Structural steel tolerances come in three layers: mill tolerances on the rolled shapes, fabrication tolerances on the finished pieces, and erection tolerances on the standing frame. In Canada, CSA S16 sets the fabrication and erection tolerances for steel buildings, and the project specification can tighten them. In the US, the equivalent rules are in AISC 303, the Code of Standard Practice for Steel Buildings and Bridges. The two documents cover the same ground, but the numbers are not interchangeable, so a Canadian job should be checked against the S16 edition its drawings name.
This post explains how the layers fit together, gives the published AISC values most people search for, and covers what tolerances mean for the trades around the steel.
Three kinds of tolerance
Every piece of steel picks up small deviations at each stage. The standards control each stage separately:
| Layer | What it covers | Where the limits come from |
|---|---|---|
| Mill tolerances | Cross-section dimensions, flange tilt, length as rolled, sweep and camber of straight shapes | The material standard: ASTM A6 for most rolled shapes; CSA G40.20, the general requirements standard that goes with G40.21 grades, for Canadian-specified steel |
| Fabrication tolerances | Cut length, straightness after fabrication, specified camber, hole and plate locations, built-up member dimensions | CSA S16 in Canada; AISC 303 in the US; plus the project specification |
| Erection tolerances | Plumbness of columns, position at the base and splices, elevation and alignment of beams | CSA S16 in Canada; AISC 303 in the US; plus the project specification |
The layers stack. A beam can be within its mill tolerance and within its fabrication tolerance and still need adjustment on site if the anchor bolts below it are off. AISC 303 states the principle directly: the accumulation of mill and fabrication tolerances must not cause the erection tolerances to be exceeded. That is why connections often include slotted holes, shims or adjustable details where steel meets other materials.
CSA S16 and AISC 303: which one applies
In Ontario, the 2024 Ontario Building Code requires structural steel to conform to CSA S16, and the Code's fire-resistance supplement (SB-2) refers to columns "fabricated and erected within the tolerances stipulated in CSA S16." So for a permitted building in Ontario, S16 is the baseline, and the edition is the one the code and drawings reference. Our guide to Canadian structural steel codes explains which edition that is.
In the US, AISC 303 is the industry's code of standard practice. It is written into most steel contracts and specifications, and its Section 11 (in the 2022 edition) is the tolerance section people mean when they search for "AISC tolerances."
On a Canadian project, AISC 303 only governs if the contract documents say so. You will still see it quoted, because its values are freely published and well known. Treat those numbers as a reference point for a conversation, not as the requirement, unless your specification adopts them.
Two other points apply in both countries:
- The specification can be tighter. Engineers set closer tolerances where the design needs them, for example around elevators, curtain wall, precast cladding or architecturally exposed steel. AISC 303 has a separate section for architecturally exposed structural steel (AESS) for that reason.
- Tighter costs more. Every tolerance below standard practice takes extra checking and fitting time. Specify it where it matters, not across the whole frame.
AISC 303 fabrication tolerances
For reference, these are the main shop tolerances in ANSI/AISC 303-22. Values are given in AISC's metric and imperial forms.
| Item | AISC 303-22 tolerance |
|---|---|
| Overall length, members with both ends finished for contact bearing | 1 mm (1/32 in) |
| Detailed length, other members framing to steel, up to 9.1 m (30 ft) | 2 mm (1/16 in) |
| Detailed length, other members framing to steel, over 9.1 m (30 ft) | 3 mm (1/8 in) |
| Straightness of straight members | Per the applicable ASTM standard, unless the contract documents are tighter |
| Holes in base and bearing plates, relative to column centrelines | ±3 mm (1/8 in) |
| Specified camber, beams up to 15 m (50 ft) | Minus 0, plus 13 mm (1/2 in) |
| Specified camber, beams over 15 m (50 ft) | Minus 0, plus 13 mm (1/2 in) plus 3 mm (1/8 in) for each 3 m (10 ft) or part over 15 m |
A few rules sit alongside the table:
- Beams without specified camber are to be fabricated so that any natural camber from rolling or fabrication ends up upward after erection.
- Specified camber is measured in the shop, unstressed. It cannot be checked after erection because self-weight and connection restraint change it. AISC also provides that a beam ordered straight, which arrives from the mill with 75% of the specified camber already in it, needs no further cambering.
- Welded built-up members take their profile tolerances from the welding code.
Camber is a topic of its own. See steel beam camber tolerances and natural mill camber and our comparison of heat cambering and cold cambering.
AISC 303 erection tolerances
Erection tolerances are measured at working points: the centre of a column at each end of its shipping piece, and the centreline of the top flange at each end of a horizontal member.
| Item | AISC 303-22 tolerance |
|---|---|
| Column plumbness, individual shipping piece | 1/500 of the distance between working points |
| Column position at the base | 6 mm (1/4 in) in any direction |
| Column offset at a splice | ±6 mm (1/4 in) |
| Exterior columns, first 20 storeys | 25 mm (1 in) toward and 50 mm (2 in) away from the building exterior |
| Columns next to elevator shafts, first 20 storeys | 25 mm (1 in) from the building column line |
| Member connecting to a column, working point to the column's upper splice line | Plus 5 mm (3/16 in), minus 8 mm (5/16 in) |
To put 1/500 in context: a column with 3 m between working points may lean up to 6 mm. Over a tall building the envelope limits for exterior and elevator columns control, which is why the facade and elevator trades care about them.
Temperature matters too. AISC's commentary notes that steel expands or contracts about 3 mm per 30 m for each 8.3°C change, so plumbing surveys on long frames have to allow for the temperature at the time of measurement.
The work around the steel: anchor bolts and bearing
A large share of tolerance problems start before the steel arrives. Anchor bolts, embedded plates and bearing elevations are usually set by the concrete or general contractor, not the steel fabricator.
AISC 303 assigns this to the owner's construction representative (usually the general contractor) and sets these limits:
- Anchor rod top elevation: ±13 mm (1/2 in)
- Anchor rod horizontal position: 6 mm (1/4 in) for 19 and 22 mm rods, 10 mm (3/8 in) for 25 to 38 mm rods, and 13 mm (1/2 in) for 44 to 63 mm rods
- Bearing devices such as levelling plates: ±3 mm (1/8 in) on elevation
- An as-built survey of anchor rods and embeds before erection, with the engineer's guidance if anything needs correcting
Canadian contracts divide this scope in their own way, so read yours. Whoever owns it, the advice is the same: set anchor bolts from a template, survey them before the steel is fabricated if you can, and send the survey to the fabricator. Base plate holes are normally oversized to absorb placement tolerance, as covered in our guide to ordering steel plate cut to size, but there is a limit to what an oversized hole can fix.
What tolerances mean for your project
For general contractors and owners:
- Check the specification for tightened tolerances before pricing. AESS, glazing and elevator support steel are the usual places.
- Make sure the anchor bolt and embed survey is someone's job, with a date.
- Give the cladding and glazing trades the steel tolerances so their connections allow enough adjustment.
For erectors: the erector plumbs, levels and aligns the frame to the erection tolerances before final bolting or welding. Our guide to hiring steel erectors in Ontario covers what to confirm with them.
For renovation and residential work: a basement beam or a lintel in an existing house is rarely governed by tight frame tolerances, but the existing structure often is not where the drawings say it is. Measure on site before the steel is cut, note any out-of-level bearing points, and tell the fabricator which dimensions are critical. Field measurement saves far more trouble than any tolerance table.
Steel built to your drawings
We prepare shop drawings in-house, then cut, fit, weld and mark every piece to match them, including cambered beams where the design calls for camber. If your specification sets tolerances tighter than standard practice, send it with the drawings so we can account for it in the quote.
Send your drawings and specification for a free take-off and quote, or see our fabrication and installation services.
Frequently asked questions
What are the fabrication tolerances for structural steel?
They limit how far a finished piece can vary from the shop drawings: overall length, straightness, camber and hole locations. In Canada the tolerances in CSA S16 and the project specification govern. In the US, AISC 303 sets them; for example, a member up to 9.1 m long that frames to other steel may vary in length by up to 2 mm (1/16 in).
What is the plumbness tolerance for steel columns?
Under AISC 303, a column shipping piece may be out of plumb by up to 1/500 of the distance between its working points, with extra limits at the base, at splices and on exterior and elevator columns. On Canadian projects, use the values in CSA S16 and the specification rather than assuming they match.
Does CSA S16 include tolerances?
Yes. CSA S16 contains fabrication and erection requirements, including tolerances, and the Ontario Building Code refers to steel fabricated and erected within the tolerances stipulated in CSA S16. The project specification can tighten them for particular elements.
Who is responsible for anchor bolt tolerances?
Usually the party that places them, normally the general contractor or concrete contractor, not the steel fabricator. Under AISC 303 the owner's construction representative sets the anchor rods to approved embedment drawings and surveys them before steel arrives. Check how your contract assigns it.
