Primary Framing
Rigid Frames for Pre-Engineered Steel Buildings
Definition: A rigid frame is the main structural portal of a pre-engineered building, made of columns and rafters joined by moment-resisting connections. It carries roof, wall, wind, seismic and crane loads to the foundations while providing a column-free interior across the building width.
Key facts
- Frames are usually spaced at regular bays along the building length, commonly in the range of 6 to 9 m.
- Members are typically built-up I-sections that taper to follow the bending moment diagram.
- Clear-span frames give a column-free interior; multi-span frames add interior columns for wider buildings.
- Knee and ridge joints are normally bolted end-plate moment connections made on site.
- Column bases are often designed as pinned, which reduces foundation moments.
Role in the structure
Rigid frames are the backbone of the load path. Roof and wall loads collected by panels, purlins and girts are delivered to the frames, which resist them through bending in the columns and rafters and transfer the resulting forces, including horizontal thrust, to the anchor bolts and foundations. In the transverse direction the frames provide stability on their own; in the longitudinal direction they rely on the bracing system.
What types of rigid frames are used in metal buildings?
The most common form is the clear-span gable frame, with two columns and two sloping rafters meeting at a ridge. Where widths become large or interior columns are acceptable, multi-span frames introduce one or more interior columns that shorten the effective rafter span and lighten the steel.
Single-slope frames have one high eave and one low eave and suit buildings that drain to one side or adjoin another structure. Lean-to frames lean against a main building and share its columns. Multi-gable frames repeat the gable profile across several modules for very wide footprints.
The choice is driven by the required clear width, internal use, crane or mezzanine loads, drainage and future expansion. The project engineer compares options and confirms the final arrangement to the governing code.
Why are rigid frame members tapered?
In a portal frame the bending moment is largest at the knee and much smaller near the base and at points along the rafter. Tapering the web depth places material where the moment is highest and removes it where it is not needed.
Because the flanges and web are cut and welded from plate, the depth, flange width and thicknesses can all change along a member. This is the main reason pre-engineered frames are often lighter than frames built from uniform hot-rolled sections of the same span.
How do rigid frames resist wind, seismic and crane loads?
Lateral loads across the building are resisted by frame action: the moment connections at the knees and ridge make the portal act as a single stiff unit. The frame is checked for strength and for lateral drift so that cladding, doors and crane runways perform properly.
Crane wheel loads are introduced through brackets or stepped columns, and add both vertical and horizontal forces. In seismic regions, the seismic force-resisting system, detailing and connection requirements follow the applicable seismic provisions, and the engineer documents the system used.
How are rigid frames delivered and erected?
Each frame is shipped as a set of column and rafter pieces sized for transport. Pieces are match-marked and bolted together on site at the knee and splice end plates.
Erectors normally start with a braced bay, erect the first pair of frames, install the bracing and secondary members, plumb and align the bay, and then extend frame by frame from that stable core.
Materials & coatings
- Built-up plate members in structural steel grades with a minimum yield commonly around 345 MPa (50 ksi class), selected by design.
- Hot-rolled sections for small frames, endwalls or local members where efficient.
- Shop primer as standard; hot-dip galvanizing or multi-coat paint systems for corrosive environments.
- High-strength structural bolts for moment connections, specified by the engineer.
Design considerations
- Clear width, eave height and roof slope required by the building use.
- Dead, collateral, live, snow, wind and seismic loads to the governing code, plus crane and mezzanine loads where present.
- Lateral drift and rafter deflection limits suited to cladding, doors and cranes.
- Stability of compression flanges, provided by flange braces from purlins and girts.
- Base fixity assumption and its effect on foundation design.
- Provision for future expansion at the endwalls.
Fabrication
- Flanges and webs are cut from plate, then fitted and joined by automated welding to the approved shop drawings.
- End plates, stiffeners and clips are welded and holes drilled or punched to template.
- Members are cleaned, primed or coated, and piece-marked for erection.
- Dimensional checks and weld inspection follow the inspection and test plan.
Erection
- Check anchor bolt positions and elevations before steel arrives.
- Erect the braced bay first and make it plumb, square and stable before adding frames.
- Tighten moment connection bolts to the method specified for the project.
- Keep temporary bracing in place until permanent bracing and sheeting are complete.
Inspection & quality
- Verify frame pieces against the erection drawings and piece marks.
- Inspect welds and bolted joints according to the approved quality plan.
- Check plumbness, alignment and ridge line before cladding.
- Record coating damage and repair it before handover.
What to include in your RFQ
- Building width, length, eave height, roof slope and bay spacing.
- Frame type preferred and any limits on interior columns.
- Governing design code and site-specific loads.
- Crane, mezzanine, solar or other collateral loads.
- Drift and deflection criteria if stricter than code minimums.
- Coating system and site exposure.
3D model & IFC of this component
Rotate the model, isolate the component and download its IFC4 file for your BIM coordination model.
IFC entity: IfcBeam. Illustrative geometry for coordination and learning. Final member sizes come from the project design.
Frequently Asked Questions
How wide can a clear-span rigid frame be?
Clear spans of several tens of meters are common in pre-engineered buildings. Very wide spans are possible, but steel weight, deflection and transport rise with span, so the engineer will compare clear-span and multi-span options for the specific loads.
Are rigid frame bases fixed or pinned?
Pinned bases are typical because they simplify foundations. Fixed bases may be used to reduce drift, for example with heavy cranes, but the foundation must then be designed for moment. The project engineer decides.
Can a rigid frame building be extended later?
Yes. If expansion is planned, the endwall is often designed as a full rigid frame or for removable endwall framing so new bays can be added with minimal disruption.
Engineering content is general guidance. Every World Steel building is designed by qualified engineers to the code and loads of the project.
