Synopsis
Additional strains occurring in unbonded tendons during the loading of prestressed beams or slabs cannot be calculated by simple methods. Therefore in the national codes some simplified design formulae are proposed. However, the validity of such approaches is very limited. Analogously to the simple kinematic model of limit state design, in the present paper a single-crack element with real physical properties is developed. To calculate the additional strains in unbonded tendons of prestressed beams or slabs from the rotations of these elements, the single-crack elements have to be related to a compatible kinematic model. As shown in the paper, the calculated strains based upon this model agree well with the measured ones observed in bending tests on unbonded prestressed concrete slabs.
