Execution Classes — EN 1090-2 Table 1

Most building structures in Europe are EXC2 or EXC3. EXC4 is rare and typically specified only for special structures.

Weld Quality Levels — EN ISO 5817

EN ISO 5817 defines three quality levels for weld imperfections:

For EXC2 buildings, quality level C is typical. For EXC3 bridges, quality level B may be required.

Visual Inspection (VT) — EN 17637

Visual inspection is the primary NDT method and is required for all execution classes.

Visual Inspection Checklist

Ultrasonic Testing (UT) — EN ISO 17640

Ultrasonic testing is required for full-penetration butt welds in EXC3 and EXC4 structures.

UT Acceptance Criteria per EN ISO 11666 (Level C)

UT is typically calibrated using a reference block with 3 mm side-drilled hole (SDH) per EN ISO 17640.

UT Coverage Requirements

Magnetic Particle Inspection (MPI) — EN ISO 17638

MPI is used for detecting surface and near-surface cracks in ferromagnetic steel.

MPI Acceptance Criteria — EN ISO 23278 (Level C)

• Linear indications > 3 mm for t < 10 mm — not permitted
• Linear indications > 0.3 × t for t ≥ 10 mm — not permitted
• Rounded indications > 4 mm — not permitted
• Any crack indication — not permitted

MPI is particularly important for:

• Weld toes of cruciform joints
• Tack welds incorporated into final welds
• Root passes of butt welds (with MPI access)
• Flame-cut edges in tension zones

Radiographic Testing (RT) — EN ISO 17636

RT is an alternative to UT for butt welds, typically used where UT geometry is difficult.

RT is slower and more expensive than UT but gives a permanent record (radiograph). UT is preferred for shop fabrication in European practice.

Inspection Personnel Certification

All NDT personnel must be certified per EN ISO 9712:

For EXC3 structures, UT must be performed and interpreted by Level 2 personnel. For EXC4, Level 2 with specific product endorsement is required.

Worked Example — NDT Acceptance for EXC3 Bridge Girder

Problem: A steel bridge girder (EXC3 per EN 1090-2) has 30 m length with multiple full-penetration butt welds joining S355J2 flange plates, 40 mm thick. The project specification requires 100% UT of all butt welds per EN ISO 17640. Determine the NDT scope, acceptance criteria, and personnel requirements.

Step 1 — Determine NDT Scope per EN 1090-2 Table 26: For EXC3 structure:

• Butt welds (full penetration): 100% UT coverage
• T-joints (full penetration): 25% UT coverage
• Fillet welds: 10% MPI on surface
• All welds: 100% visual inspection (VT)

Step 2 — UT Acceptance Criteria per EN ISO 11666 Level C: For 40 mm plate thickness (range 30-60 mm):

• Maximum indication length: 12 mm
• Maximum amplitude: Reference level -6 dB
• Reference block: 3 mm SDH per EN ISO 17640
• Probe frequency: 4 MHz, angle: 70° for flange-to-web welds
• Scanning speed: ≤ 150 mm/s
• Couplant: water-based gel, applied to clean mill scale surface

Step 3 — Visual Inspection per EN 17637: For EXC3 and quality level C (EN ISO 5817):

• Undercut: ≤ 1.0 mm for t = 40 mm
• Excess weld metal (butt): ≤ 1.0 + 0.1 × weld width (typically ≤ 3 mm for 15 mm cap width)
• No cracks, overlap, or incomplete fusion permitted
• Surface porosity: ≤ 2% of surface area in any 100 mm² zone
• Weld dimensions: throat within -10% of specified value (no reduction permitted for partial penetration)

Step 4 — Personnel per EN ISO 9712:

• UT operators: Level 2 minimum (perform + interpret)
• VT inspectors: Level 2 minimum
• Procedure approval: Level 3 (develops NDT procedures, signs off reports)

Step 5 — MPI of Cruciform Joints per EN ISO 17638: For EXC3, 25% of full-penetration T-joint length receives MPI + 10% of fillet welds:

• Linear indications > 0.3 × t = 12 mm — not permitted (per EN ISO 23278 Level C)
• Rounded indications > 4 mm — not permitted
• Any crack or crack-like indication — reject and repair per qualified WPS

Step 6 — Documentation per EN 1090-2 Clause 12: Record for each weld: weld identification number, welder ID, NDT method, acceptance standard, result, inspector ID and certification level, date. UT records must include scan plan showing probe positions and indication locations.

Result: NDT scope satisfies EN 1090-2 EXC3 requirements. All butt welds achieve EN ISO 11666 Level C acceptance.

Frequently Asked Questions

What NDT is required for a standard steel building frame per EN 1090-2?

For a standard EXC2 steel building frame, EN 1090-2 requires visual inspection (VT) of all welds — no ultrasonic or MPI is required for routine connections. For EXC3 buildings (e.g., high-rise > 50 m, theatres, stadiums), full UT coverage of butt welds and partial MPI of T-joints is required per Table 26.

What is the acceptance level for weld imperfections in building structures?

For EXC2 building structures, use EN ISO 5817 quality level C as the acceptance criterion. Level C allows minor imperfections (undercut ≤ 0.5 mm, limited porosity) but prohibits cracks, incomplete fusion, and lack of penetration. For EXC3 and EXC4, level B (stringent) applies.

Related Pages

• Welding Procedure — EN 1090-2 WPS requirements
• Weld Capacity — Fillet weld per EN 1993-1-8
• Weld Symbols — EN 22553 weld symbols
• All European References

Educational reference only. NDT requirements per EN 1090-2:2018. Acceptance criteria per EN ISO 5817. Verify execution class and NDT scope per project specification. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent verification.

Design Resources

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• Weld Symbol Generator

Design guides

• Weld Design Checklist