Experimental Study On Axial Compressive Behavior Of Stiffened Square Concrete-filled Steel Tubular Columns

There are many merits in square concrete-filled steel tubular（CFST）columns,such as simple construction in the joint,convenience in construction,connecting beams easily.However,the bearing capacity and the ductility of square CFST columns are much lower than the circular ones.It is an effective way to improve the overall behavior of square CFST columns by stiffening the cross-section of the columns.So far,the research on the stiffened square CFST columns has been attracted more and more attention and been one of the hot issues on the steel-concrete composite structures.Experimental study on different types of stiffened square CFST columns has been carried out in this paper.The effects of the popular five stiffening measures on the axial behavior of the square CFST columns have been carefully studied,including ultimate bearing capacity,stiffness,ductility,and local buckling behavior etc.Based on the experimental study.the axial load behavior of stiffened square CFST columns by different measures has been carefully compared and evaluated to provide references for the application of these colmuns in engineering practice.Based on the studies above,the main conclusions can be drawn:（1）Compared with normal square CFST columns,the ultimate bearing capacity of the stiffened square CFST columns has been increased differently.At the same steel consumption,adopting binding bars,adopting tension steel plates,setting three internal longitudinal ribs,setting three external longitudinal ribs,setting inclined tension bars,setting single internal longitudinal rib,wrapping CFRP jackets and CFRP strips have increased the ultimate bearing capacity of the square CFST columns by 43%,38%,36%,34%,31%,30%,12% and 4%,respectively.Particularly,the binding bars as a stiffening scheme is the most effective measure to increase the bearing capacity,adopting CFRP strips can increase the ultimate bearing capacity slightly.In addition,adopting tension steel plates,steel ribs,inclined tension bars,and CFRP jackets can also increase the bearing capacity of the columns significantly.（2）At the same steel consumption,the ductility of square CFST columns stiffened by the inclined tension bras,binding bars,tension steel plates,CFRP strips and single internal longitudinal rib has been improved respectively by 15.49 times,10.47 times,10.11 times,5.28 times and 60%.Namely,adopting inclined tension bras is the most effective measure to improve the ductility of square CFST columns,followed by using binding bars,tension steel plates and CFRP strips.However,the ductility of square CFST columns stiffened by three internal/external longitudinal ribs,and CFRP jackets decreases slightly compared with normal square CFST columns.（3）At the same steel consumption,the load-overall longitudinal strain curves of stiffened square CFST columns can be classified into four types.The curves of CFST columns stiffened by longitudinal ribs belong to the tape of "ascending-descendingplateauing".The curves of CFST columns respectively stiffened by tension steel plate,CFRP strips and CFRP jackets belong to the tape of "ascending-descending-ascending slowly".The curves of CFST columns stiffened by inclined tension bras behave as the third tape "ascending-descending-ascending slowly-descending".And the curves of CFST columns stiffened by the binding bars behave as the fourth tape "ascending-descendingundulation slowly-descending".（4）Local buckling of normal square CFST columns occurred severely in the middle of the columns and the local buckling half wavelength is approximately equal to the width of the cross-section.However,local buckling of stiffened square CFST columns occurred more slightly in both the middle and the top of the columns with smaller local buckling half wavelength.Hence,the local buckling mode of CFST columns has been changed due to the stiffening measures,which improves the local stability of the steel tube,increases the confining effects of the core concrete and delays the occurrence of local buckling of the steel tube.Particularly,the square CFST columns stiffened by the tension steel plates and single internal longitudinal rib locally buckled the most slightly.（5）Compared with normal square CFST columns,the initial stiffness of stiffened square CFST columns increased differently.At the same steel consumption,the initial stiffness of CFST columns respectively stiffened by three internal steel ribs,single internal steel rib,tension steel plates,CFRP strips,three external steel ribs,inclined tension bras,binding bars and CFRP jackets increase by 26%,20%,14%,13%,10%,7%,6% and 3%,respectively.The initial stiffness of square CFST columns stiffened by three internal steel ribs is the largest while that of the ones stiffened by CFRP jackets is the lowest.（6）Compared with normal square CFST columns,even if the steel consumption of the inclined tension bras is reduced by 75%,the ultimate bearing capacity and the ductility of square CFST columns stiffened by the inclined tension bras is still increased by 33%and 24%,respectively.When the spacing of inclined tension bras remain unchanged,with the amount of steel consumption increases,the ultimate bearing capacity is almost unaffected,but the ductility is greatly improved.（7）Compared with normal square CFST columns,even if the steel consumption of the binding bars is reduced by 75%,the ultimate bearing capacity and the ductility of square CFST columns stiffened by the binding bars is still increased by 15% and 6.06 times,respectively.With the amount of steel consumption increases,the ultimate bearing capacity and the ductility have been further improved.（8）The comprehensive index β was initially proposed to evaluate the stiffened square CFST columns.At the same steel consumption,it is shown that the overall behavior of the square CFST columns stiffened by inclined tension bras is the best,followed by ones stiffened by binding bars,tension steel plates,CFRP trips,steel ribs,and CFRP jackets,respectively.