Touching the void at Glasgow School of Art

Glasgow School of Art’s Reid Building features three concrete cylinders – or driven voids – designed to bring light into the building. As Stephen Cousins reports, the challenges were many. Photographs by Alan McAteer.

It’s hard to think of a more intimidating construction project to take on than the £50m Reid Building in central Glasgow. Not only was it designed by New York’s Stephen Holl, one of the world’s most creatively ambitious architects, it is also located directly opposite a masterpiece of modern architecture, namely Charles Rennie Mackintosh’s Glasgow School of Art building.

Built in two phases between 1897 and 1909, the Mack, as it is affectionately known, is considered by many to be one of the most influential buildings ever constructed in the UK, characterised by its asymmetric facades of solid sandstone, intricate detailing and dark wooded interiors. The building achieves much of its drama by being perched on a steep hill, the verticality accentuated by soaring gridded oriel windows on the west facade, which introduced a new style in 20th century European architecture.

The Reid Building, named after the GSA’s former director Seona Reid, stands on the other side of Renfrew Street and extends the same length as the Mack and to roughly the same height. It was built by a team headed up by main contractor Sir Robert McAlpine and multidisciplinary engineer Arup, to rehouse the school’s design department. Like its illustrious neighbour, it is focused on providing light and spacious studios.

Rather than attempting to emulate Mackintosh’s architecture, Stephen Holl’s design, developed in collaboration with Glasgow-based JM Architects, aims to achieve “complementary contrast” with the original building, which he describes as “a symbiotic relation in which each structure heightens the integral qualities of the other.”

Thus the Mack’s light steel structural frame and heavy stone facade is “mirrored” in the Reid Building by a very robust cast in-situ concrete frame covered by a translucent skin of glass, which wraps around the studios and extends over the external walls as a rainscreen. Where the Mack is characterised by its intricate details and ornament, the new building presents block-like glass facades to the street and monolithic exposed concrete walls inside.

Construction of the concrete frame was extremely challenging, thanks to a complex design where every space is unique, and wall lengths, widths and floor heights vary across the plan. Most complex of all was a row of three huge 5m-wide concrete cylinders that penetrate through the five storey frame on a sloping incline from the ground floor right up to roof level. Described by Holl as “driven voids of light”, the cylinders are designed to bring light into the centre of the building, while also helping boost the building’s natural ventilation and providing structural support to resist vertical forces and wind loading. In the basement, the voids also function as large cylindrical water tanks for the sprinkler system.

Complex geometry

“The geometry of the driven voids was very complicated,” explains Peter Unwin, project manager at Sir Robert McAlpine. “Each one rises from ground level on a 12 degree incline from vertical towards the Mack to the south, and in the basement cranks in the opposite direction at the same angle. The walls include a series of jagged holes, as if cut out by scissors, and Holl wanted each driven void to have a completely smooth concrete surface free of formwork marks or tie-holes, which meant devising an innovative method of casting.”

The project followed a traditional procurement route. Steven Holl led a design team, including Arup, to win the original RIBA competition. Subsequently, Sir Robert McAlpine beat off bids from five other firms, including BAM and Laing O’Rourke, to win the construction contract and were appointed directly by the GSA as a traditional contractor under a Scottish Building Contract. Piling work started on site in January 2012.

Clockwise from top: In total the frame used 3,000 cum of concrete; the pre-cast concrete “slow stair” meanders through the centre of the building; the cut-out holes had to align perfectly to the radius

“We chose McAlpine because it understood quality was going to be an important element, especially on the massive in-situ concrete structure, and when we marked it through the tender process it was obvious it had some experience in that area and its design team was pretty clear in terms of what was necessary,” said Douglas Anderson, project manager for GSA.

In section, the building comprises two long blocks located each side of a central space – a stack of studios on the north side with angled windows designed to maximise access to diffuse north light, and a stack including a refectory and office spaces located on the south side facing the Mack.

The entire central zone is designed as a social space to encourage “creative abrasion” between students. A network of galleries, ramps and meandering staircases rise through it, occasionally intersecting with the driven voids to provide views of the changing southern light reflecting off the curving walls, or of creative work going on inside the studios.

The western end of the building was complicated by Holl’s decision to retain the early 20th Century Assembly Building, a three-storey stone building that has provided a long-term home for the student union. As a result, the upper storeys of the Reid Building dramatically railroad over the top of it in a partial cantilever. 

Optimal angle

The layout of the building’s concrete frame is based on a 15m grid, interrupted by the three driven voids, which are inclined at an optimal angle to allow southern light to bounce down into the building, even on overcast days.

But the inclined tubes, in combination with a cantilevered overhang on the top floor of the southern facade, cause the whole building to slew forwards under its own weight towards the Mack.

“It took a great deal of thought and analysis to understand the convoluted load paths and restrain the building from toppling forward,” explains Derek Roberts, lead structural engineer at Arup. “The solution uses the walls on the north side of the building as a restraining arm to pull the driven voids and cantilevers back and prevent them from falling forwards. The huge overturning thrusts from the overhangs are taken through the perforated driven void structure and into the rear Vierendeel trussed walls – all this needed to be designed and constructed as a monolithic whole to work as intended.”

The driven voids were on the critical path for the construction programme and each floor-to-floor section had to be poured and cast in advance of any abutting walls or the floor slab above.

“Each driven void required eight pours, so 24 in total, and it was like a game of chess moving the shuttering around as we rose through the building,” says Sir Robert McAlpine’s Unwin.

Above: The Reid Building’s core is a robust cast in-situ concrete frame. Below: bespoke steel formwork used to make the voids

The complex geometry of the driven voids and the need for a blemish free surface led SRM and specialist concrete subcontractor Cidon to develop bespoke rigid steel formwork. This system comprised three steel sections, two for the inner surface and one for the outer, which were erected and filled with self-compacting concrete, then dismantled and erected again in stages to create each floor-to-floor section of driven void.

The rigidity of the steel negated the need for through ties connecting the outer shutter to the inner shutter, as used in a traditional formwork wall, and ensured that no tie holes were visible in the finished structure. In addition, casting the concrete from floor-to-ceiling meant horizontal joints between floors were concealed behind the floor slabs.

“The formwork was reminiscent of shipyards in terms of the shapes and sizes of steel used. It was incredibly rigid so both the inside and outside shutter could stand independently without the need for ties,” says Unwin. “We were concerned that the technology would work and the finish would be perfect, so we built a full-sized mock up in our yard, which worked perfectly.”

The cut-out holes in the walls of the driven voids are freeform in shape and had to perfectly align to the radius, which meant rejecting traditional timber void formers in favour of precision cut CNC-manufactured ones made in a factory in the Midlands directly from Arup’s 3D Rhino model.

“The architect pushed us as much as they could on the driven voids, to deliver their vision and the permeable connectivity between the creative spaces without weakening the structure, which was central to the client’s aspirations for the building,” says Roberts. “However, there were some pinch points where clever reinforcement detailing was needed to get the forces to balance out. Steven Holl had a very clear strategy as to how they wanted the geometry to provide the holistic ventilation and daylighting aspects of the driven voids, and this really pushed the structure, resulting in a strong collaboration with Arup and Sir Robert McAlpine when planning the structural build.”

In contrast to the driven voids, the vertical walls in the building have a rough finish, created by nailing 2-inch deep softwood battens horizontally to the shuttering. The porous nature of the wood helped emphasise the grain on the surface of the wall and also smoothed the consistency of the concrete, unlike plywood formwork, which would have created pock marks in the surface.

A hard stair

The scheme’s main pre-cast concrete element is the “slow stair”, which meanders up sporadically through the centre of the building at a 19 degree angle. McAlpine decided to separate the stair treads from their supporting spine beam, developing an elemental system in collaboration with the precast subcontractor and Arup. Adopting this method meant the spine beam could be prefabricated as a unique element, with the treads formed separately as identical zig-zag profiled units, each providing four stair treads prefabricated in a repeated mould and then bonded to the spine beam on site with a special epoxy mortar. This brought a greater cost efficiency to the units and eased cranage restrictions.

The complexity of constructing the Reid Building’s many concrete structures took its toll on the programme for the frame which was ultimately delayed by about eight weeks. “We had underestimated just how complicated it was going to be, although we did have some rubbish weather which increased downtime,” says Unwin.

Nevertheless, the finished building delivers a wonderfully bright and intriguing complex of spaces, which open up on each floor and around every turn to provide inspiring views into and through the driven voids, as well as opportunities for creative encounters with other students that Holl intended.

“We are delighted with the finished building, which has created a huge amount of local interest,” says GSA’s Anderson. “It’s unique, exciting and inspiring, ideally suited for an artistic community, which complements the Mack, rather than trying to outshine its brilliance,” he concludes.