The promise of BIM (Building Information Modeling) is that project data can reside in the model and can be extracted by anyone in a form for every purpose. The reality is that useful data extraction is entirely dependent on valid data input.

Models Start Generically

BIM is built with electronic objects representing building functional elements. Design teams select and place generic elements to begin to create the building model. These generic elements establish the volume required for each element and the relationship to adjacent elements – not the detail of the final construction.

When an exterior wall object is first placed in a model, the designer will not know the ultimate design solution. The designer may know the wall is about 15 inches thick – a thickness sufficient to allow for several different design solutions.

The exterior wall object will show the element boundaries, the exterior face, the interior face, the height, and the length. The BIM object can identify other important data. The designer can ensure the object knows it is an “exterior wall.” Using Revit, the assignment is simple. Set the assembly code in the family properties window to start with B2010 – Exterior Walls.

BIM Assembly Codes

The Revit assembly codes are an 8-character, alpha-numeric designation based on UniFormat™. UniFormat is a classification system for functional elements originally developed by estimators and promoted by CSI’s PPDFormat to organize Preliminary Project Descriptions (PPDs).

The first five characters in a Revit assembly code are a UniFormat element number. The last three characters are user defined to allow many configurations of the same basic functional element.

Every BIM object can be classified by UniFormat elements. Each Level 1 element is further divided into more narrow scope Level 2 and Level 3 elements corresponding to the level of detail needed to produce Schematic Design and Design Development cost estimates before detailed material take-offs are possible.

                           Level 1 Elements
A Substructure E Equipment and Furnishings
B Shell F Special Construction and Demolition
C Interiors G Sitework
D Services Z General

PPDs Provide BIM Data

Generic BIM objects lack data. That’s right! And wrong! And that is exactly the point. The BIM object can remain generic to allow the design team to explore alternative design solutions for the particular project. There is no need to load the model object with specific data about the materials composing the object. The correct assembly code provides all that is needed.

The assembly code links the BIM object to external data contained in a PPD. The PPD defines that “about 15 inch thick exterior wall,” even if the design solution is unknown. PPDs, written according to PPDFormat, describe performance requirements, design requirements, components, and attributes for each functional element. Additionally, PPDs allow the design team to document potential options for the ultimate design without modeling each option.

BIM and PPD – A Powerful Combination

Consider the possibilities. Develop generic models. Define model objects, externally, in a PPD that may serve as an office standard. Both activities can proceed concurrently, and somewhat independently. Thermal resistance, air leakage, water penetration, and wind resistance performance requirements for the exterior wall are not dependent on a specific design solution. Performance can be defined, regardless the design solution, and can be defined before any wall object is placed in the model.

Linking BIM to a PPD via UniFormat gives the design team flexibility. The only data needed in the BIM object is a correctly assigned assembly code. If designers and BIM technicians are uncomfortable making the assignments, specifiers can help. Specifiers can provide the list of assembly codes for each project and can insert the correct assembly codes in each model object.

Alternative Solution Analysis

Design teams may wish to consider alternatives and may be forced to do so because of value engineering. Having the PPD building element descriptions and the construction cost estimate organized by the same UniFormat classification system make analysis much easier.

The design team will be able to rely on building element descriptions and costs being reported in only one place for each element. For exterior walls, all the components forming the wall element will be accounted for in a single PPD description and a single cost estimate line item. Consider alternative components within the system and adjust the element costs to suit. Or consider a completely different design solution and simply compare the line item cost.

PPDs – The BIM to Specs Rosetta Stone

BIM is objects representing functional elements comprised of multiple materials and products. PPDs describe functional elements and their components including materials and products. Construction specifications define many materials and products used to accomplish the work results required for completing construction.

The BIM to PPD link is a one-to-one relationship. A direct link from BIM to Specs requires a one-to-many relationship. These many and varied relationships are difficult to establish and manage in BIM as design changes during the iterative process.

Consider removing the difficulty. Link BIM to PPDs, a one-to-one relationship. Allow the project specifier to manage the one-to-many links between the PPD and the construction specifications. Links are dependent on the components used to form each functional element. Specifiers know the required relationships between PPD components and construction specifications.

Links to construction specifications for each PPD component can be predefined, as part of a master document. Links from BIM directly to construction specifications will require each BIM object to be fully defined before the link can be made.

Additional Resources

PPDFormat – a Guide for Developing Preliminary Project Descriptions – This Prezi is available as a live WebEx presentation for AIA CES credit.


Building Design and Documentation are Iterative – This blog post explains how PPDs document design intents through the iterative design process.

Why are Preliminary Project Descriptions (PPDs) Important – This blog post shows how text and graphics can be combined to explain design intent.

PPDFormat and UniFormat – Both documents are published by the Construction Specifications Institute.