BIM is promoted as the future, the 'new way of doing things'. There is an assumption by BIM evangelists that the BIM model will be the receptacle of all information about a building, making drawings redundant. But is this really true? Is it even possible?
TRADITIONAL PRACTICEFor a contractor to build a building they need to know what they are building. The various sub-contractors and individual trades need enough information to first price their work, then to do their work. But these people do not just follow a set of instructions, they bring their own expertise. The vast majority of building trades require years of training, they are experts in their own right.
So what information do they need? They need to know the Design Intent of what they are doing. They need just enough information to provide a framework, working out the rest themselves. Provide any more information and it is likely the results will be sub-optimal and inefficient.
Current practice is to provide this information as drawings, schedules and specifications. Only relevant information is provided in these documents. If the location of something is not critical it is not dimensioned. If something has to be provided that complies with a description it is not necessarily included in drawings or schedules. If something is included in a schedule, it may not be included in drawings. Therefore, although these documents have never been a full description of the building, they are adequate to build it.
ENTER THE BIM MODELWith a BIM model the building has to be completely modelled. You can't leave out a facade just because it is part of a design construct contract, you have to model something. And if something is in a schedule it has to be in the model. Unless every screw, bolt, flashing, seal etc. is modelled in 3D it is not possible to explain how things go together like a drawn detail does.
But most importantly, how do you explain Design Intent when all you can do is create a virtual object? How do you explain which attributes of the virtual object are critical, important, for guidance, or don't matter at all? For example every object has a precise location in the model, yet the precise location may not be important, and may actually be unpredictable because it depend on decisions that can only be made on site.
LOD (Level of Development) tables are an attempt to overcome this shortcoming. But they are not really up to the task. On the one hand an LOD table is not precise enough (how do you use an LOD table to explain that the height of (some) power outlets are critical, but the location along walls is not?). On the other hand, even with this cut down amount of information, LOD tables are becoming far too detailed and complex to be of practical use. The number of element types in a building run into the thousands, let alone the number of actual objects. To assign a LOD to every one, and then track that it is being followed in the model, for each stage of a project, is just not practical in the real world.
BIM TECHNOLOGYNone of this should be a surprise to those that work in the AEC industry. Certainly the authors of BIM software are aware of it.
Take Revit as an example. The model is displayed to us via 'views'. We then add notes, dimensions and other annotation to explain our intent. These views are then placed on sheets that can be printed to paper of electronic format (DWF, PDF etc), and exported as 2D CAD files. Revit utilizes traditional practice to solve the problem of communicating design intent.
Bentley are going a step further. They have an initiative called 'Hypermodel' (as in Hypertext) that shows references to other drawings and documents in the 3D model. It is not so much a method of providing design intent information in a BIM model, more like a hybrid solution, providing access to traditional means of communication directly from within a 3D model (as opposed to just from views, as Revit does).
To go back to the original question - is it possible that BIM models will replace drawings - the answer is yes and no. Yes BIM models will one day be able to communicate design intent, but no, they won't be BIM models as we know them now. There will have to be some sort of added technology. And it is not just a technological problem. If every BIM software vendor has a different approach we will be no better off. It is not realistic to expect contractors and trades people to be familiar with a whole range of software products just to get the information they need to build a building. There has to be some kind of commonality - like drawings, schedules and specifications.
There are probably technologies out there that could do the job now, but with nothing definitive in sight I would say for the foreseeable future it will still be drawings, schedules and specifications. So what does that mean for the BIM models we do now?
SO WHAT IS A BIM MODEL?Another way to look at it is to define a BIM model as something that is not intended, by itself, to communicate design intent or how to construct the building.
Rob Snyder, who is involved in Bentley's Hypermodel initiative, has succinctly defined the problem. To him the BIM model is the 'environment'. But this environment is not meaningful unless there are 'statements' about it. For example a drawing with notes, dimensions, details is a 'statement'. It only shows the portion of the 'environment' that is relevant, with added 'statements' to point out things that need to be communicated.
So a BIM model is not another way of documenting a building project. It can not communicate in it's own right. It's purpose is as a resource to create deliverables. To create construction documentation, to run structural, thermal and other analysis, to participate in FM, and a whole range other other uses.
PRACTICAL CONSEQUENCESBy treating a BIM model as a thing in itself confusion is avoided about where information that can be used to construct the building resides. This also clarifies where your contract documentation deliverables can be found.
That said, a BIM model may well be a client deliverable, but make it clear it is only provided as a resource for others to produce their deliverables. So by all means provide your BIM model to others outside the design team, but strip out all traces of documentation - views, sheets, schedules.
You can do this very quickly in Revit:
- Create a 3D view only showing what you want to export.
- Place that view of a sheet.
- Find the sheet in the Project Browser, right click on it, select "Save as file...".
And make sure project BIM plans are only be about the BIM model. List documentation (production of drawings & schedules) as a BIM Use, but don't include anything about drafting standards or drawing production. These can be referenced in the BIM Plan, but should be completely separate documents.
So don't treat your BIM model as part of your documentation deliverable, and be clear to everyone, from your boss & colleagues to clients, contractors and trades, of this fact.