Punching Shear Strengthening

Beta Scope

Evaluation of flat slab punching shear repair options including FRP, Hilti Post Installed Studs and traditional post installed shear anchors
Recommending the most practical option for punching shear repair considering the programme
Designing the required post installed studs, suitable diameter and spacing
Construction method statement including specifications of grout and torque method

Background

Beta Design Consultants were commissioned by the Principal Contractor, Beard Construction, to provide the specialist repair design to address punching shear failure in the flat slab. The project was the addition of a new storey for the Kennedy Institute of Rheumatology. When checking the existing building, the Engineering consultant for the University of Oxford (AKS Ward) identified an issue in the punching shear of the flat slab. Beta Design Consultants were appointed to evaluate various options for the repair and to recommend a suitable solution. The institute was to remain operational without any disruption and all repair works had to be carried out from above.

Beta Solution

Beta DC Engineers evaluated the available options and the following options were considered:

Using fibre reinforced polymer FRP plates or rods was seen as an ideal solution. However, this was dismissed as the grid of required FRP clashed with the positions of steel columns that needed to be added.
Using the post installed Hilti anchors was also considered. This was later eliminated due to the need for access from below the slab.
Beta DC Engineers proposed a method that they had previously used. The method is derived from basic engineering principles and is based on working out the required shear reinforcement and then post installing it. This method allowed the columns installation for the new storey to proceed and it required access from above only.

Practical Details

Beta Design Consultants solution was to drill the slab and install specially designed studs, with a domehead and a nut washer assembly. These studs were installed vertically from above in special grout. Following grout curing, the studs were torqued. The studs were checked for steel resistance, bond strength and pull-out strength and a sufficient number of studs were used to counteract the shear force that exceeded the concrete shear strength without reinforcement.

Few concerns had to be addressed on site and these included:

Risk of hitting existing rebar
Risk of overheating in the grout due to the relatively large volume of grout used.
Risk of piercing through the slab and disturbing the operational health facility.

Beta Design Consultants worked with Beard Construction to prepare a method statement that addressed all these risks. A ferro scan was carried out to ensure position of rebar was established prior to specifying studs locations. A trial stud installation was carried out and witnessed by Beta DC Engineers. The installation sequence allowed for the measurement of existing slab thickness prior to drilling to avoid piercing through the slab. The repair was carried out successfully by specialist concrete repair contractor CRL LTD allowing the programme to progress.