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Inconsistent IFC BREP export with IfcRelVoidsElement

Wilt
Participant

According to the IFC specification here:

https://standards.buildingsmart.org/IFC/DEV/IFC4_2/FINAL/HTML/schema/ifcproductextension/lexical/ifc...

The following rules applies:

 

An IfcOpeningElement has to be inserted into an IfcElement by using the IfcRelVoidsElement relationship. It may be filled by an IfcDoor, IfcWindow, or another filling element by using the relationship IfcRelFillsElements. Depending on the type of the IfcShapeRepresentation of the IfcOpeningElement the voiding relationship implies:

  • if the IfcShapeRepresentation.RepresentationIdentifier = 'Body', then the Body shape representation of the opening has to be subtracted from the body shape representation of the voided element - implicit Boolean difference operation.
  • if the IfcShapeRepresentation.RepresentationIdentifier = 'Reference', then the Reference shape representation of the opening is not subtracted, it is provided in addition to the hole in the Body shape representation of the voided element.

But when we in ArchiCAD 26 export walls with the export gross geometry box marked the opening voids are not labeled correctly with 'Reference' but with 'Body'. Hence the exported file suggests the boolean operations still have to be performed. While in the Archicad documentation here it clearly states this is not needed:

https://help.graphisoft.com/AC/26/INT/_AC26_Help/121_IFC/121_IFC-37.htm

 

Export gross geometry of elements (Export holes separately)

Elements containing Doors, Windows and/or Openings are exported with their gross geometry. For example, a host Wall is exported with its gross geometry, without considering the hole created by a window.

Use this option to ensure accurate cost calculation using the IFC model, where the opening geometry will be considered separately.

Note: Openings are exported by checking IfcOpeningElement in the Model Filter for IFC Export. This is generally checked by default.

If the Export gross geometry option is unchecked, then elements cannot be created using extrusion. In this case, extruded elements with holes are exported as BREP!

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To summarize, if the extruded elements with holes are exported as BREP the RepresentationIdentifier for the ShapeRepresentation of the voids need to be set as 'Reference' and not as currently exported 'Body'.

The receiving side will try to subtract the shapes again from the walls leading to unnecessary CSG operations and inaccuracies in the resulting geometries.


Note: This issue applies also to IFC2x3 and older ArchiCAD versions.

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