1-2-3 Revit: Parametric Building Modeling, Part 2

14 Mar, 2005 By: AIA ,Rick Rundell Cadalyst

Why this type of modeler is so critical for a BIM solution.

In my article last month (for archives, click here), I looked at the technology behind parametric building modeling and provided several tests for determining if a BIM (building information modeling) solution is using a true parametric building modeler.

This month I'll explore why parametric building modeling is so critical for BIM and is the de facto underpinning of a purpose-built BIM solution like Autodesk Revit. BIM solutions that use parametric building modelers provide building information that is more coordinated, more reliable, of better quality and more internally consistent than object-CAD software that has been repurposed for BIM.

Parametric Modeling for Building Design
We've already seen that parametric modeling brought the design model concept to fruition for the mechanical design world, creating a digital model that's used to support design, analysis, manufacturing and so on. But as I discussed in last month's article, MCAD parametric modelers generally use fully constrained models. These work for the mechanical design world because the product, manufactured from raw chunks of material, has to be precisely defined -- unlike a building, which is generally a collection of prefabricated components with relatively few constraints that really matter.

The technology at the heart of a parametric building modeler -- and that made parametric modeling work for building design -- is the context-driven change engine used in Autodesk Revit, which uses a partially constrained building model with a network of building element relationships.

Why Parametric Building Modeling Matters
Why is parametric building modeling so vital to BIM? BIM is an approach to building design that's characterized by the continuous and immediate availability of building information that is high-quality, coordinated, and internally consistent. Reliable building information is the essential feature of BIM and its digital design processes.

Purpose-built BIM applications that use a parametric building modeler, like Revit, deliver this kind of information by design, through the natural operation of the software. When using a CAD or Object CAD BIM solution, the graphical presentation of information (such as drawings or renderings) may look similar to the output of a purpose-built parametric building modeler, but is it "high-quality, coordinated, internally consistent and reliable"?

CAD-based technology is rarely used for BIM, due to the extremely high level of effort required to include and coordinate actionable building information such as schedule, cost, design scope, building performance and so forth.

More sophisticated, Object CAD systems store some (nongraphical) data about a building in a logical structure with the 3D building graphics. Users can extract this data to provide information about quantities and attributes, just as they extract the 2D drawings from the 3D graphics. But Object CAD systems remain anchored to graphics. So additional tools (and effort) are required to keep the graphical and nongraphical data in sync -- to ensure the integrity and coordination of Object CAD models and to deliver the benefits of BIM. One example of such a tool is Solibri Model Checker, which is designed to identify inconsistencies and errors in data produced from object-CAD models before the data is used for other purposes. The larger the project, the greater the effort required to keep the data coordinated and the greater the risk of inconsistencies.

A parametric building model combines a design model (geometry and data) with a behavioral model (change management). The entire building model and complete set of design documents is in an integrated database, where everything is parametric and everything is interconnected.

The analogy of a spreadsheet is often used to describe parametric building modeling. A change made anywhere in a spreadsheet is expected to update everywhere automatically. The same is true for a parametric building modeler -- real-time self-coordination of the information in every view of the model. No one expects to have to manually update a spreadsheet. Similarly, no one has to manually revise a document or schedule from a parametric building modeler.

This bidirectional associativity and immediate, comprehensive change propagation results in the high-quality, consistent, reliable model output that is key to BIM, facilitating digital-based processes for design, analysis and documentation.

The Essence of Design
Parametric building modeling captures the true essence of design -- the designer's intent. It's a natural and intuitive way to describe a building using a computer, simplifying the creation of buildings in software. And the ease of parametric editing allows a more thorough examination of the design, which results in a better building design.

Revit's parametric building modeler also supports design optimization, letting architects develop and study multiple design alternatives simultaneously within a single model. Design options can be toggled on and off in the model for visualization, quantification and what-if analysis. The system keeps track of all the relationships within the design versions, and changes are effortlessly propagated throughout the model and all the design versions within the model.

Design Analysis
In current practice, many digital building models don't contain sufficient information for building performance analysis and evaluation. As with traditional physical models and drawings, evaluating building performance based on the graphic representations of conventional CAD or object CAD solutions requires a great deal of human intervention and interpretation, which renders the analyses too costly and/or time-consuming.

In a parametric building model, much of the data needed for supporting design analysis is captured naturally as design on the project proceeds. The model contains the necessary level of detail and reliability to complete these analyses earlier in the design cycle, and makes possible routine analysis done directly by designers for their own baseline energy analysis, providing immediate feedback on design alternatives early on in the design process.

Figure 1. Parametric building modeling allows firms to embed design and detailing decisions into the digital building model, like this one from Australian firm Architectus, so that the designer's vision is carried through into the construction documents.

It is my opinion that only a purpose-built data architecture built around a parametric building model can provide the immediate and fully coordinated set of accurate and reliable conventional documents. A BIM solution that can coordinate changes and maintain consistency at all times lets users focus on building design versus change management. This built-in change-management capability is critical to the disconnected building process, which is still heavily dependent on construction documentation, providing confidence in drawing deliverables.

While the notion of a digital building model is not new, BIM is creating renewed interest in the purposeful exchange of digital building information to support greater efficiencies in business processes. But just because an authoring tool can produce a digital model doesn't mean it's suitable for BIM. A BIM solution using parametric building modeling ensures you have reliable digital building information feeding those business processes.

About the Author: AIA

About the Author: Rick Rundell

Rick Rundell