Most of the articles on building information modeling (BIM) published in recent years focus on the generation of buildings and the design of the systems within. The creation of the components that make up those systems is all but ignored. As a result, finding, judging, and improving the quality of existing BIM content quickly is becoming a valuable skill. This article is intended to help you gain that skill.

Many BIM content features and tips are software-specific. Although this article discusses Autodesk Revit content, some of the takeaway points can be applied to other software, including Bentley BIM applications, ArchiCAD, and CADmech, as well as models of HVAC components created for in-house use.

Where to Find BIM Mechanical Content
Finding high-quality BIM content for mechanical components can be challenging, as many manufacturers do not provide building information models of their products, and generic content may be difficult or impossible to modify geometrically. Further, the creators of BIM content may not be intimately familiar with how building information models work or know the needs of BIM-software users.

In Revit, a model of a component is called a family. For Revit families, there are several sources of content. Free content is available from:

• Manufacturers' Websites.
• Autodesk Seek (http://seek.autodesk.com).
• RevitCity (www.revitcity.com).
• McGraw-Hill Construction Sweets Network (http://construction.com/BIM/).

Content is available for purchase from:

• Broutek (www.broutek.com).
• SMARTBIM (www.smartbim.com).

With Bentley BIM, product models are embedded in a database. Based on generic representations, the majority of the models are manufacturer-specific. To create custom equipment, a user must apply a set of geometric and non-geometric attributes to a parametric model provided by Bentley. Although, often, the data required to define the attributes are available in catalogs, contacting the manufacturer or manufacturer’s sales representative is the best way to ensure an accurate model.

Minimal Geometric Detail Is Essential
With the incredible amount of information required to model a building, BIM software generally is not known for speed. Thus, even with an extremely fast computer, users must be cognizant of creating content with enough detail to represent a product, but not so much as to slow down the software unnecessarily.

Low geometric detail is necessary to keep response time fast. Fewer surfaces makes the removal of hidden lines much simpler, allowing faster panning and zooming and the like.

Generally, building equipment should be modeled as solids. Components inside of equipment, such as cooling coils, fans, and internal dampers, should not be modeled. If modeling is required, the components should be modeled in plan view with two-dimensional lines. Semi-internal components, such as condenser fans and compressors, should be modeled with minimal detail, if they are modeled at all. For example, condenser fans should be modeled as simple cylinders or even just circular lines, without details such as fan blades.

Content creators commonly include manufacturers’ logos on models. This superfluous geometry can impact performance. Generally, logos should be deleted from downloaded content.

As a general rule, a single Revit-family file should be no more than 1.5 MB in size. Simpler, boxier families, such as those representing water-source heat pumps, should be no more than 0.5 MB in size.

FIGURE 1: From left: Fine, medium and coarse levels of detail.

With some content, the user can change the level of detail (Figure 1). The coarsest detail shows the bare minimum of geometry, while the finest may contain additional geometric features, such as compressors, electrical boxes, external dampers, and access doors. Some objects, such as water-cooled chillers, which naturally have more complex geometry, require greater detail.

High-Quality Content Includes Clearance Information
Most high-quality BIM content includes data related to required service and operating clearances (Figure 2). Clearance spaces may be represented two-dimensionally and viewable only in plan and elevation views. Higher-quality depictions also include three-dimensional geometry.

FIGURE 2: From left: Three-dimensional, elevation, and plan clearance depictions of a HVAC unit.