Understanding Column Reactions in PEB Design

Column reactions are the PEB frame speaking to the foundation. After analysis, each base has forces and, depending on fixity, moments. Civil engineers will design footings, piers and slabs from the table you send. If that table is a screenshot, an unexplained envelope, or a mix of two model revisions, the foundation is no longer part of the same building as the steel.

PEB reactions often originate in metal-building software printouts, then migrate into Excel, then into a civil drawing. STAAD.Pro check models produce another format. The numbers are only useful if marks, axes, units, combinations and base conditions travel with them. Reading STAAD.Pro support reactions covers that environment. MBS Reactions covers MBS-style output. This article is the PEB-centred view: what the components mean and how to issue them.

Nothing here invents combination factors. Which cases are included is a design-basis decision. The job of the reaction package is to report what the agreed model actually produced, with enough annotation that a stranger can use it. StruTools MBS Reactions is one way to turn a printout into a reviewable table; the engineering checks in this guide still apply.

What a column reaction represents — and what it does not

A support reaction is the force (and moment, if the support can take it) that the structural model needs at that base to equilibrate the frame under a stated load case or combination. It is not a geotechnical bearing pressure, not a pile design, and not a slab check. Those require the civil model, soil data and the way the base plate actually spreads load. Sending “max axial” without context invites civil to treat it as all of those things.

Reactions also do not replace connection design at the base. A large shear with a thin plate and an optimistic grout gap is still a steel problem. Keep the base detail and the reaction table in the same revision as the portal frame elevation.

Components: axial, shears and moments

Most PEB tables report vertical load, major-axis shear, minor-axis shear, and moments about those axes when bases are fixed or partially fixed. Uplift is not a separate physics; it is axial tension, and it must be visible. Horizontal shears in braced bays can govern ties and pedestal reinforcement even when axial looks modest.

Pinned versus fixed bases

If the analysis released moments, the table should not show leftover moments from a previous fixed-base run. If the analysis fixed the base, the drawing must not show a thin pinned typical. The design workflow treats this as a freeze, not a preference.

Envelopes versus individual combinations

An envelope is a set of maxima and minima that may not occur together. Civil sometimes needs coincident components for a single combination, not a Frankenstein of max axial from one case and max moment from another. State which you issued.

Where PEB reactions come from in software

Metal-building programs print reactions in their own column order. STAAD.Pro prints supports in global directions unless you specify otherwise. Excel then becomes the meeting place. The conversion step is where signs flip and columns shuffle. Build a template that includes an axis sketch and a mapping from software labels to civil labels, and do not casually paste into last job’s workbook.

If MBS is the sizing engine, extract from MBS for issue, and use STAAD only if it is a true check of the same bases. Two reaction sets for one revision is a coordination defect, as noted in the software guide. StruTools MBS Reactions exists to make the MBS extract inspectable; still read the mapping.

Checks before the table leaves the steel office

A reaction table is an issued engineering document. Treat it like a drawing: date, revision, author, and a check. Spot-check a corner column and a braced-bay column against the model. Confirm uplift appears where wind should lift a light eave. Confirm crane combinations exist if the building has a crane.

1. Marks match the current approval base plan.
2. Units printed on every sheet of the table.
3. Sign convention sketch included.
4. Envelope versus coincident combination stated in a sentence.
5. Base fixity matches the model and the elevation.
6. Special columns (crane, wind, stepped) are not averaged into a typical.
7. Superseded tables marked superseded when a new analysis revision is issued.

Braced bays, expansion joints and other non-typical bases

Bracing dumps shear into specific columns. Expansion joints create two buildings that look like one on a marketing rendering. Corner columns see two wall directions. If your table only lists “typical interior,” civil will under-design the interesting bases and over-design the rest.

When architecture moves a door and you relocate a bracing bay, the reaction table is stale the same day. Re-issue it with the revised approval set, not as a quiet Excel email two weeks later.

Issuing PEB reactions with a matching plan

The table and the base plan are one deliverable.

1. Freeze frame geometry, base fixity and the combination list from the design basis.
2. Run analysis and review supports for obvious sign or unit errors.
3. Extract reactions into a table that preserves combination names.
4. Add marks, units, axis sketch and fixity notes.
5. Spot-check at least one typical, one corner, one braced and one special column.
6. Issue the table with the same revision letter as the approval base plan.
7. Send civil the PDF or controlled workbook, not a chat image.
8. When the model changes, supersede the table even if “only a purlin moved” — confirm it really did not affect bases before you skip this step.

Engineering tips

• Put the analysis filename and date in the table header.
• Use the same column mark style as the steel plan; do not invent civil-only names.
• If you issue both service and strength, label columns so they cannot be concatenated.
• Watch uplift on windward light frames; a table of compression-only “max” is a red flag.
• Keep crane-on and crane-off stories distinct if the project requires it.
• When using extraction tools, print a control total (sum of verticals versus total gravity) as a sanity check where equilibrium allows.

Reaction-table mistakes that become foundation errors

Pasting MBS numbers into last project’s Excel with different axis labels

The cells fill, the formulas look alive, and FX is now FY. Rebuild the mapping or use a checked template.

Sending node numbers from STAAD without a mark map

Civil does not know node 187. They know column C12 on grid D.

Issuing reactions from a fixed-base model beside pinned-base drawings

Moments in the table have nowhere to go on the plate, or the plate is huge for a pin. Pick one system.

Quietly updating steel without re-issuing reactions

A moved brace bay changes shears. Foundations already in the ground are not a reason to hide the new table; they are a reason to stop the steel change or redesign the base.

PEB reaction issue checklist

If any line fails, the table is not ready for civil.

• Revision matches the current PEB base plan.
• Every column on the plan appears in the table, and vice versa.
• Units are stated.
• Sign convention sketch is on the first page.
• Base fixity is stated per column type if it varies.
• Combination names match the design basis document.
• Envelope policy is explained in one sentence.
• Uplift / tension is visible where it occurs.
• Braced-bay columns are identifiable.
• Crane and mezzanine inclusion is stated.
• Author and checker are named.
• Superseded tables are marked if this is not Rev A.

Frequently asked questions

Should PEB reactions be maximum envelopes or per load combination?

Follow what the foundation engineer and the design basis need. Envelopes are convenient but can mix non-coincident components. If in doubt, ask civil which format their foundation model requires, then issue that format from the same analysis revision.

Why do MBS and STAAD.Pro reactions differ on the same PEB?

Different geometry, releases, secondaries, combination lists or sign conventions. Treat the difference as a model problem until those match. Do not average the two tables.

Do pinned-base PEB columns really have zero moment at the foundation?

In the analysis idealisation, a true pin reports no moment. Real base plates, grout and anchors have some stiffness. The issued table should follow the agreed idealisation, and the base detail should be consistent with it. Deviations are a design judgement, not a rounding error.

Can I send reactions before approval drawings?

You can send a preliminary table labelled preliminary. Civil should not construct from it. Once geometry and bracing bays freeze, issue a revision that matches the approval plan.

Reactions are a drawing, even when they live in Excel

A PEB column-reaction table is part of the building definition: marks, axes, units, fixity, combinations and the analysis revision they came from. Treat it with the same control as an approval sheet. Foundations designed from an unexplained envelope are not coordinated; they are lucky or unlucky.

Continue with STAAD.Pro support reactions and MBS reactions if you need environment-specific reading. Combination factors and foundation design remain project-specific. StruTools provides educational process and optional extraction tools; it does not certify geotechnical or structural adequacy.