Stresses in unbonded prestressing tendons at ultimate: Recommendation

The use of unbonded tendons is common in prestressed concrete structures such as two-way slab systems, nuclear vessels, and external prestressing. The analysis of members prestressed with unbonded tendons and subjected to loading up to their ultimate behavior is more difficult than that of those with bonded tendons. Indeed, the stress increase in unbonded tendons under load, unlike bonded tendons, is member-dependent instead of section-dependent. Thus, the stress in unbonded tendons depends on the deformation of the entire member and is assumed to be uniform at all sections. It cannot be directly determined from the analysis of a beam cross section by conventional strain compatibility, as can be done with bonded tendons. Rather, the stress increase in the tendons must be determined from the analysis of the loading-induced deformations of the entire structure. This is true for the elastic, inelastic, and ultimate limit states. To determine the nominal moment resistance of beams prestressed with unbonded tendons, and thus verify the ultimate-strength limit state as is required in most codes, there is need to predict the stress f_ps in unbonded tendons at ultimate or nominal bending resistance. This can be done using nonlinear analysis and numerical solutions techniques. However, a simplified method or a prediction equation for f_ps is needed for code purposes. Herein, the authors, who are members of the Subcommittee on Stresses in Unbonded Tendons of Joint ACI-ASCE Committee 423, Prestressed Concrete, recommend a procedure believed to represent the state of the art and the best tradeoff at this time.