Brave new re-generation: PRS Architects Coventry’s Herbert Gallery’s new wood-glass gridshell atria
PRS Architects brave new re-generation

Pushing the envelope of massive timber and glulam in city cultural regeneration projects has become London’s PRS Architects watchword. With the 2008 opening of the Herbert Gallery the practice added a gridshell to this agenda

1) PRS Architects
Of all the early British practices experimenting with different engineered timbers in the early 21st century, London’s Pringle Richards & Sharratt Architects – shortened, for the last few years, to PRS Architects or just PRS – stand out in the UK as both the first to use massive cross-laminated timber, this mainstay of continental engineered woods, and one of its more ardent advocates. During the course of their thirteen year existence, the practise has completed an impressive series of buildings all of which repeatedly demonstrated engineered timbers. initially glulam, though from early on, cross-laminated woods – versatility and use in some of the most compelling built environments to have been completed in the decade since the millennium. In the last year, their Coventry Herbert Museum extension confidently extends, underlines and partially underlines a trajectory which began with one of the first in a new wave of major glulam projects in the country, the Sheffield Winter Garden building. The extension, which reaches out from the original museum building, is a light an airy atria space, a project which in timber terms is PRS Architects’ first foray into working with a variant of a gridshell canopy – a diagrid. With the building complete, evolution from the Winter Garden’s glulam to the Herbert museum’s dia-gridshell feels natural and almost inevitable.

Along with Bill Dunster, PRS Architects are graduates of one of the scions of the sixties architectural establishment, Michael Hopkins Architects. Or at least two of the three partners are. John Pringle and Ian Sharratt met while working as partners within the large London practice. Penny Richards, who previously worked in the heritage department of the Victoria & Albert Museum, is married to Pringle. Striking out in the summer of 1996, the threesome got off to a flying start, landing the Sheffield Winter Garden project within two months of forming as a practice. Casting an eye back over the previous twelve years of running a practice, Pringle, who I met at the Gallery in September 08 a few weeks before it was to be handed over, traces his interest in wood back to working on Hopkins’ Portcullis House (2000), the MP’s office building opposite Westminster. Here again, an internal foyer atrium included an oak membered diagrid thrown over a courtyard space.

Though Sheffield was the first major engineered timber structure on their books, this was complemented and overtaken by the music auditorium building the practice designed for Shrewsbury boys public school. Here PRS Architects began their pioneering use of solid timber, almost certainly the first time the material was used in Britain. Pringle traces his first awareness of massive wood to a project in Berlin he was working on, where he was introduced to the material by the then independent Southern German timber firm Merk. “A really exciting technology, recalls Pringle, the practice learnt just how much that could be done with massive wood at Shrewsbury, and as they continued their partnership with Merk, were wholeheartedly converted to the material by the Germans. “It was, recounts Pringle, “a real experience, they had real enthusiasm and an entrepreneurial can-do mindset at the time.

Shrewsbury is one of the country’s oldest public schools – its alma mater range from Michaels Heseltine and Palin respectively, to the current astronomer royal, Martin Rees and the late John Peel; the latter recounting his various school years unhappinesses on the radio over the years. The auditorium serves what sounds on first reading a diverse spread of musical activity, though it would be interesting to have known what Peel would have made of it. Designed in an ellipse, the three-storeyed building is pushed up vertically by its central cupola, itself partially determined by the careful attention to acoustics required. Having studied the design, the engineers Arups Acoustics, came back to PRS saying that the auditorium’s roof needed to be a particularly high volume – impossible to achieve within the £1.7 million budget using a normal steel frame and purlins. Working with the German timber engineer’s Merk design director at the time Michael Keller, the team came up with a within-budget solution utilising solid timber’s acoustic properties to the full. Cut into the school’s sloping grounds the performance and theatre space can hold an audience of two hundred, and ringed by a circular corridor, it has offices along the external wall, each looking out on the grounds.

As far as the use of wood is concerned, the massive timber is only one of several contexts in which different woods have been applied; a shingle roof and western cedar cladding add visible timber credentials along with window and door joinery in larch. Add Merkâ’s cross-laminated ‘Dickholz’ (thick-wood), as the massive wood panels were called at the time, to cover both office and theatres ceilings, and it becomes one of the more interesting timber buildings to come on line in the early days of the timber resurgence of 2001. The auditorium also engaged relatively early with the sustainability agenda in the form of passive heating and ventilation. So even if contemporary accounts focused on the acoustics issues, both materials and site were brought to the fore.

Today when Pringle compares Shrewsbury with Sheffield, Shrewsbury comes out as the major of these two early PRS projects, while Sheffield is the less adventurous of the two – glulam being well known by the time Pringle and colleagues sat down to design the Winter Garden. Yet if it was the Winter Garden which garnered the architectural media’s attention, it is also the most immediately comparable PRS project to their work in Coventry. Both apply timber to culturally-hued structures in the midst of dense, urban rather than rural sites; both are varieties of atria; both exercises not only in timber but in glass-timber hybrid canopies, both finished with carefully executed steel detailing. And finally and more widely, both are using timber in the service of regeneration of comparable midland city centres, each bearing long industrial traditions, which again comparably in more recent years, have been in eclipse and decline; the automobile and steel industry respectively.

2) Sheffield’s Winter Garden glass-glulam catenary greenhouse

As with Coventry, Sheffield’s Winter Garden’s timber structure was envisaged as part of larger major regeneration, which has been ongoing in the city centre under the project title, ‘Heart of the City.’ Much like the Herbert Gallery, PRS won this early project to introduce a special and spectacular regeneration showcase into the larger regeneration programme. They decided on a glass-timber hybrid catenary design to emphasise a city moving away from steel, into a new age of a new material, engineered timber.

Built on the ground of the old town hall offices, the winter garden’s walk through airy atria links the city centre to the heart of its cultural district; including the city library on one side, with the Peace Garden and commercial development, including a hotel, on the other western side. Connected though to one side, are the Millennium Galleries, also part of the PRS design, a series of art spaces, which aim to complement the cultural zoning of the public space. The seventy metre long by twenty two metre wide garden contain five plant beds, which are planted with a variety of species more normally found in Mediterranean climates.

These range from cactus, to ten Trachycarpus fortunei palm trees, as well as the tallest plants in the gardens, Norfolk island pines which are over six metres in height. In all, around 150 different species of plants can be found in the winter garden beds. To accommodate these differing specimens, particularly the tall pines, PRS and the project engineers Buro Happold decided on a greenhouse structure, which would rise taller at its centre. The resulting timber-glass house, begins at eleven metres at the entrance and climbing through three further heights until the three central bays reach up over twenty two metres. The use of glazing is again comparable with Coventry, although the majority (80%) of the 1400 glass panels are identical, divided by the ten elegantly curving glulam arches, each sitting 10.7 metres apart.

Taken together, the timber-glass hybrid makes for a striking contemporary greenhouse while also foreshadowing the more complex Herbert Gallery atria. According to the Buro Happold engineers, the inverted catenary structure – the natural form or shape a chain or piece of string makes when loose to dangle freely between two hands – is well suited performance-wise to such a building-type. It provides the largest volume while using the least mass. And as the material is glulam the engineered wood is untreated, a precondition for a structure holding plant-life. All the glulam arches, purlins and struts originally came from Polish larch, and were prefabricated at Merk’s German factory. The largest was 24 metres long, and 900 mm deep, a scale which required following European technical guides as at the time this was outside UK guidelines. Sitting on a concrete floor slab, the glulam arches join to the ground with relatively simple galvanised steel connectors, such steel-timber connector details being a PRS speciality.

Aside from the showcase element, PRS and Buro Happold’s decision to apply a catenary structure came down to its engineering efficiency, along with the need to maximise space. At the time, Matthew Lovell, Buro Happold’s project design engineer, was quoted in the company promotional material as stating how, ‘the catenary arches were developed to achieve predominantly axial forces with low bending movements.’ This gives an extremely efficient structural form, reduces the material used and results in a structure that is ideally suited to engineered wood. This was borne out in jointly funded sustainability research on the building by the DTI and Buro Happold, which compared using glulam to steel and concrete. The research results demonstrated that the Winter Garden used 95% less energy than either of the other materials, while the lightness of the glulam was 65% the weight of steel and 15% of concrete, reducing costs in materials, transportation, including those needed for the foundations.

Today the Winter Garden remains one of the highlights in the complete upheaval and remaking of much of Sheffield’s city-centrescape. The glulam arches rise up boldly when the visitor steps inside to walk among the tall exotic plantlife. However with building having continued apace in the city centre, until the economic downturn in 2008, the open public space surrounding the Winter Garden has been eaten away; and now this timber greenhouse seems a lone oasis in a desert of concrete and steel mid and hi-rise offices. Its relation to the site has disappeared, and the sense that timber might have been a future in this archetypal British steel town, has all but disappeared.

But that was almost a decade ago, for the building and for those involved in its design and realisation. What seems clear is that the Winter Garden foreshadowed much of what the Herbert Gallery and the Coventry masterplanning is doing nine years on. There is much that is comparable. Pringle says that the Herbert Gallery diagrid is significantly more complex than Sheffield, in part because the uses for the Herbert Gallery are that much more complex, with its bridging function between two other structures, the external facing post and beam reading room, and inside, the pre-cast concrete galleries.

3) The Herbert Gallery gridshell atria

The Herbert Gallery, originally part of Coventry’s ‘brave new reconstruction’ is a post WW2 avant-la-letter regeneration designed by Herbert’s relative, Albert Herbert, an architect based in Leicester. In the years since the 1960 opening, the Herbert Museum has become a significant civic feature in efforts to raise a new post-war heart to Coventry’s city centre from after large sections of the city were flattened by the waves of airborne bombings. The weighted language of late twentieth century peace iconography is present – the city is twinned with Dresden and Hiroshima, providing a sense of solidarity to the cities of the world razed to the ground by machinery of war. The most dramatic mark of this half-risen phoenix from the rubble and the ashes of Second World War bombings, overwhelming in its proximity, is Basil Spence’s remarkable post-war Cathedral; which turned a sacred burnt-out ruin into a defining exemplar of iconoclastic modernist gothic. Today the ruins shell remains next to the new Cathedral.

The refashioned Peace Garden also relates to this memory of modern devastation lodged between the extension’s new historical resource and reading room, and the old Cathedral approach; within it a two hundred year old Olive tree growing. The Peace Garden is shadowed by far older history, the brown bricks of Bayley Lane’s far wall traces back to medieval times. The pathway dividing reading room from Peace Garden is filled with rust laden, oddly angled Cor-Ten semi-sculpture, cutting into the ground recording various historical figures who lived in the lane.

PRS’s aim was to turn this part of the Gallery, in effect the rear of the original museum into a new entrance and approach. The key to this strategy is the diagrid arcade, fifty metres in length and 12 metres high, drawing people in from the revitalised University Square in front of the Cathedral. A collaborative effort, the square itself was designed by Coventry City Council architects, while Edward Hutchison Landscape Architects focused on the Herbert Gallery’s forecourt and the Peace Garden. The square’s redesign features new fountains and lighting, as well as street sculpture, and is one of three recreated squares the city council have brought back to life. The arcade itself is one of the three core elements to PRS’s extension, bridging between the new two storeyed galleries and up to the minute hi-tech reading room. Pushing out from behind the original Herbert Museum, the new gallery spaces are complemented by educational, office and other new spaces in the 1720 m sq refit. This has gone up on unused and reused ground; a mix of brownfield and the Mandela Building, a piece of 1960’s brutalism which has disappeared, Pringle and his team emphatic that it related to neither cathedral or museum.

Both the new version of the museum and the recreation of the public space between Cathedral and gallery is part of a larger redevelopment of Coventry, masterplanned by international regeneration specialists Jerde. As the eleventh largest city in the country, yet ranked 43rd for retail trade, Coventry feels that it is in need of a massive makeover; the cultural regeneration around the Herbert and Cathedral is only one of several projects underway, complete or about to break ground. Arup Associates are finishing a new computer centre for the University, while MacCormacJamisesonPrichard have rethought one of the Cathedrals’ other approaches. For Pringle, his involvement with the Herbert sounds hard enough, when recounting the funding journey the project has endured; the word “tortuous passes his lips several times. Beginning in 2002, the funding streams were principally from the Heritage Lottery, ERDF, the regional development agency – Advantage West Midlands, Coventry City Council – (who pulled out their funding at one point, but returned), the DCMS, the Wolfson Foundation, plus two HLFL backers. Originally part of a four-phase body of work, with initial interior modernisation and roofwork done in phase 1 by Howarth Tompkins, what began as the remaining three phases gradually coalesced into one overall final phase, a piece of work worth £11 million for PRS Architects.

The two storeyed ground and first floor gallery spaces, draw the original museum gallery outwards towards the Cathedral and square, with the glazed arcade court running along its side – the timber shell canopy used to great effect. Arcing over the atria, the barrel-vaulted diagrid reaches its full 12 metre height before rolling down and morphing and meshing with the buildings remaining section, the Archive and Reading Rooms post and beam structure. Pringle describes the extension as an opportunity to use both massive wood and glulam together. “It’s a hybrid structure, which is much more interesting than an absolute pure structure. At the meeting point between the two the glass wall partitioning arcade from reading rooms – the falling gridshell is met by curved LenoTec massive timber, folding to complete the second part of the roof’s S-shape which turns away to join with the reading room’s portal frame roofing. Where the shell meets the glass partition to the reading room, a series of four tree-like clusters of larch posts hold the weight of the shell’s downward force. On the reading rooms external edge the spruce glulam struts pull out at 45 degree angles cantilevering the portal modern ‘catslide’ frame.

The simple elegance of how the diagrid changes into the curved LenoTec panelling could not been achieved without the development of the curved cross laminated panelling by FinnforestMerk at their South German centre, Aichach. The result transforms the potential for fluidity of form, while also demonstrating what technology can give to such fluidity. This is the first time the curved LenoTec has been used in Britain, although the Finnforest material has already been applied at both the recent Charles De Gaulle airport concourse building near Paris, and Finnforest’s own Tapiola Headquarters outside Helsinki. Pringle acknowledges that inevitably, this variant curved cross-laminated panelling is more expensive, while adding how it has allowed for much more interesting architectural expression.

The diagrid shell itself makes for a chequerboard of diamond shaped glazed and portal spaces, which become increasingly interrupted by the panels. At the arcade’s university square entrance each of the diamond’s are glass. Quickly, however, solid boards are introduced, gradually increasing in number in relation to the glazed transparent boards, as one moves closer in towards the museum’s formal interior entrance, until, at the crossover entrance into the old building, the boards have become completely panelled. The service engineers SVM and lighting consultant from Lichttechnik, Martin Klingler, have had fun here, careful computer analysis ensuring that the most is made of natural sunlight pouring through the open glazed sections; achieving significant solar gains. Light is filtered down from 70% penetration at the entrance to 10% at the gallery end. Why PRS were especially interested in a diagrid for the arcade is partially for reference value to Spence’s Cathedral, which contains some of the more astonishing concrete diagrids completed by Ove Arup’s engineers.

Underlining the buildings civic qualities, as well as the common use of engineered wood with glass, Pringle is unequivocal in comparing the extension to Sheffield’s Winter Garden. The argument is familiar; the expressiveness and engaging qualities of engineered timber works well with civic buildings; the public is drawn in rather than pushed away. The aim of extending the open external public space of the Peace Garden immediately outside flows through into the internal shared space of the arcade. People, and here he recounts the usual perceptual elements, respond very well to timber; its colour, feel and warmth are all characteristics that work well with public spaces. This is reinforced by the glazing – which makes it all the more inviting as a public space.

Inside the arcade the airy openness of the gridshell diamond’s structure is immediately felt, added to by the spacious light feel of the space. If the Herbert shell doesn’t contain the same symmetrical feel of Sheffield’s Winter Garden – where the glulam beams are geometrically equal “there’s something comparable in being immersed in a timber structure in the midst of a larger stone and brick city. Both buildings represent and are vehicles for a cultural dimension, while standing close to City Centre shopping and the like. And there’s implicit acknowledgement of this in Pringle’s talking both of ‘user friendliness,’ and in a slightly odd turn of phrasing, “pedestrian friendliness. Indeed, an obvious next further use would be for shopping arcades, if a particular developer/retail group proved willing to stump up the extra funding needed. Asked about this, Pringle relates the arcade typology back to the extraordinary Vittorio Emanuele Galleria in Milan, a maze of covered shopping arcades built towards the end of the nineteenth century. Whether the atmosphere of buildings such as the Herbert gridshell with its light and airy feel would change significantly were they used for purposes at the more profane commercial end of this typology spectrum, compared to its usage here, something of a sacred space which arts and culture buildings are so identified with, is a point which could be long argued over. The religious undercurrent involved in Destination Art can, and does, easily lend itself to our other modern religion, that of consumption.

As with many of PRS other building’s there’s a tried and tested pragmatism to the practices’ sustainability. Massive cross-laminated timber may now be a core part of PRS’s materials vocabulary though it only one amidst a larger materials palette. Look at the care with which the steel-timber detailing interface is managed, part of the PRS trademark. And also the delicate use of the light white washed concrete for the gallery walls and ceilings. More complicated are the inner gallery spaces of the Herbert, as with the entire museums typology, a less than straight-forward building type. As containers of many old and sensitive objects, museum buildings often require artificial controls, meaning that they and other public arts buildings are generally not particularly effective in sustainable terms. At present, says Pringle, there is no common standard, and usually the best such arts buildings achieve BREEAM (the British Research Establishment’s environmental code of standards) won’t come up above the ‘very good’ category. This said, as with the Shrewsbury music auditorium, the Herbert Museum public areas are naturally ventilated and reliance on mechanical systems is minimised; primarily the result of using thermal massing to closely control the air. Both arcade and gallery walls use a high-end precast concrete Hibex, while the arcade floor is terrazzo from Malta, because, says Pringle disarmingly, it is cheap.

The buildings sustainability strategy, Pringle says, is in significant part, the result of research carried out by Professor May Cassar on the sustainable dimension of both older and contemporary buildings. Cassar, who is head of the Sustainable Heritage Department within the Museum and Galleries Board, found that using mechanical or centrally controlled heating was akin to using a steamroller to crack a nut, the rate of temperature and humidity change was more significant than keeping temperatures at an absolute and unvarying levels, which the mechanical heating control systems attempted to do, in expensive and not always completely effective ways. The result being the level of investment in kit was completely out of proportion to the variations in temperature – often only half a degree. Given that these were major parts of capital costs, the fact that Cassar’s research showed it is possible to reduce the reliance on mechanical systems is in many museums and arts buildings a persuasive option in many instances.

Before Cassar’s and colleagues research became known, the primary approach to temperature and ventilation was to use sophisticated, centralised and hi-tech building control systems in museum buildings. Although relatively effective in preserving temperatures at various preset levels, they were not flexible when it came to varying temperature and varying ventilation due to external weather. Providing scant if any thermal resistance, arts buildings have invariably overheated in summer while freezing during winter months. With buildings such as the Herbert Gallery the natural ventilation integrates inbuilt self-regulation, a simpler but effective system. “Its a solution we really like – and an interesting building type.

The Herbert Gallery, the first gridshell, adds to the practices increasingly lengthy list of timber projects which integrate timber into their mainstream and pragmatic sustainable approach. At present the practice are completing a new generation of London bus garages for Transport for London. When completed West Ham garage, part of the build up for the 2012 Olympics, will be, the PRS website claims, the largest bus garage in Europe. The garage includes widespan glu-laminated timber roofs, as well as various other sustainable features “ biomass boilers, a heating and power plant and a wind turbine – which together is over the 20% onsite renewals requirement from the Mayor of London’s office. Half of the roofs upper side will be given over to a plant life, part of the garage’s SUDS system and improving the site’s biodiversity. Other cultural regeneration projects similar to Sheffield and Coventry also in the pipeline, are the Hull History Centre which is onsite, and the Newlyn Fish Market and Processing Centre in the West Country. Both extend aspects of the glass-timber arcade typology or use of cross-laminated timber present in Coventry. Pringle and Coventry’s project architect, Malcolm McGregor have also used timber panels on two smaller housing projects; for Pringle in Carlisle Lane, Waterloo and for McGregor in Muswell Hill. So far the practice, which has veered towards public projects has not used the material in any larger housing projects.

While the Herbert extension’s atria courtyard is indeed a new gridshell, Pringle, rightly sees the Coventry arcade as separate to the lineage that begins with Otto in Mannheim, continuing through Weald and Downland, and up to the present day with Savill Gardens. The museum fits into the larger body of PRS’s cultural and arts projects, which in turn hark back to Pringle and Sharratt’s early professional origins at Michael Hopkins. There may be sizeable differences, but the similarities in both the tastefulness and detailing skill by which Hopkins projects can be identified, has been extended by PRS. Hopkins may have always erred towards the exclusive end of potential clients, while PRS are known for specialising in the public sector (even if their Shrewsbury music centre saw the practice working for one of the most elite parts of British society); but both Hopkins and PRS are cut from the same cloth when it comes to detailing, particularly the fine weave of joining such a new old material as timber with modernist steel and aluminium. Within the Herbert Museum’s gallery this mastery is tellingly on display. By contrast both Mannheim and the Weald and Downland Museum gridshells, adhere to a certain primitivism. And while Pringle acknowledges their contribution to the increasingly sizeable vocabulary of gridshells – which can be extended by the arcade’s hybrid form, with its first application, at least in the British contexts, of curved LenoTec panels, it is the mix of signature detailing of industrial materials with industrialised timber aligned to undeniably modern urban contexts which places PRS Architects experiments with shell structures in a category of their own.

Oliver Lowenstein – November 2009

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© Annular 2009