CROSS Safety Report
Cantilever bridge parapet falsework failure
A reporter has experienced the failure of cantilever parapet falsework twice, for different reasons, using two different products during bridge construction.
Key Learning Outcomes
For contractor’s supervisory staff:
- Adhere to the requirements of BS 5975:2019 – TC Code of practice for temporary works procedures and the permissible stress design of falsework
- Ensure a temporary works designer is appointed to supply engineered solutions
- Ensure a temporary works coordinator is appointed to co-ordinate all temporary works activities
- Ensure inspection and test plans are in place for all temporary works
- Be aware of TWf guidance including Information Sheet No 6 - The safe management of temporary works
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A reporter’s firm has experienced the failure of cantilever parapet falsework twice, for different reasons, using two different products, in a period of 6 years. The first was the unintentional removal of a male threaded tie from a female threaded anchor point and the second was the failure of welded studs anchoring falsework. The reporter says falsework failures can result in falls of shutter panels with danger to people and property.
In the first case the tie rod/anchor connection was out of sight below the formwork so disengagement could happen in error without anyone noticing. The reporter says there was no fail-safe mechanism. The second case involved the failure of welded studs which anchored the falsework to steel bridge beams. Upon investigation, it transpired that the same person had both welded the studs to the bridge beam and tested the studs. There was no independent check of the welds which could have discovered that they were poor.
there was no independent check of the welds which could have discovered that they were poor
The reporter goes on to say a three-point support for each panel of falsework would make the works robust against either failure type and that the introduction of a mechanical fail safe for the threaded tie could prevent the first failure type. However, better testing and an inspection and testing plan should have averted the second failure. Alternatively, instead of welded studs a different fixing detail giving some redundancy could have helped says the reporter.
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Expert Panel Comments
An Expert Panel comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-AUS Expert Panel page.
The reporter is to be commended for highlighting these temporary works failures which appear similar to failures known to have occurred elsewhere. The welding of shear studs is an operation that needs very careful monitoring, and the reporter rightly points to the importance of inspection and test plans in this regard. Failures, as described, may occur because controls are not adequate or not adequately applied. The reporter goes on to describe the lessons that were learned which can be applied to many temporary works situations.
Design and management of temporary works
Generally, all temporary works should be designed, checked and managed in accordance with the contractor’s temporary works procedures which should accord with BS 5975:2019 – TC Code of practice for temporary works procedures and the permissible stress design of falsework. The Temporary Works forum (TWf)) has published much relevant guidance including Information Sheet No 6 - The safe management of temporary works. This guidance (aimed particularly at small and medium-sized enterprises) provides a brief overview of key factors in the safe management of temporary works, including reference to the all-important key roles of temporary works designer and temporary works coordinator that should be fulfilled.
The report refers to a critical connection that was out of sight. Designs that include critical connections which are not ‘inspectable’ bring risks that should be very carefully considered and designed out where it is reasonable to do so.
critical connections which are not ‘inspectable’ bring risks that should be very carefully considered
Finally, the reporter uses the phrase 'fail safe'. There are a number of phrases like this such as 'defence in depth', 'preferred modes of failure', 'common cause of failures' and 'single point failures', all of which are aids to thinking about attributes that make a system safe or unsafe. It is helpful for designers and others (whether of permanent or temporary works) to reflect upon and appreciate the meaning of such phrases.
CROSS published report 1146 Control of temporary works excavation in 2022 which concerned the design and risk assessment of temporary works.