A numerical method for improving stiffness of the finite element with linear displacement functions is proposed. This method is based on the Hellinger-Reissner principle in which compatible displacement components and stress components expressed in term of either Maxwell's stress functions of Morera's stress functions are employed as the independent functions. A criterion for determining the base function used to express displacement components and stress functions is proposed. Several numerical examples of plates and shells are solved to investigate the accuracy of the solution of the present method. The results show that the proposed method is similar in accuracy to the method using an approximate solution of the displacement differential equations.
