Title:
On the Stress in Unbonded Tendons at Ultimate: Critical Assessment and Proposed Changes
Author(s):
Mohamad Harajli
Publication:
Structural Journal
Volume:
103
Issue:
6
Appears on pages(s):
1-10
Keywords:
prestressed; plastic hinge; unbonded tendons
DOI:
10.14359/18231
Date:
11/1/2006
Abstract:
Evaluation of the stress increase Dfps in unbonded tendons at ultimate flexural strength of post-tensioned members has posed a great challenge over the years, and numerous design equations for calculating the stress have been proposed. Despite the extensive experimental and analytical research invested on this topic, most of the proposed expressions and code design equations still differ substantially in the way the important parameters are accounted for; they also encounter significant scatter in the prediction of test results. In this paper, a comprehensive assessment of the main parameters that influence the stress in unbonded tendons at ultimate is undertaken, and the reasons behind the scatter in the predictions of test results are discussed. Using a physical model of Dfps, together with a large database corresponding to simply supported and continuous members prestressed with internal or external unbonded tendon systems, an accurate expression for evaluating the equivalent plastic hinge length, which has a great influence on Dfps , is generated. Based on this expression, three possible design alternatives were introduced for calculating the stress, and the sensitivity of each to commonly used simplifying assumptions is illustrated. Also, based on comparative assessment of design equations, it was reiterated that the ACI Building Code Eq. (18-4) and (18-5) ignore most of the critical parameters, leading to inaccurate predictions of Dfps . The fact that the inaccuracy of the ACI Code approach is shielded by being overconservative does not prevent unsafe calculation of Dfps , particularly for continuous members. While the AASHTO LRFD equation is more rational than the ACI Code equation, the closed-form solution proposed in the Commentary of AASHTO LRFD code for more accurate evaluation of Dfps is simply unconservative.