Abstract: Based on the basic principle of warping displacement function setting， a series of parabolic warping displacement functions that meet the basic warping mode of concrete composite box girders were selected for the proposed approach. The energy variation principle was applied to establish the governing differential equations and corresponding natural boundary conditions in consideration of the effect of shear lag. Then the analytical solutions to typical simply supported composite girders under mid-span concentrated load were derived. The static calculation examples verified the rationality and applicability of selecting quadratic parabolic warping displacement functions. Meanwhile， the measured results and the analytical solutions were compared with the finite element calculations so as to confirm the effectiveness of the approach. The calculation formula based on energy principle can meet the practical needs in engineering and the calculation is simple.

Key words: bridge engineering, steel and concrete composite box girder, energy variation principle, shear lag effect, warping displacement function, shear lag coefficient