| With the development and maturity of analytical theory and technical standard,concretefilled steel tube(CFST)structures have been widely used in engineering practice.In contrast,to overcome the disadvantages of single CFST in long-span and high-rise engineering structures,CFST latticed members,which are composed of CFST chords and steel braces,have the characteristics that application precedes theory obviously.Currently,there are only static design methods and few seismic design articles or regulations of CFST latticed members in the relevant domestic engineering construction standards,which not only restricts the application and development of CFST latticed members in earthquake region,but also leads to the lack of reliable basis for the maintenance and performance evaluation of such composite members after earthquake.Therefore,it is of great scientific significance and practical value to study the characteristics and laws of earthquake damage of CFST latticed members and further propose the corresponding seismic design principles and methods.Based on the above considerations,the experimental and theoretical researches on the seismic behavior of four-legged CFST latticed members are carried out,and the restoring force model which can be used for the elasticplastic seismic response analysis of such members is further put forward.The specific work and main conclusions are as follows:(1)Based on the general software ABAQUS,the refined finite element analysis(FEA)models for seismic behavior analysis of two-,three-,and four-legged CFST latticed members were established with the reasonable considerations of the mixed hardening as well as ductile damage of steel and plastic damage as well as crack section effect of concrete,and then the accuracy and rationality of the established FEA models were verified by the comparison between the simulated and tested failure patterns,load-deformation curves,stiffness degradation process and mechanical indexes of CFST latticed specimens under the combination of constant axial compression and horizontal cyclic loading.(2)The pseudo-static tests of 12 four-legged CFST latticed specimens(6 specimens with circular CFST chords and 6 specimens with square CFST chords)and 4 four-legged steel tube latticed specimens(2 specimens with circular steel tube chords and 2 specimens with square steel tube chords)were carried out,and the influence of section form of chords,diameter(width)-to-thickness ratio of chord steel tube and axial compression ratio on failure patterns,deformation development,load-deformation curves,bearing capacity,stiffness and accumulated energy dissipation of the specimens were investigated.The results show that,the section form of chords and diameter(width)-to-thickness ratio of chord steel tube have great effect on failure patterns of four-legged CFST latticed specimens.The bearing capacities,initial stiffness and ductility of four-legged CFST latticed specimens generally decrease with the increase of diameter(width)-to-thickness ratio of chord steel tube and axial compression ratio.The accuracy and rationality of the established FEA models for seismic behavior analysis of four-legged CFST latticed members were further verified based on the comparison between the above tested results and simulated results by FEA models.(3)Based on the main characteristic points on the load-deformation curve of typical fourlegged CFST latticed members under cyclic loading simulated by the FEA model,the deformation development law of chords and braces,stress distribution of chord/brace steel tubes and core concrete in the chords,horizontal shear distribution between chords and braces and damage evolution of chord/brace steel tubes and core concrete in the chords were comprehensively analyzed.The results show that,the relative deformation of adjacent chords of four-legged CFST latticed members under cyclic loading is limited but there is comparatively obvious in-plane or out-of-plane bending deformation of oblique braces.The stress distribution of chord/brace steel tubes and core concrete in the chords exhibits the characteristics of members under compression-bending-shear.The damage of chord/brace steel tubes concentrated at oblique braces or chord steel tubes in the KT-joint zones between chords and braces while that of core concrete concentrated at the root and/or top of the chords.The influencing analysis of diameter-to-thickness ratio and yield stress of chord(brace)steel tube,compressive strength of concrete in the chord,axial compression ratio and equivalent slenderness ratio on working mechanism of four-legged CFST latticed members under cyclic loading was further conducted.(4)The influence law of diameter-to-thickness ratio of chord(brace)steel tube D/T(d/t),yield stress of chord(brace)steel tube fy(fyw),compressive strength of concrete in the chord fcu,axial compression ratio n and shear-span ratio λm on the load-displacement skeleton curve and loading/unloading paths of load-displacement hysteretic curve of four-legged CFST latticed members was discussed.The results show that,D/T(d/t),fy(fyw),fcu,n and λm have great influence on the load-displacement skeleton curve of four-legged CFST latticed members,while D/T(d/t),n and λm have great influence on the loading/unloading paths of four-legged CFST latticed members.The load-displacement restoring force model of four-legged CFST latticed members was proposed based on the influence law of parameters,and then the accuracy and rationality of the established load-displacement restoring force model was verified by comparing the load-displacement hysteretic curve and skeleton curve determined by the restoring force model with those simulated by FEA models and measured in the tests. |
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