Optimal performance of FRP strengthened concrete columns under combined axial–flexural loading

Amongst various methods developed for enhancing the strength and ductility of various reinforced concrete (RC) members, external confinement of the members with fiber reinforced polymers (FRPs) has proven to be an effective and convenient method. The study reported herein is conducted on the optimal use of FRP for RC columns under combined axial–flexural loading. In this paper, the fiber method modeling (FMM) together with a nonlinear finite element analysis (FEA) are utilized to determine the capacity of such columns. The analysis starts from a fundamental virtual work equation and uses standard finite element procedures. Also, the Optimal Criteria (OC) method is employed to deploy the least amount of FRP material. Therefore, the design parameters considered are the length and thickness of FRP. The optimization problem is formulated as the minimization of a Lagrangian function expressing the FRP cost and design parameters after determining the sensitivities of the column capacity. A Gauss–Seidel technique is finally applied to solve for the design variables. The thickness of FRP jackets and the covered length of the column are optimized for rectangular pin-ended columns under axial loading together with end moments rotating in the same sense. The mentioned loading was chosen to represent the straining actions of columns in buildings subjected to lateral loading. The study was performed to investigate the effects of various parameters on deploying the least amount of the FRP material for the mentioned column. The parameters used in the investigation include the unconfined column strength, type of FRP used and the original thickness of FRP covering the whole length of the column. The study also provides engineers with charts determining the optimal length and thickness at different load levels that represent different points on the interaction diagram. Results show that as the axial load on the interaction diagram increases, the optimal FRP length increases. Also, it was concluded that greater FRP strap lengths were needed at the point of pure bending than at the balanced point. However, in the compression zone of the interaction diagram, optimal FRP volume is controlled by both, the FRP stiffness and the FRP confining effect.