Classification, IfcBaseQuanties and Schedules

[Tintifax2]

A recuring issue with quantity surveys via IFC Base quantities is the following:

(All screenshots are from the virgin Archicad 28 template) 

This is a rant - bear with me.

1. Creating some 2x2m slabs and classifying them as Foundations according to all available Subclassifications for "Fundamente".

To us German speaking folk this implies that all listed Elements under "Fundamente" will be exported as IfcFootings.

But "Fundamente" and "Footings" are different conceptual sets to different languages. Hence Archicad allows for individual predefined typing. Turns out that to some building traditions "Fundamentplatten" are "Slabs" not "Footings". -- > great feature, all cultural misunterstandings solved. 😈

 Since IFC export works correctly those "Slabs" become IfcSlabs - not IfcFootings.

Their BaseQuantities will add accordingly: 

Most users generate their quantities in Archicad from schedules not IfcBaseQuantities

Such "discrepancy" confuses the hell out of people, especially when unaware of the backstage mechanics.  The remedy is simple. List for IfcTypes instead:

Personally, I'm a big fan of USERDEFINED Types.

Which are NOT accessible in schedules! Please make ALL IFC classification aspects accessible to schedules. We need to be able to cross-check and verify those IfcBaseQuantity results in Archicad. (Who can afford Solibri anyway?)

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"Volume" is the only Quantity, that will consistently corelate with its exported BaseQuantity. (Sometimes there are minimal geometric differences due to intersections .. but those are irrelevant)

Also Volume is available for all 3D Archicad elements. Most other BaseQuantities are NOT. When forced to reclassify ArchiCAD Elements (Vaults, domes, arches ... do exist, regardless of BuildingSmart's acknowledgement) quantities are dropped.

Trying to change IFC Geometry Export setting results in unavailability of those changed settings:

In Schedules those "IfcSlabs" generate the following Dimensions:

So why are available Dimensions not exported to IFC BaseQuatities?

Test suggest that each ArchicadElement has a predefined set of exportable Dimensions, regardless of Classification. Morphs will only export Volume to IFC.

One can easily calculate an average thickness via an expression (d=Volume/Area) and map the result to a custom IfcParameter ... Thus simply exporting all human errors made in expressions to IFC

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IfcBaseQuantities and the precise dimensions they represent: such is an enigma wrapped inside a riddle ...

Most quantity deviations are due to false/unintentional attribution (see re-classification above)

Followed by inexperienced modelling (openings in slabs, roofs) Also Archicad official modelling guidelines are deceiving. Modelling for precise quantities is different from just generating floorplans.

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Summary of issues:

• Enabling schedules to reproduce IFC quantity surveys (full access)
• Improving both interfaces for Classifications and Type mapping (see IFC Export & Predefined Types 
• Export options / preview for Base Quantities

[Tintifax2]

Postscript

The "confusion" over IfcElement attribution got fixed in Arch27.

Until then, IfcType was determined by the "genus" in the classification manager list, regardless of IFC Type mapping in export settings. (Anything listed under "Foundation" became IfcFooting).

Since all Templates up to Archicad 26 suggested otherwise, (see Slab example in my original post) WE got confused. 

 

[Tintifax2 · Accepted solution]

Correction:

Classification will also limit BaseQuantity output.

e.g. IfcCoverings (regardlesss of the Archicad element used) will not export Volume to IFC

Hence: I'm dropping BaseQuantities for good --> ArchicadQuantities instead!