Frame type
A clear span frame is the default mental image of a metal building: two tapered columns, two rafters, one moment connection at each knee, and nothing at all standing between the walls. It is also the most expensive way to cover width, and the premium grows faster than the building does.
Practical span range
20–300 ft
Where this configuration is normally used
A single rigid frame carrying the full width with no interior columns. Maximum usable floor area and total layout flexibility, paid for in steel weight that rises faster than the span.
Skip it when: The interior can tolerate a column line and the width is over about 100 ft. You are paying a premium for clearance you do not need.
The knee joint is the whole mechanism. Bolt a column and a rafter together so the connection transfers moment rather than just shear, and the pair stops behaving like a post carrying a beam and starts behaving like a single bent member. It can then carry the roof to the walls without help, which is why nothing needs to stand in the middle.
That is paid for at the haunch, where the bending moment peaks. Frame depth at the knee runs to roughly span/20, so a wide clear span carries a very deep member exactly where the sidewall is, and clear height against the wall is meaningfully less than the eave height on the drawing. The frame also pushes outward at the base, and that horizontal thrust has to be resolved into the foundation — tie rods, hairpins, a grade beam, or footings big enough to lean on the soil.
Bending moment grows with the square of the span, so a deeper section is needed; the deeper section is itself heavier, which adds to what it carries; and the taller haunch needs more material at the knee. Each effect compounds the last. Double the span and the steel roughly triples.
Below about 80 ft of width, clear span is free. An interior column at that width relieves a rafter that was never the expensive part, and the footing, base plate and connection it costs cancel the saving out. The frame selector picks clear span below the crossover for exactly that reason.
Above 80 ft the two curves separate and never come back together: roughly 5% at 100 ft, 14% at 150 ft, 22% at 200 ft, and a third of the package price by 300 ft. Somewhere in that range clear span stops being a default and becomes a purchase, and the test is whether you can name what the clear floor is for. A wingtip, a crane runway, an arena surface and a hangar door are all good answers. "It seemed tidier" is not.
Standard single-span rigid frames run to about 150 ft. Custom engineering extends that to roughly 300 ft, which is the widest single span most manufacturers will quote at all.
Planning-stage estimate calibrated to published industry ranges. Not a substitute for a stamped design by a licensed engineer. How the figures behind those percentages were derived.
Uses whose profile names this configuration by default: Aircraft Hangar, Crane Bay / Manufacturing, Riding Arena, Church / Worship, Agricultural.
The widths this configuration reaches that have a page of their own, with what a clear span at that width costs and what a column line would save against it. Where the saving is meaningful, that number is the case for or against this configuration in one figure.
| Width | Clear span steel | Clear span package | Modular saves | What changes at this width |
|---|---|---|---|---|
| 40 ft | 2.04 – 2.97 lb/sf | $13.45 – $16.33/sf | — | Light frames. Span is rarely the cost driver at this width - eave height and snow load matter more. |
| 60 ft | 3.00 – 4.20 lb/sf | $15.00 – $18.00/sf | — | The practical floor for commercial work. Below this width, small-building suppliers price better. |
| 80 ft | 4.07 – 5.57 lb/sf | $16.63 – $20.79/sf | — | The crossover band. Clear span and modular start to diverge in cost here - price both. |
| 100 ft | 5.26 – 7.32 lb/sf | $18.35 – $22.66/sf | 5% | Haunch depth begins to govern interior clearance. Check usable height, not just the eave. |
| 120 ft | 6.41 – 8.92 lb/sf | $19.43 – $24.82/sf | 9% | Deep haunches and heavier anchor-bolt patterns. Foundation cost stops being a rounding error. |
| 150 ft | 8.26 – 11.56 lb/sf | $21.98 – $28.57/sf | 14% | The practical ceiling for a standard single-span rigid frame. Past this you are buying custom engineering. |
| 200 ft | 11.55 – 16.42 lb/sf | $26.85 – $35.80/sf | 22% | Custom engineering, shipping splices, and heavier erection equipment. Modular usually wins on cost alone. |
| 250 ft | 16.10 – 22.54 lb/sf | $32.44 – $44.02/sf | 28% | Specialist territory. Justified by use, not economics - hangars, arenas, column-free process floors. |
| 300 ft | 21.76 – 31.28 lb/sf | $39.53 – $55.11/sf | 33% | The upper limit most manufacturers offer. Expect a bespoke frame design and a long lead time. |
Each row is priced at the case on that width's own page — the eave and length that actually get built there — so the rows are comparable to those pages rather than to each other. Planning-stage estimate calibrated to published industry ranges. Not a substitute for a stamped design by a licensed engineer.
One or more interior column lines split the width into modules, so each rafter spans only a fraction of the building. The savings grow with width and become decisive past roughly 150 ft.
60–480 ft →One continuous slope from a high eave to a low eave, with no ridge. Drains to one side, presents a taller street face, and fits sites where a gable would break a height limit.
20–200 ft →A single-slope module framed off an existing sidewall, with no independent ridge. The cheapest square footage on any project, provided the host frame can take the added load.
10–60 ft →Two or more gable modules side by side sharing interior columns, covering great width with short, cheap spans. The tradeoff is a valley - an interior gutter line you must detail and maintain.
80–600 ft →Built-up members whose web depth follows the moment diagram - shallow at the base, deep at the haunch. The standard configuration for clear span, and the reason a rigid frame is the lightest way to cover a wide bay.
40–300 ft →Constant-depth columns. Heavier than a tapered equivalent, but they give a flat plane for racking, liner panel, masonry wainscot, and interior partitions, and they simplify sidewall framing.
20–150 ft →Not sure which one you need? The frame selector takes width, eave height, crane load and whether a column can stand inside, and tells you which configuration fits — and what the runner-up would have cost. Or go straight to quotes.