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Safety concerns over concrete pours

Poor practice increases risk of punching shear, warns CROSS

Mark Moran
12 May 2020

The designers of buildings need to inspect concrete pours on flat slabs in order to prevent failure, a structural safety body has warned. Confidential Reporting on Structural Safety (CROSS) has published an engineer’s concerns about incidents where flat slabs had not been inspected.

Inadequate treatment of the connections between columns and slabs can cause a form of structural failure called punching shear, which was implicated in the partial collapse of a multi-storey car park in 1997.

CROSS shares anonymised concerns about problems in the structural engineering sector.

In this case, the reporter was assessing candidates for membership of a professional institution. Two candidates reported  separate experiences of observing the omission of design punching shear reinforcement in a slab pour about to take place. The candidates were the structural engineers monitoring the progress of projects on sites.

Given that the candidates were from different companies on different projects, the reporter felt their comments worth reporting in case it is a trend.
One of the candidates was particularly experienced, having visited many sites and said that omission of punching shear reinforcement is a ‘watch-it’ item within their team.

In its report, CROSS said designers of flat slabs should make it their business to conduct site inspections, or have them conducted, before concreting.
CROSS said issues about shear reinforcement in slabs, and particularly flat slabs, have been around for many years and were highlighted by the failure of the

Pipers Row car park in Wolverhampton, which dated from 1965. A 120-tonne section of the top floor collapsed during the night of 20 March 1997. Investigations identified the cause as being an initial punching shear failure that developed into a progressive collapse.

“Designers should know that the critical connection on any flat slab is the shear resistance around its supports,” advises CROSS. “Part of a structural engineer’s skill set is to know what to look for and to create a structure that is capable of being strong enough even before starting calculations. These skills are only acquired by practice and under supervision.

“However, as such reinforcement is a critical factor in the safety of flat slabs, the importance of it being in place should be known to constructors and supervisors. Designers of flat slabs should make it their business to conduct site inspections, or have them conducted, before concreting.”

CROSS a joint initiative set up by the Institution of Structural Engineers, Institution of Civil Engineers and the Health and Safety Executive. These organisations hopes by disseminating lessons learned from reporters and its expert panel it will encourage industry to take steps to prevent future failures and collapses.
CROSS Newsletter 58, published last month, also contains reports about structural issues with cladding and concerns about post-tensioned slabs.

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