How to define USERDEFINED types in RelatedIfcEntityNamesList

[araccaine]

As I understand the documentation, the field RelatedIfcEntityNamesList can have multiple related IFC entitites.

• Without any type would be e.g. "RelatedIfcEntityNamesList": [ "IfcBeam" ]
• A IfcBeam with predefined type LINTEL would be e.g. "RelatedIfcEntityNamesList": [ "IfcBeamLINTEL" ]

But how do we define a IfcBeam with the USERDEFINED type MyBeam? Do we need to define a new relation with IsChildOf or something like that?

[atomczak]

You're correct with the "RelatedIfcEntityNamesList": [ "IfcBeamLINTEL" ] example. You can't use USERDEFINED because that is not a defined class in IFC dictionary. I know there is a dichotomy between official IFC4.3 and what we published in bSDD.

That is because we are trying to educate people not to use USERDEFINED and NOTDEFINED options, and instead simply leave the predefined type empty and assign a class (IfcClassificationReference) to a registered term. This way we increase consistency of the information.

To answer to your use case, I recommend modeling this as an IfcBeam without predefined type, with an IfcClassificationReference to MyBeam from your dictionary. And yes, MyBeam can be registered with relation IsChildOf IfcBeam. The added value of this approach is that one element can have multiple class references (for example if you are using multiple systems), which was not possible with predefined types.