Research On Seismic Behavior And Shear Capacity Of Steel Upper Column-SRC Lower Column-RC Beam Composite Joints

The steel structure indoor substation building is widely used in China.The typical structural form is steel reinforced concrete column-reinforced concrete beam composite frame structure at the bottom and steel frame structure at the upper part.Because the steel structure and concrete structure can be constructed in sections,the construction efficiency is greatly improved.In addition,the construction process of steel structure meets the requirements of low pollution,which is the future development direction of substation.The connection between steel columns,steel reinforced concrete columns and reinforced concrete is the key and difficult point in practical engineering design in steel structure indoor substation.At present,there is a lack of research on seismic performance of steel reinforced concrete upper column-steel reinforced concrete lower column(that is,steel reinforced concrete variable column)connection joints at home and abroad,the related mechanical properties are not clear,and there is a lack of targeted specifications to guide practical engineering.In this paper,according to the actual engineering structure,a new type of composite joints connected with steel upper column,steel reinforced concrete lower column and reinforced concrete beam are proposed.The anti-seismic performance and shear capacity of the composite joint under low cyclic loading were studied with the combination of experimental research,finite element simulation analysis and theoretical analysis,to fill the theoretical vacancy in the field of engineering research and provide theoretical guidance for practical engineering construction and design.The main research contents and conclusions are as follows:(1)Four new-type composite joints were designed for low cyclic loading test.The results showed that the failure process of all joints could be divided into elastic stage,working with cracks stage and failure stage.The failure modes of the joints were basically the same,both flexural failure and shear failure occurred in the core area.When the stirrup ratio was small,the flexural failure in the core area of the joints was more serious.The average ductility coefficient of joints was between 3.4 and 4.1,which indicated that the ductility of composite joints was better than that of ordinary reinforced concrete joints and close to that of ordinary steel reinforced concrete joints,and the positive ductility of joints was better than negative ductility.The equivalent viscous damping coefficient of the joints under the maximum load was between 0.17 and 0.19,which indicated that the energy dissipation capacity of the joints was between ordinary reinforced concrete joints and ordinary steel reinforced concrete joints.(2)The finite element model(FEM)of the new composite joint was established,and the accuracy of the model was verified by comparing with the test results,and the validated model was used for parametric analysis.The results showed that the bearing capacity would be reduced with the increase of axial compression ratio,and the failure forms of joints under high axial compression ratio need further study.The change of concrete grade has little influence on the failure mode of joint specimens,which can be ignored.The joints with larger stirrup ratio in the core area have better ductility.(3)The shear contribution value of each shear member in the core area of the joint was studied,and it was determined that the shear force of the joint was mainly borne by the concrete,steel web and stirrups in the core area,and the flexural failure of the concrete in the core area would reduce the contribution of the concrete to shear resistance.In this paper,the adverse effects of flexural failure were reflected by reducing the compressive width of concrete diagonal compression bar,and the shear influence coefficient of concrete in the core area was introduced to reflect the decrease of shear bearing capacity of joints with the increase of axial compression ratio.Finally,the calculation formula of shear bearing capacity of joints was put forward.It has been verified that the difference between the theoretical value of the shear bearing capacity calculated by this formula with the experimental results and the simulation results was about 5%,which indicated that the calculation formula established in this paper has high accuracy.