AIA Florida/Caribbean, Merit Award, 2020
Situated in the heart of Central London, neighbouring notable landmarks like Trafalgar Square and the National Gallery, Hobhouse is a mixed-used development that involves retaining, restoring and redeveloping existing buildings along Whitcomb Street, including the Grade II listed building attributed to the famous Georgian architect John Nash and its subterranean masonry vaults.
Redeveloping in a densely compact, historic area presented a highly-technical balancing act that involved creating new basement areas, refurbishing and restoring existing structures, constructing new structures, realigning the streetscape and creating public spaces, all completed in tandem with preserving the 200-year-old vaults.
A Digital-First Mindset
Our engineers relied on few valuable digital tools to help the team make informed decisions and keep the project on track:
- We used a 3D model to help visualize the complex geometries involved with the new and existing buildings, as well as Rhino models from the architect and our own Grasshopper scripts for setting out and coordinating the columns to the inclined façade.
- The survey company translated the existing structures into a 3D environment to illustrate how key new structure interfaces with the existing structure.
- The team used Autodesk BIM360 allow us to work with data in real time to ensure areas could be coordinated against the latest information.
- We heavily relied on Konstru, which allowed for data synchronisation between building design and analysis applications without requiring rework.
Ease of Constructability in a Complex Footprint
The client’s aspiration for differing building uses, maximised lettable area and clear span floor plates whilst working within the remits of the existing listed structures presented a significant technical challenge and potential risk to the project. Close collaboration with the project team early within the design process was essential to understand the constraints and limitations on what could be achieved.
We used a detailed series of surveys and testing programmes to understand the condition and capacity of the retained building and vaults so we could develop the design accordingly. The ability to retain and reuse existing structures vastly decreased the demand for material, cost and programme for the construction of an all new development. The project team estimates savings to be approximately 1100m3 of additional demolition spoil and the construction of approximately 900m3 of new RC basement.
The new development construction had to be carefully balanced against the existing vault structure to minimise structural interventions and preserve its heritage. The client’s aspiration for clear span floor plates for the new floors above the vault concentrates load into a limiting number of locations, which isn’t suitable for masonry construction. The solution: a ground floor transfer grillage that evenly spreads the weight of the building over the vaults. No new columns or foundations - with the exception of the new lift pits - were constructed through or within the basement area in order to preserve their historic fabric.
Structural zones to the floors and walls were minimised, not only to limit storey heights and maximise internal spaces, but to also reduce and minimise material. A slim floor system that fit within the structural slab depth utilising fabricated shallow beams and “plug composite action” enabled a very compact structural zone whilst achieving up to 9.5m clear spans, meeting the market demand for open areas with substantial floor to ceiling heights.
Lastly, the façade for the new-build elements features handset bricks and punched windows. As the building rises the brick cladding moves to a faceted shape with many different angles, minimising the massing on the streetscape and allowing pedestrians to see more of the sky. Using prefabricated, structurally-insulated panels as the inner leaf contributed not only to the load carrying system for the facetted handset brick façade but also its thermal performance.
A New Benchmark for Low Embodied Carbon Buildings
The design team and client agreed to the goal of developing a low-embodied-carbon building by combining material retention with efficient design:
- The new building’s external wall buildup, which consisted of prefabricated lightweight insulated panels to minimise the number of required material layers, helped reduce the embodied carbon total.
- Retaining and reusing the existing building and vaults vastly decreased the demand for new construction materials.
- The concrete replacement material from the recycled demolition spoil contained 60 to 80 percent ground granulated blast-furnace slag, similar to fly ash.
Our carbon calculator has measured the embodied carbon of the building’s structural elements as 375kgCO2e/sm and the overall building figure at 495kgCO2e/ sm. The building’s embodied carbon fell below the London Energy Transformation Initiative’s (LETI) proposed 2020 targets, even though we conceived its design much earlier, in 2013. The project also pursued BREEAM green building certification and achieved a rating of “Excellent,” receiving all six available credits for embodied-carbon reduction.
Hobhouse is an ideal example of how you can creatively reinvent, regenerate and repurpose existing buildings, using a mixture of technologies to create an expressive and efficient development.
The new buildings have created a positive economic and social contribution to the historic area while the retention and re-use of the existing structures have allowed these “lost” spaces to be re-opened to the public for gallery and retail use. The creation of new public spaces and the realigned streetscape has rejuvenated the area. Among the many project successes includes the Royal Watercolour Society - who previously occupied the building for 115 years - returning to their original base once the development reopened.