work flow (behind the scenes process)
In implementing BIM, wether the job is a remodel, new construction, as built documentation, or a model translation (2d to 3d documents); you need to evaluate three key questions:
1) First, what are you modeling?
2) Second, why are you modeling it?
3) Third, when should it be modeled?
In addition to this you could ask how do I model this? Because there are several (or dozens, or sometimes hundreds) of ways to accurately represent any one thing, its best left to another document.
Point 1: what are you modeling?
It is important to understand what needs to be modeled at any stage of the building. The idea behind the BIM model is that you can accurately represent all pertinent information for the phase of design or construction that your project is in. At the first conceptual design phase the building may be best represented as a bubble diagram in plan, and a rough massing model in elevation and section. The tool for this may be something as simple as zones. During the schematic design and design development, we should be looking at the building systems represented with walls, floors, roofs, doors and windows.
It is important to model all components as accurately as possible, leaving room for flexibility. An abstract concept of this is to be vaguely precise. For example, the windows may not represent exactly the size, operation, material, color or trim that you need for the final result until the design development phase. But it is important, if the window is mulled (or ganged) with other windows or doors that those settings are dealt with early on. Another example is with plate heights and floor depths. We may not know what these numbers are until we are in schematic or design development. It is critical, however that the wall ID, layer, and relative position are accurate in order to facilitate any story adjustments efficiently. Roof position is critical also, we may not know our overhangs until later in the job, but we must start with an accurate overhang initially so that if it needs to be adjusted in or out 6” it can be done universally without checking and rechecking every piece at every change. Gable end walls should be on a proper location, story, have a appropriate height and be properly trimmed.
This list can keep growing, but this begins to get into the question of “how do I model this?”
Point 2: why are you modeling it?
This is more to the point of both the progressive refinement of the model and the goal for final results. A BIM model should not and can not be so complex that it it impossible to modify, alter or even navigate. A large cumbersome model often needs to be stripped down to be worked within the future, which essentially doubles the work or makes much of the initial work on the model pointless and a waste of time.
Some examples of things that may be obvious to this point are structure, trim, cabinets, interior and exterior 3d detail, material and colors and landscape and other entourage items.
If we are not drafting structure, there is rarely a case for modeling it. Some critical beams may be modeled and footings should always be modeled to show an accurate section. Beyond this, it usually becomes a coordination issue and a waste of time.
Trim is better modeled than drafted, but if it doesn’t show up in an interior elevation, section and plan then it may not be needed. Cabinets may start off as a vague 2d outline or a simple cabinet profile, but eventually it is best to show them accurately to get a design sign off. Exterior details need to be evaluated on a case by case basis. If it is assumed that the overhangs will have soffits, then modeling rafter tails is time consuming and complicated without a purpose. If a material or color is not critical to the design or client sign off, then it is a fruitless exploration to fuss with photoshop for a day or more to get it right. Lastly the only time entourage components and lamps should be used is for final quality renderings, marketing images or other sign off documents.
The underlying question behind why is it modeled is to allow objects to be represented, documented and scheduled. You may ask yourself, do I model or draft this? The answer lies in where the object shows up. The power in BIM is in representing a building component once and showing it multiple times.
The analysis I do in determining to model or not to model is a review of where and how many times an object appears in any phase of documentation. Interior trim, for example, will show up in interior elevations and sections (modeled). If structural is done out of house, rim boards will show up only in section (drafted). Plumbing fixtures show up in plan, interior elevation, section and sometime schedules (always modeled). Steel connections may only show up in details or occasionally in section (never or rarely modeled).
Point 3: when should it be modeled?
This is the most critical evaluation to make to have an effective BIM workflow for design and construction processes. We can take the first two points as a lead into understanding the when. If the object to be modeled (what) needs to show up multiple times for the phase the project is in (why), then it should be modeled at that time.
An example was given above of zones for a bubble diagram during predesign. During schematic design, these zones need to be recycled as room labels and zones for scheduling. During design development these schedules need to be reviewed and coordinated with all known information.
The when of a BIM model is really about the refinement of the model into a final product, and ultimately final documents. If you show exterior trim during a massing model study, you will likely need to spend hours fixing it or deleting and starting over with every minor change to the model. During schematic design it is more effective to show the exterior trim as components of the roof (appropriate roof depth), windows and doors adding only necessary band trim or corner boards to get the ideas across.
Putting interior trim on a model in key places may be done during design development to sell an idea to a client, but trimming out every room may be a waste of time and certainly complicates any modifications to the plans and sections.
We may know that we have a certain material we want to use, or even have an eave detail figured out before the design development is complete. There are places to note and document these ideas without fully applying them to a model. We may use the project notes, project info, 2d detail markers, work sheets or a post it stuck to the side of your monitor to keep track of this until an appropriate time to apply those ideas to the model.
The same question posed for point two, “how many times will this appear (during this phase of the project)” can apply to point three ‘when’. During predesign, we are really looking for bubble diagrams and volume studies. Windows, doors, roofs and trim are usually unnecessary. During schematic design we begin to give the building shape and start to study fenestrations, roof overhangs, and some basic material components. During design development we are setting the project up for final coordination and documentation; we need as much detail as can be added while maintaining a flexible model. During construction documentation all represented objects should be in place with accurate size, material, element information and object ID. At this point in the model we should be able to quickly schedule or represent all components of the building.
