Issue: Volume: 23 Issue: 6 (June 2000)


by Jerry Laiserin

One of the key differences between art and design is that design tends to be defined by constraints. Artists might be constrained by the limitations of their materials and media, but designers often are constrained by the parameters of their design problems, such as the minimum thickness of a flange or the optimal slope of a ramp. Thus, some computer tools commonly marketed to designers, such as paint and drafting programs, which lack the ability to define and manipulate design constraints, are ill suited to highly constrained design disciplines such as industrial design or architecture.

When constraint-based modeling, or parametric design, was introduced to the mechanical design market, design engineers flocked to the new tool for the leverage and efficiency it offered. For example, someone designing a one-liter container could specify that volume as a constraint or parameter of the model, and then explore the form interactively while the software managed the details of volumetric relationships. This ease of use enabled companies such as PTC, with its Pro/Engineer program, to quickly achieve significant market share among its target audience of mechanical designers.
All views of a Revit building model, whether in plan, section, elevation, schedule, or annotation, have full two-way associativity with every other view (even views that have been rendered with the included Accurender).

Now, Pro/Engineer is being joined in the constraint-based, parametric CAD marketplace by Revit, a new 3D-based parametric building modeler for architectural designers. Available from Revit Technology Corp. (which was formed by two members of the original Pro/E development team), Revit automatically reflects any de sign change throughout an entire plan, managing all CAD chores related to a project so that architects can con centrate on the plan's intent.

Dozens of CAD tools have been offered to architects over the past 20 years, with varying degrees of success. Nearly all architects have adopted at least 2D CAD drafting, and a strong and growing minority of design firms have opted for more advanced 3D building design packages, of which Autodesk's Architectural Desktop, Bentley Systems' Microstation TriForma, and Graphisoft's ArchiCAD are the current market leaders.

These packages maintain unified 3D building models from which 2D views can be extracted. They also incorporate "intelligent" objects-software representations of building com ponents that include non-graphical attributes such as cost, construction classification, and fire rating. However, constraint-based design functionality in these programs, if available at all, typically is limited to subsets of tools for specific design elements, such as stairs, which have elaborate design constraints from external sources such as building codes.

Revit, approaching the issues of architectural design software with no legacy of 2D drafting or backward compatibility, sweeps away the often-rickety underpinnings of most prior object-oriented offerings. In Revit, everything-even the user interface-is a parametric object that can be individually constrained, or locked to other objects.

What's more, the default set of constraints in Revit matches most architects' expectations of model behavior. For example, dimension strings are locked to the entities they measure, allowing for associative dimensioning (move a wall, and the corresponding dimension notations recalculate and display accordingly).

Although many architectural CAD programs provide associative dimensioning, Revit's constraint-based approach allows bi-directional associativity: Editing a di mension will move or resize a door as easily as editing the door will change the dimension. This two-way associativity applies throughout every element of a Revit model, governed by a "parametric change engine" that tracks which elements are locked to which other elements and then ripples user-initiated changes to any element throughout all affected parts of the model.

Revit's default constraints conform to architects' typical expectations, so that locking a window to the wall plane into which it is inserted means that the window moves when the wall moves. However, users also can examine any lock and unlock it if they want (subject to multi-level security/administration controls that support office/project management).

Because everything in a Revit model is a parametric object with two-way associativity, long lists of traditionally laborious and error-prone architectural chores vanish. For instance, the paper-based process of architectural design documentation entails the manual creation of numerous, highly stylized, orthogonal views linked only by the architect's mental model of the building being described. Making changes, especially at later design stages when many drawings are well advanced, is a brain-wracking burden of checking and cross-checking to ensure that any change is reflected in all relevant views.

Revit's parametric change engine powers through these inefficiencies by making it equally convenient for architects to work in 2D views or 3D models, and by propagating changes in any view automatically and reliably to all other views. Because only the changed elements in a model must be updated, Revit's on-screen performance is as fast or faster than many of its competitors.

Revit, the company, has devised a distribution model that is tailored to architects' business practices. The software is priced on a monthly subscription basis per user seat. Because architectural employment fluctuates widely with general economic conditions and the real estate market, the ability to turn Revit licenses on and off via the Internet offers architects a flexible management tool for matching CAD costs to design revenues. Every paid-up seat license entitles the user to all upgrades while the license is in force, plus unlimited access to Revit's online training and support. Among the first announced upgrades is full integration of McNeel Accurender, merging that package's raytracing, radiosity, and texture mapping with Revit's interactively editable parametric models.

Revit's overall balance of powerful design features, ease of use, efficient change management, cost-effective licensing, and integrated upgrades make it a serious contender for any architect or allied professional evaluating or reevaluating CAD software.

Jerry Laiserin, FAIA is a member of the College of Fellows of the American Institute of Architects. He can be reached at

Price: Internet subscription at 175 per seat per month
Minimum system requirements: Windows 98/NT/2000; 366MHz or faster Pentium II CPU; 64MB of RAM; 50MB of hard-disk space
Revit Technology Corp.
Waltham, MA