On The Behaviors Of Reinforced Concrete Frame

In reality,if the strength grade differences of reinforced concrete between frame beams and columns are pretty large,usually the column concrete will be poured before the beam concrete will be.At present,it is widely used in many projects that column concrete,which has the same strength grade as the column does,is allowed to flow into the beam at the joints,and that is considered safe enough.However,such method changes structure design to some extent,and forms a strength interface within the beam(the column concrete with higher strength grade is poured at first and then it flows into the beam.After the beam concrete with lower strength grade is poured,a interface will appear between former column concrete and latter beam concrete consequently within the beam.The strength interface here is defined to be near the joints of columns and beams.).It will cause indefinite factors to structure performances.Combined with theoretical analysis and numerical simulation,a calculation model of 6-deck frame is established in this article.According to regular methods in practice,there are 4 models designed here: frame without the strength interface(Model-000,Simultaneous pouring of beam concrete and column concrete),and frame with strength interfaces(Model-200,Model-500 and Model-800,meaning column concrete of higher strength grade flows into the beam for 200 mm,500mm and 800 mm separately.).Firstly,pushover,a static analysis method,is applied to look into how the place of strength interface will influence where plastic hinge appears.Then a numerical analysis model of strength interface is established and compared with experiments in documents,in order to verify the suitability of the dynamic analysis model.Finally,based on the numerical analysis model after verification,time history analysis is used to research the dynamic response differences between models,so as to provide references for future project design.Main research conclusions are listed below:(1)The place of strength interface will directly influence where plastic hinge appears.Plastic hinge will appear further from column side wall in frames without strength interface,which means better structural seismic performances.(2)With Model-500 as standard,all indicators of frames without strength interfaces are superior to ones of frames with strength interfaces,including bearing capacity(yield load,peak load and ultimate load),rigidity and ductility,etc.For frames without strength interfaces,its yield load is 31.54% higher,peak load is 16.06% higher,rigidity is 5.53% higher and ductility coefficient is 3.5% higher than that of frames with strength interfaces.So the existence of strength interface is adverse for structure seismic.(3)Based on the research of the mechanical indexes of 3 kinds of frames with strength interface,It is that with the increase of column high strength concrete,the aseismatic performance of frame increases slightly;therefore,when frame with different strength grades is constructed,It is advisable to construct the node area with different strength grades as a whole.when it is not satisfied with the construction conditions at this time,the length of column concrete is allowed to flow into the beam should not be less than 1 times the height of the beam and the length is best within the range of 2 to 3 times the height of the beam.(4)Based on the research of force-transferring mechanism on the strength interface,a numerical calculation model of strength interface is established.It considers the strength interface as a weak layer with some thickness(maybe 10mm),and defines different material constitutions for interface and non-interface areas.The calculation results of the model in this article match existing experiment results pretty well.So the suitability of the model is verified.(5)Dynamic analysis models of frames with and without strength interfaces(KJ-500 as an example)are established and analyzed separately.Then the first natural frequency of frames without strength interfaces is 0.95768 Hz,and the one of frames with strength interfaces is 0.94857 Hz.So the former is higher than the latter,which means the lateral rigidity of frames without strength interfaces is much higher.(6)According to the dynamic response analysis for frames with and without strength interfaces,top displacements,interstory drifts,accelerations of vertex and base shears are compared between them.Dynamic response of structure is found to be sensitive to seismic wave spectral characteristics input.As for the frame without strength interface,its top displacement and interstory drift is smaller,while its acceleration of vertex,rigidity is larger.The strength free interface frame is higher than the strength interface frame in the aspect of base shear.As a result,the seismic capacity of frames without strength interfaces as a whole is better than that of frames with strength interfaces,according to dynamic analysis;The longer the distance between the high strength concrete and the high strength concrete frame,the better the seismic performance of the frame.