Erection tolerances

Why tolerances matter

A pre-engineered building is a bolted kit: if anchor bolts, base plates and frames are within tolerance, the purlins, girts, sheeting and accessories fit without site modification. If they are not, holes are reamed, sheets do not line up, doors bind and crane runways must be shimmed. Tolerances are also a structural matter – the design assumes columns are close to plumb and initial imperfections are within limits that the stability analysis has allowed for.

The values in this article are typical values drawn from the standards named. Always check the edition and the project specification, which may be more stringent – particularly for crane runways and buildings with sensitive finishes.

Anchor bolts and foundations

In North America, MBMA common industry practices refer to AISC 303 (Code of Standard Practice) for anchor rod placement. Typical AISC 303 values are:

AS/NZS 5131 and EN 1090-2 give comparable limits for holding-down bolt position and foundation level, typically in the order of ±3 to ±6 mm for bolt position within a group and ±5 to ±10 mm for bearing level, with oversize holes in base plates and grout space to accommodate them. See base plates and anchor bolts and foundations interface.

Columns, frames and secondary members

Purlins and girts are aligned so that sheeting lies flat; the MBMA manual and cladding manufacturers give limits on purlin straightness and on step between adjacent purlins, as excessive misalignment causes visible waviness and fastener stress. Rod and cable bracing should be tensioned to remove sag but not so tight that it pulls the frame out of plumb.

Crane runways

Crane runways are the most demanding part of many industrial buildings. Rail span, rail alignment, level and level difference between the two rails are controlled tightly because they govern wheel wear and crane performance. Typical references are AISC Design Guide 7 and CMAA 70/74 in North America, AS 1418.18 with AS/NZS 5131 in Australia, and EN 1090-2 (with EN 1993-6) in Europe. Runway tolerances for heavy-duty cranes are often in the order of a few millimetres over the runway length, so the runway is surveyed after the frames are final and before the crane is commissioned. See crane runway beams and brackets.

Record the as-built survey of anchor bolts, column plumbness and runway geometry. It protects all parties if a fit-up or serviceability question arises later.

Secondary members, cladding and openings

Tolerances for the envelope are less formal than for the frame, but they decide how the finished building looks and whether it stays watertight. Typical checks (values indicative – the cladding or accessory supplier's limits govern):

In Europe, EN 1090-4 sets execution requirements, including tolerances, for cold-formed thin-gauge structural members and sheeting; elsewhere the cladding manufacturer's installation manual and the project specification are normally the governing documents.

Fabrication and erection tolerances together

Erection accuracy depends on fabrication accuracy: member length, end-plate squareness and hole positions are checked in the shop (see quality control and inspection). World Steel fabricates to EN 1090-2 EXC3 and AS/NZS 5131 CC3 and provides anchor bolt plans with tolerance notes and, where contracted, erection supervision including survey of the first braced bay. The erection sequence article explains at which stage each check is made.