CROSS Safety Report
Further issues with freezing and galvanised hollow sections
This report is over 2 years old
A reporter was involved in correcting a cold formed galvanised square hollow section (SHS) four storey balcony structure where longitudinal splits formed in at least two of the four columns.
The believe the issue was that SHS balcony support posts filled with water and then split on a cold night.
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
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A reporter writes in response to report 253 which discussed how freezing split a galvanised rectangular hollow section (RHS) member. They say that in the course of their work for a warranty provider, they were involved in correcting a cold formed galvanised square hollow section (SHS) four storey balcony structure which had longitudinal splits formed in at least two of the four columns. The issue was that SHS balcony support posts filled with water and then split on a cold night. This echoes very well the issue reported in report 253, says the reporter.
Sections of the SHS were assessed by an expert who indicated that some SHS, CHS and RHS members are susceptible to splitting under freezing water conditions dependent on the thickness of tube and the shape. Water entered the tubes through what appeared to be poorly designed or detailed spigot and socket joints which were not adequately sealed.
The steelwork designers, fabricators, erectors, and the contractor’s staff had not realised the hazard and the possibility of it causing a failure. The reporter and other engineers involved had not seen the phenomenon before and initially there was unjustified concern over the quality of the steel. The reporter concludes by saying that the four columns were replaced, and care was given to sealing the tubes to prevent water ingress in the future.
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This report is interesting, but I wonder if freezing is the only cause? The second photo (of a non-galvanised section?) shows cracking in the middle of the side, not at the corners. I would expect freezing to cause bending in the spans before enough tension developed to split the corner. My suspicion is that the corners may have been cracked in the galvanising (Liquid Metal Assisted Cracking) and the freezing has just opened them up. On cold rolled RHS, the corners are work hardened so vulnerable to LMAC. If I remember right, LMAC was first observed on the corners of RHS - they were posts for motorway barriers and had split from the end like banana skins. The cracks were not initially obvious but the posts would not fit in their sockets.
Any other comments on this phenomenon will be welcome.
Expert Panel Comments
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It is an eternal truth that water will get in anywhere and one solution is that it should be permitted to drain away.
There have also been instances of similar problems on small section motorway gantry legs where small holes and unsealed joints can lead to a differential pressure sucking moisture into the internal voids in steelwork. This was then frozen and thawed leading eventually to splitting. A positive venting system (a modest sized hole) low down has been successful in avoided this happening again.
A paper entitled internal resistance to corrosion in steel hollow sections by Michel Tournay in 2002 gives information about moisture in tubes with a number of examples of corrosion over time.