My last post introduced the notion of a BIM object’s GR or “level of detailed geometry in its 3D graphical representation.” Let me explain more, to clarify the advantages over LOD that the GR approach to levels of detailing offers in terms of both efficiency and data usefulness.
What’s of value for getting the benefits of BIM is not achieving some general level of development for a model (something most seem to agree is quite meaningless), but rather detailing specific elements and assemblies at a level appropriate for what you want to do with those BIM objects.
Consider a logistics centre that has four kinds of forklifts, several types of chairs and phones, a myriad of diverse power and communications sockets, and half a dozen air-handling units on the roof. According to the standards for facility management, these objects should be modelled at LOD 500, with full detailing of wires, electronic circuit boards, screws, bolts, buttons and holes.
Nobody does that, of course, since the file with your BIM model would be so huge that you wouldn’t even be able to open it. And much of the data would never be used. But that’s the standard. And the better you fulfil it, the slower your model will be.
Maybe we need a standard that reflects the way smart people actually work, and keeps the inexperienced from going overboard with detailing that only slows everybody down?!
When you look at what’s needed for specific BIM uses (tasks and processes), there are really only two relevant types of element geometry: approximate dimensions (GR1) and precise dimensions (GR2). There are also only two relevant types of assembly geometry: those containing at least some GR1 approximate elements (GR3), and those with only GR2 precisely-dimensioned elements (GR4).
So for BIM elements we have two GR levels:
GR1: A 3D model with approximate dimensions suits two types of BIM objects:
- Solid components for use as prototypes before GR2 (plywood plates , brick mass, concrete components, steel components, metal plates);
- Ready-to-use items (things you buy from a store or equipment supplier, like chairs, telephones and most machinery). Your model doesn’t need full detailing here. If it’s an air-handling unit, for example, for interference check you need the height, length, and width of unit, and for duct and pipe routing include the corresponding connectors with precise detailing from the manufacturer at exact positions. But don’t bog your model down with all the details for fans, electronic circuit boards, screws, bolts, buttons, and so on.
GR2: Solid components ready for cutting, milling, drilling, formwork, moulding and casting, CNC production, 3D printing, etc. Here you do need all geometry in very precise dimensions.
And for assemblies we have two GR levels:
GR3: A 3D assembly of BIM objects where at least some are GR1. An example might be a reinforcing mesh cage as an assembly of approximately-dimensioned (GR1) reinforcing bars and connections. (Theoretically you could model rebars at GR2, with precise ribbing details, but that would add no value for BIM uses and would overload your model.)
GR4: A BIM assembly of precisely-dimensioned (GR2) objects which is ready for fabrication on-site or off. This might be a timber frame wall panel, for example.
To summarise, here is a simple IF…THEN formula for deciding which GR is needed:
IF somebody has to make component from your design, at a factory or on-site, THEN use GR2, GR3 or GR4.
IF you’re using components in your design process that are ready-to-use THEN the appropriate level of detailing for them is GR1.
The key benefit of the GR 1-4 approach over LOD 100-500 is it avoids an attitude of rote progression and unthinking accumulation of data. It does so by immediately tying the level of detail to intended use. That makes doing BIM simpler and more efficient. In my next post, I’ll illustrate the intuitive interplay between GR levels and BIM uses over the lifetime of a building project.
