Stresses in the box-beam subjected to the transverse load are analysed by dividing into stresses of engineering beam theory and of residual. The residual is called "deviation stress" in this paper.
The relation between the deviation forces at three adjacent rib stations is expressed by an equation in the similar form to the wellknown three moment equation.
A set of these deviation equations for a multibayed box is easily solved by the successive approximation. It is the feature that the first approximation for these equations is in good approximation to the final solution.
Hence in the light weight structure design, it is possible to estimate directly the optimum size of each component in satisfactory accuracy by considering the above first approsimation.
It is found that the engineering beam theory can be called the zero approximation.
Effect of the fixing end condition is treated also.