Hobhouse is a 7,000-square-meter mixed-use development near London’s Trafalgar Square. Two of the site's four original buildings were listed by the Historic Buildings and Monuments Commission for England (Historic England). The project demolished three of these buildings to ground level. Thornton Tomasetti provided structural and façade engineering for the refurbishment of the remaining building, the addition of two new floors at its roof level, and the construction of a new six-story building above the historic subterranean vaults of the demolished structures.
We sat down with Simon Cross, a senior associate in our Structural Engineering practice, and Duncan Cox, a senior associate in our Sustainability practice, to discuss the project’s progressive use of embodied-carbon reduction strategies.
Embodied carbon – the carbon footprint of building materials – is a new concept in sustainable building design. Why did the design team decide to pioneer embodied-carbon reduction strategies on this project?
SIMON Structural engineers have an enormous impact on embodied carbon. So as project manager, I felt we had a moral obligation to strive to meet climate-change targets. From the outset, our approach was to reuse and refurbish as much of the original 200-year-old structure as possible. Through careful detailing and design, we can conserve and redevelop historic buildings to reduce their embodied carbon.
DUNCAN This iconic structure sits right in the heart of London, next to Trafalgar Square. Historic England has granted the building “protected” status, meaning that it could be refurbished only if historic elements such as the vaults – once used for storing wine – were retained. This created a favorable condition for the reduction of embodied carbon, as it meant we had to avoid adding lots of material to reinforce the substructure.
This project added new floors above the historic section. How did the team minimize the amount of material placed atop the substructure?
SIMON The design of the additional levels required careful consideration to minimize structural intervention to the vaults, so as not to add more loading and create undue stress. Working with the architect and English Heritage, we developed a philosophy early on that achieved their project aspirations while preserving the building's historic fabric.
DUNCAN We designed a very lightweight frame, which had the double benefit of reducing material in both the substructure and the overall building. We also employed castellated beams – essentially “Swiss cheese” versions of steel beams – which increased the beam capacity by 60 percent while using less material. By placing the slab within the beams, we achieved the required floor heights, but with a lightweight structure.
So by reducing the quantity of materials, you reduced embodied carbon. Did the project use any other embodied-carbon reduction strategies, and was that a specific certification goal?
SIMON Most embodied carbon in concrete originates from its cement, so we used concrete with a high cement replacement. It contained 60 to 80 percent ground granulated blast-furnace slag, which is similar to fly ash. The predominant brick façade is low in embodied carbon, compared to metal rainscreen cladding and other common materials. The external wall buildup, which consisted of prefabricated lightweight insulated panels that reduced the number of required material layers, also helped reduce embodied carbon.
DUNCAN The project pursued BREEAM green building certification and achieved a rating of “Excellent.” It received all six available credits for embodied-carbon reduction.
Seeing the almost-finished renovation and reuse of the Hobhouse development, I was amazed at the quality, complexity, history and character of the space. After the tour, I found your presentation and the discussion on carbon emissions in the construction industry, and the Thornton Tomasetti embodied-carbon reduction project, most inspiring.
Is it now standard practice in the United Kingdom to consider embodied-carbon reduction in sustainable design?
SIMON When the Hobhouse project began seven years ago, people weren’t really talking about embodied carbon, but now there’s a lot of interest. Clients are better educated about it, and they’re taking a proactive stance on energy efficiency. It’s talked about on all projects now, not just the large ones.
DUNCAN In the United Kingdom, we’ve seen cement replacement in most concrete mixes for some time, which is bringing down the carbon footprints of projects. Our challenge now is to reduce this impact further by increasing cement replacement, improving design efficiency and considering the use of structures as carbon sinks, absorbing more carbon than they release.
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