Seismic Performance And Retrofitting Methods Of Earthquake Resilient Precast Steel-Concrete Composite Joint

Under the goal of carbon peak and carbon neutrality,one of the development directions of China’s construction industry is to develop industrialized,energy-saving and post-earthquake restorable prefabricated buildings.Connection failure and insufficient ductility are important factors limiting the improvement of seismic performance of structures.The research and development of building structural connections with good seismic performance is of great significance to promote the wide application of prefabricated buildings.In this paper,an earthquake resilient precast steel-concrete composite joint was proposed,and its seismic performance under seismic loads was studied.The post-earthquake retrofitting methods of the proposed joint under different damage degrees were proposed,and the post-earthquake damage assessment methods of the joint and its key part were carried out.The main research contents and results of this paper are as follows:(1)An earthquake resilient precast steel-concrete composite joint was proposed.The precast concrete-filled steel tube column and RC beams strengthened at the beam end are spliced together by unbonded prestressed tendons and ED steel bars.As a simple energy dissipation device,the ED steel bar could be installed and replaced conveniently.The prestressed tendons could reduce the residual deformation of the structure under seismic loads.The concrete-filled steel tube column and RC beam strengthened at the beam end could reduce the damage of the joint core area.The proposed joint meets the design concept of strong column and weak beam.Eight specimens of the proposed precast joints with different parameters were designed and fabricated,and the low cycle loading test was carried out.The failure mode of the proposed precast joints was revealed through the test phenomenon.According to the obtained hysteretic curves,the seismic performance indexes such as ductility,stiffness and energy dissipation capacity of the specimens were compared and evaluated.The tensile force of prestressed tendons,the strain of precast beam reinforcement and the composition of column top displacement were analyzed.The test results showed that the damage degree of other parts was low except the ED steel bar and plastic hinge area of beam.The hysteretic curves were full and had the characteristics of anti “S” shape.The displacement ductility coefficient of all specimens was greater than 5,which was better than that of the steel-concrete composite structures.The energy dissipation capacity could reach more than twice that of RC beam-column joints.The proposed precast joints generally showed good seismic performance.(2)Through finite element simulation,the failure mode and the stress,deformation and damage distribution of each part of the earthquake resilient precast steel-concrete composite joint were deeply studied,and the influence of various factors on the mechanical performance of the joint and each part was studied through parameter analysis.The detailed finite element model established by using solid elements considered the material nonlinearity,application of bolt preload,contact between the members,application of initial prestress and so on.The comparison between simulation and test results showed that the proposed finite element model could simulate the failure characteristics and load-displacement relationship of the precast beam-column joints.The internal force distribution,deformation distribution and damage degree of each part of the precast joints were further analyzed.The effects of different axial compression ratios,diameters of ED steel bar,wall thicknesses of steel tube,concrete strengths of precast beams,initial prestress of prestressed tendons and types of the precast columns on the seismic performance of the precast beam-column joint were systematically analyzed.The results showed that the axial compression ratio,the concrete strength of precast beam and the diameter of ED steel bar would affect the failure mode of the precast beam.The wall thickness of steel tube and the type of the precast column could directly affect the shear deformation of joint core area.And the increase of initial prestress could significantly improve the bearing capacity of precast joint.(3)Based on the test and simulation results of the earthquake resilient precast steel-concrete composite joint,the calculation methods of bearing capacity under the feature points,skeleton curve model and restoring capacity model of the precast joint were proposed.According to the failure characteristics and force transfer mechanism,the ultimate bearing capacity formulas under the feature points were deduced,and the three fold skeleton curve model of the precast joint was proposed.Based on the test results,the hysteretic rules were formulated according to the hysteretic characteristics.The stiffness degradation law was obtained by fitting method.The restoring force model of the precast steel-concrete composite joint was established.By comparison between theoretical and experimental results,it indicates that the bearing capacity calculation methods and the restoring capacity model of the proposed precast joint had relatively high accuracy,and could predict the force-deformation relationship of the precast steel-concrete composite joint under seismic loads.The proposed theoretical model could provide theoretical basis and design guidance for engineering design.(4)Based on the philosophy of earthquake resilience,the rapid restoring method of the proposed precast joint with slight damage and four different reinforcement methods of the joints with moderate or serious damage were proposed and studied,and the specimens before and after retrofitting were tested and comparative evaluated under low cycle loading.The precast steel-concrete composite joints with slight damage could be restored rapidly by replacing the ED steel bars.The results showed that the rapid restoring work could be completed within 30 minutes.The specimens after restoring still had the same bearing capacity as before,and the ductility and energy dissipation capacity had been significantly improved,which verified the feasibility of the proposed rapid restoring method.The seismic performance of moderately and severely damaged precast joints before and after steel reinforcement,CFRP reinforcement,composite plate reinforcement and combination reinforcement of CFRP and composite plate were compared.The results showed that the seismic performance of the composite plate strengthened joints was basically the same as that of the original joint,the other three reinforcement methods had significantly improved in bearing capacity,ductility and energy dissipation capacity,which proved that the proposed precast joints had good earthquake resilience and application prospect in seismic area.(5)Based on fiber grating and ultrasonic guided wave sensing technology,the damage assessment methods of the earthquake resilience precast steel-concrete composite joints and the ED steel bars were proposed,which could guide the formulation of retrofitting scheme of the post-earthquake precast structure.The FBG sensors arranged on the ED steel bars to realize the deformation monitoring of the ED steel bar in the structure under earthquake.It could assess the stress state of the ED steel bars after earthquake and whether the joints need to be restored.Based on ultrasonic guided wave nondestructive testing technology,firstly,the finite element analysis of the ED steel bar was carried out.The propagation characteristics of guided wave in the ED steel bar were analyzed,and the influence of different type damages on guided wave received signal was explored.Secondly,the damage monitoring test of the ED steel bars in the precast joints under low cycle loads was carried out.The assessment methods of the damage indexes based on correlation coefficient and energy were proposed.The relationships between damage indexes and drift ratios were established,and the damage diagnosis and assessment of the ED steel bars and the precast joints were realized by setting the damage limiting values.It also could provide the reference for the post-earthquake retrofitting scheme of precast steel-concrete composite joints.