When the White Collar Factory was completed in 2017, it offered a glimpse of the future of office working: a model low-carbon workplace in the heart of London’s Old Street tech district that mixed smart building systems with old-school warehouse elements. Now developer Derwent has evolved the model with another BREEAM Outstanding, LEED Platinum, concrete-framed office, just a few doors down.
Designed by Morris+Company, the Featherstone Building incorporates five more years of collective knowledge and understanding of material-efficient design and intelligent systems. Its embodied carbon of 496kgCOe/m2 from stages A1 to A5 is the lowest of all Derwent’s new-build projects to date. Designed in 2016-17, it beats LETI’s 2020 target, and represents a 380kgCOe/m2 reduction on the White Collar Factory’s footprint over the same stages.
The Featherstone also appears on first glance to take a different approach to workplace aesthetic. Whereas the AHMM-designed White Collar Factory rises 17 storeys from a small, irregular-shaped site, the Featherstone is an 80m-long block, articulated as four roughly square volumes, two of five storeys, one of 10 and one of 11.
And whereas the White Collar Factory is sheathed in sleek glass and hole-punched aluminium, the facades of the Featherstone pay homage to the area’s Victorian warehouses, with broad piers of dark-grey and buff brickwork and scalloped, rough-textured concrete lintels.
Yet both buildings are born out of the same philosophy: a “long-life, loose-fit, low-carbon” approach developed over many years by Derwent in collaboration with its different design partners. Morris+Company fondly refers to the Featherstone as the White Collar Factory’s little sister, and the two buildings share the Derwent hallmarks of high ceilings, large-span spaces, exposed finishes, generous outdoor areas and large, openable windows.
Both buildings also have an in-situ reinforced- concrete frame, made to work as hard as possible – not just as structure and finish but also an integral part of the heating and cooling system. Design work on the Featherstone was just getting under way as the White Collar Factory was completed, and Derwent was keen to adopt many of the same sustainable technologies.
This included “activating” the slabsby casting in a network of plastic pipes, through which chilled or heated water can flow, supercharging the concrete’s thermalmass. Sensors monitor the slab temperature and air humidity and feed this data back to the building management system, whichcontrols the water temperature.
It is easy to see why thermally active slabs appeal to Derwent. They are both sustainable and unobtrusive, leaving expanses of clearsoffits while lowering operational energy use – by a claimed 25% at the Featherstone. They enhance the building’s stripped-backaesthetic and accentuate the 3.125m floor-to-ceiling heights.
But this apparently simple passive solution took a lot of very active structural engineering to achieve. As with the White Collar Factory, the grid has to be organised in a way that balanced the requirements for large spans and a lean structure, while still being able to incorporate the pipework. And as with the previous project, they had to work within awkward local constraints, the footprint of the building exceeding the space available for foundations.
The Northern Line and Network Rail Northern City Line tunnels run adjacent to the entrance facade. With a 3m exclusion zone around the tunnels, structural engineer Heyne Tillett Steel needed to set back the piles from the perimeter, effectively cantilevering the whole building. HTS’s solution was to use the basement slab as a transfer structure, increasing its depth to 1.5m. “We looked at different options, but actually doing everything below ground was the most efficient way without compromising the internal layouts,” says Robert Mills, senior associate at HTS.
Above ground, the challenge was to create column- free spaces without increasing the depth of the slabs. The four interconnected volumes are about 21m x 21m, which would have lent itself to 10.5m spans. Post- tensioning was ruled out as a means of providing a leaner structural solution – with 60km of pipework needing to be embedded in the structure already, a traditional approach to reinforcement reduced the perception of risk. Instead, HTS refined the design, adjusting column positions, analysing stresses and modelling deflections to create a 9m x 9m grid, with some variations. “That still gives a big, open, flexible space,” says Mills, “but it meant we were able to engineer the slabs so they are only 325mm deep.”
The pipework has been designed to avoid the most heavily stressed areas of the structure, such as around column heads, as well as built-in “soft spots” to enable future flexibility. It was laid between two mats of reinforcement and fully tested before the slab was poured. (Main contractor Skanska issued follow-on trades with short 19mm drill bits to make sure that no one accidentally penetrated the pipes.)
The soffits have a matt finish from the paper-faced MDO formwork, and were also “sanded” to remove blemishes. The mix contained 30% GGBS, although higher proportions were used elsewhere, including 50% in the columns, 70% in the piles and 75% in the pile caps, where it helped to slow the curing of the thicker concrete elements, reducing the need for anti-crack rebar.
There is a 15m x 9m core on the east side of the building and a smaller 9m x 9m core on the west side, both of which provide further expanses of exposed concrete. The mix here contains 20% GGBS but also includes a 2.5% addition of silica fume, a by-product of the computing industry. This helped to give the walls a slightly darker tone.
Each jump-formed floor was cast in a single pour, partly due to programme constraints but also to avoid day joints in the exposed surfaces. “Because it was all visual concrete, the pour had to be continuous, well vibrated and controlled,” says Steve Arthrell, project director at Skanska. “It would take a whole day.” The team spent a lot of time testing the mix, he adds, as the balance between the silica fume, which is an accelerator, and the GGBS, a retardant, was critical to both the initial setting and curing times. They also had to take the location of the tie-holes into account and keep them in alignment: these are a recurring motif.
The cores were cast using phenolic ply formwork. Although this leaves a shinier finish than MDO, it can be reused more times. The exterior, on the other hand, shows very few signs of its construction method. The masonry facades may look traditional but are actually supported between perimeter columns like a curtain wall. This lightweight unitised system is made from glass-reinforced concrete (GRC), albeit with high-quality material finishes instilling a strong sense of depth and solidity.
Facade engineer Eckersley O’Callaghan had explored a number of options, including handlaid brickwork with precast lintels and fully brick- faced precast panels. However, the constrained inner-city site presented a logistical headache, with little room to install scaffolding or to manoeuvre the large elements that would be needed to work with the 9m grid and 3.8m floor-to-floor heights. The lean in-situ frame was another factor: “The slabs were as thin as they could be,” says Arthrell. “Their dead and live load deflections were quite high, which is always a sign that it’s designed as efficiently as possible.”
Skanska was also keen to explore the efficiencies of design for manufacture and assembly (DfMA), which led the team to consider GRC panels, fixed to the frame using a cassette system. This had the added benefits of being fully demountable at the end of its life and potentially reducing the facade’s whole-life carbon (see box). “It was quite bespoke at the time,” says Arthrell. “Together with the manufacturer, we worked up a design that made the facade work and the client loved the idea.”
A 1:1 mock-up demonstrated that the GRC system could deliver the required architectural quality. There were three main elements to the system: the piers, made with half bricks on 82mm-thick GRC panels; openable aluminium windows; and a T-shaped unit comprising a slender GRC mullion and the scalloped lintel. Variations include the use of a double scallop on the upper level to define the crown of the building.
The scallops were cast with a Reckli mould to create a hammered effect – a faint echo perhaps of the White Collar Factory interiors, which also use a flexible form liner to recreate a traditional effect, in that case timber board marking. It’s as if both buildings are paying their respects to a shared heritage, before striking out on their own.
Face off: comparing the carbon of two facades
At the time that the Featherstone Building was being designed, there was no standardised methodology for calculating the embodied carbon of different facade options. But Eckersley O’Callaghan (EOC) found itself in a unique position to do some numbers of its own. Having already worked up a detailed design in precast concrete and brickwork before switching to unitised glass-reinforced concrete (see main copy), the engineer realised it was able to carry out a detailed comparison.
“When the project finished, we thought it would be great to compare both options over a lifespan of 60 years and see whether or not we made the right choice,” says Florence Li, senior engineer at EOC. “We felt that the embodied carbon in offsite assembly is not well documented.”
Using product data from the supply chain as well as a breakdown of energy use in production and installation, the team initially expected the GRC option to involve more upfront carbon. “You’ve effectively got a panel with another layer of cladding on top, and there are also a lot more metal components. The bricks came from Belgium and had to be shipped to the manufacturer in Poland,” Li explains.
Yet one surprising result was that the unitised solution involved less embodied carbon from stages A1 to A5 (extraction, manufacture and installation), estimated at 95kgCOe/m2 compared with 104kgCOe/m2 for the brick laid on site. This was partly because the impact of site waste for the bricks was greater than the transport involved in the GRC panels.
But where it gets really interesting is when carbon emissions are extrapolated over a 60-year life. The unitised solution has a shorter lifespan, with the gasketry needing to be inspected and potentially replaced after 30 years. “We assumed that the whole facade would be dismantled to allow us to replace the gaskets, install new glazed units and then reinstall it all on site,” says Li. With the brick, on the other hand, EOC assumed that the punched window elements could just be reinstalled, which meant a lot less activity during the life of the building.
Factoring in the shorter lifespan, the unitised solution was now calculated to be slightly more carbon-intensive: 140kgCOe/m2 compared with 130kgCOe/m2. The D module for recovery and reuse also favoured the precast option in purely carbon terms, although Li points out that the metal unitised components are directly reusable.
It’s also easy to envisage a future where shorter- term facade options play a role in the expanding retrofit market, with property owners simply switching building skins every few decades to refresh their offer. So did EOC make the right choice? “It was nice, in a way. We landed in a place where both solutions were roughly the same – we went full circle.”