Ultimate Steel Stresses in Unbonded Prestressed Concrete

Current design practice in the United States for unbonded post-tensioned flat plate structures is partially based on the assumption that the stress in the post-tensioned tendons at the time of flexural failure can be predicted using the equation given by the American Concrete Institute (ACI) Code for stresses in beams. However, data from recent tests of a number of thin flat plate structures and of beams having comparable span-depth ratios show that this is generally not valid if the span-thickness ratio is about 45, or more. The purpose of this paper is to present the test data and to demonstrate that an additional factor, the span-depth ratio, l/d, needs to be introduced into the prediction equations for ultimate steel stress in unbonded post-tensioned members.