40 Grosvenor Place – Adaptive Reuse – Punching Shear Strengthening

Interspan developed an alternative punching shear strengthening solution for the adaptive reuse of 40 Grosvenor Place in central London.

The accepted design combines a 60 mm high-performance concrete overlay, post-installed punching shear reinforcement and bespoke high-strength concrete column collars across 54 column locations.

Project Overview

40 Grosvenor Place is a 30-year-old reinforced concrete office building in prime central London undergoing adaptive reuse for Cleveland Clinic. The building is being transformed into a state-of-the-art cancer care facility planned to provide approximately 7,500 m² of space for medical oncology, haematology and radiotherapy services.

The existing structural frame comprises typical 290 mm thick reinforced concrete slabs supported on circular reinforced concrete columns ranging from 400 mm to 600 mm in diameter. As part of the change in use and increased loading, strengthening was required to improve slab capacity against punching shear and hogging moments.

Main Contractor – Byrne Demolition

Consulting Engineer – Waterman

Strengthening Required Across 54 Column Locations

Waterman identified 54 column locations across multiple floors that required strengthening. Once the slabs were exposed during the building soft strip, extensive cracking was also observed around a number of columns, indicating that punching shear distress had developed during the building’s previous use as an office.

The strengthening design therefore needed to address the increased future loading while also responding to the condition of the existing slabs. The works required a coordinated solution combining crack repair, slab strengthening and reliable transfer of forces between the existing concrete and the new strengthening elements.

Resolving a Complex Constructability Challenge

The original strengthening concept incorporated steel collars fixed to the tops of the circular columns, post-installed punching shear reinforcement and near-surface-mounted carbon fibre reinforcement rebated into the top of the slab.

This detail presented significant constructability and coordination challenges. Fixing steel collars to the existing circular reinforced concrete columns required intrusive drilling while avoiding congested reinforcement. The multiple systems also increased the complexity of installation, interfaces and sequencing within an active adaptive reuse programme.

Alternative High-Performance Concrete Solution

Interspan proposed an alternative design based on a 60 mm high-performance concrete overlay combined with a reduced amount of post-installed punching shear reinforcement. Bespoke headed terminations were anchored into the overlay, creating an integrated strengthening system above the existing slab.

Mini shear connectors were installed to develop composite action between the original reinforced concrete slab and the new overlay. The strengthening zone was mechanically prepared and roughened, with pull-off testing and sand patch testing used to verify the required substrate condition before placement of the high-performance concrete.

The alternative solution was reviewed, accepted and adopted by Waterman. It could be incorporated within the new 200 mm floor build-up required as part of the building conversion, avoiding a separate strengthening zone that would conflict with the architectural and services design.

Bespoke Concrete Collars and Shear Transfer

Twenty locations originally requiring steel column collars were redesigned using high-strength concrete collars. These were tied into the existing structure using a bespoke detail that reduced the need for intrusive drilling into the reinforced concrete columns.

High-strength concrete collars and purpose-designed shear keys were used at critical interfaces to transfer forces effectively between the existing structure and the new strengthening system. This approach simplified the connection geometry around the circular columns and reduced reinforcement coordination risk.

Greater Programme and Cost Certainty

By replacing the complex combination of steel collars, rebated carbon fibre and punching shear studs with an integrated concrete overlay solution, Interspan reduced the complexity of the site works and minimised intrusive drilling into the existing structure.

The revised design provided Byrne Demolition and the project team with improved constructability, clearer sequencing and greater programme and cost certainty. It also supported the retention and reuse of the existing reinforced concrete frame as the building is transformed for a substantially different and more demanding use.

Project-specific carbon savings were not quantified. However, strengthening and adapting the existing frame avoids the impacts associated with wholesale structural replacement and demonstrates how targeted engineering can support the continued use of established building assets.

Interspan Scope

• Alternative punching shear strengthening concept and detailed design
• Detailed crack survey and crack injection repairs
• Mechanical substrate preparation and roughening
• Pull-off testing and sand patch testing of prepared concrete surfaces
• Post-installed punching shear reinforcement with bespoke headed terminations
• Mini shear connectors to develop composite action with the existing slab
• Placement of a 60 mm high-performance concrete overlay
• High-strength concrete column collars and bespoke shear keys at critical interfaces