High-Tech and Highlands

The Macallan looking westwards – Photo RSH+P

RSHP’s Las Arenas stadium deck, Madrid – Photo RSH+P

Scotland’s high-tech, timber-shell deck marries sophisticated engineering and computing power with geometric and timber complexity. Whether the well-publicised gridshell roof is a true grid or not is another matter.

Many architectural practices have wanted to design gridshells, the elegance of their design engineering idea casting a seductive hold over the architectural imagination. Yet, when it comes to realisation, what might be called ‘true’ or ‘pure’ gridshells’ are few and far between. Rogers Stirk Harbour + Partners (RSH+P) is a case in point.

Although, material-wise, the one-time Richard Rogers studio are the archetypal high-tech steel and glass practice, the use of timber has increasingly appeared across their more recent works, dating back to the turn of the millennium and the Welsh Parliament building completed in 2005 – a part of negotiating a move into eco-tech. For the most part, their large-scale wood projects have deployed timber for roof structures, integrated into the trademark steel and glass. If the flagship Mossbourne Community Academy (2004) is really an exception, one can look to Antwerp Law Courts, Madrid’s Barajos Airport terminal 4 (both 2005) and the Las Arena’s Bullring Stadium (2011) with its timber dome, dropped from on high, over the Barcelona arena. But up to this juncture there hadn’t been a gridshell amongst RSH+P’s portfolio. Not, maybe, for the want of trying.

This changed in 2012 with the announcement that the international studio had been chosen as architects for a new, twenty-first century distillery in Northern Scotland. From the outset onwards, the Macallan Distillery has been described as a gridshell structure, continuing with its promotion since completion in 2019 and the project winning a string of awards and written up as one of the most complex pieces of timber engineering of the last decade.

Yet the description is a sleight of hand. If one defines a genuine gridshell as a form that structurally supports a building (or structure) through a single grid form which combines walls and roof assemblage, then the Macallan visitor centre isn’t a gridshell. Despite the popularity of the form, there have been very few ‘true’ gridshells at scale. First, Frei Otto’s pioneering Mannheim Multihall (1975), followed by the resurrection of the form by Edward Cullinan Architects at the Weald & Downland Museum (2000) a quarter of a century later. Since then, the term has been applied to larger projects such as Glenn Howells’ Savill Gardens visitor centre and WilkinsonEyre’s University of Exeter Forum building, while the designs stepped away from the original gridshell structural principals. Here, latitude in defining gridshells often appears, and behind the latitude there is also acknowledgement of just how complex the original, ‘pure’ gridshell form is to realise. When the point is put to those involved, this is very much the response the Macallan engineering team adopt. Paul Edwards, the lead Arup engineer on the project, calls it “a form of a gridshell.” Similarly, like Edwards, Johannes Rebhahn, head of engineering at Austrian timber engineers, Wiehag, equivocates regarding Macallan’s relation to the genuine article of a self-supporting gridshell structure. He talks of there being many architectural gridshells, describing the shell deck as a grillage, before adopting a more colourful turn of phrase: “It’s a waffle.”

From above – Photo RSH+P

Domes over the mountains – Photo RSH+P

“It took a while to get there,” says Edwards, who took on the lead engineer role after joining Arup’s Building & Engineering 3 (BE3) team in early 2013. Edwards, with RSH+P’s project architect, Toby Jeavons, began looking at “gridshells and all kinds of related forms” and worked on the internal connections, explaining “the key drivers were to achieve the clear spans.” Awarded the competition in 2012, the project went quiet for about a year before getting going again in early 2013, with work on the roof concept continuing through the year and into 2014.

The original design envisaged the double curvature of the gridshell using curved beams. This would have produced an integrated deck comprising two interconnected layers. It was also an unenviably complicated structural solution. After considerable work though, and with challenges still unresolved, Graham Stirk – the S in RSH+P and senior architect responsible for the project – decided to cast aside the original gridshell design and modify the deck by turning it into a faceted deck system. Separating the roof structure from the lower shell structure helped simplify the upper roof design. Out went the structurally feasible, curved primary and secondary glulam beams – both structural challenges and also influencing the cladding design and its two-way curvature. In came reworking the faceted deck system design for straight glulam beams. The separation of the primary and secondary layers simplified the overall design considerably, each becoming more independent engineering sections. But any pretence that this was a pure gridshell also disappeared.

The shell domes continued to provide an elegant roof structure strategy over the heart of the whisky distillation process. The five equidistant and symmetrical timber dome mounds – 11.5 metres tall and 36 metres in diameter – run the full course of what is, for the most part, a glorified twenty-first century, high-tech industrial building, even if sparkling, complicated and new, high-tech. Four of the dome mounds form the shell-like roofs above the two-storey production level, under three of which stand eight copper fermentation stills organised in circular arrays on the upper floor. Larger steel washback vessels rise up from the ground floor below, within which malt ferments over a 55-hour cycle before being released into the stills. It’s here inside these dramatic copper onion drums that the malt matures. Above the shell domes are roof opening vents for fermenting air to escape. It is a potent brew, literally. At the far, northern end, the fourth shell sits over the mash tun, in effect a seventeen-tonne machine where the delivered barley sits for two days decomposing, before being crushed and mixed together ready for the next washback stage.

The fifth shell is at the building’s southern entrance end, and is significantly taller than the others, arching over the public visitor centre. A fire-resistant glass wall runs crosswise, marking the boundary between the building’s public and working sections. The glazed wall – which is 10 metres tall by 40 metres across, with the glass separated from the timber deck by cantilevered steel – features a high-tech and bespoke water drenching system to cool the water and prevent it cracking. The deck consists of two layers, running the entire course (207 metres by 63 metres) of the rectangular grid strip. The upper layer holds the relatively light, grass roof and provides space for insulation; the lower timber grillage is separate. Viewed from inside the building, the shell-like grillage’s underside is the heart of the timber drama. The undulating shell domes rhythmically punctuate the whisky fermentation stills, ending with the taller dome mound over the visitor centre. Yet the deck’s wavy grid underside was arrived at after endless engineering hours, including the reset to its design principles. 

Renders of the waffle shell structure – Photo Wiehag/Arup

The five shells – Photo RSH+P

Two domes up – Photo Angus Bremer/RSH+P

The broader implications of a pure gridshell weren’t solely structural or limited to detailing. Fire risk required consideration, given the distillery was to be a functioning industrial building. Apart from the glazed wall separating visitor from work-site areas, gases employed in the brewing process means the centre required explosive gas category classification. A web of safety measures, including different types of fire detectors and related systems which are normally attached and integrated into roofs, have been designed into less visible parts of the building.

As it was, Stirk’s decision, rationalising and embracing the faceted deck system, eased meeting the fire requirements, but it also sidestepped the question of how a continuous integrated deck negotiates the walls within the public and factory areas inside the structure. For Edwards, and here he likely speaks for the Arup team, a ‘true’ or ‘false’ gridshell is beside the point: “The key thing is to use timber. It is an historic and known construction material…and we’re interested in what it can do.”

By 2015, the Austrian timber engineer, Wiehag, had been brought into the timber engineering, appointed over the other two major continental timber players on the final short list, Zublin and Hess. Edward’s notes how, once integrated into the team, useful Wiehag’s structural team were, particularly with the parametric modelling – another factor influencing committing to the facet design. RSH+P had already been using BIM on aspects of the structural geometry in the design’s early stages, while Wiehag had already developed parametric design, working with Arup on the Fosters Crossrail station gridshell in Canary Wharf. This was built on for Macallan across the structural design, geometry and building physics.

While the faceting helped disconnect the roof structure from the grillage, much of the complexity of the design and engineering remained. An immediate focus was the newly straight-faceted glulam beams, from which the roof deck was composed. Organised around a uniform 3 metre by 3 metre grid, running east to west, the deck comprised two layers of primary and secondary beams, in tension and compression respectively, and with spans reaching up to 30 m2. Given the changing roof slope across the deck, these beams are varied in length: the longest, 3.7 metres, where the slope reaches 35 degrees. A mix of 200-millimetre wide and 750-millimetre deep, plus 930-millimetre deep beams for heavy load areas, the beams have been manufactured by joining glulam cores to LVL boards stuck onto each side. In addition, hybrid beams, prepared for high compression, integrate long steel rods where forces and geometries were more complex. The beams create the roof’s dazzling honeycomb web effect, but overall, according to Rebhahn, the deck was “not so important.” To the north, its edge-points land on the steel frame running along the rear of the building and cover a one-way ramp and delivery/pick-up bay for lorries and other goods. On the eastern face, the deck joins a concrete retaining wall, and on its south face, lines a one-dimensional steel frame integrated with supporting steel pod beams. Again brought on by the design rethink, Wiehag worked to develop an engineering solution, integrating the angular steel tubes with the timber beams that jutted out as part of the canopy overhang, above the external passage along the side of the glazed façade. On the western face, the deck’s timber grillage extends outwards, supported by a sliding bearing at the retaining wall, which helped with early preparatory earthworks on the site. All throughout, the deck’s glulam beams are consistently the same lengths, but required individual preparation as each respond to specific forces.

Connector testing at Wiehag – Photo Wiehag

From above – Photo screengrab from RSH+P video

Render Wiehag – Photo Angus Bremnar/RSH+P

Finnish triangle in time – Photo screengrab from RSH+P video

Under construction – the glulam roof – Photo Angus Bremnar/RSH+P

The upper roof deck is covered in removable, triangular, Finnish Metsawood Kerto LVL cassettes, with two filling cassettes taking up a single panel in the grillage. The cassettes are made up of a 9-millimetre thick top layer, 75 millimetre by 200 millimetre softwood joists and a 21-millimetre thick, LVL soffit. This simpler panel system, enabled by the facet design, is all out of view of the undulating lower grillage. The Kerto also acts as a vapour control barrier and can, if needed, be removed and recycled after 25 years. The panel cassettes still required bespoke milling to individual dimensions – determined not only by their position on the deck, but because each panel went through a building physics, number-crunching exercise to ensure snow-loads, wind-drift and other variables were accounted for.

Wiehag’s waffle grillage sits on the underside of the roof deck and is supported by a steel structure. The focus was on ensuring the beam’s efficiency by optimising the different geometries and forces of the rigid connections at each end of the beams, and on getting as close to a point of inflection as possible – although to maintain consistent detailing, the connections didn’t always quite meet the zero-moment lines. A variant of a Wiehag double bevel was developed and tested before being used as the main connector node. In places, modified beams with a CLT core and LVL cheeks are employed, where support for heavily-loaded sections of the grid is required – part of a broad repertoire of timber materials including hybrid steel-timber, gable and external beams. In all: 1,798 single beams (with 742 different geometries), 493 facetted beams, 1,305 straight beams and a rather astonishing total number of single beam (including bolts) elements – 262,756.

Even so, the complexity of the structure had been significantly paired down. Each section of the domes went up one by one, running north to south, and are structurally separate.

Impressive as this may be, the timber doesn’t carry the entire deck. Each dome is supported by steel ring beams, steel portal frames held up by V-columns, which also carry roof forces down onto rigid concrete supports. Although the engineers needed to know the roof’s forces, calculating every element in tension or compression, the faceted design led to fewer, simpler and separated out connections. This, again, was another element allowing the roof’s upper layer to sit separately from the grillage. In Edwards’ words, “we were working the complexity away from it…generating a solution that can be constructed from a single structure.” A further consequence was that there was comparatively little form-finding to arrive at the shell’s geometries.

Arup worked iteratively on the steel and tension ring beam connections to timber, to meet at the most suitable inflection points, where forces in in-compression – the shells – met those in tension – known as the zero-moment lines of the dome.

Literally hundreds of thousands of timber components were needed: 380, 000 is the figure provided – leading to the claims that the Macallan building is the most complicated recent example of timber engineering. Edwards prefers ‘sophisticated’, and notes that, while the numbers are accurate, there were only a small number of timber systems, for instance five or so triangular, Kerto panelling systems. Despite these boggling numbers, the main timber work, which began in March 2016, was complete within six months. Both Edwards and Rebhahn report the surprise among some at the speed of this stage of the building programme.

Photo Angus Bremnar/RSH+P

The undulating waffle diamonds in their full glory – Photo RSH+P

Photo – RSH+P

Though the grid deck is a technical and visual centrepiece, timber is only one material among many deployed by RSH+P at the Macallan. There’s certainly a lot of concrete, and the high-tech steel and glass signature is immediately recognisable, though with the added engineered-timber dimension. How much the Macallan’s timber waffle grid brings to both the UK’s ongoing dalliance with the gridshell form and, more generally, the twenty-first century developments of timber, is an open question. It was massively expensive, massively complex and maybe a massive distraction. It is certainly very much a complete one-off. There are not too many timber build contexts out there where budgets of the kind available to the Macallan team exist to pour into such involved and complicated projects. The complexity – or what Edwards would like to describe as sophistication – is very much in the Rogers/Arup eco-tech tradition, and closer, in some respects, to the high-tech industrial buildings both practices are identified with, than with the current edges of twenty-first century timber architecture. The building also continues the engineered, gridshell-variant lineage seen in Glenn Howells’ Savill Garden and WilkinsonEyre’s Exeter University Forum projects. Like the Macallan Distillery Centre, neither of these were ‘true’ gridshells in, what might be called, the classical sense, but they have found their place in ‘the cult of the gridshell.’

For Edwards and Rebhahn, and indeed all the main parties, the opportunity to work on such a one-off project appears to have been a challenge to be relished. Rebhahn describes his involvement as “a once in a lifetime opportunity.” With responsibility for one of the largest and most elaborate timber deck structures realised in Britain, this was a unique project, and “the only gridshell-type timber dome Wiehag have done.” Edwards, choosing his words carefully, suggests that the Macallan “satisfied the cult without being in full obeisance to the consequences of the form.”

Where this strand of faux- or gridshell-lite goes, is an open question. As Glen Howells observed some years ago, “in material terms, gridshells are incredibly lean. In terms of time, design and commitment, they’re very hungry.” The considerable early design and engineering hours, including the rethink, on the Macallan project is evidence enough. Arup’s Edwards is making a similar point when he suggests there aren’t exactly a surfeit of private organisations looking for supersize, timber gridshell architecture. The material and engineering concept may be sustainably elegant, but other factors rather mitigate any rush for projects in the next years. So, despite architects’ and engineers’ fascination with the form, the next chapter in the story is likely to be another deep-pocketed, big project somewhere far away, rather than any regional routes into mainstreaming. Still, this likely won’t stop professionals’ visits to Scotland’s High North and the Macallan Distillery further feeding this arcane fascination. The cult of the gridshell lives on.