Energy balance method for modeling ultimate strain of confined concrete

An energy balance method has been widely used in extant literature to derive the ultimate axial strain of confined concrete columns. Researchers have noted that the method yields unrealistic results compared to experimental data when applied to fiber-reinforced polymer (FRP) confined concrete columns. Through theoretical analyses and extensive case evaluations, this work discloses that the original equation of energy balance is invalid, not only for FRP-confined concrete columns but also for concrete columns confined by any material including steel stirrups. Rational reasoning shows that to achieve the energy balance, a modification factor related to confinement should be applied. Using the modified energy balance method, a new model for the ultimate axial strain of FRP-confined circular concrete columns is developed. Compared with the existing models, the new model has a smaller error and better correlation with test results.