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CROSS Safety Report

Square Hollow Section (SHS) expansion due to freezing

Report ID: 641 Published: 1 April 2017 Region: CROSS-UK

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Overview

A reporter has come across problems in British Columbia (temp range -30oC to +40oC) of cracking to square hollow sections (SHS) due to the ingress of water and freezing.

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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Further to report 579 which discussed the freezing of water in steel hollow sections, a reporter from Canada has come across a similar problem in a few locations in British Columbia (temp range -30oC to +40oC). Two examples are shown in attached photos. One was a painted 2" (50mm) square hollow section (SHS) on a sloped pedestrian railing, and the other is an epoxy paint coated 1" (25mm) SHS on a stair handrail also on a slope (Figures 1 & 2). In neither case were drain holes provided yet both hollow sections split due to frost action.

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Figure 1: split bottom square hollow section (SHS) rail
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Figure 2: split square hollow section (SHS) handrail

The reporter typically specifies drain holes in all areas of SHS members where moisture can collect, and checks are made to see that such holes are present before approving work. In the opinion of the reporter, although this is a dryer region of the province, there is still a moderate amount of moisture at night (condensation). This repeatedly finds its way into the internal space of the SHS where it collects over time (perhaps over several years). Even pin holes appear to provide enough opening for moist air to get into SHS members.

The reporter typically specifies drain holes in all areas of SHS members where moisture can collect, and checks are made to see that such holes are present before approving work

Thereafter, moisture accumulates faster than it can escape and builds up in non-draining areas, possibly over several cold weather periods/years. The SHS member finally cracks and releases the moisture. Also, corrosion build-up is taking place inside the SHS contributing to strength loss. Corrosion filled moisture may react differently when frozen than straight water in a clean test sample, suggests the reporter. Heat (direct sun exposure up to 70oC) may also affect the area in some way. We do not currently perform any testing on these items other than try to avoid them. Interesting article and good work in general.

Expert Panel Comments

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This phenomenon may well pose a bigger risk in climates with very cold nights and relatively warm days or where bright sunlight is enough to raise steel temperatures above zero thereby generating repeated freeze thaw cycles. CROSS has had several reports of hollow sections cracking, usually related to water inadvertently trapped inside the tubes. In some cases, tubes have had drainage holes.

Sections which are detailed with drain holes will allow a path for moist air to enter and hence for corrosion to take place which can cause damage. In other cases, it is water expansion, as water undergoes significant expansion during the phase change to ice and this causes splitting.

Fully sealed hollow sections are likely to be more resistant both to corrosion and to freeze/thaw damage. The more cycles there are, the more likely it is that cracking or splitting will occur. Freeze/thaw damage has also been reported in temporary pockets cast into concrete structures to take handrails.

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