The strength calculation of Vierendeel beam is very troublesome. At the initial design, the estimation of the maximum stress and deflection to occur being of great importance, it is very desirable that its calculation can be made as easily and breifly as that of simple beam. In order to apply the simple beam calculation method to Vierendeel beam one needs its "Equivalent moment of inertia" and "Equivalent section modulus" which are equal to the moment of inertia and section modulus of simple beam substituted for it. These are giben by I_e=η_1I^^- and Z_e=η_3Z^^- where I_e and Z_e are the above mentioned equibalent moment of inertia and section modulus η_1 and η_3 coefficients for effectiveness, I^^- and Z^^- moment of inertia and section modulus of Vierendeel beam, respectively, about its neutral axis when it is assumed to behave as a perfect simple beam, i.e., I^^-=2(I_1+1/4F_1h^2) and Z^^-=I^^-1(h/2), in which I_1 and F_1 refer to moment of inertia and sectional area of its horizontal member and h to its height as shown in Fig.1. This porblem has already been treated by a few persons, who, however, neglected axial and shearing forces of each member (i.e., horizontal displacement of each joint). But in this respect the authors have paied consideration. In this paper η_1 and η_3 are calculated theoretically. As numerical examples shown in Fig.5, η_1=0.356, η_3=0.350 for a case of supported ends, and η_1=0.116, η_3=0.153 for fixed ends.
