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CROSS Safety Report

Freeze and thaw of another RHS

Report ID: 485 Published: 1 January 2015 Region: CROSS-UK

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Further to reports about the freezing of water in hollow sections a correspondent has sent photos of a section of mild steel RHS that was removed from a bridge and shows the bulging that can occur.

Key Learning Outcomes

For asset owners and managers:

  • Regular inspections and maintenance can help keep a structure and its elements safe and detect any obvious safety issues

  • Consider including a risk assessment for internal corrosion in the inspection and maintenance regimes for external hollow section members

  • A check for internal corrosion should be carried out by a suitably qualified person where internal corrosion has been assessed as a significant risk

For civil and structural design engineers:

  • Where there is a potential risk of moisture build-up in external hollow section members, consider using a different section type that may be more appropriate for the given environment

  • When carrying out structural inspections, be aware that water build-up in external hollow sections is a possibility and, where appropriate, consider specific investigation

Full Report

Find out more about the Full Report

The Full Report below has been submitted to CROSS and describes the reporter’s experience. The text has been edited for clarity and to ensure anonymity and confidentiality by removing any identifiable details. If you would like to know more about our secure reporting process or submit a report yourself, please visit the reporting to CROSS-UK page.


Further to reports about the freezing of water in hollow sections a correspondent has sent photos of a section of mild steel rectangular hollow section (RHS) that was removed from a bridge travelling maintenance gantry in 1995 (Figure 1 & 2). The photos show the bulging that can occur when an RHS is not drained and ice forms.

Figure 1: bulging of rectangular hollow section (RHS) – plan view
Figure 2: bulging of rectangular hollow section (RHS) – side view

The lack of splitting of the section may be because this steel is softer than the current hot rolled and cold rolled sections. When drilling into this section to confirm the presence of water a couple of litres poured out. The section was sealed at its top, but it was not clear whether the water had been let in before sealing or some form of capillary action had occurred.

It is likely to have suffered a number of freeze/thaw cycles before it was removed. This reporter still has problems even where there are drain holes, as these are typically small (about 5-6mm) and easily get blocked by corrosion products.

Experience also shows that future clearing of such holes, as a maintenance activity, cannot be relied on. It is considered that a hole size of at least 10mm diameter is required. It is important to remember that the problems can occur in aluminium alloy sections as well as steel.

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