I was wrong.
WHY IT IS WRONGFirstly there are technical reasons 'padding' quantities is bad practice. In my previous post I tried to show that it didn't matter for quantity take-off. But accurate quantities are important for other purposes as well.
As Tim Froise pointed out in an LinkedIn discussion:
' But there are occasions where the volume is important. If the wall component is considered for its U-value, there is an additional 36% of plasterboard material, which is significant. 'He also points out it will affect acoustic and thermal mass calculations.
Indeed he is right. Materials in Revit have values assigned to them that can be used for these types of calculations.
And materials with padded thicknesses make a difference.
I can't justify rationalising away all these valid uses for accurate data.
But perhaps the most pertinent comments were about why I though it OK to mess with the BIM model for my own purposes.
From Tim McDougald:
' I watched this argument at my old firm between two different Architects within the same building. One wanted everything drawn "real" and the other rounded everything off. 'I recognised myself as one of those architects. And it is true, there is no consensus between architects (which is why I started analysing this issue).
The view held depends on what particular problems an architect is grappling with. For example at design stage the aim is to please the client, so architects tend to minimize construction allowances so higher usable and lettable areas can be achieved. During documentation the aim is constructability, construction allowances have to be found to ensure what has been designed can be achieved in the real world.
But this has nothing to do with the BIM model. It has to do with the competencies of the architects involved.
Embedding the solution into the BIM model risks reducing the designer's obligation to directly deal with their professional responsibilities. Architects who think their BIM model will 'take care' of constructability will feel confident they can ignore construction limitations. Engineers who let their software come up with solutions don't feel the need to explore alternatives.
And this is what my last post attempted to do. Shift architect's responsibility to consider constructability on to the BIM Model.
Another comment, this time from David Conant:
' I think the pressure to make a 114 wall at 120 is really another manifestation of "don't make me change the way I do things" without questioning why. 'The error I made is all too common. An approach that tries to embed too much into BIM models. To use them as much as possible to solve problems we encounter, ignoring how this may effect others using the model.
What we need to appreciate is that there are limits to what a BIM model (and hence BIM processes) can do, and to modify our expectations accordingly.
WHAT CAN A BIM MODEL DO?On "questioning why", as David suggested, I recognised the traditional purpose of dimensioning is to provide the contractor with clear instructions, or requirements, of where things are located. By rounding dimensions we are attempting to make it easier for them through expressing requirements in a clearer manner.
This is because in traditional construction documents the purpose is to provide the contractor with sufficient information to fulfil requirements that designers (i.e. architects and engineers) identify.
This often leads to a battle between contractors and designers, where designers try to provide as little information as possible, and contractors demand specific 'how to' instructions. The architect provides a note - "fix securely", the contractor wants a drawing showing the location of every screw.
Now using BIM doesn't (or hasn't yet) overcome this. Contractors still want more in the model than designers think is necessary or reasonable (and to be honest practical). On the other hand Architects, and especially engineers, don't see why they need to model enough to create a coherent virtual building.
But the significant point here is the different ways traditional documents and BIM communicate information.
As explained above traditional documents attempt to explain what the requirements are. Those who receive them expect those documents to directly communicate requirements without interpretation.
A BIM model is a facsimile of requirements. It is a representation of the final product of those requirements (i.e. a virtual model). And as such can only every reflects what the real world requirements are, it does not specifically spell them out.
For example traditional documents might have all fire rated compartments identified on drawings with a heavy dashed line. This directly tells the contractor where fire compartments are located, and by inference which walls and doors have to be fire rated.
In a BIM model those walls have their materials and construction modelled as virtual walls and doors that meet fire rated performance requirements, they may even have data associated with them stating their required fire rating. But it will be up to the contractor to extract information from the model that identifies their location, and hence extent of fire compartments (contractors need to know this because it is not just materials that create fire isolation, it is also the way they are put together).
TAKING RESPONSIBILITYSo to go back to my dimensioning example.
So yes, architects should represent walls correctly without padding, and not worry about dimensions that are unobtainable in the real world.
The consequence of this is that dimensions will not be as clear and instructive as found in traditional documents. The contractor will not be able to merely read off dimensions and use them directly, dimensions will have to be interpreted to make them useful on site, in the real world.
I see this as another consequence of utilizing BIM, an example of roles and responsibilities changing. Is it a bad consequence? I think not. The contractor is best placed to make decisions about what is best on site. And honestly, if architects were any good at it I wouldn't be spending so much time trying to find a fix for it!
The bigger picture here is that a BIM model - a virtual building - does not communicate information in the same way traditional documents do.
Traditional documents are designed to spoon feed relevant information, to highlight what is critical and what is not. BIM models just provide information. True, much more information, but not all of it is relevant to a particular recipient. It is the recipient's responsibility to extract the information they require, and if necessary manipulate it to make it useful.
For contractors no more "we didn't do it because you didn't specifically tell us to", with BIM it is "you should have identified the need from the model", (although "we didn't do it because it wasn't in the model" is still fair enough).
I've used contractors as an example, but the same applies right across all AECO processes. We all need to recognise that BIM is not the same as traditional delivery, and that it does not provide information in the same manner.
THE PRACTICAL REALITYGreat, glad we got that sorted. But what does that mean, how should this insight effect what we do now?
As I have written before in my post Can BIM alone be used for Construction, there is currently no practical way to deliver a project purely using BIM. We are in transition. We use BIM where we can but are still expected (and contractually required) to provide traditional documents.
This is where I came unstuck with dimensioning. Traditional documentation calls for rounded, padded dimensions. But we are using BIM software, and it is not designed to do this (nor should it be).
So on the one hand I'm trying to fulfil my contractual obligations, on the other trying to produce good quality BIM, both to help my processes and for others to utilize.
What is the solution? All I can think of is a hybrid.
Use real thicknesses and unapologetic actual dimensions, then add 'clear' or 'min./max.' dimensions where there are critical requirements to be met.
If you really feel the need to pad walls do it be adding a 'tolerance' layer.
But there is nothing preventing walls used at early design stages, when their construction is unknown or undecided, from being thick enough to incorporate tolerances.
For example use 150mm or 130mm for generic internal walls instead of 100mm. This is where BIM helps, it is a trivial exercise (at least in Revit) to change walls from one type to another.
CONCLUSION'Why bother' you might ask. If we are not doing real BIM why try and do things in a BIM like manner?
The reality is BIM is not an all or nothing proposition. A lot of money is always at stake on a building project, involving lots of people with differing views. The risks are enormous, so BIM will be used where there are perceived benefits and low risk, BIM by itself will never be the most important driver.
We users of BIM know its benefits, but must be prepared to demonstrate them. And the best way to do that is to be prepared. To embed BIM practices in what we do even when we don't need to. And where necessary resist following traditional practices where they conflict with good BIM practice.
A BIM model is not, and can never be, 'Real'. It is repository of data that creates a facsimile of the real world. It is up to us, the creators and users of that BIM model, to use that facsimile to inform what we humans need to know to create the real thing, in the real world.
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