UK Weld Inspection — BS EN 1090-2 Visual, Ultrasonic, and MPI Testing
Weld inspection requirements per BS EN 1090-2:2018 Execution Classes 2, 3, and 4 for UK structural steelwork. Covers visual inspection (VT), ultrasonic testing (UT), magnetic particle inspection (MPI), and radiographic testing (RT). Quality levels B, C, and D per BS EN ISO 5817, acceptance criteria, and NDT procedure requirements for UK fabrication and site welding.
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Inspection Framework per BS EN 1090-2
BS EN 1090-2 divides steel structures into four Execution Classes (EXC), each with different inspection and NDT requirements:
| Execution Class | Typical UK Structure | Inspection Level | NDT Requirement |
|---|---|---|---|
| EXC1 | Agricultural buildings, light industrial | Basic visual | None |
| EXC2 | Standard buildings, low-rise frames | Visual + random NDT | Standard UK — 10 % UT on butt welds |
| EXC3 | High-rise buildings, long-span roofs, hospitals | Visual + specified NDT | 20 % UT on butt welds, MPI on fillets |
| EXC4 | Stadiums, bridges, nuclear, hazardous | Full visual + comprehensive NDT | 100 % UT + MPI + RT as specified |
Visual Inspection (VT)
Visual inspection (VT) is required for all welds in EXC2, EXC3, and EXC4. It must be performed by a Level 2 or 3 weld inspector per BS EN ISO 9712.
Checks performed:
| Check | Acceptance | Notes |
|---|---|---|
| Weld profile | Convex ≤ 0.1 × leg + 0.5 mm | Undercut ≤ 0.5 mm for static loads |
| Weld size | ≥ specified throat thickness | Measured with weld gauges |
| Cracks | None permitted | All cracking is cause for rejection |
| Porosity | ≤ 2 % of projected area | No linear porosity |
| Slag inclusion | None (butt), limited (fillet) | Per BS EN ISO 5817 Level B/C |
| Undercut | ≤ 0.5 mm (dynamic), ≤ 1.0 mm (static) | Sharp notches not permitted |
| Stop-start craters | Filled, no cracks | Craters to be ground smooth |
Ultrasonic Testing (UT)
UT per BS EN ISO 17640 is the primary volumetric NDT method for UK steelwork:
| EXC | Butt Welds (UT) | T-joints (UT) | Fillet Welds (UT) |
|---|---|---|---|
| EXC1 | None | None | None |
| EXC2 | 10 % of welds by length | None | None |
| EXC3 | 20 % of welds by length | 10 % of welds | As specified |
| EXC4 | 100 % | 100 % of critical | As specified |
UT testing levels per BS EN ISO 11666:
| Quality Level | Acceptance Defect Height | UK Application |
|---|---|---|
| Level 2 | ≤ 0.5 mm for t < 15 mm | EXC3 bridge welds |
| Level 3 | ≤ 1.0 mm for t < 15 mm | EXC2 building welds |
Magnetic Particle Inspection (MPI)
MPI per BS EN ISO 17638 is used for surface-breaking crack detection on fillet welds and at weld toes:
| EXC | Fillet Welds — MPI | T-joints — MPI | UK Standard Location |
|---|---|---|---|
| EXC2 | Not required | 5 % of joints | Moment connection welds |
| EXC3 | 10 % of welds | 10 % of joints | All full-strength welds |
| EXC4 | 20 % of welds | 20 % of joints | All complete joint penetration welds |
Radiographic Testing (RT)
RT per BS EN ISO 17636 is specified for:
- Complete joint penetration (CJP) butt welds in EXC4 structures
- Welds where access limits UT (e.g., T-joints with limited UT access)
- Verification of UT findings in disputed welds
RT is typically not required for EXC2 building structures unless specified in the project quality plan.
UK NA Modifications — BS EN 1090-2
The UK National Annex to BS EN 1090-2 confirms:
- EXC2 is the default for UK building structures
- EXC3 is required for structures with more than 5 storeys (or 15 m height)
- EXC4 is required for structures with more than 15 storeys (or 50 m height), major bridges
- NDT personnel must be certified to BS EN ISO 9712 (CSWIP or PCN certification is UK standard)
- All NDT procedures must be qualified per the project quality plan
Weld Defect Acceptance Criteria
Common BS EN ISO 5817 defect limits (Level B — strictest, Level D — most lenient):
| Defect Type | Level B | Level C | Level D |
|---|---|---|---|
| Crack | Not permitted | Not permitted | Not permitted |
| Lack of fusion | Not permitted | Not permitted | Not permitted |
| Undercut (static) | ≤ 0.5 mm | ≤ 1.0 mm | ≤ 1.5 mm |
| Undercut (dynamic) | ≤ 0.3 mm | ≤ 0.5 mm | Not applicable |
| Porosity (butt weld) | ≤ 1 % area | ≤ 2 % area | ≤ 5 % area |
| Excess weld metal | ≤ 1 + 0.1b (mm) | ≤ 1 + 0.15b (mm) | ≤ 1 + 0.25b (mm) |
| Excess penetration | ≤ 1 + 0.3b (mm) | ≤ 1 + 0.6b (mm) | ≤ 2.5 mm max |
Where b = weld width.
Design Resources
- UK Steel Grades Reference — EN 10025-2 grade selection for UK projects
- UK Steel Mechanical Properties — fy, fu, and elongation tables
- UK Universal Beam and Column Sizes — UB/UC section dimensions and properties
- UK Bolt Capacity Tables — Class 8.8 and 10.9 bolt resistance
- UK Beam Design Guide — EN 1993-1-1 flexure, shear, and LTB
- UK Connection Design Guide — EN 1993-1-8 bolted and welded joints
- All UK Steel Design References — complete library
Frequently Asked Questions
What NDT is required for a standard UK building frame (EXC2)?
For EXC2 building frames: 100 % visual inspection (VT) of all welds by a competent person, plus 10 % ultrasonic testing (UT) of butt welds in primary members. Fillet welds do not require routine UT or MPI in EXC2. However, the project specification may increase NDT requirements for critical connections. NDT must be performed to BS EN ISO 9712 by certified personnel (CSWIP or PCN).
What is the difference between CSWIP and PCN certification for UK weld inspection?
CSWIP (Certification Scheme for Weldment Inspection Personnel) and PCN (Personnel Certification for Non-Destructive Testing) are both recognised UK certification schemes. CSWIP is managed by TWI (The Welding Institute) and is the most common for welding inspection. PCN is managed by BINDT (British Institute of Non-Destructive Testing) and covers NDT methods specifically. Both meet BS EN ISO 9712 requirements. The choice depends on project specifications — some clients specify CSWIP, others PCN. Both are equally valid for UK structural steelwork.
What are the UT acceptance criteria for butt welds in UK building steel?
Per BS EN ISO 11666 and BS EN 1090-2: for EXC2 building structures, UT acceptance is Level 3 (maximum allowable defect height ≤ 1.0 mm for t < 15 mm, or 1.5 mm for t ≥ 15 mm). For EXC3, UT acceptance is Level 2 (stricter — ≤ 0.5 mm for t < 15 mm). The probe calibration, scanning pattern, and reporting must follow the qualified NDT procedure. Defects exceeding these limits require repair or rejection.
When is radiographic testing (RT) required instead of UT?
RT is required when: (1) the weld geometry prevents full UT coverage (e.g., complex T-joints, limited access), (2) the material is austenitic stainless steel (coarse grain structure limits UT), (3) the project specification explicitly requires RT (common for EXC4 structures, bridges, and pressure equipment), or (4) verification of UT findings is needed. For standard UK building structures (EXC2), RT is rarely required — UT is the preferred volumetric method.
Related Pages
- EN 1993 Steel Design Overview
- European Steel Properties
- EN 1993 Beam Design Guide
- EN 1993 Column Buckling
- EN 1990 Load Combinations
- UK Steel Chemical Composition
- UK Steel Charpy Values
Educational reference only. All design values are per BS EN 1993-1-1:2005 + UK National Annex and BS EN 10025-2:2019. Verify all values against the current editions of the standards and the applicable National Annex for your project jurisdiction. Designs must be independently verified by a Chartered Structural Engineer registered with the Institution of Structural Engineers (IStructE) or the Institution of Civil Engineers (ICE). Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent professional verification.