BIM model gives us quantities

David Miller

The second article in our Digital Futures series looks at how BIM and digital design can take the drudgery out of estimating quantities and give architects another service to offer clients

The quantification model for a typical flat in an off-site modular social housing project.
The quantification model for a typical flat in an off-site modular social housing project. Credit: David Miller Architects

There are many possible futures with digital design, some of them closer than you might realise. Through the process of realigning our design teams with a BIM level two workflow, we at David Miller Architects have moved from a descriptive narrative of the project to a more prescription virtual representation. We therefore no longer expect the constructor to interpret information and fill in the gaps. We have a clear move from the ‘descriptive’ to the ‘prescriptive’. This offers benefits beyond the obvious ones of more accurate and coordinated construction information; it creates the opportunity to interrogate the model for quantity.

But before exploring the opportunities (and challenges) this presents, a little more about our BIM workflow. It has made a leap. Before BIM it was a descriptive narrative of our, and our consultants,’ designs, made up from disparate drawings, diagrams and specifications. These were all coordinated to a greater or lesser extent depending on the time and skills available.  

Our new mode is a more prescriptive virtual representation of a project, made up of coordinated 3D geometrical models, structured schedules and non-graphical information that follow the agreed conventions and rules. These are all brought together using processes that are embedded in the collaboration platform that is used to manage the project. 

This has allowed us to expand our services to include digital quantity take-off. Our models are now assembled out of virtual components that ‘know what they are’. They can be organised and sorted just like any other database items. Once we have a library of virtual components (these use agreed naming and classification systems), it is possible to automatically sort the information generated by the model. Information about components is diverted into the appropriate pricing book and appropriate rates can be applied. The cost manager can then build up the overall figure by combining the package costs. Because this is automated, changes can be tracked and it becomes reasonable to review costs much more often – and to track the cost implications of changes, which makes value engineering a science rather than an art.

The model needs to be a genuine virtual version of the project that is going to be built. This does require more work and that needs to be factored into any fee proposal

Not everything is modelled, though what is has to be a true representation of what is actually going to be built. The quality of the cost estimate now depends on the quality of the model. The model has to be reliable, it’s not enough to model simply to produce drawings. It needs to be a genuine virtual version of the project that is going to be built. This does require more work and that needs to be factored into any fee proposal. For example, if there is a change of brick specification below DPC, the two brick types need to be modelled separately. Similarly, if you aren’t going to plaster above a suspended ceiling the plaster should stop at ceiling level in the model.

It’s important to agree what is actually going to be modelled; this should be confirmed and recorded early in the design process. This is because quantities can be generated in a number of ways and be the responsibility of different members of the team:

  • Extracted quantities are derived from actual objects within the model, for example number of doors, areas of plasterboard etc.
  • Interpreted quantities are taken from the model, but not necessarily modelled. An example could be skirting boards which could be generated automatically by applying a formula, for example perimeter of the rooms minus the width of the doors = length of skirting.
  • Assumed quantities are identified by using traditional QS techniques.

Another technique being employed is to include simple pieces of abstracted geometry in the model to represent a component without fully modelling it. This allows the components to be automatically scheduled, but keeps the model light and avoids excessive complexity. A good example of this would be cavity barriers behind rainscreen cladding or EPDMs around windows. Getting this balance right will evolve over time and will be different from project to project.

The implementation of these techniques could have a very positive effect on procurement strategies and create a leaner workflow. As the quantity information is more defined, there is more confidence in the numbers. This leads to a different approach to risk. Instead of offloading the risk of material costs down the supply chain, clients or tier one contractors can more confidently take the risk on materials themselves or even ‘free issue’ them avoiding waste [checking].

A final thought is to recognise that the model is only generating quantity information, it is not interpreting it. For example a tall thin wall will cost more than a low wide wall, but will have the same number of bricks, so the role of professional cost managers will still exist. Their time can be spent on high value services rather than the mechanical task of taking off quantities, while there is a potential for the architect to provide those quantities.  

David Miller is director of David Miller Architects. He is creative director of the RIBA’s 2017 Guerrilla Tactics



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