Study On Overall Buckling Behavior Of Steel Columns Reinforced With Pre-stressed CFRP Strips

Many collapse accidents are caused by the buckling of steel columns,which occur quickly with serious consequences.Thus,avoiding the buckling of steel columns is an important concern in the design of steel structures.Applying fiber reinforced polymers(FRPs)in strengthening/reinforcing steels,has a number of meaningful advantages,which have been proved by many studies and engineering projects.This is due to the fact that FRPs,especially carbon fiber reinforced polymers(CFRPs),have excellent properties of high strength,light weight,good fatigue and corrosion resistance.Current technologies being used for FRP strengthening/reinforcing steel against buckling include gluing FRP on surface of steel and using FRP tube with filling material around steel.This research study proposes a novel reinforcing method that reinforces a steel column against overall buckling using a prestressing system(PS system),which includes pre-stressed(PS)CFRP strips,anchorages and PS chairs.This method can be applied to new structures directly or strengthening existing old structures,which achieves high efficiency,light weight,easy construction and good corrosion resistance.Specifically,a PS system can be used to prestress CFRP strips without using a hydraulic jack.This method can be used for reinforcing of existing steels or constructing new steels for high-rise buildings,long-span bridges and other structures in outdoor spaces.The main work of this research includes but not restricted to the following:(1)The conception of the novel reinforcing method is firstly proposed and developed in this research.The composition and the construction technology of PS-reinforced steel columns are designed,and the principles of the effective anchorage and PS chair are explained.Then measurement of geometrical initial imperfection of steel column is conducted.The results show that the initial imperfection can be reduced after reinforcing by PS CFRP.Finally,a long-term monitoring of CFRP prestress is conducted.The prestress loss is caused by material relaxation,temperature change and anchorage slip.It shows there is only 5.1% prestress loss on average after 42 days,most part of which is finished after 17 days.(2)Axial compression tests have been conducted for nine steel columns of three different slenderness(i.e.,105,140,200)including pure steel columns,steel columns with CFRP and steel columns with PS CFRP.The results show that the buckling load of the PS-reinforced specimens can be increased significantly while that of steel column with CFRP has very limited improvement.Specifically,the buckling load of the PS CFRP-reinforced specimens is at most 2.5 times greater than that of an un-reinforced specimen,and the failure mode of the PS CFRP-reinforced specimens is symmetric in most cases,but ranges between a symmetric and antisymmetric overall buckling mode when the supporting length is large.(3)Eccentrical compression tests including three different eccentricity ratios(i.e.,1,2,3)have been conducted for PS CFRP-reinforced steel columns.All PS CFRP-reinforced specimens under eccentrical compression exhibit a good reinforcing efficiency and show asymmetric deformation after buckling.It is also seen that as the eccentricity ratio increases,the reinforcing efficiency increases.(4)A relatively accurate finite element model(FEM)for steel columns with PS CFRP under axial compression and eccentrical compression is built.Through 255 examples,the reinforcing efficiency has been further proved.Then parametric studies are conducted considering prestress value,supporting length,steel yield point and slenderness,etc.Finally,the influence of the above parameters on buckling loads and reinforcing efficiency is given.(5)A theoretical method is proposed to calculate the buckling load of steel columns with PS CFRP under axial compression and eccentrical compression considering real conditions including initial imperfection and material plasticity.It is revealed that the buckling of such columns is caused by the yielding of the margin of steel or the relaxation of CFRP in the concave side.Based on it,optimal prestressing value is obtained to be the one which makes the above two states of buckling happen at the same time.Thus,a design method towards least reinforcing material and most efficient reinforcing is given followed by a design example.