Research On Seismic Performance Of Four-tube Variable Cross-sectional Concrete Filled Steel Tubular Laced Columns

Concrete Filled Steel Tubular(CFST)laced columns has the advantages of good stability,large bearing capacity and stiffness,high ductility,etc.Comparing with uniform-section CFST laced columns,variable cross-section CFST laced column doing better in absorbing and dissipating seismic energy and reducing the seismic response with lower center of gravity.Thus it is widely used in long-span structures and high-pier bridges which located in seismic areas.The present studies usually focus on seismic performance of uniform-section CFST laced columns,while there are few reports on variable cross-section CFST laced columns.Therefore,this paper aim to study the hysteretic behavior of variable cross-section CFST laced columns and the restoring force model for practical engineering purpose is put forward.(1)Horizontal low cyclic loading test of variable cross-sectional Concrete Filled Steel Tubular(CFST)laced columns are carried out with experimental parameter of longitudinal slope and arrangement type of lacing tubes,and the seismic performance of specimens was researched,including the rules on hysteresis loop,ductility,energy expenditure,and stiffness degradation of elements,then failure mode was discussed.Test results indicate that all specimens have good seismic performance,their hysteresis loops are full without obvious shrinkage.With the increase of longitudinal slope,the horizontal carrying capacity increases,and there is slightly increase in stiffness degradation.The influence of arrangement type of lacing tubes on displacement ductility is big.Compare with CFST laced columns with flat lacing tube,CFST laced columns with inclined lacing tube has larger stiffness and peak load,smaller energy dissipation capacity,and more obvious strength degradation.(2)Finite element models based on OpenSEES software verified against the test results was created.Based on that models,the effects of longitudinal slope,axial compression ratio,slenderness ratio,area ratio of lacing tubes and longitudinal tubes,arrangement type of lacing tubes,yield strength of steel,strength of concrete on seismic performance of four-tube variable cross-section CFST laced columns are Investigated.It is found that longitudinal slope,axial compression ratio,slenderness ratio,area ratio of lacing tubes and longitudinal tubes,yield strength of steel,strength of concrete are the main influence parameters on seismic performance of variable cross-section CFST laced columns.The arrangement type of lacing tubes has little iMPact on skeleton curve of specimen.The influence of different parameters on variable cross-section CFST laced columns and uniform section CFST laced columns have similar laws.(3)On the base of these numerical results from FEM analysis,the calculation formula of skeleton curves characteristic value(include elastic stiffness,Horizontal peak load and its displacement,fall-period stiffness)and restoring force model of four-element variable cross-sectional CFST laced columns was obtained referring to the calculation frame of equal sectional concrete filled steel tubular columns and equivalent length method,then these calculation formula are verified by engineering example.Finally the practical method to calculate the restoring force model of four-tube variable cross-sectional CFST laced columns is provided,and these research results can provide theoretical basis for seismic design of CFST laced column and the further perfection of calculation theory of CFST structures.