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Much of a building is no longer made on site. Structural steel, ductwork, pipework, precast concrete and curtain walling are made in factories, often straight from a fabrication model (or shop model) built by the fabricator's detailers. The design model is their starting point. Your job as the BIM person is to make sure the two models meet: the fabricator starts from good information, and what comes back is checked before anything is cut.
A design model shows the size and position of elements. A fabrication model adds everything needed to make and install them: connections and bolts, plate thicknesses, hanger positions, joints, cut lengths and a part number for each piece. Machines in the factory can read it directly.
The ductwork is coordinated against the structural design model and signed off. Meanwhile the steel fabricator deepens a transfer beam for its connections and has its shop model approved by the engineer. Nobody federates it. On site, the ductwork contractor finds 150 mm of duct running into the beam's bottom flange; the duct is remade.
The BEP says approved shop models are uploaded to the CDE and federated in place of the design model for their zone. The coordinator's weekly clash test picks up the deeper beam against the duct. The mechanical engineer drops the duct within the ceiling void a week before fabrication of the ductwork starts.
Who models what
Who models what?
Pick who usually produces each piece of information on a project with prefabricated steel and ductwork.
- Frame layout, member sizes and design loads
- Steel connections, plates and bolts
- Duct routes and sizes for the design
- Duct fittings, hangers and part numbers
- The federated model and the clash tests
Fixing design before fabrication
| Step | What the BIM person checks |
|---|---|
| Before detailing | The design model in that zone is coordinated, and its fixed information is listed |
| Shop model received | Federated, checked against neighbouring models and against the design intent |
| Shop model approved | Replaces the design model for that zone; noted in the model register |
| After approval | Changes go through the change log, not straight into either model |
The clinic's steel and ductwork are both to be prefabricated. Write the three rules you would add to the BEP so that the shop models and the coordination stay in step.
Key takeaways
- Fabricators work from their own detailed models; the design model is their starting point, not their product.
- Agree which design information is fixed before fabrication starts, and log every change after it.
- Shop models come back into the federated model and are checked like any other model.
- Approved shop models replace the design model in that zone; don't coordinate against both.