Abstract: Because the planesection assumption is not applicable to tendon in unbonded prestressed concrete beams and slabs under loading, the constant stiffness method and momentcurvature nonlinear analysis method were used to compile the calculation programs of the law with serviceability and ultimate carrying capacity. Based on results of the model experiment and a large number of simulations analysis, the law of some basic factors such as nonprestressed reinforcement index βs, prestressing reinforcement index βp, ratio of span to depth l/h, loading type, distribution of prestressed reinforcements and depth from concrete extreme fiber to centroid of the prestressing steels hp affecting stress increment of tendons in simple beams and slabs were presented, and then calculation formulus of stress increment of tendons at serviceability and ultimate carrying capacity state in unbonded prestressed concrete beams and slabs were proposed.

Key words: engineering mechanics, prestressed concrete, simple beams and slabs, unbonded tendon, stress increment