Strengthening of defected circular steel columns using CFRP

Most members of structures whose useful life has elapsed need to be repaired. These members may be damaged by a variety of factors. Due to the high cost of reconstruction, a large portion of countries’ development budgets are spent annually repairing and rehabilitate these structures. Compressive members such as columns are one of the most important components in a structure that play a major role in bearing and transporting all the vertical and lateral loads of the building. Basically, no column can bear to its fullest capacity and is failed by buckling. As a result, many researchers are interested in retrofitting and increasing column strength using new materials and methods. In this investigation, damaged circular hollow section steel columns with vertical and horizontal notches and different percentages of 25, 50, 75 and 100% were examined, also the effects of Carbon Fiber Reinforced Polymer (CFRP) for strengthening has been studied. 26 specimens of steel Circular Hollow Section (CHS) column with the same height and different damage dimensions under compressive load were analyzed by ABAQUS 2016 software. The main problem with slender columns is the global buckling under compressive loads. In order to improve the accuracy of the analysis, a combined method was used to study the post-buckling of the plastic zone. For this purpose, the specimens were first subjected to elastic buckling analysis and then Riks non-linear analysis with global and local imperfections was conducted. The results showed that the defect reduces the bearing capacity and rigidity of the steel columns and horizontal defect is more effective in reducing ultimate load in compare to vertical damage. Horizontal-defective columns experienced ‎significantly lower load bearing capacity than ‎vertical-defective columns and can reduce final load up to 52% ‏ in 100% damage, which this reduction indicated that by increasing damage along the perimeter of the column section, final load decreased sharply. The results also showed that it is critical when the deficiency zone is entirely destroyed, while the effect of damage less than 25% was maximum 2.66%. Columns failure occurred in the form of global and local buckling; in all cases global buckling emerged in the form of the column bending, but the local buckling was different according to the type of the damage. ‎Failure modes of the control column is global ‎buckling with focus on the middle of the ‎column, for non-strengthened specimens with ‎horizontal and 100% damage, local buckling‎‏ ‏is shrinkage of ‎notch edges and for vertical notch is defect ‎edges opening. In specimens with a lower percentage of damage, local buckling occurred for horizontal defects in the form of the inward buckling on the middle and for the vertical ones was outward buckling. ‎Strengthening of columns retrofitted with CFRP presented that these kind of fibers have a positive effect on significant gaining ultimate load capacity, delaying defect buckling, controlling fractures and reducing stresses at the damaged area. CFRP strengthening of defected cases using 4 layers, restored the reduction of ultimate load up to 51%, which shows the proper performance of the fibers in retrofitting.