Proposed Modification of AASHTO-LRFD for Computing Stress in Unbonded Tendons at Ultimate

In 1998, AASHTO-LRFD adopted a new equation for evaluating the stress $f_{ps}$ in unbonded tendons at ultimate. In explaining the development and applicability of the new AASHTO-LRFD equation, some investigators concluded that the new equation is conservative and more rational when compared with the ACI building code approach for computing $f_{ps}$. However, in a comprehensive study carried out by the writer, it was found that while the AASHTO-LRFD equation is indeed more rational than the ACI building code approach because it accounts for continuous members, it still encounters significant scatter in predicting test data. Also, using conservative collapse mechanisms that involve loading a single span in a multispan member for all loading cases as proposed in AASHTO-LRFD leads to design inconsistencies. Based on the results of this assessment, supported by a large body of test data, an expression for evaluating the equivalent plastic-hinge length in unbonded members at ultimate was developed. The corresponding expression can be used in conjunction with the concept of a collapse mechanism for developing a general strain-compatibility model and a direct equation for evaluating $f_{ps}$ in unbonded externally or internally posttensioned continuous members. This paper illustrates the development of the strain-compatibility model and the direct-design expression and proposes that they replace Eq. 5.7.3.1.2-1 of AASHTO-LRFD for computing $f_{ps}$. A design example is provided to illustrate the use of the proposed approach.