Abstract
In this paper, we propose that a optimum design system to aid structural design in practice should present sensitivity derivatives of an objective function with respect to the parameters of structural performances. It is shown that the sensitivity derivatives of the objective function are evaluated inexpensively by Lagrange multipliers which are by-products of the optimization scheme. And, we study the dual problem which has close relationship with the sensitivity analysis of the optimum design to present that the balance between the cost and the performance which structural designers are concerned about is represented by inequalites between the derivatives of the cost and the performances with respect to the design variables.
