Closed form constitutive relationship for concrete filled FRP tubes under compression

Fiber-reinforced polymer (FRP) tubes filled with concrete are very effective structural elements when loaded under axial stress. The exterior jacket of FRP, in fact, confines the inner concrete and significantly enhances both its resistance and stiffness. On the other hand FRP protects the inner core from temperature and humidity effects, and may also be used both as formwork and as reinforcement. Nonetheless, stability problems arise when FRP tubes are used as slender structural elements. Although the physical behavior of a straight structural element is simple, state-of-the-art models on one side do not deal explicitly with instability effects, on the other do not offer closed form solutions for the longitudinal constitutive relationship. Both aspects are solved and validated in this work. First a closed form constitutive relationship for concrete filled FRP tubes is developed. Departing from this solution, the critical load is computed in closed form. Secondly, the constitutive models for squat and slender tubes are validated against experimental results and compared with other state-of-the-art models. Design charts for FRP tubes under compression including buckling are obtained as a function of the most important variables and used to verify example elements. The proposed model shows a good capability of describing the behavior of FRP-concrete tubes, while being simple to use, expressed in a closed form and hence computationally fast.