Numerical procedure to generate interaction diagrams for circular concrete columns wrapped with FRP

Fiber reinforced polymers (FRP) is an attractive material to the field of strengthening and confining new and existing structures. FRP is usually used to wrap columns to increase the ultimate strength and strain of the concrete through confinement. Existing columns typically have spiral steel reinforcement (SS) in the section when wrapped with FRP. The nature of the problem becomes totally different since there are two systems with different behavior engaged in confinement. Several models were proposed to depict the behavior of the FRP alone in confining concrete. On the other hand, the literature has limited studies assessing the behavior of FRP and SS working on confining concrete simultaneously. This paper proposes a model addressing the two materials engagement in circular columns. The development of the effective lateral confinement pressure is based on Lam and Teng model for FRP action and Mander Model for SS action. It also introduces the force eccentricity as a new parameter that plays an important role in estimating the amount of confinement involved. Hence, the level of strength and ductility vary based on the eccentricity. The proposed model considers the fully confined curve as an upper bound curve with zero eccentricity and the unconfined curve as a lower bound curve with infinite eccentricity. In between these two curves, infinite numbers of stress-strain curves can be generated based on the eccentricity. Generalization of the moment of area approach is utilized based on proportional loading, finite layer procedure and the secant stiffness approach, to achieve equilibrium points of P-ε and M-φ diagrams up to failure. Finally the model is validated by showing good conservative correlation to experimental data.