Unbonded Tendon Stresses in Continuous Post-Tensioned Beams

An evaluation procedure for predicting the unbonded tendon stress in the continuous post-tensioned beams was performed and calibrated through analyzing experimental data from 16 two-span specimens under static loading up to failure. As the relationship between the increase of the tendon stresses and the change of the tensile nonprestressed reinforcement stresses in a same critical section of a post-tensioned beam, response is nearly linear at various levels of loading such as zero, elasticity, cracking, and before yield of nonprestressed tension reinforcement. The non-prestressing bonded steel satisfied strain compatibility. Thus, the increase of the tendon stresses at the service load may be determined through linearly reducing the change of the tensile steel stresses. A model for predicting the ultimate stress of unbonded tendons treats the prestressing reinforcement index and the global reinforcement index as the two independent variables. This and the other variables, including the ratio of the two global reinforcement indexes of support and midspan, and span-depth, affect the rotation capacity of plastic hinges. This prediction model was derived from the equilibrium of the ultimate flexural capacity, and was calibrated by current testing data. Compared with the ACI code and China specification, the proposed equation of the model was validated and was found to provide better accuracy for predicting the ultimate stress of the unbonded tendons in the continuous post-tensioned beams.