Abstract: In order to accurately calculate the beams and griders compact-bending effects,which is applied to longspan bridges,an equation is obtained in this paper on the assumption that the beams and griders shift on the basis of a 3-degree distribution of longitudinal displace ment along transverse direction.Thus a set of differential equations and boundary conditions are derived according to the energy equation considering the compact-bending effect,and the solutions of simple bearing and cantilever girders with uniform and concentrated load are achieved.Verifications of these methods show that they are dependable.