Seismic Design Method Of RC Beam-column Joints Strengthened Using Near-surface Mounted CFRP Strips

Reinforced concrete(RC)frame structures are one of the most common forms of industrial and residential buildings.Under horizontal earthquakes,the joint zone is subjected to complex forces and belongs to weak parts.The RC frame joints designed according to the earlier codes have deficiencies,such as insufficient stirrups in the joint core,which do not meet the requirements of the current seismic design code,and are prone to collapse under earthquakes.Therefore,it is necessary to strengthen these substandard beam-column joints.Fiber-reinforced polymer(FRP)is widely used for structural strengthening because of its high strength-to-weight ratio and excellent corrosion resistance.However,there are still some problems for its application.For example,the externally bonded and wrapping FRP method has deficiencies,such as premature debonding of FRP composites from the concrete surface,beam hinges at the column face,and not available for 3D frames due to the presence of transverse beams and slabs.Based on the plastic hinge transfer theory,seismic behavior of RC joints strengthened by near-surface mounted(NSM)CFRP strips was studied.The main research contents and results are as follows:(1)Based on the existing test results of research group,with the strip spacing and the transverse beam as the main parameters,an experimental research of the joint test was carried out,and the performance indexes such as failure modes,the ductility,the energy consumption,the stiffness degradation,the strain distribution and development of beam longitudinal reinforcements and CFRP strips were paid more attention.The test results showed that the use of NSM CFRP strips in beams and joints was found to effectively improve the seismic performance of the joints,and to relocate the plastic hinge away from the column face,thereby leading to a ductile failure mode(beam flexural failure)through reasonable parameter design.(2)Nonlinear finite element models of the joint specimen were established by ABAQUS and verified using the test results.And due to the limitations of the parameter numbers and their variation ranges,the finite element model was used to study the effect of the primary retrofit design parameters such as the strip area,the extension length and the strip spacing.The results show that the capacity increases with the increase of the strip area and concrete strength,and decreases with the increase of the strip spacing.(3)Based on the code design formula,with the design concept of equivalent stirrups,thecalculation method for CFRP strips was proposed,and the influence coefficient ? was introduced to consider the effective strain.On this basis,the seismic design method of RC beam-column joints strengthened using near-surface mounted CFRP strips was proposed,which can be adopted for application.