The stress fps in the prestressing steel at nominal flexural strength of partially prestressed concrete unbonded members is investigated using a nonlinear analysis. Many different parameters are explored and a rational analytical model explaining the mechanism of member span-depth ratio and its effect on the predicted fps is presented. The present investigation, supported by experimental observations, shows that the type of load application, the loading pattern in continuous members, the amount of ordinary tension reinforcement and span-depth ratio have a significant effect on fps in addition to the parameter pp/fc currently adopted in the ACI Code. While the current study supports the recent ACI adoption of member span-depth ratio parameter in computing fps , it shows that its implementation is unwarranted in the present form. Based on an extensive parametric evaluation, recommendations are given and unified design equations for computing fps in both bonded and unbonded members, expressed as a function of familiar design parameters, are proposed. Numerical examples illustrating the use of the proposed prediction equations are provided in Appendix B.
