Severe undercut on Structural Steel BMU Bracket
Severe undercut was observed along the toe of a fillet weld connecting a structural steel bracket. The undercut reduces the effective thickness of the parent material and creates a stress concentration that may initiate cracking or reduce the connection’s static and fatigue resistance. The bracket should be quarantined and assessed by a competent welding inspector against the approved welding procedure, specified execution class and acceptance criteria before being accepted or repaired.
The relevant execution standard is BS EN 1090-2:2018+A1:2024. BS EN ISO 9606-1:2017 concerns welder qualification, while finished-weld imperfections should also be assessed against the specified quality level in BS EN ISO 5817:2023.
- NCQLE reference
- NCQLE-2026-00010
- Quality topic
- Defects and corrective action
- Lesson type
- Failure
- Evidence rating
- Experience
- Published
- 31 July 2026
- Last reviewed
- 1 August 2026
Evidence basis: Supported by relevant professional experience. This learning is provided for professional consideration and does not replace project-specific competent advice, statutory duties or contractual requirements.
Context
During an inspection of a stainless-steel structural bracket forming part of the Building Maintenance Unit (BMU) safety system, the welded connection was reviewed before the system was placed into service. As the bracket formed part of a safety-critical load path used during façade access and maintenance operations, the weld profile, continuity, reinforcement and visible imperfections required verification against the approved design, welding procedure and inspection criteria. The stainless-steel bracket appeared to have been TIG welded without adequate filler metal or weld reinforcement. Severe undercut was visible along the weld toe, where the parent material had been melted away and not sufficiently filled. This produced a reduced section and an irregular weld profile at a structurally important connection. No evidence was available at the time of inspection to confirm compliance with the approved welding procedure or specified acceptance criteria.
The transferable lesson
Structural stainless-steel brackets must be welded using an approved welding procedure specification by a suitably qualified welder. TIG welding without filler metal must not be used unless the procedure specifically permits an autogenous weld and demonstrates the required strength and quality. Weld toes, profile, reinforcement and undercut must be inspected against the specified acceptance criteria before the component is installed or loaded.
Recommended action
Confirm the approved welding procedure, filler material, welder qualification and inspection requirements before fabrication begins. Introduce a hold point for visual inspection of structural welds before components are installed, coated or loaded. Measure any undercut and assess it against the specified BS EN ISO 5817 quality level and project execution specification. Quarantine non-conforming brackets and obtain an approved repair or replacement procedure from the responsible structural engineer and welding coordinator.
Limitations and application
The acceptability of the weld depends on the bracket material grade, thickness, loading, execution class, approved welding procedure and specified BS EN ISO 5817 quality level. A photograph alone cannot confirm weld penetration, internal defects, filler use or structural capacity. Final acceptance, repair or replacement must be determined by a competent welding inspector, welding coordinator and responsible structural engineer, with non-destructive testing where specified.