Assuming the availability of a more general building data model, one must define life and fire safety features of a building before any automatic checking can be performed. Object oriented data structures are beginning to be applied to design objects, since they allow the type versatility demanded by design applications. As one generates a functional view of the main data model, the software user must provide domain specific information. A functional view is defined as the process of generating domain specific data structures from a more general purpose data model, such as defining egress routes from wall or room object data structure. Typically in the early design phase of a project, these are related to the emergency egress design features of a building. Certain decisions such as where to provide sprinkler protection or the location of protected egress ways must be made early in the process.

The majority of architectural programs teach technology topics through content specific courses which appear as an educational sequence within the curriculum. These technology topics have traditionally included structural design, environmental systems, and construction materials and methods. Likewise, that course model has been broadly applied to the teaching of computer aided design, which is identified as a technology topic. Computer technology has resulted in a proliferation of courses which similarly introduce the student to computer graphic and design systems through a traditional course structure.

Inevitably, competition for priority arises within the curriculum, introducing the potential risk that otherwise valuable courses and/or course content will be replaced by the "'newer" technology, and providing fertile ground for faculty and administrative resistance to computerization as traditional courses are pushed aside or seem threatened.

An alternative view is that computer technology is not a "topic", but rather the medium for creating a design (and studio) environment for informed decision making.... deciding what it is we should build. Such a viewpoint urges the development of a curricular structure, through which the impact of computer technology may be understood as that medium for design decision making, as the initial step in addressing the current and future needs of architectural education.

One example of such a program currently in place at the College of Architecture and Planning, Ball State University takes an approach which overlays, like a transparent tissue, the computer aided design content (or a computer emphasis) onto the primary curriculum.

With the exception of a general introductory course at the freshman level, computer instruction and content issues may be addressed effectively within existing studio courses. The level of operational and conceptual proficiency achieved by the student, within an electronic design studio, makes the electronic design environment selfsustaining and maintainable across the entire curriculum. The ability to broadly apply computer aided design to the educational experience can be independent of the availability of many specialized computer aided design faculty.

One cannot conduct such studies on real cities except, perhaps, as a point of departure at some specific point in time to provide an initial layout for a city knowing that future forms derived by the studies will diverge from that recorded in history. An entirely imaginary city is therefore chosen. Although the components of this city at the level of individual buildings are taken from known cities in history, this choice does not preclude alternative forms of the city. To some degree, building types are invariants and, as argued in the Appendix, so are the urban typologies into which they may be grouped. In this imaginary city students of urban history play the role of citizens or groups of citizens. As they defend their interests and make concessions, while interacting with each other in their respective roles, they determine the nature of the city as it evolves through the major periods of Western urban history in the form of threedimensional computer models.

My colleague R.J. van Pelt and I presented this approach to the study of urban history previously at ACADIA (Seebohm and van Pelt 1990). Yet we did not pay sufficient attention to the manner in which such urban models should be structured and how the efforts of the participants should be coordinated. In the following sections I therefore review what the requirements are for three-dimensional modeling to support studies in urban history as outlined both from the viewpoint of file structure of the models and other viewpoints which have bearing on this structure. Three alternative software schemes of progressively increasing complexity are then discussed with regard to their ability to satisfy these requirements. This comparative study of software alternatives and their corresponding file structures justifies the present choice of structure in relation to the simpler and better known generic alternatives which do not have the necessary flexibility for structuring the urban model. Such flexibility means, of course, that in the first instance the modeling software is more timeconsuming to learn than a simple point and click package in accord with the now established axiom that ease of learning software tools is inversely related to the functional power of the tools. (Smith 1987).