This research extends Maisel's (1982) methodology, the generalization of Vlasov's, simple beam theory, and its extension to evaluate torsional, distortional and shear lag effects in simply-supported girder consisting of a single cell uniform box section. The computer program developed can analyse simply supported box sections with trapezoidal and rectangular sections, any material properties, for a unit eccentric loading at midspan, formulate the bending, shear, torsion, distortion and warping stress at different locations of the section. Two separate computer programs have been developed, the first dealing with shear flow, shear stresses, combined shearing stress, torsional warping stress, distortional warping stress, maximum transverse bending stress, etc. The second program includes shear lag only. Torsional warping is treated using the method of Kollbrunner and Hajdin and Heilig (1966), while Sedlacek's (1971) method is used to account for the distortional effects