Anatomy of a Pre-Engineered Building: Main Parts

The anatomy of a pre-engineered building describes its main systems: primary rigid frames, secondary purlins and girts, bracing, the roof and wall envelope, and accessories such as doors, skylights and canopies. Each system has a defined role in carrying loads to the foundation and keeping the building weathertight.

Key facts

Primary framing: rigid frames, endwall columns and endwall rafters.
Secondary framing: purlins, girts, eave struts, sag rods and base angles.
Bracing: rod, cable or angle bracing and portal frames for longitudinal stability.
Envelope: roof and wall panels, liners, insulation, trims and gutters.
Accessories: openings, skylights, ventilators, canopies, fascias and mezzanines.

Primary framing: the main load carriers

Rigid frames span across the building width and are spaced along its length at the bay spacing. Each frame is made of columns and rafters joined at the knee and ridge with bolted end-plate connections, and fixed to the foundation through base plates and anchor bolts.

At the ends of the building, endwall framing closes the structure. It may be a full rigid frame, which allows a later extension, or a lighter post-and-beam arrangement of endwall columns and rafters.

Secondary framing: supporting the envelope

Purlins run along the roof and girts run along the walls, spanning between frames. They support the panels, transfer wind and gravity loads to the frames, and restrain the frame flanges through flange braces.

Eave struts sit at the junction of roof and wall, connecting frames and supporting the edge of both roof and wall sheeting. Sag rods or bridging keep purlins and girts straight, and base angles fix the bottom of the wall panels to the slab.

Bracing: stability along the length

Frames are stiff in their own plane but need help in the other direction. Roof bracing in the plane of the rafters and wall bracing in the sidewalls form a load path for wind on the endwalls, seismic forces and crane surge.

Where diagonal bracing would block a door or opening, a portal frame can be used instead. Purlins and eave struts act as struts that tie the bracing bays to the rest of the building.

The building envelope

Roof panels may be through-fastened profiled sheets or concealed-clip standing seam panels. Wall panels are usually profiled sheets or insulated sandwich panels, sometimes with liner panels on the inside.

Insulation, vapor retarders, trims, flashings, closures and sealants complete the weather barrier. Gutters and downspouts collect rainwater, and translucent panels, ridge ventilators and louvers bring in daylight and air.

Accessories and special structures

Framed openings take roll-up doors, sliding doors, personnel doors and windows. Canopies, roof extensions, fascias and parapets shape the appearance and shelter entrances.

Mezzanine floors, crane runway beams, roof monitors, catwalks, stairs and handrails can be integrated with the main frames. These items add loads that must be included in the frame design from the start.

Frequently Asked Questions

What is the difference between purlins and girts?

Both are secondary members, usually cold-formed Z or C sections. Purlins support the roof and span between rafters, while girts support the wall cladding and span between columns.

Why does a PEB need bracing if the frames are rigid?

Rigid frames resist loads in the plane of the frame only. Bracing provides stability along the building length and carries forces such as wind on the endwalls to the foundations.

What does an eave strut do?

It runs along the top of the sidewall, connecting the frames and supporting the edges of both the roof and wall panels. It also acts as a strut in the bracing system.

Engineering content is general guidance. Every World Steel building is designed by qualified engineers to the code and loads of the project.