02 June 2014

BIM is not so Real after all

In my last post, Keep your BIM Model Real, I explored the issue of relating a BIM model to the real world. One of my conclusions was to "actively include tolerances in our BIM Models".

I was wrong.


Firstly 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.


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).


So 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.


Great, 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.


'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.

Bored with BIM?
Need a present for that special woman in your life?
The Lost Woman series follows the adventures of Christina as she makes her way through a world of design, fashion, new media and ... men.
Book one of the series, "Awakening the lost woman", is available now on Amazon,  Google Books,  Kobo  and  iBooks.


  1. Are rounded/padded dimensions a common deliverable specification for you, I'll say I have never come across this at all.

    I have come across contractors who scant regard to dimensions of any kind, but not who stipulate they have control over the design.

    1. Jonathon, I didn't say rounded/padded dimensions are a deliverable. Contractors don't request rounded/padded dimensions, they say they want easy to measure dimensions. They want to be spoon fed with easily digested information.
      So designers oblige.

  2. Antony, a great read and very thought provoking.
    I have never really understood why anyone from whatever discipline or type of industry feels the need to round up dimensions or details. Surely it only takes the same "effort" to type in an accurate dimension as an inaccurate one. Certainly there are a million occasions when a tolerance factor needs to be applied to a dimension and it's here that the conscientious drafter/modeller comes into his own rights as a true professional. Without doubt as you have stated, the likes of Revit is perfect for building in "padding" or LOD.
    Determining a true dimension or increment depends on many factors. For example, stock sizes in sheet metal for ventilation ducting, or dare I mention positioning windows and doors on an architectural model to coincide with actual brick or block dimensions; but again this is surely down to the mentality or mind-set of the author and their willingness to go that little bit further.
    This mind-set applied across the BIM model surely can only enrich the model itself and ultimately strive towards the ethos of BIM.
    And yes everyone has their own expectations and levels of detail or requirements but perhaps everyone needs to think a little bit more about BIM and what it means to themselves and how it fits in with the BIM ethos.
    Having said all that, perhaps it boils down to an individual's definition of BIM. From what I have seen, that seems to a vast array of thoughts.

  3. As a highly specialized subcontractor I am surprised that even here "tolerance" is not well understood. They can be found in the specs. Every dimension has a stated tolerance, be it for a material (waviness of the flange of a beam, bowing of precast) or for a location in space (column-column is x feet +/- 1/4" at the base but there is also a tolerance for plumbness, etc.). The issue here is not simply means and methods, but what is in the model. Designers should understand tolerances because otherwise the design includes details that can be contractually acceptable in terms of stated tolerances but yet not right aesthetically or functionally. The windows&brick example is a good one. There are two very distinct issues here - what can BIM software do and what the creator of a BIM has in terms of knowledge of actual tolerances and their relevance.

  4. One unintended side effect of using padded thicknesses at design then changing to actual later is if walls/elements are carefully aligned at design, when "unpadded", the elements will become unaligned. Given the dimensions you are talking about, the offsets would be slight...

  5. One of the great maxims I have always used when modelling in Revit is - "Build it in the model the same as you build it in the real world" ie, if a part has two pieces, model two pieces, don't 'weld' them. Sure you can still use one single family, but the parts might be two subfamilies. The caveat is "within reason" - I'm not going to model individual bricks!

    The same maxim also implies "Don't cheat". Rounding dims is a form of 'cheating' - you are making a false representation of what the real world actually is. OK, it is "traditional" to "make it easy for the contractors" but there is not much excuse today in a world of laser distos, site ruggedised ipads and digital plans. The dimension is what it is.

    Other industries 'got over' this issue long ago - would you step onto an aircraft built with +/- 20mm tolerance? What about +/- 5mm tolerance? Of course aircraft are built to tolerances as well (very small ones!) but the point is BUILT to tolerances - not designed to tolerances.

    The is the same point made by planetcommercialconstruction above - tolerance is applied on the building site, NOT the design drawings. If a dimension on plan is stated at 1033mm, then it is perfectly acceptable (and reasonable) for the contractor to construct the dimension at the easy-measure 1035mm - well within the tolerance stated in the contract (let's say +/-5mm for example).

    If the designer rounds up to 1035mm, then the contractor builds 1040mm now we have a problem - the true dim is +8mm - outside the tolerance.

    It's the contractor's job to work the tolerances, not the designer's.

    From experience, I can tell you that rounding dimensions (it wasn't me!) produces an almighty mess when you're working on a build 350m long!

    From my distant youth I recall a certain German engineer from a major conveyor manufacturer responding to my query about the tolerance on their setout drawings. I was coordinating these to Australian concrete peno plans, with the typical Australian "she'll be rite mayte!" +/-50mm(!) concrete tolerance:
    me: "OK, so what tolerance are you allowing for the penetration locations?"
    German engineer: [stunned silence] "TOLERANCE?! Zere ist NO TOLERANCE!"

    1. Adrian, I like the point you make - as designers we express the actual in our model, but be aware that the contractor will not necessarily exactly follow it.

      The consequence is our design (arrangement of elements) must not depend on contractors exactly achieving what is in our model. Designers must be aware variances will likely occur, and +/-5mm probably a reasonable assumption.

  6. German engineer: [stunned silence] "TOLERANCE?! Zere ist NO TOLERANCE!"
    Draw what you mean. No fudge factors.