Computer-Aided Design

Computer-aided design allows for architectural drafting and design through the use of computers. The Sketchpad developed by Ivan Sutherland in 1963 is the first computer-aided design (CAD) system aimed at creating a geometrical building model through a graphical computer interface. At first, CAD was mainly used as a computer-supported drawing system, which allowed for easier updated drawing compared to manual drawing. To speed up the process even more, catalogues of two-dimensional (2D) building details were developed for virtually any building product. When the computer capacity increased, steps were made by CAD developers into three-dimensional (3D) modeling. Although this seemed a simple extension of the 2D system, in reality the complexity of the CAD system increased dramatically. Whereas the 2D computer screen resembled the sketch plane, for 3D modeling, completely new interface metaphors and operations needed to be developed. Initially, 3D modeling tools were completely devoted to visualization and as such not integrated in the design process. Only for the final presentation were 3D models created to generate realistic renderings. Since the initial extension of 2D CAD into 3D was not particularly successful, new CAD systems were developed around 3D parameterized building objects. These so-called semantic objects represent existing building objects such as doors, floors, walls, and so forth. When they are inserted in the model, they perform specific behavior. This behavior takes care that unwanted changes to the default shape are prohibited and that objects will fit in a constructive, correct manner. Moreover, these building objects include material properties. Instead of drawing lines in 2D CAD building, objects are modeled in 3D CAD.

The semantic enrichment of the building model started off a new, strongly related development in building information modeling (BIM). Whereas CAD supports the creation of 3D models, BIM supports the exchange of models between different disciplines. After a semantically rich CAD model is created, it allows for reuse of the information included in the model for different purposes, such as finite element methods (FEM), cost estimation, and construction planning, among other things. Therefore, the model is transferred digitally to another party in the building project using a standardized or de facto file format. Consequently, modern CAD systems have facilities for versioning and for checking differences between models. 3D CAD systems support different design phases and levels. The most basic 3D model is a mass model that represents space functions and volumes. Although simple, these models are very useful for global estimates of cost, gross floor area, and other relevant [Page 84]information. In the next phase, parameterized building objects are inserted from libraries. These objects are not yet completely materialized, but the main dimensions are set. Object libraries usually include multiple zoom levels. Each level shows its own detailing, thus preventing cluttered images. During the design process, geometric changes are made, and more information is added by the user while the intelligent model maintains consistency. Finally, when the model is complete, that is, when all objects in full detail are specified, cross sections and elevations are made for construction on site, since shop drawings are still needed there. The 3D model is also used for visualization purposes using the onboard renderer of the CAD system.

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