Rigid frame glossary

Rigid frame

The primary structural unit of a pre-engineered metal building: a pair of columns and rafters joined by moment-resisting connections so the assembly stands on its own without interior support. Because the knee joint transfers moment rather than only shear, the frame can clear span the full width of the building.

Also called: Main frame, primary frame, red iron frame

Primary framing

The rigid frames, plus the endwall framing, that carry the building. Everything a manufacturer calls primary is load-path structure; it is where the engineering and most of the cost sit.

Also called: Main framing

Secondary framing

Purlins, girts, eave struts, and bracing — the members that span between rigid frames and carry panel loads back into them. Secondary is not a synonym for minor: on a heavy-snow building it can be where the spacing decision is won or lost.

Clear span

A frame configuration with no interior columns, so the full width of the building is unobstructed. Standard single-span rigid frames reach about 150 ft; custom engineering extends this to roughly 300 ft, which is the widest most manufacturers will quote.

Also called: Single span

Modular frame

A frame with one or more interior column lines dividing the width into modules, so no single rafter spans the whole building. Cost divergence from clear span becomes noticeable near 80 ft of width and is usually decisive past 150 ft.

Also called: Multi-span frame

Haunch

The knee of a rigid frame, where the column meets the rafter, and the deepest point of a web-tapered member. Bending moment peaks here, which is why the steel is deepest here and why haunch depth — not eave height — often sets your real interior clearance at the sidewall.

Also called: Knee

Web-tapered member

A built-up plate girder whose web depth varies along its length to follow the bending moment it carries. The wedge shape of a rigid frame is not styling; it is the member using steel only where the moment demands it, which is what makes rigid frames the lightest practical way to cover a wide bay.

Also called: Tapered column, tapered rafter

Eave height

The vertical distance from finished floor to the top of the sidewall column, measured at the eave. It is the input owners most often underestimate: a taller eave lengthens the column, raises the moment at the haunch, and grows the whole frame, so it is rarely just extra wall panel.

Bay

The space between two adjacent rigid frames, measured centre to centre. 25 ft is the industry default because it sits near the cost optimum for most snow loads and divides cleanly into common building lengths and rack modules.

Also called: Bay spacing, frame spacing

Purlin

A secondary member spanning between rigid frames to support the roof panel, almost always a cold-formed Z or C section. Purlin depth is driven by bay spacing and roof load, which is the mechanism by which snow load decides bay spacing.

Girt

A secondary member spanning between columns to support the wall panel. Girts also brace the column against weak-axis buckling, which is why removing one for a large opening is a framing change, not just a panel change.

Eave strut

The member at the junction of roof and wall that supports both panels and closes the corner. It carries load in two directions and is a common leak point if the detail is compromised for a gutter or a fascia.

Single slope

A frame with one continuous roof slope from a high eave to a low eave and no ridge. Drains to one side, presents a taller street face, and fits sites where a gable would break a height limit — but the high eave carries the whole frame up with it, so it gets expensive on wide buildings.

Also called: Shed roof frame

Lean-to

A single-slope module framed off the sidewall of an existing building, with no independent ridge. The cheapest square footage available on any project, but it always requires a check that the host frame can take the added reaction.

Multi-gable

Two or more gable modules side by side sharing interior columns, covering great width with short spans. The tradeoff is a valley — an interior gutter line that drifts under snow and is the first place a very wide roof gives trouble.

Also called: Gable-on-gable

Bracing

Diagonal members, usually rod or cable in the roof and wall planes, that carry wind and seismic load to the foundation. Bracing lives in specific bays, so wider bay spacing gives fewer places to put it and can force a braced bay into a wall where you wanted a door.

Also called: X-bracing, wind bracing

Ground snow load

The mapped snow load on the ground at a site, in pounds per square foot, written as p-g. It is not roof snow load: the roof value is derived from it, and drift against a wall, a valley, or a roof step routinely governs over the balanced flat-roof case.

Also called: pg

Basic wind speed

The mapped 3-second gust wind speed at 33 ft above ground in Exposure C, for the building's Risk Category. Wind mostly shows up in uplift, bracing, and connections rather than in rafter size, which is why a high-wind site and a high-snow site are expensive in different places.

Also called: Design wind speed, V

Seismic Design Category

A letter from A to F that sets how much seismic detailing a building needs, derived from mapped ground motion, Site Class, and Risk Category. Because Site Class comes from soil conditions, a geotechnical report can change the category — and therefore the connection design — after the frame has been priced.

Also called: SDC

Risk Category

A classification from I to IV reflecting the consequence of a building's failure, which scales the design loads applied to it. An ordinary warehouse is II; a building with an assembly occupancy is III; an essential facility is IV. Moving up a category raises wind, snow, and seismic demand together.

Also called: Occupancy Category

Anchor bolt

The cast-in bolt connecting a column base to the foundation. Anchor bolt patterns for a wide clear span frame get large and precise, and the setting plan arriving late or wrong is one of the more common causes of a delayed erection.

Also called: Hold-down bolt

Framed opening

A reinforced opening in a wall or roof for a door, window, or louvre, with jambs and a header replacing the interrupted girts. Openings that land on a column line cost more than openings between frames, which is why door layout and bay spacing are worth deciding together.

Building package

What the manufacturer ships: primary and secondary framing, panels, trim, fasteners, and typically anchor bolts. Commonly $14–22 per square foot. It is not the installed cost, and confusing the two accounts for a large share of the surprise in metal building budgets.

Also called: Kit price

Erected cost

Installed cost including foundation and erection, before interior buildout — commonly $22–42 per square foot. Roughly 1.55 to 1.9 times the package price, and the number a budget should actually be built on.

Also called: Installed cost

AC472

The IAS accreditation programme (IAS AC472) for metal building systems manufacturers, covering engineering and manufacturing quality. Many jurisdictions and specifications require an AC472-accredited manufacturer, so it is worth confirming before comparing prices.

Also called: IAS AC472

MBMA

The Metal Building Manufacturers Association, which publishes the Metal Building Systems Manual — the reference that governs much of how these buildings are designed and how loads are applied to them.

Missing a term? The frame selector and clear span calculator explain most of these in the context where they matter, which is usually easier than a definition.