Report 950 reinforces the need for Building Control Inspections and the creation of a documented audit trail.
Report Overview -Remedial works were required in a number of residential buildings after inadequate punching shear reinforcement was identified.
Report Content – This 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 un-related difficulties with the private building control 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. They had therefore to assess firstly whether the design of the frames was in accordance with Part A of the Building Regulations and secondly to determine whether the frames had been built in accordance with the drawings.
The LA were 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 Building Regulations. Unfortunately, despite assurances of a comprehensive set of inspection records, very 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 building control 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 structural engineer and they were unable to provide any verification or comfort that the structure had been built in accordance with their drawings.
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 LA. These records however were not considered impartial by the LA.
One of the LA’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 5 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 photos showed that the as-installed punching shear reinforcement to these columns is 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 learned from this project, but perhaps the greatest would be for rigorous checking of critical structural elements; in this case punching shear reinforcement.