Primary and Secondary Members in Pre-Engineered Steel Buildings

Every PEB drawing set quietly depends on a classification: primary members versus secondary members. Primary members are the frames and major elements that define the building’s skeleton. Secondary members are the repetitive cold-formed items that support cladding and, just as importantly, restrain those frames. Mix the two up and you will analyse a building that cannot be built, or detail a building whose rafters were never restrained the way the calculations assumed.

The split is not a status ranking. A purlin is not unimportant because it is secondary. In many PEB systems the rafter’s compression flange is only stable because purlins and flange braces exist at the spacings used in design. Remove them on a drawing to “simplify the elevation” and you have changed the primary member. That is why purlins and girts and bracing systems belong in the same conversation as portal frame components.

This article is a working map of PEB member families, how they appear in analysis versus on approval drawings, and how marks should stay consistent through fabrication. It assumes you already know what a PEB is. Tools such as StruTools MBS Approval Package help present the members; they do not classify them for you.

Why PEB offices bother with the classification

The classification drives three practical decisions: what is in the analysis model, what is on the approval elevation, and what the shop cuts as a unique plate versus a repeating cold-formed length. It also drives procurement. Built-up primary frames come from the welding shop; Z-purlins often come from a roll-former. If a wind column is accidentally ordered as a girt, the error is not semantic.

A fourth reason is restraint. Design of slender built-up rafters depends on how often the flange is braced. That brace might be a purlin, a flange brace down to a purlin, or a dedicated fly brace. Calling those items “miscellaneous steel” on a drawing is how they disappear from both the model and the bill of materials.

Primary members: the skeleton the analysis is about

If you stripped cladding, purlins and girts and the remaining steel could still be described as the building’s frames, you are looking at primary members. They take collected loads to the foundations and they dominate column reactions.

Main frames

Interior and typical rigid frames — tapered columns and rafters, knees, ridges and splices — are the PEB trademark. Endwall frames may be rigid or may be post-and-beam bearing frames. The choice changes how longitudinal and lateral loads travel, so it is a primary-system decision, not a cladding decision.

Wind columns, portal beams and crane runways

Endwall wind columns, spandrels, crane beams and heavy mezzanine girders behave as primary members even when they look like “extras.” They attract load, they need connections designed for that load, and they often change adjacent girt and purlin layouts.

What primary members need on drawings

Marks, overall depths, taper points, splice locations, base type and the grid they sit on. Depth “similar to typical” is not enough when the next frame is a crane frame.

Secondary members: cladding support and primary restraint

Secondary members repeat, but they are not decoration. Roof purlins span frame to frame and carry sheeting. Wall girts do the same on walls. Eave struts sit at the intersection of roof and wall and often participate in both systems. Sag rods, bridging and anti-roll clips keep slender cold-formed sections from rolling. Flange braces connect primary flanges to secondaries so the restraint assumed in design exists in the building.

Layout is a structural decision

Purlin spacing is driven by sheeting capacity, by purlin capacity and by the brace points the rafter needs. Changing spacing to suit a skylight without checking those three is a design change. The dedicated purlins and girts article covers layout mechanics.

Openings punch holes in the secondary system

A large door removes girts and may remove a bracing panel. Trimmers and wind posts then become local primary elements. Draw them as members with marks, not as lines on an architectural elevation.

How the classification should appear across models and sheets

Analysis software, approval drawings and shop bills often use different names for the same piece. The office needs a translation table. A rafter that is MEMBER 12 in STAAD, RF-3 on the approval elevation, and mark R3-1 in the shop must still be recognisable as one object when a comment arrives.

Load path from cladding into primary frames

Wind on wall sheeting goes into girts, into wind columns or main columns, and then into bracing or frame action. Gravity on roof sheeting goes into purlins, into rafters, into columns, into bases. If a drawing shows sheeting spanning the wrong way, or purlins stopping short of a frame because of an undocumented opening, the load path on paper is fiction.

• Confirm sheeting span direction before locking purlin and girt spacing.
• Check that eave struts can actually receive both roof and wall sheeting as assumed.
• Do not assume a masonry wall braces a PEB column unless the interface is designed.
• If purlins are nested or continuous, say so; simple-span capacity is not continuous-span capacity.
• Flange-brace length and orientation must reach a secondary that can take the force.
• End bays often differ: closer purlin spacing, different girt conditions, wind columns.
• Mezzanine beams that sit on PEB columns are primary for those columns even if someone listed them on a “miscellaneous” sheet.

Detailing and fabrication implications of the split

Primary built-up members generate unique nested plates, weld maps and splice design. Secondary cold-formed members generate repeating lengths, punching patterns and clip standards. The shop needs both identities. If flange-brace holes are omitted from purlin punching because “secondaries are standard,” the erector will drill on the ground and the restraint you designed will be in the wrong place.

When primary and secondary marks collide (both called “C1”), procurement and erection suffer. Reserve prefix families: frames, columns, purlins, girts, braces. Then keep those prefixes stable from approval through shop drawings.

Classifying members on a new PEB job

Do this once at kickoff and the drawing legends write themselves.

1. List every steel item on the architectural and owner briefs, including cranes and walkways.
2. Tag each item primary, secondary, stability fitting, or by others.
3. Decide which primary items are in the analysis model as members versus as loads.
4. Decide which secondary items provide flange restraint and therefore cannot be “optional.”
5. Assign mark prefixes and stick to them on plans, elevations and reaction tables.
6. Place specials (crane beams, mezzanine girders) on their own sheets so they are not lost among girts.
7. Review one typical bay as a full-depth section: cladding, purlin, brace, rafter, girt, column.
8. Issue the classification with the first approval set so consultants use the same language.

Engineering tips

• If a member takes foundation-level load or supports other steel, treat it as primary until proven otherwise.
• Never hide flange braces only in a typical that is not referenced on the frame elevation.
• Endwall posts that carry girts are primary enough to deserve marks and base details.
• When a purlin also acts as a strut in a bracing bay, say so on the roof plan.
• Keep cold-formed grade and coating on the secondary schedule; they are not interchangeable leftovers.
• If analysis omitted secondaries, still draw them before you claim the frame design is finished.

Classification errors that corrupt PEB design

Calling flange braces miscellaneous and omitting them from the BOM

The rafter design may have counted on them. If they are not procured, the as-built restraint is not the designed restraint.

Analysing secondaries as simple spans while drawing them continuous

Continuity changes moments and interior reactions on purlins. The drawing must match the assumption or the assumption must match the drawing.

Using the same mark for a main column and a wind column

Erection and reaction tables become ambiguous. Civil will apply the wrong load to the wrong footing.

Treating crane beams as secondary because they are “not the PEB frame”

Crane runways load the frames, the bracing and the bases. They are primary for everyone who has to live with the deflection.

Deleting girts at a door and forgetting the wind post

The opening still collects wind. Without a designed post and header, the wall load path is broken.

Primary / secondary review checklist

Use on the first approval issue and again before shop release.

• Every frame line has a unique mark used on plan, elevation and reactions.
• Wind columns and endwall rafters are marked, not only drawn as thin lines.
• Purlin and girt spacing matches sheeting and brace-point assumptions.
• Flange braces are shown where the rafter or column design requires them.
• Eave struts are identified and their role in roof and wall systems is clear.
• Openings have trimmers and posts with marks.
• Crane beams and mezzanine girders are classified primary and included in analysis if they load PEB columns.
• Secondary schedules list section, length rule, coating and punching typical.
• Bracing members are not left as anonymous dashed lines.
• Prefix legend appears on the cover or general notes.
• No two different members share a mark.

Frequently asked questions

Is an eave strut a primary or a secondary member?

It is usually specified with the secondary system, but it often carries both roof and wall sheeting and may act as a strut. Treat it as a named member with a defined section, not as leftover purlin stock, and check any axial role in bracing.

Do secondary members need to appear in the STAAD or MBS model?

Not always as finite elements. They do need to appear in the design intent: as loads, as restraint spacing, and on drawings. If you omit them from the model, document how their load and restraint were applied.

Why do flange braces sit between the two classes?

They are small fittings, but they enforce the unbraced length of primary flanges by connecting to secondaries. Miss them and a “secondary” omission becomes a primary-member stability problem.

Can hot-rolled members be secondary?

Yes. Some walls use hot-rolled girts, and some roofs use hot-rolled purlins for long spans or heavy hanging loads. Classification follows function and load path, not whether the piece came from a mill or a roll-former.

Name the members the way the load path works

Primary versus secondary is a load-path language. Frames and major beams take collected load to the ground. Purlins, girts and eave struts deliver cladding loads and restrain those frames. Flange braces and bridging make the assumed unbraced lengths real. If the names on your drawings do not match that physics, the shop will still build something — just not the building you analysed.

Continue with portal frame components, purlins and girts and bracing systems. Member capacities and restraint rules remain project-specific. This classification guide is educational and does not replace the responsible engineer’s design.