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CROSS Safety Report

Common use of S235 cold rolled steel instead of S355 hot rolled steel

Report ID: 740 Published: 1 April 2018 Region: CROSS-UK

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Overview

It has come to a reporter's attention that some local fabricators are purchasing S235 cold rolled hollow sections rather than the specified S355 hot rolled sections, as they are easier and cheaper to obtain.

Key Learning Outcomes

For construction professionals:

  • Substitution of construction elements and material grades should not be made without approval from the design engineer

  • Consider introducing a quality assurance process that covers the correct use of products, components, and materials on site

For civil and design engineers:

  • Share your knowledge of the behaviour of products, components and materials and routinely raise the risks associated with substitutions to contractors and the wider project team

  • If you are concerned that a lower grade material may have been installed, consider asking the contractor for the material certificates

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It has come to a reporter's attention that some local (CE marked) fabricators are purchasing S235 cold rolled hollow sections rather than the specified S355 hot rolled sections, as they are easier and cheaper to obtain. This is also an issue with the main local stockist (who supply local fabricators) who say that there is no demand for the higher-grade steel; even though they fabricate from the reporter's drawings which specify S355 hot rolled steel. They tell us that they buy from Europe and it would be uneconomical for them to stock any S355 tubular products.

They suspect that this is happening nationally and has been happening for some time. This is a surprise as they expect with the use of CE marking and execution classes, that there would not be this basic lack of understanding about the implications of incorrect substitutions. The reporter is not sure where this leaves them for work that has already been completed and what they might use in the future where a tubular product is most suitable if they cannot be sure that the correct grade will be used.

This is a surprise as they expect with the use of CE marking and execution classes, that there would not be this basic lack of understanding about the implications of incorrect substitutions

Expert Panel Comments

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This report is in many ways very similar to report 634 which discusses how a contractor installed the incorrect steel grade. The allegation is not quite clear as to whether products are being incorrectly CE marked or whether contractors are substituting lower grade material without authorisation. Neither action is acceptable. As for report 634, no changes should be made without sanction from the designer.

Major asset owners have had problems with the wrong (i.e. cheaper) grade of steel being used. A fundamental problem is that many within the supply chain do not appreciate that such product substitution results in a completely different product being used – a product with different design properties.

Hot v. cold formed hollow sections

For example, one could not simply substitute a hot rolled product for the same section thickness cold formed product – the equivalent cold rolled section would need to be thicker. This is because of the stresses induced in the steel during the manufacturing process – such stresses would not be induced in hot rolled steel. Hence, different manufacturing process, different stresses, different design properties. Also, substituting hot rolled for cold would be a serious problem in areas where the section would be subject to fatigue loading. Another implication would be welding – completely different welding procedures would apply.

For example, one could not simply substitute a hot rolled product for the same section thickness cold formed product – the equivalent cold rolled section would need to be thicker

Reliance on CE marking with no independent review may not be sufficient, and substitution must be guarded against. The Standing Committee on Structural Safety (SCOSS) Alert Anomalous documentation for proprietary products - February 2013 warns of this practice. There are additional  reports on the subject such as in  510 Policing of CE marking on steelwork.

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