CROSS Safety Report
Wall reinforcement cages collapse
This report is over 2 years old
Overview
A contractor says that they have experienced two cases of wall reinforcement cages collapsing.
Key Learning Outcomes
For the construction team:
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Having a competent temporary works designer/adviser in place to supply an engineered solution can ensure all temporary works, such as the stability of rebar cages, are carefully considered and planned
For civil and structural design engineers:
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If there is a risk of temporary instability to rebar cages, noting this on the drawings and risk register can ensure these are addressed on site
Full Report
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A contractor says that they have experienced two cases of wall reinforcement cages collapsing. In both cases, the walls were part of the construction of new reinforced concrete (RC) water tanks where the height of the walls exceeded 6m and the vertical reinforcement was relatively light (12mm or 16mm bars). In both cases, the site teams had identified a risk of collapse, but the temporary works put in place to restrain the cages proved inadequate in both design and management.
One of the collapses happened over a weekend during which high winds were experienced. The other collapse appears to have been instigated by pulling sideways on the partially completed cage while the reinforcement was being fixed. Both of the reinforcement cages were inherently unstable in the temporary condition before shutters were erected. In both cases, although the subcontractor responsible for erecting the reinforcement had identified the risk of collapse and taken steps to mitigate the risk by providing props, these were ineffective.
Both of the reinforcement cages were inherently unstable in the temporary condition before shutters were erected.
The following advice has been given internally to mitigate the risk of further collapses:
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Where wall reinforcement is more than 3m high, the temporary stability of the reinforcement should be managed as a temporary works item
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The stability of reinforcement should preferably be assessed by calculation. Alternatively the reinforcement should be assumed to need propping and suitable propping should be designed and installed.
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Propping designs need to be reviewed by the responsible Temporary Works Coordinator and any deficiencies such as a lack of information on connections and fixings corrected prior to use
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The propping should be installed either before fixing starts or incrementally as fixing proceeds but the required sequence needs to be briefed to all involved with the construction activities
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Generally the propping should be on one side only to permit erection of formwork on the other side of the wall
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The method statements for propping installation, reinforcement fixing and formwork erection should include hold points for the Temporary Works Coordinator to inspect the installed temporary works at suitable points in the construction process
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Expert Panel Comments
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There have been many cases where rebar cages have collapsed, sometimes with fatal consequences, and it appears to be a risk about which there is limited awareness. Report 327 suggests that recommendations are not readily available. In general, it should be self-evident when there will be temporary stability issues during construction. However, it may be that the expectations of each party in relation to the skill and understanding of others is not always justified. How much should the designer expect the contractor to know? Who in the contractor’s supply chain should have the experience? When does expert knowledge take over from common sense?
Experience on site
Experienced steel fixers might know when a cage is stable as a result of familiarity and on-the-job training. However, the steel fixers on a project with unusually high wall or column lifts might not have had previous relevant experience and not recognise the risk of instability. Even if some risk is seen, the knowledge as to how to stabilise a cage may not be there.
Who takes responsibility?
Similarly. those others who are engaged in the design and construction process may not have the specialist knowledge to recognise when stability is becoming of concern. Or they may believe that is in the hands of experts who know what they are doing. Who is responsible for which actions in that difficult transition that occurs in the temporary stages of permanent construction? Is this a role for the Temporary Works Coordinator? These questions cannot be answered here but the industry as a whole should initiate steps to promote education and give advice.
Temporary works
In the UK, reinforcement cages, prior to concreting, may be considered to be temporary works and their management should comply with BS 5975:2008+A1:2011 Code of practice for temporary works procedures and the permissible stress design of falsework. The Temporary Works Forum (TWf) has also received concerns regarding reinforcement cages and is currently writing a good practice guide.
The Temporary Works Forum (TWf) has also received concerns regarding reinforcement cages and is currently writing a good practice guide
The Washington State Department of Labor & Industries published a narrative report on the death of a steel fixer when a rebar cage collapsed (SHARP Report No; 71-100-2011) which included the following requirements and recommendations:
Ensure that reinforcing steel for columns, walls, and similar vertical structures is guyed or supported to prevent collapse
Make sure there is a programme to addresses hazards and abatement methods when installing reinforcing for columns, walls, and similar vertical structures
Ensure that the inspection of rigging and equipment is done by a qualified rigger
Use bracing that is in good working order
Frequently inspect and replace defective equipment and material
Make sure that bracing and guying are able to support the forces imposed
Ensure that loads are secured and will not inadvertently become displaced when released
A qualified person should design methods of bracing reinforcing steel when being placed into position