As its name suggests, Cambridge University’s West Hub is primarily a place for connections to happen.
The £40m, three-storey facility houses a library, cafe, seminar rooms, offices and work space. So far, perhaps, not that unusual. But West Hub represents the start of a radical transformation of this research campus to create a new quarter in the city, dubbed the West Cambridge Innovation District.
The buildings here have to walk the talk in terms of inventiveness, sustainability and cutting-edge technology – so the 4,700m2 West Hub is rated BREEAM Excellent, and features a ground-source heat pump, a blue (water-retaining) roof, and internal gardens to promote wellness via biophilia. And the whole and its parts are carefully organised to foster links between the university and the town, and between business and academia. This desire for user-friendly openness informed the whole design, including the choice of concrete for the structure.
“The building is open to the public,” says architect James Tatham, a director with Jestico + Whiles. “You don’t have to cross a security threshold to enter it, and in fact there is a pedestrian route through the building via its two main entrances, so we wanted it to be welcoming. We considered several options for the frame, but we wanted this place to feel different to other university buildings: not too academic or polished, but more like shared workspace with a cafe feel. So the largely exposed concrete frame, with its slightly rougher, almost industrial aesthetic, really works well for us.”
The exposed concrete is also integral to the heating and ventilation strategy: its thermal mass absorbs sudden influxes of excess heat and stores it to reduce heating demand later. “This is a building with a variable and unpredictable occupancy – you might have 200 students coming in for a lecture and then disappearing,” Tatham points out. “Compared with a more lightweight structure, the concrete helps to even out the fluctuations that can cause.”
Finally, he says, standard reinforced-concrete flat slabs of variable depths provided robust flexibility to deal with a demanding range of loading requirements: “The shelves of books in the library area, together with some permanent bespoke furniture, results in some heavy loadings. The structure needs to be able to cope with that along with the potentially large live loads caused by groups of people – queuing to get into a lecture, for example.” The building sits on an 800mm-deep raft slab made from concrete containing a high proportion of GGBS cement replacement. “We specified at least 70% GGBS for this,” says Oliver Neve, an associate at Ramboll, which undertook the structural design of the project to stage three.
“The higher content slows the curing process and helps keep down the heat of hydration during the pour.” Generating less heat, which helps to maintain the integrity of the finished concrete, is also achieved by keeping slabs thin. So why did Ramboll choose the thick raft option? “We’ve done a lot of work in this location and find that a raft is usually the best solution in terms of cost and concrete volume,” says Neve. “It sounds counterintuitive, but long piles and a slimmer suspended slab can actually use more concrete and are more time consuming and expensive to construct.”
The building was designed back in 2016, so there was no specific embodied carbon assessment as there would be now, he adds. “But clearly the GGBS content of the raft helps reduce the carbon content. We specified 20% GGBS for the rest of the building.” West Hub’s many different functions are reflected in a structure that is quite irregular, or “bespoke” as Neve puts it. “There’s a lot going on. We have spaces here providing many different functions – from library to kitchen to auditorium – and this results in different loadings and different ceiling heights.”
“The slabs vary to suit spans and loading types that vary within the floorplate,” agrees Nicholas Clegg, a structural engineer at BDP, which took the design forward from stage four. “So we have slab depths of 275, 300, 350, and 375mm – putting the strength where it is needed. It’s the same with the grid. Typically it is 7.2m between columns along the length of building. But across, it varies, being either 6.45m, 6.725m or 6.85m.” The main atrium provides visitors with an excellent view of the structure: a 275mm slab edge is clearly visible and rising from it is a 550mm-high concrete upstand.
As well as forming part of the balustrade, these 400mm-thick upstands stiffen the slab edge around the atrium, effectively acting as slab edge beams. This upstand beam arrangement is continued beyond the slab, part way along the glazed facade side of the atrium. This works both aesthetically and structurally: “The beam here is 825mm deep to match the slab/ upstand combination, and spans between columns at the facade in order to support the corner of the atrium slab that does not align with the column grid.” One corner of the building required unusually long spans:
“This is the area with the bar on the ground floor and seminar rooms above it,” says Clegg. “To keep these areas as column-free as possible, the slabs here span 10.8m from the building perimeter to support from a central column. To achieve the longer span, the slab is a thickish 350mm, but is also pre-cambered by about 10mm to accommodate the deflection.”
Tough choice: filtering the structural options
Standard reinforced in-situ concreteis seen as a safe and cost-effective structural option – but it would be wrong to assume designers simply pencil it in without giving thought to alternatives. At West Hub, the team undertook an exhaustive “filter” study to ensure that a fair hearing was given to a wide range of structural choices.
“For example,” says Ramboll’s Oliver Neve, “we looked at cross-laminated timber and glulam, but discounted them because we wanted to use the thermal mass of concrete to cope with temperature variations. The university likes to minimise active heating and cooling where possible, so that choice resulted very specifically from the client’s long-term sustainability strategy. We also considered post-tensioned concrete slabs but, on this occasion, felt that the grid arrangement was not regular enough to realise efficiencies there.”
The design didn’t involve enough repetition to make precast concrete economically practical. Meanwhile, the long spans involved ruled out a steel-andprecast combination: “They would have led to some very heavy sections and the need to design-in downstands.” Finally, metal composite decking was discounted as “failing to meet the architect’s aesthetic requirements”, or simply: “Too ugly.” In the end, says Neve, “we felt a standard reinforced-concrete frame was the most cost-efficient way of achieving the design flexibility we needed throughout the building.”
The pre-cambering was created by pouring the slab over a thin, moundshaped form. The effect is that of a very shallow arch to the underside, which disappears once formwork is removed and the concrete has settled. In accommodating the deflection, the weight of the slab also compresses its centre, helping to strengthen and stiffen the element. Most of the columns in the building are 400 x 400mm square, but here they are increased to 450mm sections to support the extra load of the longer spans. The chamfer-edged, square-section columns are visually impressive.
Around the atrium areas they rise up to two storeys and have a smooth, almost precast appearance, with little evidence of the pouring process. The smooth finish results partly from the use of metal forms: being more than one storey high, the weight of concrete could have distorted traditional timber shuttering. “Much as you can try to specify a finish, it is ultimately quite subjective,” says Tatham “So early on we visited several existing buildings to benchmark and reach a consensus with the contractor on what was acceptable in terms of quality.
” A number of test panels were constructed before final approval – a luxury afforded to West Hub by virtue of the fact that it shares both designer and contractor with the new Cavendish Laboratory, now being constructed on the next-door site. “The investment in samples would have been difficult to justify for us alone,” he explains. “But as the combined value of the projects is around £200m, we could afford to try various approaches.” In some places, he adds, post-finishing was carried out by GreyMatter, a firm that specialises in improving the appearance of in-situ concrete. “Its work is akin to art restoration – blending in any problem areas.”
The clean, pale concrete is set off by bold orange accents from the permanent furniture, and the orange-clad steel truss staircases which are attached to fixings cast into the slab edges. Moving out on to the second-floor open terrace, building users will be able to look over the supersustainable Innovation District – but may not be aware that they are actually standing on one of West Hub’s green innovations. “Being a north-facing building we were keen to allow occupants the chance to get out into the sunshine,” says Tatham. “Rainfall on the roof is drained through the terrace pavers into a 600mmdeep “blue” roof – effectively a wide, shallow, concrete attenuation tank that works to regularise runoff from the building and avoid putting any undue pressure on local drainage systems.”
The terrace is typical of how this building works. It is undemonstrative, yet subtly working hard to achieve its owner’s goals: a friendly, collaborative space, and one that treads lightly on the earth.