Geometrical tolerances – essential and functional

The vocabulary of tolerances

AS/NZS 5131 defines a tolerance as the band between the permitted upper and lower limits of a dimension (Clause 3.1.30), and Clause 3.2 then names five types. Getting these terms right avoids most disputes between fabricator, erector and certifier.

Manufacturing, fabrication and erection describe when a tolerance is checked; essential and functional describe why it matters. A single criterion, such as column inclination, can carry both an essential and a functional value.

Essential versus functional, Class 1 versus Class 2

Clause 12.1 explains the two families. Essential tolerances relate to mechanical resistance and stability and are broadly consistent with the imperfections assumed in AS 4100, AS 5100.6, AS/NZS 4600, AS 2327 and NZS 3404. Functional tolerances serve fit-up and appearance and come in two classes: Class 2 is stricter than Class 1.

• Both essential and functional tolerances must be met – the functional class is not optional.
• The class may be set in the construction specification or an application standard; where nothing is specified, Class 1 applies.
• A note to Clause 12.1 recommends that designers consider Class 2 for CC3 and CC4.
• Permitted deviations exclude elastic deflection under self-weight, which is assessed separately.
• Tolerances accumulate: fabricated pieces must meet fabrication tolerances on top of the mill tolerances of the product, and the erected frame must meet erection tolerances on top of fabrication tolerances.

The specification may tighten or add tolerances. If it does, Clause 12.1 requires it to state the amendment, the parameters and limits for any special tolerance, and whether it applies to all components or only nominated ones. Tight tolerances should be reserved for the interfaces that need them – see specification and documentation.

How Appendix F is organised

Appendix F is normative and contains 21 tables in three groups. Each table lists the criterion, how it is measured and the permitted deviation in essential, Class 1 and Class 2 columns.

Illustrative key values

The selected values below show how the system works. They are not a substitute for the tables – refer to the current edition of AS/NZS 5131 for the full requirements and measurement conventions.

Two lessons follow. First, Class 2 often halves the Class 1 value, which affects jigs, drilling and survey methods. Second, some essential limits are tighter than Class 1 – column inclination is an example – so meeting Class 1 alone does not guarantee compliance. For AESS 2–4, straightness is halved again (Clause 10.4(g); see AESS).

Measurement

Clause 12.2 points to Clause 13.4 for fabricated components and Clause 13.11 for erected steel; Appendix F adds that measurement accuracy is assessed to the relevant part of ISO 17123. For erected steel, Clauses 13.11.4–13.11.7 require methods and instruments from ISO 7976-1 and -2, chosen so their accuracy is small relative to the tolerance; temperature correction to a 20°C reference; and allowance for tape sag and slope, instrument error and column shortening under load. Each measured value must be within its permitted deviation, and the algebraic sum must not exceed the limit for the whole structure; for large or unusual structures a statistical (SRSS) approach is allowed. Measurements are taken near site connection nodes under self-weight unless specified otherwise. See erection and site work.

When a tolerance is exceeded

Clauses 12.3 and 12.4 are strict. Any deviation beyond the permitted value must be corrected until it is back within the limit. For essential tolerances alone, there is one alternative: the deviation may be accepted if the designer explicitly includes it in a revised capacity calculation to the relevant design standard. A fabricator has no authority to accept an essential exceedance itself.

In practice this means a nonconformance report, the measured values, a proposed disposition and the designer's written acceptance or a correction procedure. Corrections by heat must respect the temperature limits of Clause 14.7, and every disposition belongs in the quality records.

How a fabricator controls tolerances

Tolerance compliance is built in, not inspected in. Typical controls at a CC3 fabricator include:

• Detailing: a 3D model that sets out holes and connection positions from common datums, with allowances for weld shrinkage and specified camber;
• Production: CNC cutting and drilling for hole groups, fixtures and jigs for repeat assemblies, and welding sequences that limit distortion;
• Inspection: calibrated tapes, squares and levels, first-piece checks, and dimensional inspection reports for critical dimensions as nominated in the ITP;
• Interfaces: trial assembly at the works where the specification requires it (Clause 11.5.10), and comparison with the as-built anchor bolt survey before shipment.

For steel shipped from overseas, a fit-up problem found on site is costly. The class, special tolerances and survey data for existing supports should be agreed during detailing, not after the steel arrives.