Structural steel framing is a building skeleton made of hot-rolled steel members (columns, beams, girders, joists and bracing) that carries the floors, roof and walls and delivers their loads to the foundations. The walls and roof are attached to the frame rather than holding the building up. It's the standard system for warehouses, plants, offices, schools and mid-rise buildings, and in smaller doses (beams and posts) it shows up inside many wood-framed houses.
This guide names the parts of a steel frame, explains how roofs and floors are framed, and covers who designs, fabricates and erects it.
The members of a structural steel frame
Every steel frame is built from a handful of member types. Their names describe the job each does.
| Member | What it does | Typical shapes |
|---|---|---|
| Columns | Carry load down to the foundations | W-shapes, HSS |
| Girders | Main beams spanning between columns, carrying other beams or joists | W-shapes, plate girders |
| Beams | Carry floor or roof load to girders or columns | W-shapes, channels |
| Joists | Lighter, closely spaced members directly under the deck | Open web steel joists, small W-shapes |
| Purlins | Secondary roof members that support deck or roof panels | Channels, cold-formed C and Z sections |
| Girts | Horizontal wall members that support cladding | Channels, angles, cold-formed C and Z sections |
| Bracing | Resists wind and earthquake loads | Angles, HSS, rods, W-shapes |
| Steel deck | Corrugated sheet spanning between joists or beams | Roll-formed steel deck |
| Base plates and anchor rods | Connect columns to the foundations | Plate and threaded rod |
| Connections | Join everything together | Plates, angles, bolts and welds |
The deck isn't usually thought of as "framing," but it's part of the structure: on most steel roofs and floors it ties the frame together sideways and carries wind load across to the braced bays.
How a steel frame carries load
A steel frame has two jobs, and different parts of it handle each.
Gravity load (the weight of the building, its contents, people and snow) follows a simple path: deck to joists, joists to beams or girders, girders to columns, columns through base plates and anchor rods into the footings.
Lateral load (wind and earthquake) pushes the building sideways. A frame of simply connected beams and columns has little sideways stiffness on its own, so the engineer adds a lateral system:
- Braced frames. Diagonal braces in selected bays, often in an X, chevron or single-diagonal pattern. Common and economical in low-rise buildings.
- Moment frames. Rigid beam-to-column connections that let the frame resist sway without braces. Used where braces would block openings, at a cost in heavier members and connections.
- Shear walls or cores. Concrete or masonry walls, often around stairs and elevators, that the steel frame leans on.
The roof and floor decks act as horizontal diaphragms that collect the lateral load and deliver it to whichever system the building uses. The connections at each joint are designed to match the role of the joint; our post on types of steel beam to column connections explains shear and moment connections.
Structural steel roof framing
Steel roofs are framed in one of a few standard ways.
Girders, joists and deck
The most common arrangement for flat and low-slope roofs on commercial and industrial buildings:
- Girders run between the columns along one direction of the column grid.
- Open web steel joists span between the girders at regular spacing. They're light, long and easy to run ducts and pipes through.
- Steel roof deck is fastened to the joists and covered with insulation and the roofing membrane.
Openings for rooftop units, hatches and drains are framed with angles or channels between the joists. Joists are usually designed by the joist supplier to the loads the building's engineer specifies, so the drawings show joist depth and loading rather than member sizes. Our post on steel joists vs steel beams compares the two.
Beams and purlins
On sloped roofs, and in some industrial buildings, W-beams or rafters span between columns and purlins run across them to carry deck or metal roof panels. Purlins may be hot-rolled channels or cold-formed C and Z sections.
Rigid frames
Pre-engineered metal buildings use tapered, welded rigid frames with light cold-formed purlins and girts, all supplied as a system by the building manufacturer. That's a different product from a conventionally framed steel building; see structural steel vs pre-engineered buildings.
Steel floor framing
Floors follow the same logic as roofs, with heavier loads and tighter limits on deflection and vibration:
- Beams and girders on a column grid, often with W-shapes as the beams themselves.
- Joists for lighter floors and mezzanines, or closely spaced beams for heavier ones.
- Composite deck and concrete. Steel floor deck is laid over the framing and filled with concrete. Where the design calls for it, shear studs welded through the deck make the slab and beams act together, which stiffens the floor and saves steel.
Mezzanines inside existing buildings are a smaller version of the same system: columns, beams, deck or grating, guards and stairs.
Steel framing in houses
Most Ontario houses are wood-framed, but structural steel often carries the heaviest parts of the load: the main basement beam and its posts, beams that replace load-bearing walls, garage door headers, ridge beams for vaulted ceilings and lintels in brick walls. A full steel post-and-beam house is less common but growing, especially for open plans and large glazed walls.
Don't confuse structural steel framing with steel stud framing. Steel studs are thin cold-formed members bent from galvanized sheet, used like wood studs for walls and sometimes floors. The 2024 Ontario Building Code sends the two to different standards: structural steel to CSA S16 (Article 4.3.4.1) and cold-formed steel to CSA S136 (Article 4.3.4.2).
Why builders choose a steel frame
- Long spans and open space. Steel girders and joists span much farther than wood, with fewer columns in the way.
- Speed. Members arrive cut, drilled and welded, and a frame goes up quickly with bolted connections.
- Strength for its weight. A steel frame is lighter than a comparable concrete frame, which can mean smaller foundations.
- Adaptability. Steel can be reinforced, extended or modified later more easily than many other systems.
- Predictable material. Structural steel is made to tightly specified grades with mill certificates.
There are things to plan for, too: fire protection where the code requires it, corrosion protection for exposed or damp areas, and the lead time for detailing and shop drawings before fabrication starts.
Who designs, fabricates and erects the frame
A structural steel frame passes through several hands:
- The structural engineer designs the frame to CSA S16 and the Ontario Building Code and issues structural drawings.
- The steel fabricator prepares shop drawings for every piece, has them reviewed, then cuts, drills, welds, coats and marks the steel.
- The joist and deck suppliers design and supply their products to the engineer's loads.
- The erector sets columns on the anchor rods, then places girders, beams, joists and deck in sequence, plumbs the frame and completes the bolting and welding.
- Inspectors check the connections and work against the drawings and specifications.
The full sequence on site is covered in our post on the steel erection process. Structural steel framing in any building needs a design by a qualified designer or engineer and, in Ontario, a building permit.
Structural steel framing from Procut
We supply and fabricate structural steel to your drawings, from basement beams and posts to full commercial frames, with in-house shop drawings, CWB certified welding, and delivery across the GTA and Ontario. We provide supply and installation of structural steel, and engineering services are available for projects that need them.
Send your drawings for a free take-off and quote, or browse our steel products.
Frequently asked questions
What are structural steel members?
The main members are columns, which carry load down to the foundations; girders and beams, which span between columns and carry floors and roofs; joists, which are lighter, closely spaced members under the deck; and bracing, which resists wind and earthquake. Girts, purlins, base plates and connections complete the frame.
What is the difference between girts and purlins?
Both are secondary members that span between the main frame members. Purlins run across the roof and support the roof deck or panels. Girts run horizontally along the walls and support the wall cladding. Both can be hot-rolled channels or light cold-formed C or Z sections.
How is a structural steel roof framed?
A common flat-roof arrangement uses steel girders between the columns, open web steel joists spanning between the girders at regular spacing, and a corrugated steel roof deck fastened over the joists. The deck also acts as a diaphragm that carries wind load to the braced frames. Pre-engineered buildings use rigid frames with light purlins instead.
Is structural steel framing the same as steel stud framing?
No. Structural steel framing uses hot-rolled shapes such as W-beams, HSS, channels and angles designed to CSA S16. Steel stud framing uses thin cold-formed members bent from sheet, designed to CSA S136, and is closer to wood framing in how it is built.
