Study On Seismic Behaviors Of Existing Reinforced Concrete Frame Strengthened With Self-Centering Members

The construction of resilient city is an important development direction of urban transformation and upgrading in recent years.In order to achieve the construction purpose of resilient city,China has taken a series of measures to deal with the resilience upgrading of existing buildings.As an integral part of existing buildings,existing reinforced concrete frame structure buildings have a large number of holdings in China.Most of them are difficult to meet the requirements of resilient city due to long completion time and serious structural aging.At present,the main measures to improve the toughness of existing buildings before earthquakes are planning,monitoring,prevention and early warning.Among them,seismic reinforcement and reconstruction is a major means of pre-earthquake prevention.Under the premise of strictly following the design of earthquake prevention and disaster reduction specifications,the structural toughness is improved by strengthening the existing buildings.Although the traditional seismic reinforcement method can improve the seismic bearing capacity,stiffness and ductility of the structure to varying degrees,it is difficult to ensure that the structure is slightly damaged under the fortification earthquake or rare earthquake,and the deformation can be restored independently,so it is difficult to meet the requirements of building seismic resilience.In view of the above problems,this paper proposes a new method of seismic strengthening of reinforced concrete frames by using external assembled self-centering components to improve the seismic resilience of buildings.In this method,the frame beam and column are strengthened by increasing the cross section,and the precast beam members are attached to the side of the reinforced beam.The method of establishing prestress in the precast beam by tensile unbonded prestressed tendons can improve the stiffness and bearing capacity of the reinforced structure and reduce the residual deformation after the earthquake.Finally,the structure to be reinforced without the ability to reset can obtain a certain ability to reset,so as to achieve the purpose of improving the seismic toughness of the structure.In order to verify the effectiveness of the new reinforcement method,three single-story and single-span frame structure test specimens were designed and manufactured based on an existing building in Beijing,and the quasi-static test was carried out.Among them,one specimen was an unreinforced contrast specimen and two specimens were frame structure specimens reinforced with external assembled self-centering members.The results show that the seismic bearing capacity,ductility and energy dissipation capacity of the strengthened specimens are significantly enhanced compared with those of the unstrengthened specimens.At the same time,within a certain interlayer displacement angle range,the residual deformation of the strengthened specimens is significantly smaller than that of the unstrengthened specimens,and the reset ability is significantly enhanced.The finite element simulation analysis of three test specimens was carried out by using ABAQUS finite element analysis software,and the finite element analysis results were compared with the quasi-static test data.The results show that the modeling method can better simulate the displacement load curve of the frame before and after reinforcement,and the structural damage cloud map and stress cloud map can also be better.The observed experimental phenomena are in good agreement.In order to further explore the influencing factors of the seismic performance of the frame after the reinforcement of the external self-centering members,the variable parameter finite element analysis of the test specimens was further carried out.The results show that the external self-centering beam member can reduce the residual deformation rate of the attached frame.Increasing the initial tension of prestressed tendons in the external prefabricated beam can improve the bearing capacity of the reinforced frame and effectively reduce the residual deformation rate of the reinforced frame after the earthquake.Increasing the diameter of the energy dissipation steel bar can improve the stiffness and energy dissipation of the frame.According to the current code,the calculation formula of the ultimate bearing capacity of the normal section of the encased enlarged section flexural member is supplemented.The calculation method of the bearing capacity of the self-centering precast beam member and the calculation method of the bearing capacity of the self-centering member reinforced frame are proposed,and compared with the test specimen results.The calculation error is less than 5 %.