Abstract: Cosine functions are supposed to simulate the longitudinal displacement of thin·walled box beams when bended. The differential equations of Trapezia thin-walled box beams with constant section are derived on the basis of generalized variational principle，and the solutions of simply supported box girders subjected to vertical concentrated load and vertical uniform pressure load are obtained .The example shows that this method matches cubic·parabola—function method.

Key words: thin-walled box beams, shear lag, generalized variational principle, cosine function

摘要： 假定箱梁的纵向位移沿横截面满足余弦函数分布，考虑荷载沿横向的作用位置的影响，采用广义变分原理推 导出了等截面梯形箱梁发生对称挠曲时的剪力滞效应表达式，并与三次抛物线法的计算结果进行对比，证明了计算 假定的合理性．

关键词: 等截面梯形箱梁, 剪力滞效应, 广义变分原理, 余弦函数