30 Grosvenor Square

Overview

30 Grosvenor Square, the former US Embassy has been successfully converted to 'The Chancery Rosewood' - a highly prestigious luxury hotel located on Grosvenor Square in London’s Mayfair. The Grade II listed modernist landmark, originally designed by world-renowned architect Eero Saarinen, is the former American Embassy and underwent a highly complex renovation that focused on the preservation of history while delivering a building that could live up to the aspirations of the client Qatari Diar.

Careys was involved from the initial construction stages, directly contracted by the developer as principal contractor to carry out extensive enabling works, including decommissioning of the building, asbestos removal, temporary works, demolition, façade retention, piling, basement excavation and sensitive removal of listed heritage items for restoration and reinstallation at the end of the project. With this work complete Multiplex took over the role as principal contractor to deliver the full construction of the building all the way up to the premium interior fit-out. Careys remained on the project in a subcontractor role, utilising its experience and expertise in complex substructures and heritage façade retention projects.

The Chancery Rosewood represents a landmark achievement in the delivery of complex, design‑led heritage projects and sets new benchmarks in quality, sustainability, innovation and collaborative delivery. Among the technically demanding elements include the restoration of listed façades, large‑scale temporary works – including propping and jacking operations – the development of bespoke interfaces, and the installation of highly complex MEP and vertical transportation systems.

Quality and technical innovation were defining characteristics of the project. This was essential as the design revolved around retaining the historic façade and unique architectural elements while transforming and fully modernising the building’s interior. 

Key engineering challenges:

Careys Design Team’s façade retention scheme was designed to comply with strict AKT criteria. We created a BIM Execution Plan. We began by scanning the existing structure and creating a Revit 3D model, which was linked with the construction programme using Synchro 4D software. The scan captured the existing condition of the building (and surrounding buildings) and we then compared them to the historical information to identify any discrepancies. This was critical to the temporary works design and structural alteration sequence of the extended basement.

Once the scans and the existing structure final model was complete, we detailed the temporary works model in Tekla Structures to LOD 400 and the permanent RC formation structure in Revit to LOD 400, including rebar detailing. These models were shared with the project team in the common data environment (Aconex) The steelwork models were then sent in native Tekla format straight to the fabricator without the need for drawings. This reduced drafting time, RFI’s and questions, resulting in a 5% saving to the steelwork and a 3-week reduction in critical path activities. We continued with this level of fabrication by detailing all the reinforcing in Tekla and having the models approved by the client’s representative. This approach significantly reduced the cutting and bending of bar on site, and in turn the risk of RFIs and NCRs.

The 660 tonne system comprised both vertical and horizontal trusses to support a four-storey retained facade and a diagrid floor at first floor level. The horizontal trusses support the diagrid floor and form a ‘platform’ off which the vertical trusses are anchored to support the retained façade. Each horizontal truss is in turn supported at three locations; externally on secant piled perimeter to the new enlarged basement, at an existing building perimeter on plunge column piles, and internally on reinforced columns bearing onto ‘internal’ piles.

 

The façade was constructed using precast concrete columns, supporting precast concrete backing panels, all of which is clad in Portland Stone. It is made up of alternating O-rings with infill panels. The diagrid is a cast in-situ beam and slab design, waffle-like in its appearance, albeit at 45 degrees to the building grid. Both façade and diagrid are delicate in their construction, having been designed to tight parameters and leaving little residual strength to rely on during demolition. The temporary works system, took this fragility into account in terms of truss design, spacing, detailing and connection design. Most notably, the timber waler detail used at each O-ring anchored the façade to the steel system.

We designed the trusses to be prefabricated largely off site and installed using WK 355 cranes so as to minimise crane time and maximise on-site production. By using slightly heavier chord sections, we were able to reduce the size of the trusses so that normal lorry sizes could be used. Our bespoke trolley system allowed the main trusses (21m , 25t) to be installed in three five-tonne segments bolted together in situ. So effective was the design that these segments were simply positioned by two steel workers and a crane driver. The tender programme allowance of 26 weeks was reduced to 20 weeks on site, despite multiple challenges and downtime due to wind. We divided the building into three zones approximately aligned with the three ‘wings’ so as to allow phased completion of the steelwork installation and the introduction of load ( jacking) to the trusses. Our Design Team provided a load calculation for each jack location (120 no. in total), together with an expected jack displacement. The accuracy was such that both load and displacement were almost exactly as expected - remarkable given the variability of the building and the complexity of the load calculations. The movement experienced by the trusses, once the supporting columns to the existing structure were cut free, was 1-2 mm. Meanwhile, splitting the building’s support system into three had allowed the demolition to progress concurrently, both above and below the diagrid. We designed and detailed all waterproof concrete elements and completed all reinforcement detailing to the permanent works.

Additional challenges successfully met:

The project featured a significant amount of asbestos in unexpected places, for example, used as a cast-in socket to secure the screws to all window mullions. All was safely removed. A large number of heritage items - both internal and external - are being retained, including the structure’s façade with the window mullions and frames, the aluminium birdsmouth cornice on the roof, a large golden eagle structure on the front elevation, the Berlin Wall memorial, various statues, cruciform aluminium cladding, stonework on the entranceway walls and floors, the diagrid infill boards, slab lighting and roof lights, balustrades and handrail, and stonework cladding.

Sustainability

The project is pursuing a BREEAM ‘Outstanding’ rating and we achieved 100% diversion from landfill and recovered a total of 120,000m3 materials for reuse, We had to protect multiple ‘London plane’ trees surrounding the structure under Section 211 and in consultation with an arboriculturalist, requiring us to dig trenches between the tree-line and our works, excavating part of the exposed root ball and trimming it to distance the remainder from our utility works. Our Section 61 Agreement gave 78 dB as an action level - challenging given the acoustic mapping supplied and the demolition techniques we had agreed with Westminster at the outset. However, we met requirements and kept neighbours happy in spite of their increased perception of noise in the far quieter streets of Covid-19 lockdown.