Cross Reports: Report 950

Confidential Reporting on Structural Safety. Report 950: Inadequate punching shear reinforcement.

Highlights concerns over where the responsibility lies for inspecting flat-slab floors CROSS report 950 explores 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, owing to unrelated difficulties with the private company, the developer decided to change the building control function to the local authority (LA).

A reversion Building Regulations application was then submitted to the LA. It had to assess, first, whether the design of the frames was in accordance with Part A of the regulations and, second, whether they had been built in accordance with the drawings.

The LA was satisfied with the design but requested inspection records from the private building control company to gain confidence that the frames had been built in accordance with the regulations. Unfortunately, despite assurances of a comprehensive set of records, few inspections of the reinforcement to the flat slabs had been carried out and those were of a general nature and of non-critical areas. The private company suggested that the responsibility for inspecting the reinforcement lay with the project’s structural engineer. Unfortunately, again, insufficient inspections of reinforcement had been carried out by the engineer and they were unable to provide any verification or comfort that the structure had been built in accordance with the drawings.

The main contractor then provided pre-pour sign-off sheets of the reinforcement signed by the frame contractor before the slabs were cast, hoping that this would satisfy the LA. These records, however, were not considered impartial by the LA. One of its greatest concerns was verification that the very significant quantity of punching shear reinforcement shown on the designers’ drawings had been correctly installed. 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.

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 images 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, and 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.

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 with shear studs drilled through the slab at the required perimeters.

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

Comments

Punching shear around columns in flat slabs has been causing concern for many years, both in terms of design and execution, so 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 (Pipers Row car park, Wolverhampton: quantitative study of the causes of the partial collapse on 20 March 1997). It is concerning to think that other buildings may have deficient slab 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 more serious than others and flat slabs are 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 Reports will identify the common theme of there being a mismatch between what designers thought was being built and what was actually constructed, and this report is yet another example of that. There is a general concern that designers do too little site checking and that records are inadequate to verify construction quality. The Edinburgh school’s failure (SCOSS alert: inquiry into the construction of Edinburgh schools) and that of others, exemplify concerns such as:     

• a 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 CROSS 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 that addresses such issues is GIRI (the Get it Right Initiative) and its work is to be encouraged.  

Image Credit | Alamy