Negotiating the Level of Development*
When quoting a project with a BIM requirement, you'll often look first at the construction documents, to determine the scope of the project, and then to the BIM Execution Plan (BEP) to see the BIM requirements. Usually there is a matrix, often called a "Model Element Table", showing each trade or model element of the building with a corresponding Level Of Development (LOD). What do you see for Curtain Wall? 500! A quick search for that document's definition of LOD 500 and you'll find phrases such as "fabrication level of detail", " model all parts and assemblies as manufactured", and "include manufacturer, finish, composition, and maintenance schedule for all components".
Some correspondence with the GC or Architect (whomever is defining the BEP) is probably in order to find out exactly what is needed and what is acceptable. Modeling to an LOD higher than what is required to fulfill the purpose of the model (clash detection, scheduling, marketing, facility maintenance), but equal to the BEP requirements will drive up your fee quoted to the GC. Modeling to the level required by the purpose of the model but less than the level stated in the BEP may leave you in the position where you've expended resources but haven't fulfilled your BIM obligation.
Because of the nature of curtain wall and the typical terminology within the industry, we may be fooled into believing that the architectural requirements are higher than they might be in reality. What does it mean when the BEP specifies that fabrication level of detail is required? We in the curtain wall industry tend to think, "I need to do fabrication drawings within my model showing all of the notches, holes, clips, gaskets, sealant, boots, tape, fasteners, etc. with every unique part in the modeled, scheduled and given a unique tag number."
What does the GC/Architect actually require? Let's look at a few examples:
The Gypsum Board Contractor
What do the architect and owner need from a gypsum board contractor? Do they need to see how each piece of gypsum board will be cut, where each joint will be and where each screw will be penetrating the drywall into the studs?
The Finish Carpenter
What needs to be shown for wood trim? Does anyone need to see where each piece of crown molding will be spliced and where each nail will be placed? Does each unique length of molding need to have a different part number or does the architect really just want to have the information so that if a piece is damaged they can get a new piece that matches the rest of the molding?
Is this Level of Development required?
Is a model with this Level of Developement
more than enough?
Often, you'll find that the BEP focuses on the "M" in BIM rather than the "I" and call for every tiny component, even those too small to be modeled in many BIM software packages, to be a 3D component in the final model. A composite of all the models contributed by the different trades results in large and unwieldy files that are impractical to work with and difficult to extract information from.
BIM models shouldn't attempt to create a virtual world with all of its parts if the intent is to make sure that there is no interference between trades, and to have enough information to identify and element at a later date if needed.
What is ultimately required from a curtain wall BIM model? Usually, accurate mullion profiles, glass/terra cotta/aluminum infill panels that match the architectural design, mod lines that correctly define the unit placement, shadow boxes, anchors that don't clash with the columns and embeds that do clash with the floors. It is always beneficial on projects to open the lines of communication between the GC/Architect and the BIM providers so that scope and context can be adjusted to produce effective models that accomplish the needed objective of the architect and the owner in the most cost effective manner.
*For a clear and complete history and definition for "Level of Development" see http://practicalbim.blogspot.com.au