A hybrid solution, incorporating a steel frame supporting cross-laminated timber flooring, has been successfully used for a prestigious office scheme
Located in the heart of London’s Mayfair, 38 Berkeley Square is an 11-storey steel-framed commercial development providing more than 7,890m2 of office accommodation and approximately 650m2 of communal terraces.
The steel superstructure supports an innovative flooring solution of exposed cross-laminated timber (CLT) floor panels, a lightweight hybrid solution that offered up a number of benefits, particularly below ground.
By limiting the loads transferred to the supporting foundations, a raft solution was installed instead of traditional deep piles. This method halved the volume of concrete required below ground, reduced the construction programme and minimised the potential effect of the works on neighbouring heritage assets.
Award: 38 Berkeley Square, London
- Architect: Piercy & Co
- Structural engineer: Elliott Wood
- Main contractor: Mace Group Limited
- Client: Berkeley Estate Asset Management
The main steel frame of the project provides the strength, spanning capability and adaptability required to create generous floorplates, while responding to the restricted planning envelope and the demands of the architectural and servicing strategies.
Steel’s high strength-to-weight ratio made it possible to achieve the required spans within the restricted structural zone. Its adaptability also allowed service openings, connections, façade supports and interfaces with the floor panels to be accurately coordinated before being manufactured.
Bespoke connections were developed between the steel beams and CLT panels to provide the required structural performance, while accommodating the different characteristics of the two materials. At critical interfaces, the steelwork was over-boarded to limit heat transfer into the timber and maintain the required compartmentation between floors.
“As the CLT panels remain exposed within the completed scheme, fire performance was a major consideration,” says Elliott Wood director James Souter. “Demonstrating fire compliance for an exposed CLT and steel structure, at a time when regulations had become much more stringent and industry confidence around tall mass timber buildings was low was a challenge.”
Detailed analytical work demonstrated the performance of the structure, including the behaviour of the steel–timber interfaces and the ability of the exposed timber to self-extinguish. This provided a route to regulatory approval without costly full-scale physical testing and allowed the original construction programme to be maintained.
As well as supporting the CLT floor panels, the steel frame performs other demanding structural roles. At the upper levels, the building steps back to create terraces, supported by a number of transfer structures.
“This was an exemplary project using a hybrid steel and CLT frame, setting a
benchmark for future low-carbon construction. The judges praised the team’s collaboration and industry leadership, engaging Building Control, the London Fire Brigade and insurers, helping to shape the Commercial Timber Guidebook. Its success was driven by both a forward-thinking client and a highly-committed design team”
The erection of the steel frame was carefully phased as the site had very little ground-level storage space for materials.
Completed areas of each floor provided temporary lay-down areas as construction progressed, while just-in-time deliveries limited the amount of material held on site. Early coordination ensured the steel components arrived ready for installation, which was particularly important within the constrained central London site.
Highlighting the development’s sustainability credentials, it has achieved BREEAM Outstanding and LEED Gold, together with a NABERS UK Design for Performance target rating of 4.5 stars. Approximately 99% of the structural steel was sourced from mills certified to BES 6001, while 68% was manufactured in the UK and a further 30% in mainland Europe.
Work undertaken to establish the compliant hybrid structural solution has also contributed to the Commercial Timber Guidebook. Funded by Built by Nature, the initiative supports wider understanding of hybrid commercial construction and engagement with the insurance industry.
Produced by BCSA and Steel for Life in association with Construction Management.
