| Compared with the conventional embedded column base and exposed base plate column base for the concrete filled steel tube(CFST)piers,the anchor rebar columnfoundation connection(SAC)employs anchor rebars to be embedded into the foundation to transfer the load instead of the steel tube,which could alleviate the construction difficulties,reduce the labor-consuming,and improve the construction efficiency.To ensure a more reliable load transfer,additional shear hoop rings(SAC-H)or PBL shear connectors(SAC-P)may be adopted.Moreover,anchor rebars could be directly welded on the surface of steel tube to transfer the axial load(SAC-W).CFRP is wrapped around the steel tube to restrict the tube bulge in the case where the diameterto-thickness ratio of steel tube exceeds 120(SAC-C).To improve the load bearing capacity,stiffness,and anti-impact capacity of piers,the reinforced concrete(RC)component could be set around the CFST pier(CECC).To date,research on the unembedded column base for the concrete filled thin-walled steel tube piers is limited,and the design method is still inexplicit.To this end,the seismic performance and design method of six kinds of column base for the concrete filled thin-walled steel tube piers are investigated in this paper.The main research contents are summarized as follows:(1)Experimental studies were conducted on the seismic performance of 3 SACs,4SAC-Hs,and 1 embedded column base with the key parameters of reinforcement ratio,configuration of shear hoop rings,and axial load ratio.Based on the test results,the influences of tested parameters on the failure mode,load-displacement hysteresis/skeleton curves,load bearing capacity,ductility,strength degradation,stiffness degradation,and energy dissipation capacity were investigated.The test results indicate that the failure mode of all the column bases is characterized by bending failure mode with good anti-seismic performance.The reinforcement ratio is the main parameter influencing the load bearing capacity.The seismic performance of SACs is similar to that of the embedded column base when designed to satisfy the strength equivalence principles.(2)The finite element model(FEM)was developed on the basis of existing test results,and the predicted results were in good agreement with the test results,on the basis of which a parametric study was conducted.The analysis results indicate that the yield strength and reinforcement ratio of anchor bars,concrete strength,length-todiameter ratio,axial load ratio,thickness and height of the outer RC component,and the yield strength and reinforcement ratio of anchor rebars within the RC component are the main parameters influencing the load bearing capacity of the column base.The lengthto-diameter ratio and the thickness and height of the outer RC component are the main parameters influencing the elastic stiffness.In addition,the concrete strength,axial load ratio,and thickness of the outer RC component have significant influence on the stiffness of descending branch.(3)Modifications were made to the calculation methods for the sectional strength in existing codes,based on which the calculation methods for the sectional strength of SAC,SAC-H.SAC-P,SAC-W,and CECC were proposed.The predicted results were in good agreement with the test results and FEM results.The theoretical calculated model for the elastic stiffness of concrete filled thin-walled steel tube piers adopting these column bases was developed considering the stiffness of the cross-section,the predicted results of which coincided well with the FEM results.Based on the parametric analysis,a simplified model for the calculation of elastic stiffness was proposed.(4)A novel type of SAC wrapped with CFRP(SAC-C)was proposed for the concrete filled thin-walled steel tube pier.Experimental studies were conducted on the seismic performance of 6 SAC-Cs and 3 SACs with the key parameters of CFRP layers and diameter-to-thickness ratio of steel tube.Based on the test results,the influences of tested parameters on the failure mode,load-displacement hysteresis/skeleton curves,load bearing capacity,ductility,strength degradation,stiffness degradation,and energy dissipation capacity were investigated.The test results indicate that the failure mode of all the column bases is characterized by bending failure mode.Bulging of steel tube is observed in SACs with diameter-to-thickness ratio larger than 120,which is effectively restricted by wrapping CFRP.Wrapping CFRP could slightly improve the load bearing capacity and significantly improve the deformation behavior.The diameter-to-thickness ratio of steel tube is the main parameter influencing the load bearing capacity when the anchor rebars are designed to satisfy the strength equivalence principles.(5)The FEM was developed for SAC-Cs,and the predicted results were in good agreement with the test results,on the basis of which a parametric study was conducted.The analysis results indicate that the yield strength and reinforcement ratio of anchor rebars,concrete strength,length-to-diameter ratio,and axial load ratio are the main parameters influencing the load bearing capacity of the column base.The length-todiameter ratio is the main parameter influencing the elastic stiffness.In addition,the axial load ratio has significant influence on the stiffness of descending branch.The stiffness of descending branch is improved when CFRP is adopted,however,the influences of layers and height of CFRP are limited.(6)The existing calculation method for the sectional strength of SAC-Cs was verified against the test results.The predicted results were in good agreement with the test results and FEM results.The contribution of CFRP to the elastic stiffness can be ignored when calculating the elastic stiffness of concrete filled thin-walled steel tube piers adopting SAC-Cs.Thus,identical calculation methods for the elastic stiffness of concrete filled thin-walled steel tube piers adopting SACs could be adopted,the predicted results of which coincided well with the FEM results. |
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