Maurice Brennan, Associate Director at RSHP, reflects on the vertical Vierendeel structural frame for the Mandarin Oriental Mayfair

Nearly two years on from the opening of the Mandarin Oriental Mayfair in June 2024, the building has become part of a wider transformation of this corner of Mayfair. Together with the arrival of the Elizabeth line at Bond Street and comprehensive public realm improvements to Hanover Square, the project has helped redefine the area’s character and connectivity.

Today, the vitality of the square belies the complexity of developing 22 Hanover Square. Delivering the scheme required reconciling architectural ambition with significant physical and planning constraints on a tightly bounded site. The adoption of a vertical Vierendeel frame proved pivotal, providing the necessary structural stability while preserving architectural clarity and flexibility.

The Brief

Clivedale’s brief for 22 Hanover Square was to replace the existing office building with a mixed-use development of a grain and scale that is in step with the wider regeneration plans for this historic Westminster square and the arrival of the Elizabeth Line (Crossrail).

The response took the form of two ‘pavilion’ towers of 10 and 12 storeys, linked by a fully glazed stair and lift core. Conceived as a contemporary ‘lightwell’, this central, non-structural glass and steel element mediates between the volumes and draws daylight deep into the plan. Glass floors, bridges and double-height spaces enhance permeability and spatial continuity.

The deliberate omission of a central structural core was fundamental to achieving spatial flexibility and long-term adaptability. Early studies tested precast concrete perimeter walls, braced structural frames and alternative column grids. Each strategy, however, imposed architectural compromises, resulting in excessive structural depth, visually intrusive bracing, or distortion of established facade proportions.

The Solution

Our aim was to give Clivedale maximum flexibility in planning their layouts and the ability to respond to future changes in the London residential market, while meeting the demands of the Mandarin Oriental Hotel Group, with whom they were in negotiation. In doing so, we sought to avoid the inefficiency of central structural cores, particularly as technical requirements reduced and varied over the building’s height.

We developed an efficient base plan of two separate tower forms linked by a single external stair and lift bank serving the whole building, with an additional lift group serving the lower levels only. This created a simple, flexible floor plate free from centralised structural cores. At this stage, working with Ramboll’s Le Carpentier, we made the key decision to move the structure to the perimeter and stabilise the building externally.

Our initial proposal explored a perimeter structure of C-shaped precast reinforced concrete walls with a post-tensioned slab. However, the wall thickness was too great to accommodate insulation, thermal breaks and fire protection without compromising internal layouts. We also tested a single-column strategy and alternative braced frames, but both required substantial elevation bracing, undermining the architectural intent.

The breakthrough came when we developed a steel exoskeleton wrapping the structure around the site as a Vierendeel frame. 

This structural solution is unique for a building in the UK and remains unusual in modern engineering. The Vierendeel principle allows fixed connections at the columns, enabling the connecting beams to act as cantilevers. The frame is simply joined at mid-span — the point of contraflexure where bending moments are zero — creating an efficient and elegant structural solution.

Although the two towers operate as separate stability systems, the Vierendeel frame removes the need for diagonal or cross-bracing by resisting all lateral loads within its rigid geometry. This approach allowed the column and beam elements to retain their intended proportions while maintaining the façade rhythm developed in response to Hanover Square and the emerging residential and hotel layouts.

The cruciform steel columns, just the size of an A4 sheet in profile, are 6.5m high, spaced at 1.6m centres, with only four internal columns per tower, creating 8.2m spans and highly flexible floorplates.

The Delivery

Delivering a deep, boundary-to-boundary scheme in central London is inherently complex, but at Hanover Square the constraints were exceptional. Working within a tightly bounded urban site while navigating historic and new services, easements, and neighbouring structures required intense coordination with main contractor ISG and structural engineer Ramboll.

The structure was fully fabricated off-site. Despite controlled factory conditions, extensive prototyping and testing were required by specialist fabricator Barratt’s of Aspley. Achieving a high-quality polished concrete finish on exposed structural sections added further complexity, as the surfaces had to cure consistently despite seasonal variation and withstand long-term exposure to the city’s climate.

In total, 52 Vierendeel columns wrap the perimeter every two levels, with 684 sections across 11 types — a system Barratt’s likened to a “jigsaw puzzle.” 

At peak construction, eight two-storey columns were installed daily. Logistics were equally demanding: trailers were modified to handle point loads and deliver the columns upright for craning into position, aligning precisely with slabs installed by specialist contractor McGee.

The compact 35m × 15m site allowed materials to enter only through a single “letterbox” opening on Brook Street, leaving minimal storage space. Strategic gaps in the Vierendeel frame enabled prefabricated bathroom pods and materials to be inserted before being closed later.

The façade was erected in parallel to the frame, combining glazing with Juliet balconies and Venetian brick-faced panels formed from full-size brick “baguettes.” These were assembled into cassettes on site and craned into place independently of the structural sequence, adding another layer of coordination to an already intricate build.

The Response

The envelope extends a contemporary language that responds to the historic grain, rhythm, scale, colouration, and proportion of Hanover Square. We wanted the façade to take its cues from the traditional Mayfair house; the grid is domestic and human in scale rather than simply a function of what is technically possible.

Horizontally, the façade is articulated in two-storey strata, a device observed in neighbouring buildings and historically expressed in Hanover Square through cornice lines and double-height colonnades. This ordering principle suppresses a floor-by-floor reading, establishing a measured vertical emphasis across the elevation and lending a sense of generosity and grandeur. Its primary intent is to draw a connection between the listed No. 24 and the cornice line of Fenwick on Brook Street.

The façade is conceived as a Vierendeel structural frame with expressed columns and beams, infilled with full-height brick panels and glazed openings. 

Finished in white, the frame forms a legible lining to the reveals. Through this approach, depth is articulated in both material and tectonic terms, as projections and recesses produce shadow and relief comparable to the masonry traditions of Hanover Square.

When seen obliquely, or within the restricted views of Brook Street, the deep Vierendeel recesses turn the façade white, complementing the predominant white of Fenwick to the south. From the broader vantage points within Hanover Square, the burnt-red brick infill assumes greater prominence, read as a field framed by the white structural grid and dark-grey concrete panels.

Unlike on many contemporary buildings, we wanted the envelope and Vierendeel structure to reward and reveal more information the closer you come to it. The joining-together of the prefabricated elements, the bolt connections in their recessed sculpted pockets, and the materiality and texture of the pigmented grey concrete – all give scale and legibility to the façades.