In the narrative of industrial architecture, the “Primary Frame”—those massive, imposing I-beams and columns—often steals the spotlight. They are the skeletal giants that provide a building its shape and primary strength. However, a primary frame alone is just a cage; it cannot hold a wall, support a roof, or protect the interior from the elements.
To transform a skeleton into a functional building, we need the “unsung heroes” of steel construction: the Secondary Frames.
If the primary frame is the skeleton, the secondary frame is the muscle and connective tissue. These components bridge the gaps between the heavy steel rafters, providing the necessary surface area to attach cladding and ensuring the building can withstand the “local” pressures of wind and snow. Here is the essential guide to what secondary frames are and why they are the secret to a long-lasting steel structure.
1. Defining the Secondary Frame
The secondary frame consists of cold-formed steel members that are bolted or screwed to the primary structural frame. In 2026, these components are almost exclusively manufactured from high-strength, galvanized steel to ensure they remain rust-free for the life of the building.
There are three primary protagonists in the secondary framing story: Purlins, Girts, and Eave Struts.
Purlins: The Roof’s Backbone
Purlins are horizontal beams that span the distance between the primary roof rafters. Their job is to support the roof panels and transfer “live loads” (like snow or maintenance workers) and “dead loads” (the weight of the panels themselves) into the primary frame.
Girts: The Wall’s Support
Girts serve the same function as purlins but for the walls. They are attached to the primary columns and provide the “nailing surface” for the exterior metal siding. Without girts, a wall panel would simply flap in the wind.
2. The Geometry of Strength: C-Sections vs. Z-Sections
Secondary framing members are usually “cold-formed,” meaning they are shaped from thin sheets of steel at room temperature. The narrative of their strength comes from their shape.
Z-Sections: Shaped like the letter “Z,” these are the favorites for long-run buildings. Because of their shape, Z-sections can be “lapped”—meaning one piece fits inside the next over a support column. This creates a continuous beam that is significantly stronger than two separate pieces.
C-Sections: Shaped like the letter “C,” these are typically used for door headers, window trimmers, and the “end-wall” girts where lapping isn’t required.
3. Why Secondary Frames Matter: The Four Pillars of Function
It is a common misconception that secondary frames are just “accessories.” In reality, the building’s performance depends entirely on the precision of these components.
I. Load Distribution
A primary frame is designed to handle massive, concentrated loads. However, wind and snow don’t hit a building in a single spot; they apply “distributed pressure” across the entire surface. Secondary frames take that pressure from the thin exterior panels and “channel” it safely into the heavy primary beams.
II. Lateral Stability (Bracing)
The primary frames are strong vertically, but they can be prone to “rolling” or twisting under extreme stress. Secondary frames act as stabilizers. By connecting the rafters and columns together, they create a rigid “box” that prevents the primary members from buckling.
III. The “Skin” Interface
Modern insulation and metal cladding require a perfectly level surface. High-quality secondary framing ensures that the “lines” of the building are straight. If your purlins are misaligned by even half an inch, your roof panels won’t “nest” correctly, leading to leaks and aesthetic “oil-canning” (visible waves in the metal).
IV. Thermal Bridging Management
In the 2026 energy-conscious market, secondary frames play a role in Thermal Blocks. By placing insulation between the girt and the exterior panel, builders create a “thermal break” that prevents the cold from traveling through the steel into the interior of the building.
4. Comparing Primary and Secondary Framing
| Feature | Primary Frame (I-Beams) | Secondary Frame (Purlins/Girts) |
| Material | Hot-Rolled Structural Steel | Cold-Formed Galvanized Steel |
| Function | Main structural support | Lateral bracing & panel support |
| Installation | Requires heavy cranes | Can be installed by hand/small lift |
| Weight | Very Heavy | Lightweight but Rigid |
| Connection | Heavy Bolts / Welding | Self-drilling screws / Small Bolts |
5. The “Eave Strut”: The Critical Intersection
Perhaps the most complex secondary member is the Eave Strut. This component sits at the very corner where the roof meets the wall.
The narrative of the eave strut is one of dual-purpose: it must act as a girt for the top of the wall and a purlin for the edge of the roof. It is also the primary attachment point for the Gutters. If the eave strut is not engineered correctly, the most vulnerable part of your building—the corner—will be prone to moisture infiltration and structural sagging.
6. Installation: A Narrative of Precision
Because secondary frames are lightweight, they are often the fastest part of the construction process. However, they require a high degree of “squaring.”
Alignment: Once the primary steel is standing, crews install “sag rods” or “bridging” between the purlins to keep them from twisting during the roof installation.
Fastening: In 2026, many contractors use “auto-fed” screw guns to attach secondary members, ensuring that every fastener is driven to the exact torque required by the engineer.
The “Bypass” vs. “Flush” Choice:
Bypass Girts: These sit outside the primary columns, creating more interior space and a continuous “look” for the siding.
Flush Girts: These sit between the columns, which is often preferred in “finished” buildings where you want to install drywall directly over the steel.
Conclusion: The Secret to Longevity
The next time you walk into a massive steel warehouse or a clear-span aircraft hangar, look up past the heavy beams. Notice the grid of steel running horizontally across the ceiling and walls. Those purlins and girts are the reason the building stays square, the reason the roof doesn’t leak, and the reason the walls don’t bow during a hurricane.
The secondary frame may be “secondary” in name, but it is primary in importance. By investing in high-quality, galvanized secondary framing, you are ensuring that your steel building remains a durable, efficient, and beautiful asset for decades to come.
