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Joint detailing key to flat slab safety

Letters

As someone who has had access to all the findings from the investigation into the Pipers Row car park collapse (NCE 4 and 11/25 December 2003), may I offer a few pointers.

Pipers Row was a punching shear failure and occurred with only dead load acting, implying an apparent major reduction in strength.

The failure was initiated by local deterioration of the concrete, confirmed both by examination of the debris and by subsequent rigorous structural analysis.

The concrete in the collapsed top slab was of lower quality than elsewhere in the structure.

It was exposed to the elements for over 30 years and undoubtedly went through many freeze-thaw cycles. Frost action led to the concrete becoming friable, penetrating into the slab near an internal column, as far as the top steel, eventually leading to a complete loss of bond and anchorage.

Rigorous analysis of Pipers Row clearly showed that the undeteriorated as-built strength had a safety margin of at least 1.5 against the worst possible inservice load. This is in spite of the original CP114 design being inferior to current practice.

If anyone needs assurance about current safety levels against punching shear, they should consult fib Bulletin 12.

This objectively analyses all available international test data, and comments on safety levels actually provided by codes, including BS8110 and EN1992.

Local detailing in joints is important. For flat slabs, the key feature is to ensure that the slab bottom steel is effectively continuous through the joint, to mitigate against progressive collapse. This is a design requirement in EN1992.

Dr G Somerville, georgesomerville@ukcares.

com INFOPLUS For details of fib Bulletin 12 visit www. nceplus.co.uk

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