Model Progression Specification for BIM
Organizing the Development of a Building Information Model
Jim Bedrick, AIAVice President of Virtual Building and Design, Webcor Builders
At the core of architectural design is the process of moving from approximations to progressively more precise information. Representations of building elements in a BIM, though, are exact, whether they’re intended to be or not, and can give a false indication of the precision actually known at a given point in the design process. Add to this confusion the fact that it is possible to use a BIM for many purposes costing, scheduling, performance simulation, code checking, and visualization, to name just a few, some possibly not considered by the author of the BIM. The need for a framework for defining a BIM’s precision and suitability for specific uses becomes obvious.
Model Progression Specification for BIM
To address this need Vico Software (then a division of Graphisoft) began work in 2004 on a Model Progression Specification (MPS). Webcor Builders teamed with Vico to further develop the concept, and then brought it to the technology subcommittee of the AIA California Council’s Integrated Project Delivery (IPD) Task Force. Here the viewpoints of architects, contractors, engineers, subcontractors, owners, and software developers were brought to bear in order to broaden the applicability of the MPS. The AIA National Documents Committee has adopted the approach, provided further development, and incorporated it into the new E202, an exhibit which formalizes the processes for development and use of BIM’s for a specific project. This document will aid teams in agreeing on the purposes for which the project BIM(s) will be used, the level of detail to which specific elements of the BIM(s) will be brought at the conclusion of each phase, and who will develop specific elements of the BIM(s) to the specific levels of detail. The E202 is slated for publication in the fall of 2008.
While the MPS is extremely useful in any project using BIM, the depth of collaboration in IPD makes some kind of systematic approach like this essential. With this in mind, the MPS has been developed to address two principles of IPD:
- The requirement “that phase outcomes—milestones and deliverables—be defined succinctly” so that team members “understand the level of detail at which they should be working, and what decisions have (and have not) been finalized.” (See Integrated Project Delivery: A Guide,http:www.aia.org/ipdg, p 23)
- The idea of assigning tasks “on a best person basis, even when that differs from traditional role allocations.” (Integrated Project Delivery: A Guide, p 13)
Level of Detail (LOD)
The core of the MPS is the LOD definitions—descriptions of the steps through which a BIM element can logically progress from the lowest level of conceptual approximation to the highest level of representational precision. It was determined that five levels, from conceptual through as-built, were sufficient to define the progression. However, to allow for future intermediate levels we named the levels 100 through 500. In essence, the levels are as follows:
200. Approximate geometry
300. Precise geometry
These definitions are further developed in the context of specific uses of the model. The current state of the LOD definitions is shown in Table 1.
Table 2 shows some examples to help clarify the concepts.
The LOD definitions can be used in two ways: to define phase outcomes and to assign modeling tasks.
As the design develops, various elements of the model will progress from one LOD to the next at different rates. For example, in the traditional phases, most elements will need to be at LOD 300 at the conclusion of the CD phase, and many will be taken to LOD 400 in the shop drawing process during the construction phase. Some elements though—paint, for example—will never be taken beyond LOD 100, i.e., the layer of paint is not actually modeled, but its cost and other properties are attached to the appropriate wall assembly.
Beyond its 3-dimentional representation, there is a great deal of information that can be linked to an element in a BIM, and this information may be provided by a variety of people. For example, while a 3-dimentional representation of a wall may be created by the architect, the GC may provide a cost, the HVAC engineer a U-value and thermal mass, an acoustical consultant an STC rating, etc. To address this multiplicity of input, the AIA Documents Committee developed the concept of “Model Component Author” (MCA), who is responsible for creating the 3-dimensional representation of the component, but not necessarily for other discipline-specific information linked to it.
In a traditional project, it is likely that MCA assignments will align with the design phases—the A/E team will do all the modeling up through CD’s, and subcontractors and suppliers will do any shop-drawing modeling required. However, in an IPD project, with tasks assigned “on a best person basis,” it is likely that handoffs will occur at various points in the design process. For example, the mechanical subcontractor may take over as MCA for ductwork during the Detailed Design phase.
The Model Progression Specification
Figure 1 shows a portion of a completed MPS. While the example shows phase names, LOD progression, and MCA assignments typical of an IPD project, these entries can be changed to fit the project at hand.
The spreadsheet shown is in the public domain—the latest version can be downloaded free of charge from www.ipd-ca.net (click on “Tools/Publications”). This is a work in progress—it is likely that through use on actual projects, shortcomings will be found and improvements proposed. We encourage project teams to use the spreadsheet, modify it if desired, and provide feedback to email@example.com.
We would like to thank the members of the AIACC IPD Task Force Technology Subcommittee for their hard work in developing broad-based consensus around this process:
Paul Audsley, NBBJ Architects
Jim Awe, Autodesk
Jim Bedrick, Webcor Builders (chair)
Dan Gonzales, Swinerton Builders
Mario Guttman, HOK Architects
Atul Khanzode, DPR Construction
Greg Luth, GPLA Engineers
Tony Rinella, Anshen + Allen Architects
John Wynne, Lucasfilm (owner)