Lansdowne House Shortlisted for PTA New Build Project of the Year

Interspan is proud to have its work on Lansdowne House (Project Aleph), Berkeley Square, London, shortlisted for the PTA New Build Project of the Year at Construct Day 2026.

The £200 million Lansdowne House development is a landmark commercial project in the heart of Mayfair, comprising approximately 27,000m² of premium office, retail and restaurant space across 10 storeys.

Designed to achieve BREEAM Outstanding, the project adopts an innovative “long life, loose fit” philosophy, creating a building capable of adapting to changing occupier requirements over its lifespan.

Post-Tensioning at the Heart of the Structural Design

Unlike a conventional commercial building where the central concrete core provides much of the structural stability, Lansdowne House incorporates an innovative “soft-core” structural concept.

The main structure comprises an eight-storey reinforced concrete sway frame incorporating tree-like precast columns, cruciform precast column heads and in-situ post-tensioned floor slabs, topped by a two-storey steel crown.

Interspan was appointed by Kilnbridge to undertake the in-house design, supply and installation of the bonded post-tensioned slabs across Levels 01–08.

Design coordination commenced more than 12 months before construction, with the PT installation subsequently delivered over a 40-week programme.

12 Metre Grids with 325mm Slabs

The post-tensioned floor system was fundamental to achieving the architectural and structural requirements of the development.

Bonded post-tensioning enabled typical 12m × 12m structural grids with slab depths of only 325mm, while localised areas span more than 15 metres.

Reducing slab depth was particularly important within the townscape constraints of the Mayfair Conservation Area, helping the development accommodate 10 storeys of high-value commercial space within the permitted building envelope.

The PT solution also reduced the quantity of conventional reinforcement required, helping minimise reinforcement congestion, material consumption, labour requirements and waste.

Complex Structural Coordination

Lansdowne House presented a number of significant engineering and construction challenges.

Large cruciform column heads provide the rotational stiffness required by the sway frame, creating highly congested reinforcement zones that needed to accommodate slab reinforcement, precast column reinforcement and the PT tendon system.

At the same time, a Thermally Activated Building System (TABS) was cast directly into the slabs. Heating and cooling pipework therefore had to be carefully coordinated with tendon profiles and structural reinforcement.

Interspan worked closely with Kilnbridge, AKT II and Mace, using detailed 3D coordination during the design phase.

A full-scale installation mock-up was also undertaken before construction, enabling reinforcement, tendons, precast components and building services to be coordinated before work progressed on the permanent structure.

Engineering the Perimeter Transfer Forces

One of the project’s most technically demanding conditions occurs between Levels 01 and 02, where the building footprint reduces, and a series of raking perimeter columns introduce substantial horizontal forces into the structure.

Interspan completed the detailed design and installation of a specialist solution using post-tensioning bars, together with bespoke anchor plates integrated into the slab.

The bars were progressively stressed during construction to control tensile forces and maintain the stability of the sway frame throughout the construction sequence.

At Level 08, the PT system faced another significant challenge. The floor supports the building’s two-storey steel crown, whose columns are positioned around the midspan of the PT slab. The resulting transfer slab was increased to 450mm thick and incorporated five-strand PT tendons to accommodate these concentrated loads.

Designed for Future Adaptability

Perhaps the most distinctive aspect of Lansdowne House is that the structural frame has been designed not just for its first use, but for how the building may be adapted decades into the future.

Large 5m × 5m “soft spots” have been incorporated into the floorplates so future occupiers can create interconnecting stairs with significantly less structural intervention.

Bonded post-tensioning supports this strategy because alterations can be undertaken while maintaining prestressing force within the remaining parts of the tendon and reducing the amount of conventional reinforcement that may need to be cut.

The result is a structural system designed to accommodate change rather than become obsolete.

Recognition for Technical Excellence

Commenting on Interspan’s contribution, Ben White, Head of Project Engineering at Kilnbridge, said:

“Interspan’s early involvement was fundamental to the successful delivery of the Lansdowne House structural frame. Their technical expertise and collaborative approach, in combination with that of Kilnbridge, enabled solutions to be developed that would have been extremely difficult to achieve using conventional reinforced concrete construction.” – Ben White, Head of Project Engineering, Kilnbridge

Completed in May 2026, Lansdowne House demonstrates the role post-tensioning can play in delivering more than structural efficiency.

Through long spans, reduced slab depths, enhanced crack control, reduced material consumption and greater future adaptability, the PT solution has contributed directly to a building designed for both high performance today and flexibility well into the future.

Interspan Europe is proud to see this outstanding project recognised as a finalist for the PTA New Build Project of the Year at Construct Day 2026.