OPTIMIZATION OF STRUCTURAL FRAMES WITH ELASTIC AND PLASTIC CONSTRAINTS

Structural design methods that employ optimization have traditionally followed one of two approaches: optimal elastic design which considers constraints on elastic stresses; and, optimal plastic design, involvi ng ultimate load constraints. This study addresses elastic and plastic design, incorporated into an iterative scheme for checking both linea r elastic stresses and plastic collapse load factors. Structural optim ization of steel frames is achieved through material reallocation, whi ch is governed by a generalized stress parameter in those cases where elastic design controls, and by a reduced cost vector from a linear pr ogram when the plastic collapse load factor controls. Regression analy sis techniques among member properties are employed to reduce the numb er of independent design variables and to incorporate buckling and axi al load effects for the elastic design. The linear programming optimiz ation procedure is implemented in a computer system developed for the minimum-weight design of steel frames. Our model is applied to one sto ry-one bay, two story-two bay, and three story-five bay frames.