For the structures with a flexible floor such as the wooden frame structures, the effects of the in-plane stiffness of the floor on the static and dynamic behavior of structures have been investigated so far. Among these works, several investigations are conducted on the effect of stiffness of orthogonal wall, restoring force characteristics of orthogonal wall, etc. However, there is no investigation focused on the orthogonal wall arrangement. In this paper, A new beam model which enables us to evaluate the effect of the orthogonal wall arrangement is proposed and the effect of the orthogonal wall arrangement on the sharing ratio of lateral force is discussed by using the proposed model.
 In Chapter 2, from the viewpoint of the formulation, the beam model is employed as the proposed model and the proposed model is derived from Timoshenko beam theory: the proposed model is built by interpreting the floor as the Timoshenko beam subjected to the action of the unidirectional force and the moment.
 In Chapter 3, the numerical calculation is carried out by using the proposed model, in order both to verify the effect of the orthogonal wall arrangement on the sharing ratio of lateral force and to confirm the difference of the sharing ratio of lateral force between by using allowable stress design and by using the proposed model. As a result, major findings are summarized as follows;
 1) The change of the sharing ratio of lateral force, with the difference of the orthogonal wall arrangement, is essentially due to the change of the couple moment transmitted from the orthogonal wall.
 2) The couple moment transmitted from the orthogonal wall consists of the effects of eccentricity and the in-plane deformation of the floor. The effect of the orthogonal wall arrangement appears as the constraint effect of the in-plane deformation of the floor.
 3) According to the orthogonal wall arrangement, the sharing ratio of lateral force by using allowable stress design can be evaluated to the dangerous side.