CROSS Safety Report
Concerns over execution class categorisation of steel pedestrian bridge
A reporter is concerned in respect of the steelwork execution class, and quality management arrangements, applied to the construction of a public access pedestrian bridge.
Key Learning Outcomes
For clients specifying bridgeworks:
- Observe established technical approval processes
- Consider the advantages of a single controlling mind to oversee design
For building and bridge design engineers:
- Specify the execution class or classes for the whole structure
- Consider independent audit of fabrication for some structures
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A steel pedestrian bridge specified as requiring construction to execution class 3 (EXC3) was fabricated by a steelwork contractor certified to EXC3, however, the reporter is concerned that insufficient monitoring and due diligence were undertaken of the contractor’s work in fabricating the bridge.
The reporter is also concerned that one support for the bridge was included in an adjacent steelwork package which was undertaken by a steelwork contractor certified to execution class 2 (EXC2). The reporter believes that the whole of the bridge, which provides public access, including any supporting steelwork, should have been constructed by a steelwork contractor working to EXC3.
Expert Panel Comments
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Bridges in the UK are usually required to be constructed to execution class 3 (EXC3) whereas buildings are often required to be constructed to execution class 2 (EXC2) or indeed EXC3 for some higher-risk buildings. The execution class defines a set of quality and assurance controls for the fabrication process. The higher execution class includes more stringent requirements. Specifying the right Execution Class – newsteelconstruction.com provides a background to execution classes and selecting the right level of quality and assurance controls needed to ensure a structure meets the engineer’s design assumptions.
a controlling mind with complete oversight of a project
Most bridges in the UK will be constructed under the control of an ‘intelligent client’ who will overview a technical approval process. A technical approval process for a public highway or railway bridge in the UK is likely to cover the whole structure including supports. The whole structure will therefore be subject to consistent overview and scrutiny. Clearly, it is advantageous if there is a controlling mind with complete oversight of a project and the ability to ensure consistency of execution standards across a single structure – it appears that this may not have been the case with the bridge reported here. Where a client has less experience in a sector, then designers can help promote adequate safety by guiding the client to industry standard practice for that sector. In this case, the bridge designer could have confirmed to the client that the scope of the EXC3 works included the whole bridge including all supports.
It would be unlikely that a lower execution class would be specified or acceptable for any part of the structure. However, it is possible that a higher execution class could be considered necessary and specified for a critical part of a structure.
The report raises the issue of insufficient monitoring and due diligence of the contractor's fabrication. The specified execution class will determine the standards, checks and monitoring that should be applied to the work package. There is an argument that independent audit of fabrication (especially for EXC3 steel structures, where fatigue may be a design criterion) could be advocated. This would help guard against structures not meeting design requirements. It should also be remembered that both correct design and execution are required to achieve an adequate structure; without correct fabrication, an expertly designed structure will not meet requirements and vice versa.
without correct fabrication, an expertly designed structure will not meet requirements
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