| Transfer frames are often located in key locations of structures,bear:ing huge vertical and horizontal loads.The whole structure will capsize or collapse once the transfer frame is damaged,which could result in serious casualties and huge property losses.Under earthquake action,the discontinuity of the upper and lower structures of the transfer frame results in a sudden change of stiffness,which often leads to serious stress concentration and elastic-plastic deformation concentration.Therefore,in structural design,the requirement for the bearing capacity and seismic performance of the transfer frame is much higher than that of the conventional frame structure.Concrete filled square steel tubular(CFST)columns and steel reinforced concrete(SRC)beams have been proved to have high bearing capacity and ductility.For the existing engineering practice and literature research,there are few reports on the application of such components in the transfer frame.Therefore,the seismic performance of transfer frames with concrete filled square steel tubular columns and steel reinforced concrete beams is studied in this paper.Ten test specimens were designed and fabricated,and vertical loading and cyclic loading tests are carried out to examine the static and seismic behavior of the proposed structures.On the basis of the experimental study,the numerical analysis model of the structure is established,and the mechanical mechanism and the parameter influence are studied in depth.Based on the results of tests and numerical analysis,the restoring force model of the structure is established,and the seismic design method based on energy balance is proposed.The main research results are as follows:(1)The structural response under vertical loads is studied.The crack development pattern,steel strain variation law,structural moment distribution characteristics and deformation of SRC beams are analyzed.The results show that bending cracks first appeared at the mid-span section of the SRC beams,and then shear cracks appeared at the 1/3 beam span section.The maximum crack width is about 0.2 mm,and the deflection of the SRC beam is about 1/600 of the beam span;the strain of the steel tube is directly related to the axial loads imposed on the structure,and the greater the axial pressure ratio,the higher the load-bearing ratio of the steel tube;the measured moment of the structure is slightly different from the calculation results of the moment distribution method.The measured mid-span section moment of the beam is about 13.8%lower than the calculated moment,and the measured moment at the bottom of the CFST column is about 20%lower than the calculated moment.(2)Cyclic loading tests were carried out for 10 test specimens.The failure process and failure modes of the structure were observed.The hysteretic curve,skeleton curve,bearing capacity degradation,stiffness degradation,ductility and energy dissipation capacity of the structure were analyzed in detail.The main conclusions are as follows:1)during the loading process,shear cracks are mainly the developed cracks on SRC beams,and plastic deformations were observed at the CFST column ends after peak state.The failure modes observed among the 10 tested specimens are column hinge failure mode,column hinge-beam hinge failure mode and column hinge-column tension mixed failure mode;2)the hysteretic curves of all specimens are full and spindle-shaped.However,the influence of axial compression ratio is particularly significant,the influence of beam-column linear stiffness ratio is second,and the influence of other parameters is relatively small.The displacement ductility coefficients of all specimens are greater than 3.0,and the equivalent viscous damping coefficient is in the range of 0.258-0.463;3)the inter-story displacement angle at yielding is in the range of 1/108?1/146,and that at the ultimate state is in the range of 1/28?1/47,which is much higher than the code limit value.In addition,the mid-span deflection of the beam at the ultimate state is about 1/100?1/150 of the beam span,which shows a good anti-deformation ab ility.(3)The numerical simulation and mechanical mechanism of CFST column-SRC beam transfer frame are studied.A finite element model suitable for structural analysis is proposed.The finite element model is validated by test data in the aspect of failure mode,hysteretic curve,skeleton curve,steel strain,and hysteretic energy dissipation.Further,the strain variation law of the structure is analyzed,and the idealized mechanical analysis model of the CFST column-SRC beam transfer frame is constructed by referring to the truss model recommended in ACI 445R-99(R2015).The verification results show that the idealized mechanical model is accurate and reasonable.(4)Based on the analysis of test data and finite element simulation,the restoring force model of CFST column-SRC beam transfer frame is constructed.The calculation methods of key parameters such as elastic stiffness,ultimate bearing capacity,peak displacement and degenerated stiffness are determined,and the corresponding hysteretic rules are formulated.The restoring force model is used to calculate the skeleton curve and hysteretic curve of the structure,and the results show that the calculated skeleton curve and hysteretic curve are in good agreement with the measured curve.This proves that the restoring force model can provide reliable calculation data for elasto-plastic analysis of CFST column-SRC beam transfer frames.(5)Based on the analysis of seismic input energy and structural dissipated energy,a plastic seismic design method of CFST column-SRC beam transfer frame based on energy balance is established.Meanwhile,the relationship between component deformation and overall structural deformation is clarified by using the idea of structural deformation deconstruction.The component deformation is regarded as the control objective of the performance design.The design process embodies the idea of performance based seismic design of "low elastic bearing capacity,high ductility" or "high elastic bearing capacity,low ductility".In addition,based on the three failure modes of cyclic Loading test of CFST column-SRC beam transfer frame,four examples are designed,and the proposed seismic design method is used to design the example structure.The results show that the proposed seismic design method is efficient,convenient and practical. |
PDF Full Text Request