Abstract: In virtue of reference Cartesian coordinates, geometrical relations of spatial curved structure are presented in orthogonal curvilinear coordinates . Dynamic equations for helical girder are derived by Hamilton principle . These equations indicate that four generalized displacements are coupled with each other . When spatial structure degenerates into planar curvilinear structure, two generalized displacements in two perpendicular planes are coupled with each other . Dynamic equations for arbitrary curvilinear structure may be obtained by the method used in this paper .

Key words: curved girder, submerged floating tunnel (SFT), dynamic equations, Hamilton principle