Main Components of a PEB Portal Frame

The portal frame is the PEB’s backbone. Two (or more) columns, rafters meeting at a ridge or continuing as a single slope, haunches at the knees, perhaps a ridge haunch, bolted splices where plates would otherwise be unshippable, and bases that tell the foundation what to expect. If you can read that elevation, you can read most PEB buildings. If you cannot name the parts, you will misplace flange braces, mis-draw splices and send civil the wrong column reactions.

This article stays at component level. It does not size a haunch or pick a bolt group. Those are project calculations. It does explain what each part is for, how it usually appears on approval drawings, and where analysis models quietly omit something the shop will weld. Readers who need system context should start with what a PEB is and primary versus secondary members.

Endwalls, crane frames and multi-span interiors are variations on the same kit of parts. Once the typical interior rigid frame is clear, the variations are easier to brief. Keep this map next to the bracing and purlin guides so fittings are not orphaned from the frame they stabilize.

Reading a PEB portal frame from eave to ridge

Start at the base, not at the pretty ridge. The base condition — pinned, fixed, or partially restrained — changes moments throughout the frame and changes what civil must provide. Then read the column taper, the knee, the rafter, any interior splice, the ridge, and the opposite side. Only then overlay purlins and flange braces. Engineers who start with cladding lines tend to draw a house and forget it is a moment frame.

A single-slope frame is still a portal frame; it simply has no symmetric ridge. A multi-span PEB adds interior columns that may be pinned at the rafter or moment-connected, depending on the system. Name that interior joint on the elevation. “Looks like a continuous rafter” is not a joint specification.

Columns, rafters and the built-up taper

Most PEB main-frame columns and rafters are welded I-sections with webs that change depth along the member. The taper is not decoration; it is an attempt to put depth where bending is high and to save steel where it is not. Analysis must use that geometry. A STAAD prismatic stand-in is a different member.

Columns

Exterior columns often taper from a deeper section at the knee toward a shallower section at the base on pinned-base frames, though practices vary. Interior columns in multi-span buildings may be prismatic. Crane columns may be stepped. Each shape needs its own elevation, not a copied typical.

Rafters

Rafters carry roof load and participate in frame action. Depth usually increases toward the knee. Roof slope, haunch length and splice location all belong on the elevation because they are fabrication data and because they affect clearances to doors, cranes and ducts.

Haunches, ridges and splices — the joints people hand-wave

The knee haunch is where PEB frames earn their stiffness. It is a deepening of the rafter, the column, or both, with a web and flanges that must be designed as a connection region, not as a drawing convenience. Ridge details range from a simple bolted apex to a haunched ridge. Splices exist because a 30-metre rafter does not travel as one piece, and sometimes because the design was segmented.

Knee haunch

Show haunch length, depth at eave, and whether the inner flange is straight or broken. If analysis used a different haunch length than the shop typical, the typical is wrong for this job.

Ridge and apex

The ridge is a geometry control point for purlins, cladding and interior clearance. It is also a connection. If erection bolts a ridge plate, that plate is part of the frame, not a flashing accessory.

Field splices

Locate splices where the design allows and where shipping requires. A splice drawn at midspan because it “looked even” may sit at a moment you did not intend to connect with a standard end-plate family.

Bases and the conversation with foundations

The base plate, grout gap, anchor pattern and assumed fixity are the portal frame’s last components. They turn frame analysis into reactions someone else will use. If the elevation shows a thin pinned base while the model is fixed, civil and steel are not on the same building. Put an axis sketch on the reaction sheet that accompanies approval.

• State pinned, fixed, or the actual spring/partial model used — in words, on the drawing.
• Match base-plate size to the column flange geometry at the base, including any taper.
• Show anchor projection and whether bolts are cast-in or drilled, as the civil package requires.
• Keep column marks identical on frame elevation, plan and reaction table.
• Do not hide a holding-down bolt inside a typical that is not cut at this column.
• If a pocket or stub column exists, draw it; it changes effective height.

Fittings that belong to the frame, not to “miscellaneous”

Flange braces, knee braces (when used), crane brackets and walkway support seats attach to the portal frame and change local plates. They should appear on the frame elevation or a dedicated connection sheet referenced from it. Bracing systems covers building-level bracing; this section is about fittings that live on the frame itself.

If a rafter design assumes a flange brace at each purlin, those braces are frame components. If a crane bracket welds to the column web, the web stiffeners are frame components. Listing them only on a secondary BOM is how they are value-engineered out of existence.

When the “typical portal” is no longer typical

Stepped crane columns, monitors, canopies, lean-tos, valley conditions and frames that pick up mezzanines are still portal frames, but they need their own elevations. Copying the interior typical and stretching it is how clearances die. Give each special frame a mark series and an analysis case that includes its extra loads.

Multi-gable and corner conditions also break the simple two-column picture. The component names remain useful: columns, rafters, haunches, splices, bases. The arrangement does not. Draw the actual line; do not force a valley into a standard ridge typical.

Building a portal-frame elevation that matches analysis

A short production sequence for the first interior frame on a job.

1. Print or export the analysis geometry: widths, eave, ridge, taper points, haunch lengths.
2. Draft the centreline frame, then add depths as analysis states them, not as a remembered typical.
3. Place splices at the designed stations; confirm shipping lengths with the shop.
4. Add base condition and plate outline; confirm it matches the reaction table’s assumption.
5. Tick purlin and flange-brace locations so restraint is visible.
6. Annotate marks that will be used on plans and in reaction tables.
7. Peer-check against the model: three dimensions and one haunch length at random.
8. Only then copy the typical to other bays and edit the specials.

Engineering tips

• Dimension taper change points from a documented origin (base or eave), not from a random CAD snap.
• If the model used a prismatic equivalent, say so on the elevation until a tapered property exists.
• Keep left and right columns as separate marks even if they are symmetric; erection and reactions prefer it.
• Show roof slope as a ratio and as an angle if your audience is mixed.
• Do not let cladding lines overprint the inner haunch flange; reviewers need to see steel.
• Photograph the first fabricated frame against the elevation; it is the cheapest shop-drawing audit.

Portal-frame drawing mistakes

Drawing a haunch as a filled triangle with no plate logic

The shop cannot fabricate a hatch pattern. Provide web and flange intent, or reference a designed typical that matches this depth.

Omitting the rafter splice because CAD drew one polyline

The erector will still splice it. If the location is not designed, it will be improvised.

Using a pinned-base typical on a fixed-base analysis

Moments, plate thickness and civil design all disagree. This is a system error, not a CAD typo.

Stretching a typical frame to a crane bay

Stepped columns, brackets and different tapers make it a different component set. Give it its own elevation.

Portal frame elevation checklist

Check the PDF of the first interior frame before cloning it across the building.

• Overall width, eave height and ridge height match the input register.
• Roof slope matches cladding and drainage intent.
• Column and rafter depths at key stations match analysis.
• Haunch length and depth match the property used in analysis.
• Splice locations are shown and labelled.
• Base type is stated in words, not only drawn.
• Frame mark matches plan and reaction table.
• Flange-brace ticks are present if required by design.
• Special attachments (crane, canopy, mezzanine) are on this frame or clearly not.
• Grid bubbles match the architectural grid used on the job.
• Revision of this elevation is listed on the cover index.

Frequently asked questions

Is a PEB lean-to still a portal frame?

A lean-to often uses rafters spanning to a main-building column, with or without moment continuity. It may not be a symmetric portal, but it still has columns, rafters, connections and bases that must be named and analysed as they will be built.

Do all PEB frames need a ridge haunch?

No. Many frames use a simple ridge connection. A ridge haunch appears when the design needs more depth at the peak. Do not add one from habit; do not omit one the analysis used.

Where should field splices go?

Where the connection is designed for the forces and where pieces can ship and lift. Those two constraints together, not a preference for even-looking elevations, should set the station.

Can hot-rolled prismatic sections form a PEB portal?

Yes. Some PEB suppliers use mill sections for shorter spans or for interior columns. The component names remain useful; the taper discussion simply may not apply to those members.

If the elevation cannot name the parts, the model is not finished

A PEB portal frame is a short list of serious components: columns, rafters, haunches, ridge, splices, bases, and the fittings that restrain them. Each one has a geometry that analysis used and a mark that drawings must carry into foundations and the shop. When any of those names is missing, someone downstream will guess.

Use this map with primary and secondary members and column reactions. Component identification is not a substitute for connection design or for the project design basis. StruTools publishes this as engineering education, not as a typical that can be copied into a calculation.