Evans Vettori’s extension building for Nottingham Trent University uses cantilevering concrete to weave around the surrounding tree roots.
The Dryden Enterprise Centre at Nottingham Trent University is a building shaped by trees. Not just because its glazed upper storey can feel like a treehouse, surrounded from spring to autumn in the canopy of the neighbouring London Planes. But also because its distinctive form, with curving corners, alcoves and overhangs, is a direct response to the roots spreading below.
“The structure had to do something quite clever to dance around the root protection zones,” says John Evans of Matlock-based architect Evans Vettori. The platform for these dance manoeuvres is an in-situ concrete frame, with a central core branching into 250mm-deep flat slabs on the lower levels and 350mm slabs on areas of the third floor and roof. From this base, the building is able to leap and lunge, most notably in a 6m cantilever over the entrance facade.
This required huge upstand beams to be integrated into the flat slab, spanning nearly 6m into the building and 4.5m out to the external terrace. “When it was there in its raw form, before the masonry facade had been installed, it looked seriously imposing – just concrete flying in mid-air,” says Evans.
The university is pitching the facility as “a new home for enterprise in the city centre”, offering business support and high-quality facilities to a mix of students, graduates, entrepreneurs and SMEs. Such structural dexterity was necessary because the site was already constrained, with an existing 1970s strip-glazed library block to the north, and the architects had a lot to fit in. The four-storey building, which extends from the existing 1970s block, squeezes in 2,000m2 of floor area, from cellular offices and boardrooms to collaboration spaces and open-plan areas.
The concrete frame has already proved flexible to different uses – at one point during the development process, the building was going to be taken by a single corporate client. The structural grid allows floorplates to be configured in various ways: slabs are supported by 400mm-diameter circular columns at 5m intervals; and there is a rationalised single core, made possible through the reuse of a fire escape stair in the existing building.
The servicing is equally loose-fit, with cabling run through raised access floors and high ceilings leaving plenty of room for suspended lighting, ductwork and vertically hung acoustic baffles. The 3.7m floor-to-floor height also allows the slabs to tie in, step-free, with the floorplates of the existing building.
The frame has been left largely exposed, partly to exploit its thermal mass to reduce heating and cooling needs, and partly for aesthetic reasons. The concrete is relatively light in tone, thanks to the use of 40% GGBS, which also helped to lower the carbon content (the building has been designed to achieve BREEAM Excellent). The finish is smooth – it was cast using MDO-faced ply formwork – with neat mitred joints and small rebates at the top of the columns.
Much of the construction work took place during Covid lockdowns, and Evans says it was fortunate that the design team were using a single 3D model coordinated via BIM software, which meant they could all work on the project remotely. “Rather than having everyone traipse down to site with pieces of paper and printed drawings, trying to figure out which drawings went where, we could just boot up the model, take an extract from one of the areas and spin around it. Within minutes you could see how it was supposed to work.”
Externally, the palette changes to brick and precast concrete. Precast string courses, specified to echo the tone of Portland stone, are suspended from a bespoke masonry support system fixed to the slab edge. The English grey bricks were then hand-laid, supported by the string course. The asymmetrical curves to the projecting terraces – a nod to Hans Scharoun’s legendary 1930s Schminke House in Saxony – required the radial dimensions of the curving bricks and precast units to match to the millimetre.
The double-height external columns were precast using the same concrete as the other facade elements. These 7m-high units had to be navigated into position between the cantilevered slab above and the terrace below with pinpoint precision: a tiny 150mm-wide aperture was opened in both the roof slab and the cantilever, and a crane chain was dropped through, like a needle and thread. This was then connected to a lifting eye cast into the column, enabling the precaster’s site team to manoeuvre each piece into place.
The facade also includes large expanses of glazing, with openable windows on the lower floors and shading provided by the overhangs, particularly on the south facade. The top floor, which houses most of the executive areas, required solar-resistant glass and the building’s only air-conditioning, but the payoff is a seat amid the tree canopy. “Considering you’re in the centre of a city, it definitely feels like a lot of green,” says Evans.
There is one final 1970s-inspired touch: a polished-mirror soffit to the underside of the terrace, injecting a playful, almost disco-like vibe into the otherwise business-like exterior. “It’s almost like a reflecting pool,” says Evans, “with the effect of the columns disappearing into the building. Internally on the second floor, right beneath the cantilever, is my favourite area in the building. You get the reflection of the curving landscaped area below, and all sorts of little effects.”