Wednesday, September 04, 2013

LOD Spec: Do I have to model in progression?

Over the course of the first two years of development before the first release of the LOD Specification for BIM in 2013, the work group thoroughly discussed many topics related to the real world use of the levels of development. One such extended discussion focused on how architects or engineers might develop their model data through the various stages of design while adhering to a model element table using the standard LOD’s.

The initial assumption is to think of LOD as ‘level of detail;’ therefore, you might think you are required to model with a generic component first, and then swap it for a more detailed component later in the design phase. While this approach seems logical in accordance with much of the imagery you’ll see in the LOD Specification, it’s not how most of us work.

The correct way to think about this progression is in terms of the RELIABILITY of the information in your model. For example, you might start a project with your company’s model template that has all the standard wall types defined. You might progress through concept or schematic design using an assortment of these standard wall types, but the overall layout and scope of interior partitions is still in flux. In this case, you would define Interior Partitions to be at LOD 200 in the early stages of design, and then specify LOD 300 for the latter stages. How can this work if you’re using the same wall types through all stages?

The LOD 200 definition in the LOD Specification for Interior Partitions includes the definition: “Generic wall objects separated by type of material (e.g. gypsum board vs. masonry).”

This definition can serve as either a guide to the model element author (architect) to model to the minimum specification – or, it can serve as the definition of reliability for a recipient and end user of the model, such as an estimator. An end user may receive a model with highly detailed wall types, but because it is contained in an interim deliverable and the model element table established Interior Partitions to be at LOD 200, the recipient can only use the model data up to the level at which the partitions can be distinguished by type of material. The more detailed data is there – but it cannot be relied on just yet.

Here’s another example – in another blog post from Practical BIM, Antony McPhee uses an illustration from the AEC (UK) BIM Protocol in which a chair is shown at varying levels of detail:


Notice that the reliable properties are shown in red. Now, what if your practice only has the chair model as depicted under Component Grade G3? Can you put that component in your model if Moveable Furnishings was initially specified to be LOD 200 (the equivalent of G1 in the above image)? The answer here is YES. You can put the highly detailed chair element in your project model, but the project design has only been developed to a conceptual level at LOD 200. Therefore, even though it looks like a finished, rendered product, it can only be considered as a generic office chair.