Design shells: a formalism for prototype refinement in knowledge-based design systems Rivka Oxman Faculty of Architecture and Town Planning, Technion - Israel Institute of Technology, Haifa 32000, Israel The paper presents an approach for the development of a specialized shell for design. The concept of a knowledge-based design shell is proposed as a medium for ericoding and using structured schema of generic design knowledge. The prototype as one kind of significant class of generic knowledge in architectural design is introduced. The significance of the concept of a design shell is discussed with respect to formalization, implementation, application and operation. A generative prototype design shell is proposed, defined and elaborated. The schema of generative prototype shell provides a vehicle for the representation of both the syntactic and the semantic content in design generation. A method for representing various types of knowledge in these schema is proposed. The method exploits the characteristics of both rules and frames, and integrates them in a single schema. A plan type of a row house, as one kind of significant class of design prototypes in architectural design, a specific design knowledge in GPRS, is demonstrated in a system called PRODS. The implementation and significance of such an approach is knowledge-based design systems containing knowledge of multiple prototypes is discussed. Relevant future research topics are identified. Key Words: design shell, expert system, knowledge-based design, knowledge structures, design prototype, architectural knowledge. 1. INTRODUCTION: KNOWLEDGE-BASED SYSTEMS AND THE CONCEPT OF A DESIGN-SHELL The development of general structures for representing " and manipulating design knowledge in knowledge-based design systems was influenced directly by the concept of the expert system shell 1. A shell is an empty structure which provides means to assemble specific knowledge within its framework. The MYCIN expert system, in the field of medical diagnosis 2, was one of the first systems that demonstrated the possibility of separating specific knowledge from a general structuring framework of that knowledge. The developers of MYCIN realized that there were two distinct parts to their system: the knowledge base which had a general structure represented in the form of rules, which in turn, contained specific knowledge of medical diagnosis; and a general purpose reasoning mechanism. This distinction led to building the E-MYCIN empty shell, without its specific knowledge base, as one of the first knowledge engineering tools. The differentiation between the structure and the content as infill has significant implications for the use of what will be referred to as knowledge-based design shells in CAD systems. A basic assumption of kn0wledge-based design is that human designers manipulate information and data by utilizing higher level knowledge structures and certain types of control mechanisms. It is this fundamental assumption which makes the concept of the shell so potentially rich as a medium for design applications. Designers have broad knowledge and probably employ various kinds of Paper accepted August 1989.DiscussionclosesJune 1990. knowledge structures and reasoning processes. This requires the representation and manipulation of more complex structures than the first expert systems could afford. The initial experiments with expert systems shells in design were relatively simplistic adaptations which soo.n indicated their limitations for design needs. They were developed to carry out diagnosis using classification concepts mainly for analysis and evaluation tasks. They -could make interpretations of properties and per- formances of design artifacts, where theory by which interpretations are made is well understood. Expert systems for design analysis are well documented in the literature 3-7. These types of expert systems were also used in design synthesis where design problems can be divided into independent sub-problems. This approach has also been adapted in the development of an expert system for design analysis and design synthesis s. The limitation of current expert systems technology for design is due to the lack of a comprehensive theoretical foundation including an epistemology of design knowl- edge. As opposed to the utilization of expert systems for . design, the idea of a design shell will be based upon the • development of such a theoretical foundation. As an " efficient medium to accommodate the characteristics of knowledge structures 9 in design this new concept should enhance further developments in the field of knowledge- based design. The purpose of this paper is to justify the development of a specialized shell for design and to introduce the concept of a knowledge-based design shell as part of the ~theoretical foundation of knowledge-based design systems. First the idea of empty knowledge-based design shells and its significance is presented as a general approach. An important connection between the concept © 1990 Computational Mechanics Publications 2 Artificial Intelligence in Engineering, 1990, Vol. 5, No. 1