Abstract: Strain analysis was performed to the cross sections of prestressed concrete beams with straight line unbonded tendons.Based on the strain distribution calculation of the compression concrete fiber in ultimate state,the concrete strain at the location of the unbonded tendons was integrated to establish an equation group to calculate the ultimate stress increment.A computer program was assembled to solve the equations.To verify the accuracy of the computational method,21 specimens with straight line unbonded tendons were tested to failure by two concentrated loads at 1/3 span.Comparison indicates that the calculated ultimate stress increment had shown excellent consistency with the tested results.When the strain in the elastic area of the beam was neglected,the ultimate stress increment decreased 7.5 to 15.1 MPa,accounting for only 1.9% to 4.5% of the total stress increment,which indicates that the strain in elastic area can be safely neglected.Based on the theoretical and experimental research,the unbonded tendons of the prestressed concrete box-girders in Sandaohu middle bridge were designed.