CROSS Safety Report
Collapse of a reinforcing cage
This report is over 2 years old
The reinforcing cage to a blade pier for an overpass bridge collapsed without warning when nearing completion.
Stability of large, reinforced cages should be regarded as temporary works which should be certified by a competent and experienced structural engineer.
Key Learning Outcomes
For structural and civil engineers:
Check that large reinforcement cages will be fixed (or welded) so that they remain stable at all times during construction before and during casting of concrete
For construction personnel:
Ensure that fixing of bars within large reinforcement cages is secure throughout and has been checked as temporary works by a competent and experienced structural engineer
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A blade pier for an overpass bridge was under construction. The reinforcing cage for the pier was nearly complete when it collapsed without warning. Fortunately, no one was working on the site at the time and there were no injuries. The reporter's opinion is that the stability of large reinforcing assemblies such as this should not be the responsibility of the steel fixers.
Large reinforcing assemblies should be treated as temporary works
Large reinforcing assemblies should be treated as temporary works and should have a designed system of stability certified by a practising structural engineer. Subsequently, the reporter came across this issue in CROSS Newsletter No 30, April 2013 where it is well dealt with in reports 327 and 357. However, the reporter believes that drawing attention to this again would be a timely reminder for Australian engineers.
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As the reporter has noted this is not a new problem, but it is one that needs a regular reminder that when large reinforcement cages are to be assembled on site they should be treated as temporary works and subject to a stability design assessment prior to their assembly. The reporter also notes that CROSS Newsletter No. 30 of April 2013 contained two reports (nos. 327 and 357) on this topic. These reports highlighted that although there had been several such cases, there was a lack of awareness of the temporary stability issues involved and raised questions about where responsibility should lie. It was also noted that there was limited advice generally available at the time. However this was followed up by the Temporary Works forum (TWf) in the UK who subsequently published its TWf Safety Bulletin - Stability of reinforcement prior to concreting (TW15.116, 16.10.15).
More international concerns
The commentary to Report 357 also noted similar concerns in the USA, leading to a Caltrans research project Stability of Bridge Rebar Column Cages during Construction that contains several examples with recommendations.
In 2013 the American Society of Civil Engineers (ASCE) published the book Rebar Cage Construction and Safety: Best Practices that presents guidelines for the safe handling of steel reinforcing-bar (rebar) cages throughout the design, fabrication, and erection process. The focus is on rebar cages used for large, cast-in-place concrete columns in a variety of settings, including bridge piers, elevated highway sections, and high-rise buildings. These cages are inherently unstable, usually held together by tie-wire alone. They are challenging to fabricate, to lift from the horizontal to the vertical position, and to support in the temporary condition until concrete is cast.
Engineered temporary support systems, such as bracing or cable-guy systems, can mitigate the instability of standing rebar cages and resist lateral loads, ensuring better safety for construction workers.
Use of welded joints in Australia
The Steel Reinforcement Institute of Australia (SRIA) recommends the use of welded joints to AS/NZS1554.3, Welding of Reinforcing Steel, rather than wire tying in large prefabricated cages wherever possible, in accordance with its Technical Note 4, Fabrication and Site Handling of Reinforcing Bars. However, it is recognised that is not always possible with large cages such as bridge piers that are assembled on site. In which case there are specific types of wire tie arrangements that must be employed depending on the bar arrangement and bar size, and they must be treated as temporary structures and their stability should be properly designed to prevent collapse during site lifting, erection and concreting.