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CROSS Safety Report

Missing punching shear reinforcement in concrete slabs

Report ID: 906 Published: 1 April 2020 Region: CROSS-UK

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Overview

A reporter raises concerns of how two separate engineers observed the omission of design punching shear reinforcement prior to slab pours being undertaken on two different sites.

Key Learning Outcomes

For civil and structural design engineers:

  • Be aware that one of the critical connections on flat slabs is the shear resistance around its supports
  • If carrying out pre-pour inspections look to ensure punching shear reinforcement is installed and installed correctly in accordance with the detailed design
  • If you are unable to attend site, consider asking the contractor for site photos of the installation of critical items such as punching shear reinforcement

For construction professionals and supervisors:

  • Be aware of the importance of punching shear reinforcement and how it is a critical factor in the safety of flat slabs

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A reviewer for a professional membership institution was concerned to note that two candidates they recently interviewed reported similar site experiences. As a structural engineer monitoring the progress of their projects on site, they had observed the omission of design punching shear reinforcement in a slab pour about to take place.

Given that these two candidates were from different companies with different project experiences, the reporter feels it is worth reporting this 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.

Expert Panel Comments

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Issues about shear reinforcement in slabs, particularly flat slabs, have been around for many years and were highlighted by the failure of the Pipers Row Car Park. This multi-storey structure was built in 1965 and a 120-tonne section of the top floor collapsed during the night of 20 March 1997 (Figure 1). An initial punching shear failure developed into a progressive collapse.

Image
Figure 1: flat slab failure at Pipers Row car park (photo source: Jonathan Wood)

Critical connection of flat slabs

Designers should know that the critical connection on any flat slab is the shear resistance around its supports. 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 (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.

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