CROSS Safety Report
Critical welding of structural steelwork missed
The designer of a steelwork roof frame found a critical fabrication error when inspecting steelwork on site before it was erected.
Key Learning Outcomes
For structural and civil design engineers:
- Ensure designer’s pre-construction information is passed to the principal designer in compliance with CDM 2015
- Have in place a steel specification supported by appropriate inspection and test plans
- Communicate clearly with steel fabricators at an early stage
- Consider the safety benefits of inspecting steelwork during fabrication and/or erection
For principal designers:
For structural steelwork contractors:
- Consider quality control processes for fabrication drawings
- Seek approval of fabrication drawings before the manufacture of steelwork
For builders and contractors:
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A steelwork roof frame had been designed to support several tension columns which facilitated a large column free area at a lower level in the building. The roof frame was designed to carry significant moment, shear and axial forces. Two steel sections making up part of the frame were required to be welded together with full penetration butt welds to achieve the required load carrying capacity.
The design engineer’s drawings specified the full penetration butt welds. This requirement was commented upon by the engineer a number of times when reviewing the steelwork fabrication drawings. Despite this, during a site visit to inspect the steelwork the required full penetration butt welds were found by the design engineer not to have been made. The result was that the fabricated roof frame was not sufficient for the loadings to which it would be subjected.
The reporter considered that the cause of this error was the fabrication detailer not picking up the requirements set down on the engineer’s drawings nor acting upon the later comments provided by the engineer confirming the need for full penetration butt welds. The reporter assumes that there must have been ineffective checking of the fabrication drawings in the steelwork contractor’s office, as otherwise such a fundamental error would have been spotted.
The reporter confirms that had the structure been installed as found during their site inspection, the structure would have been structurally deficient. The reporter identified several learning outcomes:
- Even when drawings and comments are explicit in their requirements, these could be missed by third parties
- There is a need for quality assurance throughout all construction stages, particularly for key details
- Project managers need to be made aware of the risks involved with critical areas of fabrication
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As described by the reporter, it is common practice that design engineers comment upon the steelwork contractor’s fabrication drawings to ensure that the whole design has been adequately interpreted by the steelwork detailer. This is especially important when the ‘design’ is unusual or there are divisions of responsibility. The objective is to ensure that the design intent is properly carried over into construction reality. This practice often throws up connections or some other aspect, in this case, weld specifications, where the fabrication drawings require amendment or clarification. Designers and fabricators must be clear as to the exact specification of welding required. Butt welds must be fully specified. Butt weld type, the gap between surfaces, parent metal preparation and other details should be specified.
The passing of fabrication drawings between the steelwork contractor and designer is part of the dialogue between designer and fabricator. An adequate dialogue, started at an early stage, can help to ensure that the design is executed in a manner that meets design intent, proper fabrication practice and feasibility of erection. CROSS report Execution not matching design assumptions dealt with a not dissimilar case concerning detailing of a critical part. The report recommended that ‘a process be in place whereby the execution is not allowed to commence without the detailed design stage being completed and approved’. Clearly, a project process that requires fabrication drawings to be checked to assure compliance with design intent prior to execution, would be beneficial. Formal process for the sign-off of fabrication drawings between the designer and fabricator is good practice and should help derive maximum value from the dialogue. Good dialogue and effective quality control procedures within the steelwork contractor’s office should lead to the fabrication drawings being correct before the steelwork is fabricated. Good practice would also dictate that fabrication drawings are checked by another detailer within the steelwork contractor’s office.
fabrication drawings to be checked to assure compliance with design intent
The benefit of site checks by staff independent of the site team is also well illustrated by this report. In this case, the design engineer was very well placed to inspect the steelwork since this engineer would have understood the structural performance requirements in detail. Had the design engineer not undertaken the site check there could have been very serious consequences. In some cases, there may be benefits from third party inspections of critical elements.
Whilst not mentioned by the reporter, there are at least two further mechanisms that could prevent occurrences such as reported. Firstly, in cases where the design includes critical situations (for example critical elements or connections) where special attention is required, it would normally be necessary for this to be covered in the designer’s pre-construction information developed in compliance with the Construction (Design and Management) Regulations 2015 (CDM Regulations 2015). Details of the risk, the designer’s steps taken to manage the risk and how it is perceived that any residual risk will be managed during the construction phase would be set down. In projects involving critical elements, the requirements and specifications for design verification, manufacturing validation and inspection and testing would typically be stipulated by the designer. Adequate drawn information should always be provided.
The designer’s pre-construction information would be shared with the principal designer who would ensure that design risks are considered by everyone involved in the pre-construction phase. The principal designer would then liaise with the principal contractor in respect of risks that need to be controlled during the construction phase. The controls (such as inspection and testing) required for critical elements would be documented. All contractors, including any steelwork contractor, would therefore understand key areas of the construction which require scrutiny. The Health and Safety Executive provides clear CDM 2015 guidance for all parties including designers and principal designers.
any steelwork contractors, would therefore understand key areas of the construction which require scrutiny
Secondly, it is essential that the steelwork is delivered against an appropriate robust specification, such as the National Structural Steelwork Specification for Building Construction (based on execution class 2 structural steelwork), or other specifications, supported by appropriate inspection and test plans. Critical welds, should be subject to non-destructive testing such as ultrasonics. Furthermore, it is also a legal requirement for all fabricated structural steelwork delivered to site to be CE Marked (or certified to the successor scheme - UKCA Marking). To comply with the regulations, only steelwork contractors with an execution class equal to that required for a project should be considered. The CE marking factory production control requirements would cover all welding.
Learning from previous failures
The structural collapse in 1981 at the Hyatt Regency hotel in Kansas City in which 63 tonnes of walkway fell, resulted in 114 fatalities and more than 200 injured. A designed critical connection was changed by the fabricator but not formally approved by the designer; this connection failed. The original design of the connection was less practical to erect and so was amended by the fabricator. This disaster reminds all of the importance of effective dialogue between designer and steelwork fabricator.
Another case, highlighted in the CROSS Alert Structural stability/integrity of steel frame buildings in their temporary and permanent condition published in 2017, proved to be a near miss for operatives who had just left a site when there was a sudden and catastrophic collapse.