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CROSS Safety Report

Concerns about punching shear in a flat slab

Report ID: 1050 Published: 13 December 2021 Region: CROSS-UK


Overview

This report concerns potential shear failures in flat slabs. This originates from inconsistent design and detailing of the shear reinforcement where a proprietary shear link system is assumed in the design but not followed in detailing.

Key Learning Outcomes

For structural design engineers:

  • Shear failures in flat slabs are serious and can result in structural collapse
  • Flat slab shear reinforcement should be detailed exactly as designed. Proprietary shear link systems (studs/rails) should not be replaced with arrangements of traditional links
  • Shear links in flat slabs should be detailed in accordance with The Institution of Structural Engineers Standard method of detailing structural concrete

For the construction team and site inspection team:

  • The importance of shear links in flat slabs cannot be overstated and they must be fixed exactly as detailed
  • Where there are difficulties in positioning reinforcement the designer must be consulted

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A reporter is a chartered structural engineer undertaking civil and structural engineering design in the UK. Part of the workload of their practice is undertaking the review, on behalf of an Approved Inspector, of engineering designs submitted for compliance with Part A of the Building Regulations.

The reporter has seen several examples of designs and detailing of punching shear reinforcement in flat concrete slabs which have raised concern. A number of engineers have used specialist punching shear rail computer software, provided by manufacturers of proprietary systems, to design a radial punching shear solution (see Figure 1).

Illustration of proprietary flat slab shear reinforcement
Figure 1: proprietary flat slab shear reinforcement (main slab reinforcement not shown)

However, the engineers have then specified and drawn traditional shear links on construction drawings (see Figure 2). These links are detailed to a radial pattern (see Figure 3) similar to the proprietary system, even though the slab main reinforcement is orthogonal. The links do extend from the top layer 1 reinforcement to the bottom layer 1 reinforcement but do not explicitly wrap around any top or bottom reinforcement other than a lacer bar.

 

Example B for safety report 1050 V2
Figure 2: traditional shear links (shapes codes as per Standard method)
Illustration showing links in a radial layout for safety report
Figure 3: plan showing links in a radial layout (main slab reinforcement not shown)

The reporter has raised concerns about a number of aspects of this practice:

  1. The traditional links are sometimes provided with short lengths of lacer bars so that they can be tied into the main reinforcement. However, the reporter is concerned that if the traditional links are not adequately anchored to the main reinforcement, they will not carry the tension loads developed from the applied shear force.
  2. They are also very concerned about the practicality of installing traditional links on a radial pattern to the degree of accuracy that can be achieved with proprietary radial systems. The reporter is concerned that the alternative traditional reinforcement will not achieve the shear capacity of the proprietary designed system assumed in the design.
  3. They are concerned that the level of supervision on site will be inadequate to ensure that the shear reinforcement will be placed in accordance with the engineers’ drawings.
  4. The reporter does not know if the Approved Inspector has any responsibility to check the structure during construction.

The reporter has raised the matter with a number of engineers, and a typical response is that, in their opinion, it is acceptable. Also, regarding workmanship onsite, they considered that it is the contractor’s responsibility.

Given the nature of punching shear failure, the reporter felt that they should inform CROSS of their experience and concerns.

Expert Panel Comments

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Shear failures must not happen

Shear failures in flat slabs are very serious and can result in structural collapse not just of a section of the slab but of complete structures. It is concerning that software for proprietary ‘headed’ bars is being used to design traditional links. Headed bars may well have a more effective anchorage than traditional links and some software, for proprietary products, takes advantage of this either to increase the maximum shear that can be carried, or to allow a relaxation of the spacings required for traditional links. Unless it is clear that the software is fully compliant with Eurocode 2 for traditional links, it should not be used to justify the use of traditional links.

It may also be the case that shear capacity using a proprietary system is not based solely on calculation but on testing which draws in the benefit of the improved anchorage of such systems.

Detailing matters

The method of detailing has an effect on structural performance; indeed, the detailing will impact the actual capacity of the section. This is why documents like The Institution of Structural Engineers Standard method of detailing structural concrete which sets out best practice, are essential reading for those responsible for the detailing of reinforced concrete. Moving away from such proven standard details, without a full understanding of the principles these details address, may well have an impact on the safety of the whole structure.

 

The Standard method of detailing structural concrete is essential reading.

Traditional links should be fixed as per the Standard method of detailing structural concrete. Where fixing (lacer) bars are required, they should extend at least an anchorage length beyond the last link. It is not clear how this requirement has been relayed to the contractor, but the overall length of the fixing bar must be given and complied with on site. In addition, a design check is required to ensure that the fixing bar provides an adequate anchorage to the links as per Eurocode 2.

When designing flat slabs, it is also important to allow for the effect of holes immediately adjacent to the column for drainage and service risers; these holes will alter the shear capacity and require specific consideration. Detailing for holes, including links, is covered in the Standard method of detailing structural concrete.

Ensuring reinforcement is fixed accurately on site

Attempts to set out traditional links on a radial grid will result in non-compliant spacings. Such links should be set out on an orthogonal grid as recommended in the standard method of detailing

If the engineer has been checking on behalf of an Approved Inspector, then the Approved Inspector has a duty to review on site. It is however true, that the responsibility remains with the designer and contractor, and the Building Control body shouldn't be relied upon to do a contractor's QA check.

Previous reports

 

There have been previous reports about shear in flat slabs, including:

950 Inadequate punching shear reinforcement in flat slabs

906 Missing punching shear reinforcement in concrete slabs

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