BIM Steel Fabrication Workflow: From Model to Shop

A fabrication BIM workflow is not “model it and press draw.” It is a sequence that starts with the right kind of model and ends with a shop that can cut, drill, weld and ship without asking which file is current. Design models explain member sizes. Manufacturing models explain holes, copes, bolts, welds, marks and assemblies. If you skip the translation between those two, the shop inherits a coordination view dressed up as a cutting instruction. That is how NC files and PDFs disagree on a hole that nobody meant to move.

The sequence is older than the software. Input, layout, connections, approval, shop information, fabrication support — the same detailing process, run with parts instead of only lines. Drawings still exist because fitters, inspectors and many reviewers still read sheets. Lists still exist because purchasing does not walk a model. Machine data exists when the shop asked for it and when it is generated from the same revision as the sheet. The modeller’s job is to keep those three outputs on one story, not to abolish two of them.

This article stays general. It does not walk through a particular program’s commands. Whether the office uses Tekla Structures or another manufacturing modeller, the control questions are the same: which objects are fabrication-level, when drawings are issued, how fabrication drawings relate to erection drawings, and what happens when a comment arrives after plates are nested. Approval-stage sheets can still be a standardised 2D package; the MBS Approval Package sits at that earlier gate for teams whose approval set repeats from job to job.

Design model versus fabrication model

Treat the incoming design model as a background until detailing has added manufacturing information. Copying a design beam into a nested plate file because “it was already 3D” skips connection design, copes and marks. That is not a shortcut. It is an incomplete connection detailing step.

• Confirm section, grade and camber against the design drawing, not only the imported solid
• Add connection objects from an approved typical or a documented connection design
• Number parts and assemblies only after the mark system is written down
• Freeze a copy of the design import so later architectural moves are visible as diffs

Connection information in the model

If the connection is not in the model, the piece drawing is fiction. Model the plates, bolts and welds that the shop will actually put on the member, at the level the contract requires. Schematic nodes are fine during layout. They are not fine at NC release.

Shop drawings still exist

Creating drawings from a model is a report, not a review. The sheet still needs a title block, a revision letter, readable dimensions and a checker. Offices that skip drawing standards because “the model is the deliverable” discover that the shop plotted a default view with overlapping text.

Map drawing types to your existing index: piece, assembly, GA, erection. Name them the way the shop already files them. A software default layout that nobody recognises will be marked up as if it were a draft, because it is.

NC, BOM and paint or mark data

Machine files and lists must come from the same model revision as the PDF. If purchasing orders from an Excel export that someone filtered by hand, you are back to two sources. Prefer a report from the model, then a structured workbook only for print layout — see Excel Programs when the shop still wants a table on paper.

Erector information

Erection drawings are not leftover GAs. They show marks, orientation, field bolts and any temporary works the contract requires. A model can produce them; a person still decides what an erector must see at a glance. Read erection drawings explained before you assume a 3D view replaces that sheet.

Change management after nest

Once plates are nested, a model change is a commercial event, not a regenerate. Identify whether the change hits a part already cut, a drawing already issued, or only a future lot. Cloud the drawing, bump the revision, and regenerate NC only for the affected parts with a new identifier.

1. Log the comment or RFI against marks, not against “the model.”
2. Decide lot status: not nested, nested, cut, shipped.
3. Revise drawings for anything already issued.
4. Re-issue lists and NC only for the marks that changed.
5. Tell the shop which previous NC files to discard.

From model to a shop release

Use this as the outer loop. Software steps sit inside it; they do not replace it.

1. Import or lay out geometry and freeze the design background used.
2. Apply typicals and model connections to fabrication level for the release lot.
3. Number parts and assemblies to the written mark system.
4. Create drawings and run a detailing check as if they had been drawn by hand.
5. Issue approval information first if the contract splits that stage.
6. Release shop PDFs, BOM, bolts and NC from one frozen model copy.
7. Support shop queries on that revision; do not edit the live file silently.

Engineering tips

• Release in lots that match mill and nest packages, not “the whole building when the model feels ready.”
• Put the model revision identifier on every PDF title block.
• Keep a discard list for superseded NC files. Shops will run whatever is in the last email.
• If a part cannot be modelled to fabrication level yet, keep it off the nest. A hold is cheaper than a recut.

Common mistakes

Nesting from a live model

Someone will keep detailing while the machine is cutting. Freeze, then nest.

BOM in a side spreadsheet that nobody regenerates

The bill of materials has to follow marks. A side file is a second project.

Erection marks that do not match piece marks

The erector and the fitter must share a language. One mark system, two sheet types.

Changing a typical after fifty assemblies are numbered

That is a revision project. Treat it as one, with a list of affected marks.

Checklist

• Fabrication-level objects complete for the release lot
• Mark system applied without duplicates
• Drawings checked, not only created
• BOM and bolt list generated from the same revision
• NC generated only after PDF check
• Model copy frozen and archived with the issue
• Hold items excluded from nest
• Superseded machine files listed for discard

Frequently asked questions

Can we send the model to the shop instead of drawings?

Only if the shop asked for that format, can open it, and the contract still names a record. Many shops still plot sheets for the floor. Send what they can use, from one revision.

Where do approval drawings sit in a BIM fabrication workflow?

Usually before manufacturing release: geometry, typicals and reactions for the reviewer. Shop and NC come after comments. Do not nest on an unapproved typical.

What if only part of the building is modelled?

Then only that part can follow this workflow. The rest stays on its CAD process. Mixed methods are fine if marks and revisions do not collide.

Do we still need Excel?

Often, for print layouts, hold lists and registers the shop already knows. Generate numbers from the model; use the workbook as a readable sheet, not as a second database.

Where this leaves the work

A BIM fabrication workflow is a freeze-and-release discipline. The model earns its keep when drawings, lists and machine data leave together and come back together after a change. If any one of those travels alone, you are running three projects.

Keep fabrication drawings and the detailing process as the human sequence. The modeller is how you author that sequence when the job is dense enough to need it.