Mechanical Model for Unbonded Seven-Wire Tendon with Single Broken Wire

This paper presents the derivation and experimental validation of a mechanical model for unbonded seven-wire prestressing tendons with a single broken outer wire. The model has practical significance because corrosion of these tendons typically causes a single outer wire to fail first. The tendency for the tendon to deflect toward the broken wire causes strains in the unbroken wires to be unequal at any cross section. As a result, the strains in the two wires adjacent to the broken wire increase significantly due to the wire break. Equations are presented for: (1) the strains along the lengths of the broken and unbroken wires; (2) the affected length, where the broken wire can be detected because its strain is less than the strains in the unbroken wires; and (3) the prestress force remaining after the break occurs. Experimental data obtained from tests of seven-wire tendons performed on an $18.3\mathrm{m}$ $\left(60\mathrm{ft}\right)$ long strongback beam validate the model.