I first wrote about LOD (I’ll break down the acronym later) and the AIA E202 BIM Protocol Exhibit in a post from December 2008, but the topic is such an important part of doing BIM right that I wanted to bring it back for further definition. Here we are three years later and I am reflecting on the last statement of that post…
“[The BIM Protocol Exhibit]…has not yet become a formal addendum to any of our project contracts. That will likely change very soon.”
Since early in 2011, I have been participating in a work group that was spawned from the AIA and the AGC BIMForum to further expand upon the LOD definitions. That is to say, if you are a designer, engineer, or builder and you are asked to provide model elements at LOD### – do you know what you are required to provide? The group will be presenting a progress update at the upcoming BIMForum event in San Antonio, Texas, but for now, allow me to share some insight I have gained while involved with the group.
What is “LOD?”
First, let’s get our terminology straight. According to the AIA E202 document, LOD is defined as “level of development.” It does NOT mean “level of detail;” however, this is an important concept. Unfortunately, the choice of words makes for increased confusion over the acronym, but there definitely is a difference.
Level of detail is the amount of information and geometry provided by the content author or other project participant. There can always be a higher level of detail in a project model than what is realistic to be used by others in a downstream workflow. Level of development is the maximum amount of information and geometry that is authorized for use by others.
Expressed in different terms: DETAIL = INPUT and DEVELOPMENT = RELIABILITY
No Such Thing as an “LOD### Model”
I mentioned this in a recent post about the ‘fluffy kittens,’ but allow me to reiterate the point that LOD is based on assemblies – NOT entire models. In my opinion, the easiest way to understand this is to separate the milestone deliverables from your thinking about LOD’s. In other words, don’t think that schematic design = LOD100; design development = LOD200; and so on.
At any given project milestone, you will likely have a combination of LOD’s. For example, at the end of construction documents, the interior walls and doors might be at LOD300, furniture is at LOD200, and information about electrical fixtures is only supplied as an allowance per square foot; thus LOD100.
Let me explain this a little further with an example many firms are using (including some folks at HOK). Below is a screenshot from an LOD modeling guide offered by one of the participants in the LOD work group. I won’t use the person’s name or the firm name, but I certainly do not mean any disrespect by using this example (many others make the same assumption).
In this image, a progression in LOD is assumed to be aligned with the development of the project design – probably from SD to DD to CD. However, if you look closely at the difference between the 200 and 300 plans, elements like the plumbing fixtures seem to be the same model object. How can that be if the deliverables are at two different LOD’s? What if instead we label the plans as “SD Floor Plan,” “DD Floor Plan,” and “CD Floor Plan?” In the example above, you might know exactly what manufacturer and model number the plumbing fixtures will be (‘buy it’), so that assembly will be listed at LOD300 in the DD and CD deliverables.
In contemplating the LOD concept as it relates to any assembly or component in a project model, I developed the following (although I’m still working on a better explanation for 100):
- LOD100 = Interpolated calculations (estimates)
- LOD200 = Specify it
- LOD300 = Buy it
- LOD400 = Build or install it
- LOD500 = Operate or maintain it
The above list simplifies the concept of authorized uses. Essentially, what you should be able to do with an object given a certain amount of input information.
Does LOD Imply Time?
Another misconception about LOD is that it implies an ‘in progress’ state of content. For example, you might think that LOD200 means that you are placing manufacturer-specific content in your model, but you are still in an early phase of design such that quantities and locations are not finalized. In this case, the correct LOD assignment would be LOD300 with the understanding that at design milestones quantities and locations will be in flux.
Here’s another twist, if you are working on a project that will be competitively bid – essentially, design-bid-build – what LOD would you use for final construction documents? If you think about it, specifications for bid projects usually allow for (or require) a list of alternate products (Wicks Law, New York state). As such, even if you are using a model component provided by a specific manufacturer, the level of development (reliability) can only be at a generic level – LOD200. This may seem counter-intuitive, but a full understanding of these levels is going to form the foundation of ‘good input’ for the industry to start realizing improved interoperability.
I invite you to read more about LOD at the Vico Software website, BIModal Evolution blog, Revit Sticky Notes, Revit OpEd, The BIM Manager, and Q5 (Tocci). There are also some good discussions happening in the BIM Experts group on LinkedIn.