Federated IFC did not give me automated quantity takeoffs. It just looked like it did.
I federated Architecture, Structural, and HVAC IFC models, then wrote a Python parser to pull element counts and BaseQuantities.
The script ran fine. No errors. Clean output.
The problem was that the numbers were wrong.
The HVAC model was the first red flag. It had duct segments and air terminals, but no quantity sets. No length, area, or volume. The parser did not fail. It just returned empty values and moved on.
Then the counts were inflated. Some elements appeared in multiple models, like chimneys and roofs. If you just append IFC files without deduplicating by GlobalId, you can double-count things while the final QTO still looks totally normal.
The last issue was ownership. Walls were split between Architecture and Structural. So a rule like “walls belong to Arch” would quietly miss half of them.
That was the main lesson for me: federation is not the same as clean QTO automation.
Before trusting the output, you need to check:
GlobalId duplicates
missing quantity sets by discipline
element ownership across models
Otherwise you are not really automating the takeoff. You are just producing a spreadsheet that looks convincing.