This paper proposes a simplified method for the calculation of the critical load of a portal frame with fixed supports and restrained by roof sheeting. The roof sheeting is idealized as concentrated spring in the direction of the girder. The method of analysis is built on the equilibrium of moment at the connection between the column and girder. A closed from expression for the calculation of the critical load is derived. Application of this expression on some practical with increasing the elastic stiffness of the roof sheeting.