CROSS Safety Report
Slipform temporary works design
A reporter was performing a CAT III check on a 14 storey slipform design when they discovered that the temporary works design was inadequate.
Key Learning Outcomes
For clients and contractors:
- Having a competent temporary works designer/adviser in place to supply an engineered solution can ensure all temporary works are carefully considered and planned
- This report highlights the value of undertaking CAT III checks and independent design verifications to ensure temporary works are adequately designed
- The Temporary Works Forum is a good source and can be consulted for advice on such matters
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A reporter was performing a CAT III check on a Slipform Design. The core being slipformed was 20m x 20m in plan and to be slipped 14 storeys. The temporary structure was 7m high with 4m of ply sheeting around the top two levels, thus attracting a lot of wind load. The check highlighted some long sections of wall that did not have return walls to resist wind loads. The overturning forces due to the wind had to be resisted as a couple in the concrete in the bottom of the form, which due to the process would be extremely young weak concrete.
The check returned that if high winds were experienced when the concrete was less than 12 hours old that the concrete could be damaged by the overturning forces applied by the slipform structure. At this point, questions were asked of the designer as to how this load path was assumed to work. It became clear that the designer did not have any design calculations that described how the overturning force was resisted.
Overturning due to wind load not considered
Standard calculations did exist but they were found, according to the reporter, to completely ignore the issue of overturning. The designer did not have the technical knowledge to discuss the problem and find a solution. Subsequently, the designer did employ a sub-consultant to provide a design philosophy and design calculations after CAT III check comments requesting these.
It was quite surprising to find the designer struggling to explain how their proprietary system worked, and further provide standard calculations that ignored the issue altogether. It raises questions about the amount of "design" being carried out for these temporary works items, there was no evidence of standard rules that dictate the limitations of the temporary works, e.g. shapes of these dimensions cannot be formed.
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