Coordinating MBS Results with CAD and PEB Approval Drawings

MBS knows a building as analysis geometry and results. CAD knows it as layers, blocks, and sheets. Coordination is the work of making those two descriptions match: the same width, the same bays, the same column grids, the same units, the same reactions on the same supports. When they do not match, approval drawings become a third building that exists only on paper. From MBS Output Files to PEB Approval Drawings mapped which result belongs on which sheet. This article is about keeping the CAD model honest after that mapping starts.

Most PEB offices still draft approval sets in AutoCAD even if analysis lived in MBS. That is fine. What is not fine is tracing frames from a screenshot, rotating a site plan until shears point the wrong way, or typing reactions from a print that used a different origin. Read How to Read and Use MBS Column Reactions before you place a reaction table on a DWG. Use AutoCAD Layer Standards for Structural Steel Drawings so analysis notes do not land on a no-plot layer.

Automation can help after the match is defined. StruTools MBS Reactions feeds a table CAD can transcribe or import. The StruTools MBS Approval Package helps compose the set. Neither tool will notice that CAD's grid is 250 mm off the model. That check is still a person with the issued geometry summary and the DWG in the same review. Layer, block, and file habits that support the check are covered later in the AutoCAD guides.

Lock the grid before you draw the first column

If the CAD grid is sketched from an architectural PDF while MBS used a different width, every later check is theatre.

Copy bay spacing and building width from the issued geometry summary, not from a scaled PDF. If architecture and MBS disagree, stop and resolve it as an analysis issue, not as a CAD fudge factor. Grid bubbles should be blocks with attributes so a later rename does not leave dead text; see Using AutoCAD Blocks to Improve Structural Drawing Consistency.

Orientation, north, and reaction signs

Site rotation is the classic silent error. The model was along X. The CAD plan was rotated to north. Shears kept their old names.

• Write a one-line axis note on the reaction table and on the plan.
• If CAD is rotated, relabel Fx/Fy in the table language the sheet uses, or keep global names and show the rotation.
• Do not mirror a frame in CAD and keep the original left/right reaction rows.
• Confirm that 'left column looking at the frame from side A' is the same sentence in analysis and in CAD.

Units in the DWG and units in the table

A drawing in millimetres with a reaction table in kips is readable only if both units are stated. It is dangerous if one is assumed.

Keep analysis units on the table even if the drawing is metric or imperial the other way. Convert in the signed table, not with a CAD scale factor applied to text. Insertion units and dimension styles are a separate CAD-file topic; they must not become a second conversion of foundation forces.

Layers for analysis-sourced content

Reactions, design notes, and 'from MBS issue B' stamps should live on dedicated layers so they can be frozen for shop sets later without deleting the data.

• A visible layer for reaction tables and design-data notes
• A layer for analysis-issue stamps in the title block or note block
• No analysis notes on DEFPOINTS or on a no-plot layer unless that is an intentional working overlay
• Layer names that match the office standard in AutoCAD Layer Standards for Structural Steel Drawings

What CAD may change without sending work back to MBS

Not every CAD edit is an analysis change. Knowing the difference keeps the workflow from freezing, and from drifting.

Usually CAD-only

Notes, dimensions, hatching, sheet composition, title-block attributes, clouding of purely graphical errors.

Always back to MBS

Eave height, width, bays, crane capacity, frame type, support fixity, missing or added columns, large openings that change load path.

Review the pair, not the parts

A CAD check that never opens the issued MBS package is only a drafting check. A model check that never opens the DWG is only an analysis check. Approval needs both.

On each approval issue, sit the geometry summary, the signed reaction table, and the DWG together. Confirm grid, sizes, and a sample of reactions on the sheet. Number the set with the office system, including approval drawing numbering. If you also bring STAAD geometry into CAD on hybrid jobs, keep that path separate with StruTools Staad2CAD so PEB frames do not inherit the wrong origin.

A coordination pass before each approval issue

Short, scheduled, and documented. Not 'we will notice on the plots'.

1. Open the issued MBS geometry summary and the current CAD grid.
2. Confirm width, bays, eave, and slope. Stop if they differ.
3. Confirm orientation and the reaction sign note.
4. Place or update member sizes from the governing member report.
5. Place or update the reaction table from the signed extract only.
6. Stamp the analysis issue letter on the sheets.
7. Record the coordination pass on the transmittal.

Engineering tips

• Keep a 'coordination origin' sketch: model X, CAD X, north, and where grid A1 sits in both.
• Use the same grid-bubble block on every PEB job so attributes extract cleanly.
• If a frame is typical, say how many bays it covers; do not imply the whole building if endwalls differ.
• Freeze working CAD layers during the coordination pass so markup is not confused with issued geometry.
• When in doubt, send the change back to MBS. Quiet CAD edits of width are not coordination.

Common mistakes

Tracing the analysis screenshot

Screenshots are not dimensioned and they carry perspective from the view cube. Type dimensions from the geometry summary.

Different rounding on the sheet and in the table

Pick a rounding rule in the signed table and print that. CAD should not round again.

Updating only the plan, not the elevation

Eave height lives on the elevation. A coordinated plan with an old elevation is still uncoordinated.

MBS–CAD coordination checklist

Attach to the approval transmittal for the issue.

• CAD grid matches issued geometry summary.
• Eave height and slope match on elevations.
• Column marks match the reaction table IDs.
• Axis / sign note is on plan and table.
• Member sizes match the governing report.
• Reaction values match the signed table (sample check at least two columns).
• Analysis issue letter is on the sheets.
• Layers for analysis notes follow the office standard.
• Changes that affect load path were sent back to MBS, not only clouded.

Frequently asked questions

Should we import MBS geometry directly into AutoCAD?

Only if your version and office standard support a checked export. Many teams still draft from issued dimensions. Either path is acceptable if the coordination checks pass.

What if architecture, MBS, and CAD all disagree?

Stop. Pick a governing issue, update analysis, then CAD. Do not average three widths.

Can LISP help with this coordination?

Yes, for placing grid bubbles, stamping issue letters, and extracting attributes. See AutoLISP for Structural Engineers: Practical Automation Examples. It will not decide whether eave height is correct.

Where do shop drawings enter?

After approval geometry is coordinated. Shop drawings should not invent a fourth grid. They inherit the approved CAD model.

CAD is the published model of the analysis

Coordination is not a export button. It is a repeating check that grid, orientation, sizes, and reactions on the DWG still equal the issued MBS package. Fail that check and the approval set is fiction.

For steel-detailing habits inside AutoCAD itself, continue with Using AutoCAD for Structural Steel Detailing. For the package those drawings belong in, stay with From MBS Output Files to PEB Approval Drawings and the StruTools MBS Approval Package.