Study On Axial Compressive Behavior And Fire Performance Of Circular Concrete-filled Steel Tubular Columns With Built-in High-strength Steel Tube

To significantly improve the fire resistance of concrete-filled steel tubular columns and make sure that the bearing capacity of them does not decrease,this paper puts forward circular concrete-filled steel tubular columns with built-in high-strength steel tube.Under the condition that the amount of steel consumption keeps basically unchanged,this kind of column reduces the thickness of the outer tube and places a high-strength steel tube inside.Different grades of steel are used in the inner and outer steel tubes,and the high-strength steel is all put in the interior of the column.Under the double constraint of the outer steel tube and the sandwich concrete which is between the inner and outer steel tube,the strength potential of the highstrength steel can be employed more efficiently.To provide technical support for the engineering application of such columns,the main research works are as follows:1.Twelve circular concrete-filled steel tubular columns with built-in high-strength steel tubes were tested under axial compression,and the influences of the yield strength of inner tube,clearance between inner and outer tube,steel ratio of the inner tube,demolished concrete lumps on compressive behavior of this kind of column have been investigated.It is found that:(1)under the condition that the total steel consumption keeps almost unchanged,the conventional CFST columns’ axial bearing capacity can be greatly enhanced and initial stiffness can be improved a little by thinning the outer steel tube and placing a high-strength steel tube inside;(2)with the increase of clearance between inner and outer steel tube,the axial bearing capacity of this column can be further improved and the initial stiffness does not change much;(3)the increasement of inner steel ratio has no significant influence on the axial bearing capacity and initial stiffness of this column;(4)compared with built-in high-strength steel tube,built-in normal-strength steel tube not only significantly reduces the improvement of the axial bearing capacity of double concrete-filled steel tube,but also may cause the initial stiffness of this column slightly lower than that of the conventional CFST column;(5)the use of demolished concrete lumps has little influence on the axial compressive behavior of the columns;(6)the errors between the predicted and measured axial bearing capacities of this column can be remarkably reduced by using the proposed calculation method.2.Based on ABAQUS,a numerical simulation model of circular concrete-filled steel tubular columns with built-in high-strength steel tubes were modelled.The influences of the cylinder strength of concrete,the outer diameter of steel tube,the ratio of inner steel tube,the yield strength of the inner steel tube,the clearance between the inner and outer steel tubes and the initial diameter-to-thickness ratio have been analyzed.It is found that:(1)under the condition that the total steel consumption keeps almost unchanged,axial bearing capacity of this column is increased to different degrees compared with that of conventional CFST column,and the influence of the concrete strength on the lifting effect is limited.But,with the increase of the diameter of the outer tube,the lifting effect is enlarged.(2)with the increase of yield strength of inner steel tube ranging from 530 MPa to 930 MPa,the axial bearing capacity of CFST columns built-in high-strength steel tube increases significantly.(3)with the increase of clearance between inner and outer steel tube ranging from 20 mm to 50 mm,axial bearing capacity of this column could keep unchanged or may be further improved,but the overall change is limited.3.Five specimens were tested for fire resistance.The effects of the yield strength of the inner tube,clearance between inner and outer tube,steel ratio of inner tube on fire performance of CFST columns with built-in high-strength steel tube have been examined.It is found that:(1)under the condition that the total steel consumption keeps almost unchanged,the fire endurance of conventional CFST columns can be remarkably increased up to 200% by thinning the outer steel tube and placing a high-strength steel tube inside.(2)under the condition that the total steel consumption keeps basically unchanged,increasing the clear distance between the inner and outer steel tube or increasing the steel ratio of inner tube can further greatly improve the fire resistance of this column.(3)compared with built-in high-strength steel tube,built-in normal-strength steel tube has a relatively limited effect on increasing the fire resistance of conventional CFST columns.