How to Read and Use MBS Column Reactions

Column reactions are the MBS result that leaves the analysis group and enters everyone else's work. Foundation engineers size footings from them. CAD staff put them in tables and notes. Fabricators do not, but they will feel the error if an anchor-bolt plan was drawn to the wrong load case. Reading reactions is therefore not a software trick. It is an engineering handoff: identify the column, the case, the component, the units, and the sign convention before a single number is copied.

MBS reaction output typically lists support forces and moments at each column base for each load case or combination your run included. Component labels vary by version—often a vertical force, two shears, and moments about one or more axes—so use the documentation for the version you actually run. Do not assume that a column headed Fz is gravity in every file you have ever seen. Confirm the axis system against the model geometry, especially if the building was rotated to match a site grid.

This article is the working companion to Understanding MBS Output Files for PEB Engineering. It stays on reactions: how to read them, how to use them in foundation and approval work, and how not to mix envelopes with individual cases. When you are ready to tabulate, use How to Organize MBS Reaction Data into a Usable Table or StruTools MBS Reactions. The full review path is in PEB Reaction Data Workflow: From Software Output to Engineering Review.

Identify the support before you trust the number

A reaction without a stable column ID is not usable. Grid marks, frame lines, and interior-versus-endwall labels have to survive the trip from the model to the table.

• Match the support ID in the report to a grid intersection on the architectural and CAD plans.
• Separate interior rigid-frame columns from endwall posts. They are not interchangeable rows.
• Note left and right columns of a clear-span frame; wind and crane cases will not be symmetric just because the building looks symmetric.
• If the model uses a different origin than the drawing grid, write the mapping on the extract cover.

Read load cases as part of the reaction

The number 180 is meaningless until you know whether it is dead load, a wind combination, a crane case, or an envelope. The case name is half the result.

List the cases you intend to issue: gravity service, gravity strength, wind combinations that produce uplift, wind combinations that produce compression, crane if present, and any owner special case. Then extract those rows only. A dump of every combination on one sheet is how the wrong value gets circled in a meeting.

If you issue an envelope, state whether it is a maximum compression, a maximum uplift, a maximum shear, or a signed pair. How to Organize MBS Reaction Data into a Usable Table shows a layout that keeps the governing case visible instead of hiding it behind a single 'max' column.

Units, signs, and axis systems

Three silent errors account for most bad footings that started as 'correct' MBS numbers: unit mix-ups, reversed signs, and local axes that nobody checked after a model rotation.

Units

Write kN, kip, N, or lb on every column of the extract. Convert in one place, once, with a checked factor. Do not convert in CAD by memory.

Signs

Agree whether compression is positive, whether uplift is negative, and whether moments follow the model right-hand rule. Put that sentence on the table header. Foundation staff should not have to guess.

Axes

Global building axes and member local axes are not the same thing. After a building is rotated to a site grid, re-check that Fx and Fy still mean what the drawing notes claim. If they do not, relabel the table; do not relabel the footing in your head.

How foundations should use MBS reactions

A reviewed reaction table is input to foundation design, not a footing schedule. The foundation engineer still chooses pad, pier, strip, or pile and still applies their own load factors if the table was issued at service level.

• State clearly whether values are service or factored. Do not make the foundation group guess.
• Provide coincident shears and moments for the same combination, not a patchwork of unrelated maxima, unless you also provide the envelope rules.
• Call out net uplift cases so anchor-bolt tension is not designed from a gravity-only row.
• Pass crane and wind cases as themselves. Combining them in a mystery 'worst' column hides whether the crane was on the building.

How approval drawings should show reactions

Approval sheets need a readable table or a note that points to a calculation package, not a screenshot of the analysis post-processor. Coordinate the sheet with From MBS Output Files to PEB Approval Drawings.

Typical practice is a reaction table on the anchor-bolt plan or on a design-data sheet, with grid marks that match the plan. Include units, sign convention, and the analysis issue letter. If the table is too large, put governing values on the drawing and keep the full extract in the calculation set.

CAD layers and numbering should follow the office standard, including approval drawing numbering if that is how your packages are organized. The analysis origin of the numbers still has to be cited on the sheet or in the transmittal.

When a reaction should not be used

Some printed reactions are complete as text and still unusable as engineering input. Stop rather than 'just send something' to foundations.

• The run is incomplete or still carries unresolved support warnings.
• Load combinations do not match the basis of design.
• Column IDs cannot be mapped to the current grid.
• Pinned-base models show large moments you cannot explain, or fixed-base models show zero moment at a base you intended to fix.
• You cannot state units and sign convention in one sentence.

Reading a reaction extract in order

Do not start in the middle of the number block. Walk the extract the same way every time.

1. Confirm the extract belongs to the issued model and drawing issue, not a trial run.
2. Read units, axis notes, and whether values are service or factored.
3. Map each support ID to a grid mark. Separate interior frames from endwalls.
4. List the load cases you will issue; hide or archive the rest.
5. Check one known gravity case by hand against tributary roof and wall weight as a sanity test.
6. Only then copy values into the project reaction table or StruTools MBS Reactions.

Engineering tips

• Sanity-check gravity axial load with a tributary-area estimate before you believe wind uplift.
• Keep left-hand and right-hand columns on separate rows even when the frame is symmetric.
• If an envelope is issued, store the governing case name next to the value.
• Uplift and compression for the same column should not share a cell.
• Re-extract reactions after any change to eave height, crane capacity, or bracing that can move load to the base.

Common mistakes

Circling the largest number on the page

The largest printed value may be a factored combination, a local-axis shear, or a moment you do not intend to take at the base. Circling it is not a review.

Giving foundations an envelope without coincident components

Maximum axial plus maximum shear plus maximum moment from three different cases is not a load combination. Either issue coincident sets or explain the envelope rule.

Assuming pinned bases produced zero moment

Check the support definition in the model. If the support was accidentally fixed, the moments are real in the model and wrong for the intended base-plate detail.

MBS reaction handoff checklist

Use this before the table leaves the analysis group.

• Extract is from the issued run, not a trial folder.
• Column IDs map to current grid marks.
• Interior frames and endwalls are labeled.
• Units are written on the table.
• Sign convention and axis system are written on the table.
• Service versus factored is stated.
• Load cases to be used are listed; unused cases are not mixed in.
• Uplift is visible where it governs.
• A gravity sanity check was performed on at least one typical column.
• Recipient (foundations, CAD, or both) is named.

Frequently asked questions

Should I issue every load combination MBS printed?

No. Issue the cases your foundation and approval standards actually use, labeled clearly. Keep the full extract in the calculation set.

What if my MBS version uses different component names?

Follow that version's documentation for labels and axis sense. The engineering task is unchanged: identify component, case, units, and support.

Can I put reactions only in the calculation book and omit them from drawings?

Office standards differ. If drawings omit the table, the transmittal must point to the exact extract and issue letter so field and foundation teams are not guessing.

How do I convert reactions into a table without retyping?

Use a controlled extract and a tool such as StruTools MBS Reactions, then still review mapping, units, and cases. Automation does not replace the sanity check.

A reaction is a labeled force, not a leftover printout

If you can name the column, the case, the component, the units, and the sign, you can use an MBS reaction. If you cannot, you are passing a typesetting problem to the foundation engineer.

Tabulate next with How to Organize MBS Reaction Data into a Usable Table, then run the numbers through PEB Reaction Data Workflow: From Software Output to Engineering Review before they appear on an approval sheet.