Feature

MASTERS OF MASS

Few architects have contributed more than Bennetts Associates to the renewed popularity of exposed concrete. Nick Jones met directors Rab Bennetts and Simon Erridge

It may come as a surprise to learn that workplace wellbeing was born on a business park outside Coventry. It was here, in 1991, that Bennetts Associates designed what was only its second major project: a headquarters for the newly privatised energy company, Powergen.

The practice took a seminal approach to energy efficiency, pulling out all the stops to show that good design could create a comfortable environment just as effectively as heavy servicing. Narrow office floors were flooded with daylight, ceilings were high, windows were top-hung and openable for natural ventilation, and the facade was carefully designed to provide solar shading. One of the most significant elements were the profiled exposed-concrete floor slabs which, combined with night purging, stabilised internal temperatures through their high thermal mass.

Powergen set a template for much of Bennetts’ subsequent work, which in turn has had a massive influence on the resurgence of concrete as both a structural and aesthetic material – it could be argued that neither the White Collar Factory nor 45 Folgate Street, featured elsewhere in this issue, would have been designed in quite the way they have without Bennetts’ pioneering work on concrete-framed offices in the 1990s and early 2000s. It is one of the reasons why the practice’s founder, Rab Bennetts, was the obvious choice to speak at CQ’s 70th anniversary celebrations in September, and why we caught up with him and fellow director Simon Erridge at Bennetts’ Clerkenwell HQ earlier in the summer to reflect on the practice’s past, present and future.

Bennetts first began thinking about energy efficiency and thermal mass during the 10 years he spent at Arup Associates in the late 1970s and 1980s, but it wasn’t until he and his wife Denise set up their own practice in 1987 that they “started to push those ideas a bit”. When the Powergen job came four years later, it was the opportunity they had been waiting for. “That was a bit of a breakthrough, and a lot of our early convictions were born in that building,” says Bennetts. “Powergen took the view that they should be efficient with their own product, which was admirable, so we ended up looking at thermal mass in more detail.”

 

The practice collaborated with the services engineer Nigel Griffiths, who provided “real rigour in the analysis of the building, which is what we’d been missing up to that point”. This was all the more important as it was Bennetts’ first design-build project, placing the young practice under contractual obligation to prove that natural ventolation would work without reducing comfort levels. Use of computational fluid dynamics, then in its infancy, proved the cooling benefits of wind pressure across clear floorplates; equally close attention was paid to the profile of the vaulted structure, which was deepened to increase the exposed area of thermal mass and elegantly tapered towards the source of daylight.

But Powergen would not have had the impact that it did if it had just been about efficiency. Twenty years before anybody had coined the phrase “wellbeing” in relation to offices, this was essentially what Bennetts was doing. “You could only do it if it was comfortable, otherwise people wouldn’t accept it. So the analysis of temperature was massively important. Then there were visual stimulants, views, the ability to open your own window – all that stuff is now called wellness.” For Bennetts, these ideas were also in part a “Trojan horse”, in an era of postmodern glitziness and Prince Charles-inspired classical facades, for the modernist ideal of an expressed structure. Structure and quality of space were, he argued, inextricably linked. Describing Powergen in CQ in the winter of 1991, he wrote: “The deadening effects of flat suspended ceilings have been avoided and the structure will resume its natural status as the means of providing shape and form to architectural space.” This may not sound radical today, but for the times it was something like a manifesto.

The practice quickly gained an impressive client list, with whom it could continue to hone its approach. In the wake of Powergen, Bennetts adopted similar principles on headquarters buildings for John Menzies (1993) and BT (1996), both in Edinburgh, and Wessex Water (1998) near Bath. “They were all looking for this stuff and we were just about the only practice who had done it on this scale. Other architects had done good work on sustainability, but we had actually built this bloody great big building,” says Bennetts.

They explored the same techniques in other sectors: Hampstead Theatre (1994), the practice’s first theatre project, employed displacement ventilation below the auditorium seating. And at Brighton’s Jubilee Library (1999), it incorporated high levels of thermal mass into a large public space through a dramatic arrangement of concrete structural trees. Not for the first time, this project depended on Bennetts’ trademark blend of rigorous analysis and commercial nous. This was the heyday of PFI, a procurement route that tended to favour cost-cutting over good design, which meant that the developer “started chopping out the nice bits. They said, you can cut down the concrete, make it GRP, do something with the glazing. So we had to prove to them that if we did that, the building wouldn’t work. We showed that it would add more running costs, which for PFI is not good.”

If PFI clients were hard to win over, another sector was proving equally resistant to change: traditional London offices. Until the mid-2000s, Bennetts’ commercial clients tended to be end users in discrete buildings away from the urban centre. In the city, the look was still decidedly corporate speculative office market never really accepted these ideas,” says Erridge. “It has taken 20 years for them to filter down to the mainstream.”

A turning point was New Street Square (2002), the first BREEAM Excellent-rated building in the City and its largest concrete-framed development for many years. Post-tensioned concrete floor slabs offered the potential of a thermally massive structure while reducing the quantity of concrete required, and the whole scheme was designed with flexible, expansive floorplates so that the complex could be adapted to unforeseen future uses. This was a lesson that Bennetts had learned from the failures of the post-war modernists, whose rigid designs are often difficult to adapt to highly serviced, computerised workspaces.

New Street Square may have heralded a new era of sustainable speculative development, but one thing hadn’t changed. Just as at the Powergen and Wessex Water buildings, where the internal exposed structure was painted white, the concrete aesthetic was greeted with a certain degree of ambivalence. Many original tenants still opted to fit out their offices in a conventional corporate style, with high-level servicing concealed by a suspended ceiling, rather than benefit from the exposed slab. If the same tenants moved in tomorrow, you suspect they might make a different decision. Over the past decade, exposed concrete has become a desirable workplace look – and a key reason for this is the association with wellbeing and sustainability.

Perhaps unsurprisingly, the practice’s current work embraces this trend. The Royal College of Pathologists in Aldgate, east London, employs a language of crafted concrete throughout the building. Walls are lightened by ground granulated blast furnace slag and beautifully boardmarked – the handiwork of contractor Gilbert Ashe, which was also responsible for the superb finishes at Haworth Tompkins’ Stirling Prize-winning Everyman Theatre in Liverpool. And the various spaces are united visually by a deeply coffered soffit – a direct descendant of the vaulted ceilings at Powergen.“It’s picking up on the lessons we learned from those early buildings,” says Erridge. The coffers give extra height to the spaces, and increase the area of exposed concrete: “For the size of the floor plate, the building has 1.4 tennis courts more surface area than flat slab construction. So you’re able to energise that concrete, and use it through night-cooling to improve the thermal performance of the building.” Moreover, by using less concrete than a typical floor structure, “you’re able to save the weight of 51 Asian elephants”.

Impressive though these figures are, at the Royal College of Pathologists it is the character imbued by exposed concrete that is the material’s most important attribute. “They’re building a headquarters that they want to stay in forever. This is their permanent home,” says Erridge. “The building had to be contemporary but it had to have a degree of gravitas and permanence that concrete could bring, and would be difficult to do with things like suspended ceilings. It had to look like a substantial new HQ for a royal college.” For now, Bennetts seems to have won the argument about expressed structures – his Trojan horse has well and truly breached the barricades.