Getting the span (clear distance between columns) and bay spacing (frame-to-frame distance) right does more for a portal-frame building than any other early decision. These two numbers drive steel tonnage, purlin sizes, cladding economy, erection speed, and even how well loading docks or racking will fit. Choose them with the use case in mind, not just a rule of thumb.
Table of Contents
A sensible starting point
For most steel warehouses and prefabricated steel buildings without cranes in moderate wind/snow, the sweet spot is simple:
Span: 24–30 m
Bay spacing: 7.5–8.0 m
This range keeps rafters efficient and lets common Z-purlins (e.g., Z200–Z250) work economically without excessive bridging. You also reduce the total number of frames and foundations, which shortens the program.
When to tighten or widen the grid
Bay spacing isn’t only a cost lever; it protects serviceability. Larger bays mean fewer frames, but longer purlins and bigger uplift checks. Smaller bays add frames and bolts, yet keep secondaries light and stiff.
Tighten bays toward 6.0–7.5 m when:
Design wind uplift is high or you expect meaningful snow load.
The roof will carry skylights, PV arrays, or collateral loads that interrupt purlin continuity.
You need better lateral stiffness around large doors or mezzanines.
Widen cautiously to 8.0–9.0 m when:
Climate is mild and the building is very long, so cutting frame count materially reduces cost.
Interior fit-out benefits from longer uninterrupted runs and faster frame-by-frame erection.
Buildings with overhead cranes
Cranes change the conversation. Lateral drift, runway alignment, and local bracket detailing often govern.
Recommended span: 18–27 m
Recommended bay spacing: 6.0–7.5 m
Eave height: hook height + runway beam depth + 1.5–2.0 m clearance
Shorter bays keep runway beams and wall bracing better controlled. Moderating span reins in rafter depth and frame sway under crane surge. The result is a portal frame that tracks, lifts, and stops without cracked cladding or fussy alignments.
Large logistics and distribution
Where clear floor area is king—high-bay racking, fast MHE, long dock walls—two paths work:
Single-span: 30–36 m with 8–9 m bays. Great for column-free aisles; check purlin uplift carefully.
Multi-span: for example 2 × 24 m with a central column line and 8 m bays. Slightly more intrusive inside, but lighter rafters and friendlier purlins often reduce total steel.
If the target span drifts beyond ~38–40 m, consider a multi-span portal or a tapered truss rafter to cap depth and weight without compromising interior planning.
Serviceability and detailing often decide
Strength rarely kills a portal; deflection and drift do. Verify:
Rafter deflection under gravity and ponding checks at low roof slopes (1:10–1:15 is common).
Frame sway under wind—and under crane surge if applicable.
Purlin uplift and lateral-torsional buckling; don’t skimp on bridging lines.
Bracing layout that clears doors, docks, and office blocks, with a clean load path to the foundations.
Standardizing ridge/eave splices and keeping member pieces within 40-ft HQ shipping logic (<~11.8 m per piece) preserves fabrication speed and site productivity.
A quick selection workflow
Fix the functional envelope: clear height, door sizes, racking aisles, and (if any) crane hook height.
Pick a span: 24–30 m for no-crane; 18–27 m with cranes or harsh climates.
Test bay spacing at 7.5 m first; iterate 6.0–8.0 m against purlin/cladding checks.
Run serviceability: rafter deflection, frame drift, purlin uplift.
Adjust one variable at a time; if members grow, shorten the bay or temper the span.
Validate shipping and erection (lift capacity, splice positions, brace lines). Freeze the grid and standardize details.
Bottom line
For a typical portal frame steel warehouse without cranes, start at 24–30 m span and 7.5–8.0 m bay spacing. Add cranes or tougher climate, and tighten bays and temper span. If you’re chasing very wide clears, don’t fight physics—step into multi-span or trussed solutions. The right grid is the one that balances steel weight, envelope performance, and the way the building will actually be used.