On the thin-walled member, there is great difference between the torsional regidity of open cross section and closed cross section. This reason is as follow, that the condition of shearing stress is difference complytly. The purpose of this paper is to analyze the torsional problem of of the stiffened beam, which is in middle of their members, with the stiffness matrix method. The sttiffness matrix method is based on the finite element method, and in this paper, we substitute the simple polinominal for the deformed shape, agree it's coefficients with the degree of freedom, and assume that the bending rididity in thire plane is infinite. On this kind of beam, the deformation of cross section owing to the stiffened member is remarkable and the torsional rigidity decrease with this deformation. This fact shows that the old method, which is dased on the hypothese of underformed cross section, is unsuitable i. e. the method of the replacing the stiffened plate with an elastic reduced orthotropic plate equivalent. In order to check this theory, we try to experiment. Farther, we analyze the influence of boundary conditions and stiffened member upon the movement of the center of shear and the shearing flow with the stiffness matrix method and try to experiment.
