Seismic Failure Mechanism Control And Performance Indexes Of RC Frame Structures Based On Beam-column Sub-assemblages

Controlling the failure mode of the frame structure and establishing the performance index that can reflect the structural performance state are of great significance to the achieve the seismic fortification goal of frame structure.The failure mode and performance index of the frame structure are the comprehensive results of the development of the elastic-plastic deformation of each member.The deformation of different members combined in different proportions would have different effects on the failure mode and performance index of the structure.At present,the control method of the failure mode of the frame structure in the Chinese code is mainly based on the strength relationship between the beams and columns,ignoring the essential reasons for the different failure mode of the structure.Meanwhile,for the frame structure,the inter-story drift ratio of 1/50 is used as the performance index of the limit state,ignoring the influence of the deformation of the difference members on its performance.This paper takes the reinforced concrete(RC)frame structure as the research object,through theoretical analysis,experimental research,and finite element simulation analysis,the failure mode control method and performance index are analyzed based on the development of the deformation of members.The main conclusions are as follows:(1)For the frame structure with the failure mode of "strong column and weak beam",the deformation capacity and energy dissipation capacity of that are significantly improved.Meanwhile,the deformation contribution of beam occupies the main part,which is always above 45%,and it increases with the increase in the inter-story drift ratio,and is always above45%.The deformation contribution of column accounts for a relatively small proportion,which is always below 33%,and it gradually decreases with the increase in the inter-story drift ratio.For the frame structure with the failure mode of "strong beam and weak column",the deformation contribution of beam and column show opposite development trend.The deformation contribution of beam is basically about 25%,and that of column is basically about 50%.The deformation contribution of joint is small in both failure modes,and it is basically distributed between 8.21% and 12.36% when the structure failed.For the frame structure with the failure mode of "strong column and weak beam",changing the design parameters,its deformation composition,deformation capacity and energy dissipation capacity will change to a certain extent,but the change range will not be large.(2)The calculation methods for the bending deformation of beams and columns,the shear deformation of joint and the sliding deformation of longitudinal bars are developed.Considering the failure mode,the interlayer deformation of the frame structure is equivalent to the lateral displacement deformation of the beam-column sub-assemblage,and its calculation method is established considering the effect of failure mode.The theoretical calculation results are in good agreement with the test results.(3)Taking the beam-column linear stiffness ratio as the control parameter,from the perspective of member deformation,the control equations of the generalized yield mode and failure mode of the frame structure are established.The finite element software ABAQUS is used to establish the frame structure model under different seismic fortification intensities.The control equations of failure mode are used to adjust structural design parameters.It is found that the distribution of plastic hinges at the column ends of the original structure is relatively scattered,and the curvature ductility coefficient of some column ends is similar to that of the beam ends,and the structure presents a "hybrid-column hinge" failure mode.After adjustment,the number of plastic hinges at the column ends is reduced,and their curvature ductility coefficients are significantly reduced.The plastic hinges of columns are mainly formed in the 1-2 storeys,and the structural failure mode is transformed into a "hybrid-beam hinge" mode.(4)The performance level of the frame structure and the performance state of the member are determined.Considering the difference in the deformation contribution of different members under different failure modes,the performance level of the structure and the performance state of the members are linked,and the deformation development degree of members under each structural performance level was determined.Then,the structural storey performance index and the overall structural performance index under different failure modes were established.The differences between the two performance indexes in the structural performance evaluation were compared and analyzed.It is found that the accuracy of the structural storey performance index for evaluating the structural performance level is relatively low,but this index is easy to obtain and can be easily applied in practical engineering.The overall structural performance index can more clearly indicate the performance level of the structure.(5)Quasi-static tests were carried out on three RC plane frames.It is found that the specimens showed a typical "beam hinge" failure mode,and their inter-story drift ratio all exceeded 1/40.For the specimen F1 with decreased axial compression ratio and beam-column linear stiffness ratio,the inter-story drift ratio reaches 1/25.The deformation is mainly concentrated in the beam and column ends in the first storey.In addition,the deformation degree of the bottom members is generally larger than that of the upper members.With the axial compression ratio decreased,the deformation degree of the beam end increases.The deformation of the column end of the structure with small beam-column linear stiffness develops more quickly,and the ductility and deformation capacity of the structure decreased.(6)The MODF model of the frame structure is established,and it is equivalent to the SODF system.Through the single-degree-of-freedom equivalent linearization model,the calculation method of the deformation response of the frame structure under different seismic intensities is established.The influence of static and dynamic characteristics on the deformation response of the structure is analyzed.With the increase in the ductility,the deformation capacity of frame structure is significantly improved,but its relative load resistance reduces to a certain extent,and degree of dispersion caused by the uncertainty of ground motion increase.With the decrease in the initial stiffness,the deformation capacity of the structure is improved,but its bearing capacity decreases.The deformation capacity of the structure increases with the decrease in the post yield stiffness coefficient,but its bearing capacity slightly reduces.The post yield stiffness coefficient has little effect on the degree of dispersion caused by the uncertainty of ground motion.