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CROSS Safety Report

Freezing splits galvanised SHS columns

Report ID: 253 Published: 1 April 2012 Region: CROSS-UK

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Overview

Concerns were raised after several square hollow section members split vertically on a partially complete construction site.

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 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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At a construction site several ground floor square hollow section (SHS) columns had split vertically (Figure 1). The columns were 120 x 120 x 5.0mm hot rolled members and formed part of a single-storey section of a new complex. The construction was only partially complete and included the profiled metal deck roof and in situ concrete slab only, and possibly some nominal construction live loading.

Image
Figure 1: splitting of SHS column

The design was checked and found to be satisfactory and the failure mode did not follow the expected Euler buckling shape for compressive forces. The sections were galvanised and holes had been left in the cap plates for drainage as part of the galvanising process. These had been left exposed to the elements prior to concreting.

The reporter suspected that water had come in through the holes and filled the hollow sections, and that this had then frozen to exert a significant bursting pressure sufficient to split the steel along one corner, (not at the weld position).

Holes were drilled through the columns near to the base and proved this theory as water jetted out under pressure (Figure 1). This could have gone undetected if the weather had not been sufficiently cold to freeze the water within the columns but could have occurred at a later date following completion of the building with greater consequences.

Expert Panel Comments

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The Standing Committee on Structural Safety (SCOSS) biennial report for 2001 included a case where barrier posts had filled with water that had caused damage on freezing in winter. The barrier structures were only four to five years old and made of rectangular hollow sections (RHS) with fully welded joints and end plates. Visual inspection of the barriers showed no sign of any openings that would allow ingress of water (Figure 2).

Image
Figure 2: splitting of SHS barrier post

Does welding produce a complete seal?

It appears that water can be drawn into tubular steel structures of this type by capillary action. The phenomenon has been experienced in other structures such as roof trusses made from hollow steel sections, and welded box girders. Although the structural elements are manufactured as sealed units, continuous welding does not necessarily produce a complete seal and water may accumulate inside where the units are exposed to the weather.

Research into the phenomenon in Canada has shown that a partial vacuum caused when a relatively warm, imperfectly sealed steel tube is rapidly cooled by rainwater, can draw in water through small cracks and holes. Over time this can result in a considerable build-up of water trapped inside. Where such a steel tube forms a structural element that is exposed to the weather, the trapped water may freeze in winter. The element may thus be damaged as it fails to restrain the expansive action of ice formation. The resulting bulging and/or splitting may reduce the load-carrying capacity of the element substantially, thereby undermining the safety of the structure.

The issue may go undetected

CROSS draws attention to this reported experience because tubular or other sealed steel elements in crush barriers, bridge parapets and other structures may be similarly affected. The phenomenon may exist elsewhere even though it has not yet progressed to the point where bulging and/or splitting has become visible.

The presence of water cannot be detected merely by an external visual inspection until damage occurs. Inspecting engineers should be aware of this possibility and, where appropriate, consider specific investigation. For the damaged crush barriers, the reported remedy was to repair and replace damaged posts and to make drain holes just above ground level to allow water to scape.

The presence of water cannot be detected merely by an external visual inspection until damage occurs. Inspecting engineers should be aware of this possibility and, where appropriate, consider specific investigation

A borescope inspection through the drain holes enabled the internal condition of the posts to be checked for corrosion. More generally for tubular steel structures exposed to the weather and in unheated buildings, the report on the Canadian research advised either of the following:

  • Provide complete weld seals free from porosity

  • Provide some form of drainage with periodic inspection of drain holes to ensure they do not become blocked

Provision of holes also prevents the development of partial vacuum in the units and can also enable beneficial ventilation of the internal voids. Similar problems have been found on highway structures with water ingress into so called sealed steel sections used as gantry legs when there were freeze/thaw splits, and relieving holes had to be drilled and repairs carried out.

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