Ultimate Stress of Unbonded Tendons in Partially Prestressed Concrete Beams

U nbonded tendons are being used increasingly in partially prestressed concrete structures in China. This is because unbonded tendons usually have a lower unit cost compared to bonded tendons while also offering simplicity in construction. Tests indicate that the ultimate strength of unbonded beams can be increased by the addition of bonded nonprestressed reinforcement. Although several research papers' focus on the presence of nonprestressed reinforcernent, this practice is not currently recognized in many building codes 2 for computing the ultimate stress in unbonded tendons at beam failure. That is to say, although most designs employ at least a minimum amount of nonprestressed steel for distributing cracks in concrete and Iimiting their width, the actual influence of this added reinforcement on the ultimate stress of unbonded tendons is often neglected. For example, the minimum bonded reinforcement in AC! 318-83 is only 0.4 percent of the area of that part of the beam section between the flexural tension face and the neutral axis of the gross section, Since partially prestressed concrete is classified between fully prestressed concrete and ordinary reinforced concrete, the amount of nonprestressed steel can vary widely. Therefore, research must he conducted to determine the effect of bonded nonprestressed steel on the ultimate stress in unbonded tendons, as well as on the ultimate strength of the beam itself.