Why BIM?

Computer-aided design (CAD) programs are widely used in the architecture/design industry to create drawings and details. But, like the hand drawings that preceded them, CAD drawings are discrete entities. With CAD, a designer creates a series of separate drawings to depict different views of a particular project – plans to show how the project appears from a horizontal viewpoint, elevations to show how it looks vertically, and perspectives to show the project from a particular angle in space. If some aspect of the design changes, the designer has to make that change in every drawing. For example, say you’re designing an addition to a house and the owner initially asked for a 24′ x 24′ addition. You draw a set of plans and elevations for your design, but the owner calls and says she can only afford an 18′ x 24′ addition. You have to re-draw the changed walls on each of your drawings. If the owner then decides she wants a larger window overlooking the back yard, you have to create yet another set of drawings to show the change. With each view contained in a separate drawing or file, every element has to be drawn many times even though it appears in the final building only once. Whether drawings are hand-rendered or created by a CAD program, this process is extremely time consuming and highly likely to result in errors or omissions.

Bulding Information Modeling (BIM) offers a radically new approach. BIM programs (Revit is one) are not drawing programs, but modeling and information programs. Projects are built from intelligent components that are located in a 3-D model and all information about that model resides in a single file. The model can be viewed from any angle, be it plan, elevation, or perspective. A component – a window for example – is placed into the model/file only once. If you put a window into the wall of a real house, it is there and you can see it no matter where you stand in the yard so long as it is within your line of sight. In BIM, once you place the window into the model, it is as if it were installed in the house and it appears in all views. Because components are “intelligent,” they adjust to changes automatically. For example, a stairway is not a series of risers and treads but is encoded as a staircase. If the floor levels to which it connects change, the staircase adjusts to fit. There is no need to update separate drawings. BIM also collects information about your model components and makes that information available in automatically-generated schedules. For example, it keeps track of the number of windows of each type that you add and includes them in the window schedule.

The differences between CAD and BIM are more complex than this. Paul F. Aubin, in his book Mastering Autodesk Revit Building (Thompson Delmar Learning, Clifton Woods, NY 2006) states that the BIM is not simply a 3D model of a building, but a “full description of a building” (41) – a data model rather than a geometric model. All elements within the model relate to each other and retain these relationships. He describes this in an example of a change made in a schedule:

If you go into a schedule view where [a] door is listed and change it from wood door to a glass door, not only will the calculations like quantities or costs for that door change in the schedule, but also the change will be reflected in all graphical views – for example, in shaded Views, the Door will now appear transparent. Likewise, if this data is linked to cost estimation or green building calculations, the change to a glass door Type will have other important impacts as well (43).

The BIM is not perfect. The first chapter of Aubin’s book summarizes some of the trade-offs between the two systems, but I need to explore Revit further to appreciate these fully. Regardless of how BIM programs shake out in the long run, it’s clear that the industry will be moving away from CAD toward a more sophisticated and efficient approach.

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