When is a door not a door . . . . choosing the correct entity classification for quantity take off.
The old joke goes “When is a door not a door, when it’s ajar”. My dad used to get constant amusement from this joke and I perhaps now truely understand why it can be so funny but perhaps in more of a desolate manner.
Recently I upgraded my export process out of Archicad 23 from IFC2x3 TC1 to IFC4. Rather than relying on an out of the box translator I had spent the time to create a Archicad classification system for IFC2x3 and IFC4 allowing a one to one type mapping on export.
This I am finding is more and more important the further I get immeshed in the workings of IFC. Going back to my father’s often repeated joke, I now find myself asking this question over and over again for each element in my BIM model. The latest anomaly I have found is when is an IfcCovering CLADDING not an IfcCovering CLADDING but an IfcCurtainWall and conversely when is an IfcCurtainWall not an IfcCurtainWall but an IfcCovering CLADDING.
At this point because from my limited experience in estimating, it comes down to what facade is being modelled and what shape the panels take.
The cladding layout shown was originally classified as IfcCovering CLADDING.
However on closer inspection of the IfcCovering only the thickness of the cladding material is given and only when this material is ‘prismatic’, as shown in property Width, Q_Length. (My own opinion some clarification of language use here needs to be done because a lot of people are at first glance going to get this wrong).
This classification is not the appropriate one for the FC cladding shown above. A take off cannot be achieved because no width nor length is available. Note here width actually means width not thickness. Just to make it more confusing than it already is.
So I went hunting through the IFC Schema for the appropriate classification. IfcCurtainWall has the correct quantity values for take off.
Again note here width actually means width not thickness. I know lockdowns are hard but I really do we really need to make it this hard? If I wasn’t crazy already . . . . .
So the lesson I have learned is that you cannot have a predefined classification system that associates a BIM modelling tool with a specific classification in IFC because you will end up making it much more difficult to get the correct quantities you need for your take off.
So when is a door not a door . . . . .