Erection tolerances
Beam camber check against NSSS tolerance (as amended by AD 465)
- where
- camber deviation (mm)
- measured camber (mm)
- specified camber (mm)
- permitted positive deviation (mm)
- beam length (m)
- permitted negative deviation (mm)
Explain this equation
Checks a measured beam camber against the specified camber and the permitted deviation. Use it when receiving cambered beams on site, before they are erected.
c_s is the camber shown on the drawing and c_m the camber you measure with the beam unloaded. Δ is the difference. T₊ is how much over-camber is allowed and T₋ how much under-camber, for a beam of length L.
The tolerance scales with span because longer beams are harder to camber exactly, with a fixed minimum for short beams. L in metres is multiplied by 1000 to work in millimetres. Under-camber is held tighter than over-camber because too little camber shows as sag in the finished floor.
Sense check: a 9 m beam may be up to 18 mm over or 9 mm under the specified camber.
Watch out: measure with the beam on its side or supported so its own weight is not taking camber out.
Checks a measured beam camber against the specified camber using the NSSS 7th edition tolerance, as amended to allow a small negative deviation.
Enter your values to see the result.
Good to know
NSSS 7th edition clauses 7.2.6, 7.4.9 and 7.5.8 allow +L/500 or 6 mm, whichever is greater, above the specified camber. SCI AD 465 amended the negative tolerance from zero to L/1000 or 4 mm, whichever is greater, matching BS EN 1090-2 Table B.6 class 2. Measure with the beam unloaded and lying on its side or supported so self-weight does not reduce the camber.
Common mistake
Measuring camber with the beam on its supports in the frame, where self-weight deflection has already taken some out.
Limits
Fabrication tolerance for cambered members. Check the specification for any project-specific camber tolerance.
Sources
- SCI Advisory Desk AD 465: Amendment to clauses on negative tolerances on cambers in the NSSS (7th ed. cl. 7.2.6, 7.4.9, 7.5.8: +Δ = L/500 or 6 mm, −Δ = L/1000 or 4 mm, whichever is greater; matches BS EN 1090-2 Table B.6 class 2)
- BCSA National Structural Steelwork Specification for Building Construction, 7th edition, 1st revision (2023) – Section 9 accuracy of erected steelwork; Section 7 cambers; 8.6.2 reference temperature 20 °C
- BS EN 1090-2:2018+A1:2024 Execution of steel structures and aluminium structures – Technical requirements for steel structures (8.2.2 bolt protrusion; 8.5 preloaded bolts: Fp,C = 0.7 fub As; torque method Mr,2 = km d Fp,C, first step about 0.75 Mr,i, second step 1.10 Mr,i; combined method Mr,1 = 0.13 d Fp,C simplification; k-classes K0, K1, K2; Annex B tolerances)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Always work to the engineer’s drawings and bending schedules, BS 8666, BS EN 1992, BS EN 1090-2, BS 5531/the BCSA National Structural Steelwork Specification and manufacturers’ data. Lifting must be planned under LOLER by a competent person.