| Steel consumption in thin-walled concrete-filled steel tube(CFST)columns is less.Welding and construction of thin-walled steel plates can be easier.Thus,economic advantages are obvious.To facilitate the use of thin-walled and high-strength steel,enhance the confinement from square steel tubes to infilled concrete,and improve local buckling behavior of thin-walled steel tubes,this paper proposes a kind of diagonal ribs stiffened square CFST columns by welding diagonal ribs with cut-outs at adjacent sides of square steel tubes.The experiment study,finite element simulation and theory investigation were carried out.The main contents were as follows:(1)Axial compression tests on 38 diagonal ribs stiffened thin-walled square CFST columns were conducted.The main parameters were: width-to-thickness ratio of steel tube,concrete strength,diagonal rib thickness,diagonal rib details(welding position,height,hole diameter and spacing),stiffening schemes,and whether the specimens were constructed with reinforcing bars(re-bars)or not.Based on test results,the failure modes,load-displacement relationships and steel stress strain relationships were analyzed.The results showed that the diagonal ribs could improve the local buckling performance of steel tube significantly;the diagonal ribs and steel tube can work together;and the diagonal ribs could not only carry vertical force through vertical stress,but also confine concrete and steel tube through horizontal stress,and thus the failure modes,load carrying capacity and ductility were enhanced significantly;(2)Seismic tests on 8 diagonal ribs stiffened thin-walled square CFST columns of two patches were conducted.The main parameters were: width-to-thickness ratio of steel tube,axial load ratio,shear span ratio,whether the diagonal ribs were constructed with holes or not,and whether the specimens were constructed with diagonal ribs.Based on test results,the failure modes,load-displacement relationships,seismic performance assessment indexes and steel stress strain relationships were analyzed.The results showed that specimens with shear span ratio ?=3.3 experienced flexural failure next to the column end,while specimens with shear span ratio ?=1.0 experienced shear failure;the existence of diagonal ribs could significantly improve the cyclic shear behavior of stub columns;and the steel tube and diagonal ribs could not only carry shear force through horizontal stress indirectly,but also carry shear force through shear stress directly;(3)Different concrete constitutive models were compared in detail.Based on the existed models,the confined concrete constitutive model of unstiffened or diagonal ribs stiffened square CFST columns applicable in ABAQUS was proposed.The simulated results using the proposed concrete model agree well with the test results;(4)Finite element analyses on the axial compression behavior of diagonal ribs stiffened square CFST columns using ABAQUS were carried out.The main parameters were: hole diameter and spacing,welding position of diagonal ribs,concrete strength,and steel strength.Based on the parametric analyses,width-to-thickness ratio limits of steel tube,rational diagonal rib details,and thickness matching relationship between the diagonal rib and steel tube were proposed.Finally,the calculation methods were recommended;(5)Solid finite element analyses on the flexural behavior of diagonal ribs stiffened square CFST columns using ABAQUS were carried out.The main parameters were: hole diameter and spacing,concrete strength,steel strength,steel thickness,and axial load ratios.The parametric analyses results showed that the concrete strength and axial load ratios affected the deformation capacity obviously,and thus the axial load ratio limits corresponding to the concrete strength were proposed;the revised plastic stress distribution method(i.e.,the concrete stress was assumed as the confined concrete strength,and using a cross section through the holes in the diagonal ribs(i.e.,the net section)was proposed to calculate the column capacity under combined bending moment and axial compression;(6)Solid finite element analyses on the shear behavior of diagonal ribs stiffened square CFST columns using ABAQUS were carried out.The main parameters were: shear span ratio,axial load ratio,concrete strength,steel strength,steel thickness,and axial load ratios.The shear model,shear capacity calculation formulas,and shear force-shear deformation relationships were proposed,and agree well with the test and finite element results;(7)Based on the ABAQUS analyses results,fiber finite element model for the diagonal ribs stiffened square CFST columns using Open Sees was proposed.The fiber finite element results showed that the steel effective stress-strain relationships can reflect the effect of local buckling of thin-walled steel tube.Finally,the steel effective stressstrain relationships,which can consider the effect of local buckling of thin-walled steel tube,different concrete and steel strength,different width-to-thickness ratios of steel tube and hole diameter and diagonal rib thickness.Then,fiber finite element model for the diagonal ribs stiffened square CFST columns using Open Sees was proposed.The proposed shear force-shear deformation relationships were assigned to the section fiber through Command “Section Aggregator” within Open Sees framework,which could consider the shear deformaiton effectively. |
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