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AD 453: Accumulated deviations in erected steelwork

Questions about tolerances continue to arrive at SCI’s Advisory Desk – often concerning the potential to sum all the possible deviations to reach a (usually large) tolerance on the final position of a component.

The suggestion is that, (for example) the base of a column can be out of position, and the column can be out-of-plumb, and the connections for a façade beam can be out of position, and the beam itself can have a lack of straightness. Combine that situation with some fabricated bracket (with its own set of tolerances) connected to the beam and the potential for a large deviation at measured locations is obvious.

The National Structural Steelwork Specification (NSSS), which is now in its 7th Edition, deals with this by adopting a “root sum of the squares” approach. The accumulated sum of several independent sources of deviation (Δ1, Δ2, Δ3 etc) is given by:

SCI advice is that when certain locations are critical (usually at interfaces with other components), it is much better to build in provision for adjustment than to argue about tolerances later.

Contact: Graham Couchman
Tel: 01344 636555
Email: advisory@steel-sci.com

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