| Adding storey with light steel on the existing RC frame structure is one of the main methods to solve the contradiction between the growth of population and the scarcity of land resources.The connection between the upper and lower structure is the key to seismic design of RC frame light steel mixed structure.Based on the traditional planting joint,a new type of outer hoop assembly joint for this kind of mixed structure was proposed,which aims to strengthen the connection between old and new structure,reduce the stiffness mutation and enhance the overall cooperative seismic capacity of the structure.In order to study the seismic performance of this new type of joint,the low-cycle repeated load test of the new external hoop joint and the exposed rigid joint with the upper structure of box column about 1/2 scale model was carried out,and the seismic performance index was compared and analyzed.At the same time,the finite element model of this new external hoop joint under the low-cycle repeated loading was established based on ABAQUS finite element software.The calculation results were compared with the experimental results to verify the correctness of the model,multi-parameter analysis was carried out based on this model and optimized design recommendations were provided.The main research contents and achievements of this paper include:(1)Experimental study on seismic behavior of joints in light steel adding storey.A new external hoop joint(JD-1)and an exposed rigid joint(JD-2)with box column about 1:2 scale model were designed and manufactured.Its seismic performance under pseudo-static test was compared and analysed.The test results show that:①The final failure modes of the two specimens were the bending and ductile failure of the beam end.The final fracture section of JD-1 was located at the interface between the external hoop steel and the concrete beam.Compared with JD-2,the plastic hinge of the beam end was obviously moved outward.The core area of the joint was effectively protected.②The hysteresis curves of the two specimens had different degrees of "pinch" phenomenon.Compared with JD-2,the energy consumption of JD-1 was increased by 25.93%.The yielding load and limit load were increased by 12.35%and 26.38%respectively.The ductility,the degradation of stiffness and strength were better than JD-2.Overall,the seismic performance of JD-1 is significantly better than JD-2.(2)The correctness verification of the finite element model for the new external hoop joint.Based on the ABAQUS finite element software,the finite element model of the new external hoop joint was established.The low-cycle repeated load test was simulated.The seismic performance index was analyzed according to the calculation results,and the correctness of the finite element model was verified by comparison with the test results.The finite element calculation results show that:the model was consistent with the final failure mode of the test;the error of the load characteristic value and the test result of the model skeleton curve were within 10%;the development trend of the stiffness degradation curve was the same,and the error was within the allowable range.The model of the external hoop joint can better simulate the test results and can be used for subsequent research.(3)Research on related parameters affecting the seismic performance of new external hoop joint.On the basis of the finite element model verified by correctness,the original model modeling process was unchanged.The finite element model multi-parameter study was carried out by changing the axial compression ratio of the column top,adding different lengths of reinforcing steel plates and filling different heights of concrete inside the steel column.The results show that:① With the increase of axial compression ratio,the ultimate bearing capacity and initial stiffness were slightly improved,the yield displacement and the longitudinal stress of the concrete beam were significantly increased,the stress distribution area of the steel column was obviously enlarged,and the stress of anchor bolt and screw were reduced.② The bearing capacity,energy dissipation capacity and overall stiffness of the joint were effectively improved by adding reinforced steel plates on the upper and lower surfaces of concrete beams.At the same time,making the plastic hinges of the beam ends move outward.With the increase of the length of the strengthened steel plates,the plastic strain values and the Mises stress value of beam longitudinals ribs of the concrete were significantly reduced.The Mises stress of the upper steel structure and the anchor were obviously increased.③ The rigidity of the column bottom was effectively enhanced,the Mises stress of the upper steel column and the longitudinal reinforcement of concrete beam was reduced,the sudden change of the stiffness at the joint of the upper and lower structure was reduced,and the ultimate bearing capacity of the joint was improved by filling the concrete inside the steel column.The above research results fully show that:the new external hoop joint can effectively strengthen the connection between the upper structure and the existed structure,significantly improve the seismic performance of the joint.There are important reference values for its seismic performance in the overall frame and the design and construction of the actual adding project in further study.It is recommended that the axial compression ratio of the new external hoop joint is between 0.25 and 0.30,the length of the strengthened steel plate is between 0.6H～0.8H(H represent the height of the concrete beam)and the height of the filled concrete is between 8a/39～12a/39(a represent the height of adding storey),the specific value should be determined according to the actual situation. |
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