Abstract This paper describes an expandable software system for optimization-based, interactive computer-aided design of structures. This system can be used for the design of statically and/or dynamically loaded structures which exhibit linear or nonlinear response. The software is the union of (1) an interactive base program for the management of the computeraided design process named DELIGHT, (2) a dynamic nonlinear general purpose structural analysis package named ANSR, (3) a library of optimization algorithms with corresponding interfaces for use in structural design, and (4) specialized software for the design of seismic-resistance planar steel frames. Flexibility has been emphasized in the development of this system so that a wide range of structural design problems may be considered. The user may describe a particular design problem to the system by applying a minimal amount of software or by selecting software from expandable libraries.