Abstract: The influence of radial force caused by horizontal bend of prestressing tendons can’t be taken into account in planar member models. The solid models,in which the influence could be reflected,are complicated in modeling process. To solve this problem,an elasticity formula on tensile stress at medial axis is proposed in this paper,which can be applied to the top flange of continuous box girder. Then the concept of equivalent width is suggested to further simplify the elasticity formula based on the idea that the maximum tensile stress is always the control point in the design process. Finally,it is verified by numerical experiment with the aid of ANSYS software. The analysis results indicate that the errors caused by “Drawing Curves by Straight Lines” in the derivation process decrease gradually with the increase of horizontal bend radius. Furthermore,numerical solutions and formula solutions tend to be identical after a correction term is introduced to take the influence of trend change of prestressing tendons into consideration,that is,the simplified calculation formula is practical.