Abstract:This paper presents a matrix structural analysis of variable stiffness on the complete loading history of prestressed concrete grids with unbonded tendons. A model of torsional rigidity affected by moment is derived by the analogy of space truss, and the effect of torque on flexural rigidity is also studied. In order to calculate the increment of stresses in tendons, a corresponding equation is created. Comparison of calculation shows well agreement with the experimental results.