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CROSS Safety Report

Inadequate punching shear reinforcement in flat slabs

Report ID: 950 Published: 14 October 2020 Region: CROSS-UK


Overview

This report highlights how remedial works were required in a number of residential buildings after inadequate punching shear reinforcement was identified.

Insufficient site records led to questions being raised as to what had been installed.

Key Learning Outcomes

For construction professionals and building control companies:

  • It is good practice to keep detailed site records including photos of the installation of critical structural elements such as punching shear reinforcement

  • Good record keeping can lead to the resolution of any later disputes if evidence can be provided easily

  • Quality control and competent supervision on site can help to ensure that the structure is built in accordance with the design

For civil and structural design engineers:

  • If you are unable to attend site, consider asking the contractor for site photos of the installation of critical structural elements

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This report concerns the checking and inspection of punching shear reinforcement in a small number of medium-rise residential buildings with flat slab floors. The building control function was being undertaken by a private building control company up to the point at which the reinforced concrete frames were complete.

At this stage, due to unrelated difficulties with the private building control company, the developer decided to change the building control function to the local authority. A reversion building regulations application was then submitted to the local authority.

The local authority had therefore to assess the following:

  • Firstly, whether the design of the frames was in accordance with Part A of the building regulations

  • Secondly, to determine whether the frames had been built in accordance with the drawings

The local authority were satisfied with the design but requested inspection records from the private building control company. The records would allow them to gain confidence that the frames had been built in accordance with the building regulations.

Poor inspection records

Unfortunately, despite assurances of a comprehensive set of inspection records, very few inspections of the reinforcement to the flat slabs had been carried out. The ones that had been carried out were of a general nature and of non-critical areas.

The private building control company suggested that the responsibility for inspecting the reinforcement lay with the project's structural engineer. Unfortunately, insufficient inspections of reinforcement had been carried out by the structural engineer. They were unable to provide any verification or comfort that the structure had been built in accordance with their drawings.

Do pre-pour sign off sheets verify correct installation of reinforcement?

The main contractor then provided pre-pour sign off sheets of the reinforcement signed off by the frame contractor before the slabs were cast, hoping that this would satisfy the local authority. These records however were not considered impartial by the local authority.

One of the local authority’s greatest concerns was verification that the very significant quantity of punching shear reinforcement shown on the designers' drawings had been correctly installed.

Punching shear reinforcement was required in the slabs to approximately 50% of all columns and in certain locations up to distances of five perimeters out from the face of the columns.

Inadequate punching shear installation

When the frame contractors' pre-pour reports were forwarded to the LA, photos were also included of the slab reinforcement around a limited number of columns. The photos showed that the as-installed punching shear reinforcement to these columns was inadequate.

The quantity and setting out of the bars were not as per the design drawings or the relevant code of practice. The inclination of bars was parallel to any potential shear failure plane, rendering the bars installed ineffective. This major defect was pointed out to the main contractor and designer who both accepted the deficiencies.

Costly strengthening works were required

The choice was made by the main contractor and the designer and accepted by the LA, to strengthen all slabs where punching shear reinforcement had been required, as there was no easy way of verifying the punching shear provisions to any of the columns.

This involved the installation of substantial stiffened shelf angles, being fixed to the columns hard up against the underside of the slabs. Shear studs were drilled through the slab at the required perimeters. Many lessons could be learned from this project, but perhaps the greatest would be for rigorous checking of critical structural elements; in this case punching shear reinforcement.

Many lessons could be learned from this project, but perhaps the greatest would be for rigorous checking of critical structural elements; in this case punching shear reinforcement.

Expert Panel Comments

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Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.

Punching shear around columns in flat slabs has been causing concern for many years, both in design and execution. Well done to those who spotted this problem and their persistence which enabled the matter to be rectified before anything dire happened.

It is clear that insufficient inspections were carried out by various parties to spot the poor workmanship that should not have been permitted in the first place.

A dramatic example of flat slab collapse was at the Pipers Row car park in 1997 as can be seen in Figure 1.

An open-sided multi-story concrete frame car park during construction. The upper level flat slab has failed by punching through of the concrete columns. The slab has collapsed onto the floor level below it.  There are adjacent buildings to the structure with more buildings in the background.
Figure 1: flat slab failure at Pipers Row car park (photo source: Jonathan Wood - Pipers Row Car Park, Wolverhampton Quantitative Study of the Causes of the Partial Collapse on 20th March 1997)

It is of concern to think that other buildings may have deficient punching shear reinforcement at the column heads, especially as this type of failure is sudden and catastrophic. In any structural system, some modes of failure are less desirable than others and flat slabs offer a prime example of this.

Under overload, it is highly desirable that such slabs fail in bending before a support punching failure becomes critical, since the latter is brittle and gives little warning.

Regular readers of CROSS will identify the common theme of there being a mismatch between what designer’s thought was being built and what was actually built, and this report is yet another example of that.

Three safety concerns affecting construction sites

There is a generic concern that designers do too little site checking, and records are too inadequate to verify construction quality. The Edinburgh school’s failure (SCOSS Alert - Inquiry into the construction of Edinburgh Schools) and others, exemplify the concerns such as:

  • Lack of quality control in the construction process

  • The need for proper supervision by competent staff

  • A lack of independent inspections

Again, these are issues that should be addressed in actions flowing from the Draft Building Safety Bill and the introduction of the new Building Safety Regulator.

It is believed by some of the Expert Panel that a traditional clerk of works would have found the lack of rebar straight away and saved a great deal of money as well as ensuring safety. A body who address such issues is GIRI (Get it Right Initiative) and their work is to be encouraged.

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