Dimensional tolerances in structural steel fabrication are critically important for parts to fit together on site, avoid assembly difficulties and be consistent with structural calculations. Over-tolerance production adds unnecessary cost and time; under-tolerance production leads to site problems and rework costs.
EN ISO 13920 is the primary standard defining general dimensional tolerance classes for welded steel structures. EN 1090-2 references this standard and specifies the applicable tolerance class according to execution class (EXC).
EN ISO 13920 Tolerance Classes
| Dimension Type | Class A | Class B | Class C | Class D |
|---|---|---|---|---|
| Length (30–120 mm) | ±1 mm | ±1 mm | ±2 mm | ±4 mm |
| Length (120–400 mm) | ±1 mm | ±2 mm | ±3 mm | ±6 mm |
| Length (400–1000 mm) | ±1.5 mm | ±3 mm | ±4 mm | ±8 mm |
| Length (1000–2000 mm) | ±2 mm | ±4 mm | ±6 mm | ±11 mm |
| Length (2000–4000 mm) | ±3 mm | ±6 mm | ±9 mm | ±16 mm |
For angular tolerances, class E (±20') and class F (±45') are used. For straightness/flatness, classes G (tighter) and H (wider) apply.
Relationship with EN 1090-2
EN 1090-2 specifies tolerance class requirements for various structural components. The general rule:
- EXC1 — Tolerance Class D or C (wide tolerance)
- EXC2 — Tolerance Class C or B
- EXC3 — Tolerance Class B or A (tight tolerance)
- EXC4 — Tolerance Class A (tightest)
EN 1090-2 Annex D.1.1 also defines component-geometry-specific special tolerances for profile straightness, flange squareness and web flatness.
Practical note: If no tolerance class is specified on the drawing, the default class applicable to the EXC level is applied for EN 1090 work. Explicitly stating the tolerance class in the contract prevents future disputes.
Measurement Methods
Methods and tools used for dimensional inspection:
- Manual Measurement: Tape measure, digital calliper, angle gauge — generally sufficient for tolerance classes C and D.
- Laser Tracker or Total Station: Sub-millimetre accuracy for large structural members (beams, truss systems). Recommended for large structures under EXC3.
- 3D Scanning: Full surface mapping; for complex geometries and rework analysis.
Dimensional Inspection Process at ArmaWeld
At ArmaWeld every structural component passes through a dimensional inspection stage after welding. For EXC3 projects, an inspection plan is prepared before production; critical dimensions and control points are defined. Measurement results are recorded in a dimensional inspection report and delivered with the shipping document package. When a deviation is detected, the CAPA (Corrective and Preventive Action) process is initiated and the root cause analysis is recorded.
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