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AD 465: Amendment to clauses on negative tolerances on cambers in the National Structural Steelwork Specification

Engineers generally specify cambers on steel beams to reduce deflection under self-weight. However, when specifying cambers, they should carefully consider the tolerances, especially the negative tolerance (−Δ). For example, in the case of long span beams the negative tolerance can be so large that the beam loses its theoretical camber (f) and is curved in the opposite direction.

In the 6th Edition of the National Structural Steelwork Specification (NSSS), Clauses 7.2.6, 7.4.9 and 7.5.8 required the tolerances on camber to be ±L/500 or 6mm whichever is greater. During the development of the 7th Edition of the NSSS it was recognised that a tighter negative tolerance is necessary to achieve the required camber, and consequently the 7th Edition specified a zero negative tolerance on cambers. An extract from clauses 7.2.6, 7.4.9 and 7.5.8 of the 7th Edition is given below:

Deviation from intended curve or camber f at middle of length L of curved portion when measured with the web horizontal.
Δ = 0
+Δ = L/500 or 6mm whichever is greater

This is a more onerous requirement than criterion No. 4 in Table B.6 of BS EN 1090-2:2018, which stipulates the following functional tolerances:

  • For Class 1, −Δ = L/500 or 6mm whichever is greater
  • For Class 2, −Δ = L/1000 or 4mm whichever is greater

Steelwork contractors are finding that in many cases the zero negative tolerance is difficult to achieve and requires more expensive cambering techniques. For long-span beams the Class 1 negative tolerances given in BS EN 1090-2 may not be appropriate because, as explained above, it can result in the beam cambering in the wrong direction. To avoid this situation the more stringent class 2 tolerances from Table B.6 of BS EN 1090-2:2018 for the negative tolerance limit should be adopted in Clauses 7.2.6, 7.4.9 and 7.5.8 of the 7th Edition of the NSSS. The change to the negative tolerance in 7.2.6, 7.4.9 and 7.5.8 is given below.

Δ = L/1000 or 4 mm, whichever is greater

Contact: Ana M. Girão Coelho
Email: ana.girao-coelho@ steelconstruction.org

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