ULTIMATE STRESS IN EXTERNAL TENDONS: COMMENTS ON THE EXISTING TYPICAL METHODS

Three methods for the determination of the ultimate stress in external tendons are introduced and commented. They are bonded reduction methods, regression formulas related to reinforcement index and deformation-based methods. It is shown that deformation-based methods can reflect the characteristic of the coupling between the flexural strength and the entire deformation in prestressed concrete members with external tendons. In the deformation-based methods, two commonly adopted models for computing the length of equivalent plastic region are critically investigated. It is found the model based on the span-depth ratio together with loading type places much emphasis on the effects of loading type on the ultimate stress increment in external tendons. In addition, for the ultimate limit state of a highway bridge with moving traffic loads, it is difficult for designers to judge the coefficient which is related to the load type and load arrangement. In contrast, the key parameter in the model based on the neutral axis depth for computing the length of equivalent plastic region, is stable and consistent and can be treated as a constant. The present study can contribute to establishing a simple formula for the estimation of the ultimate stress in external tendons.