Understanding MBS Output Files for PEB Engineering

Once an MBS model runs, the program writes a pile of output. Some of it is geometry. Some of it is member design. Some of it is support reactions. Some of it is diagnostic text that only matters when the run failed. PEB teams get into trouble when they treat that pile as a single document, or when they pick a convenient page and ignore the rest. Output is a set of calculation artifacts, not a drawing set, and not a substitute for What Is MBS Software in Pre-Engineered Building Design?.

Exact file names and extensions depend on the software version you run. This article stays at the conceptual level: member reports, reaction reports, geometry summaries, and load-combination lists. Follow your vendor documentation for the clicks that produce each report. The engineering job is the same across versions—know which output answers which question, keep the issued set together, and refuse to copy numbers from a trial run that was never marked complete.

The rest of this guide maps common output families to the people who use them: frame designers, foundation engineers, and CAD staff preparing approval sheets. When reactions need a table instead of a printout, use How to Organize MBS Reaction Data into a Usable Table or StruTools MBS Reactions. When those numbers must land on sheets, the StruTools MBS Approval Package is the drawing-side counterpart, not a replacement for reading the output first.

Do not hunt for a secret file format

Offices waste hours trying to reverse-engineer proprietary export layouts that change between versions. You do not need a hex dump. You need a named, reviewable extract.

MBS can print or export reports as text, tables, or other office-standard formats depending on version. Whatever the container, treat it as readable engineering output: labels, load cases, member IDs, and numbers with units. If a colleague sends a file you cannot open, ask for the report they used for issue—not a scavenger hunt through program directories.

When you automate later, parse only the reports your office has agreed to keep. How Engineering Automation Can Process MBS Output Files covers that discipline. Inventing an unofficial binary specification is a maintenance trap and is not something this guide will do.

Geometry output: prove the model is the building

Before anyone believes a member ratio or a reaction, the model has to match the architectural issue that is current this week—not last month's schematic.

Print or save a geometry summary with the issued output. CAD staff should be able to read bay spacing from that summary and see the same numbers on the grid. If they cannot, stop. You do not yet have an output file worth detailing.

• Width, length, eave height, and roof slope
• Bay spacing and the identity of interior versus endwall frames
• Frame type (clear-span, modular, single slope, gable) as modeled
• Support locations and whether bases are pinned, fixed, or otherwise restrained in the model
• Openings, cranes, or mezzanines that were included or omitted

Load and combination output: the hidden source of bad reactions

A reaction table can be perfectly transcribed and still be wrong if the run omitted a crane case or used an old wind speed. The load list is part of the output, not optional cover material.

Look for the list of applied load cases and the combinations used for design and for reactions. Confirm that gravity, wind, snow or rain, seismic if required, crane, and collateral cases match the basis of design. Confirm that service and strength combinations are not mixed when you later copy a number onto a foundation sketch.

If your office issues both an 'analysis combination' reaction and an 'envelope' reaction, label them. How to Read and Use MBS Column Reactions explains why unlabeled maxima are dangerous.

Member reports: what to keep versus what to skim

Member reports can run to many pages. Issuing every page without a governing-frame summary hides the results that actually matter.

• Keep a governing-frame summary: controlling ratios, governing load combinations, and final sizes for typical interior and endwall frames.
• Keep detailed reports for frames that are not typical: crane bays, stepped-eave spans, or frames next to large openings.
• Do not archive only the last page you glanced at. A reviewer should be able to reconstruct why a rafter size was accepted.
• If secondaries were designed in MBS, store those reports separately from primary-frame reports so they are not mistaken for column design.

Reaction output: the file everyone else will ask for

Foundation engineers and CAD staff rarely want the full member report. They want column-base forces that they can check and put on a sheet.

Reaction output should identify the column or grid, the load case or combination, the force and moment components, and the units. If any of those are missing, do not send the file downstream. Organize it using How to Organize MBS Reaction Data into a Usable Table or StruTools MBS Reactions before it reaches a footing design.

Store the reaction extract with the geometry and load-combination output from the same run. A reaction file with no parent model is an orphan, and orphans get reused on the wrong issue.

Diagnostics belong with the issued set

Warnings about unapplied loads, missing combinations, or incomplete solutions are easy to skip because they are not a table of numbers. They are often the only sign that the pretty report is unfinished.

• Save solver messages and warning lists with the issued output.
• If the program reports an incomplete run, do not extract reactions from that run.
• If warnings mention supports or load cases, resolve them before CAD starts.

How to file an issued MBS output set

An issued set is a package, not a single print. Use the same package structure on every job so reviewers know where to look.

1. Create a folder named with project, frame line if needed, and date or issue letter.
2. Save the model file used for the run, or a snapshot if your office standard requires it.
3. Save geometry summary, load-combination list, member governing summary, full member reports as needed, and reaction extract.
4. Save diagnostics or a note that the run completed with no unresolved warnings.
5. Write a one-page cover: software version, code edition, who reviewed, and which drawing issue this output supports.
6. Point CAD and foundations to that folder only. Trial runs stay elsewhere.

Engineering tips

• Name output folders after the drawing issue they support, not after the time of day you clicked print.
• Keep units visible in every extract. Silent kN versus kip swaps are still the classic PEB error.
• If two runs exist for the same building, the cover sheet must say which one is current.
• Do not email a reaction page without the load-combination list from the same run.
• When automation arrives, feed it the issued report, not a working directory full of trials.

Common mistakes

Keeping only the reaction print

Reactions without geometry and load combinations cannot be checked. Six months later, nobody knows whether wind was on the building or only on a schematic model.

Mixing trial and issued files in one directory

The newest file is not always the issued file. CAD staff will pick the newest. Separate the folders.

Forwarding screenshots instead of reports

A cropped screenshot loses units, load-case names, and the column ID. Use a real extract, then a table.

Issued MBS output checklist

If any item fails, the output is not ready for foundations or CAD.

• Software version and code edition are written on the cover.
• Geometry summary matches the current architectural issue.
• Load cases and combinations are saved with the reports.
• Governing member summary exists for typical frames.
• Reaction extract identifies column, case, components, and units.
• Diagnostics were read; incomplete runs were discarded.
• Trial output is not in the issued folder.
• A reviewer name and date are on the cover.
• The package is linked to a drawing issue letter or number.

Frequently asked questions

Which MBS output file should I send to the foundation engineer?

A reviewed reaction table from the issued run, plus enough load-combination context to know what the numbers mean. Not the entire member-report stack unless they ask for it.

Are output file extensions the same in every MBS version?

Not reliably. Do not depend on an unofficial extension list. Ask for the report your office uses for issue and follow the documentation for your version.

Can I delete member reports after I extract reactions?

No. Reactions are only one product of the run. Member reports are the record of why sizes were accepted. Keep the issued set together.

What if CAD needs geometry, not reactions?

Give them the geometry summary from the same issued run, then coordinate in CAD as described in Coordinating MBS Results with CAD and PEB Approval Drawings.

Output is a package you can defend

Understanding MBS output is less about memorizing report titles and more about knowing which artifact answers which question. Geometry proves the model. Load lists prove the criteria. Member reports prove the sizes. Reactions prove what the foundation must accept.

Once you can name those pieces, run MBS Output Quality Checks Before Using Results in Design Workflows before anyone copies a number, and keep How to Read and Use MBS Column Reactions next to the extract you actually issue.