For the Avicii Arena, a new retractable roof is an acoustic game changer

Until recently, the 1989 Stockholm Globe Arena had been the world’s biggest spherical building. Designed by architects Svante Berg, Lars Vretblad and Esbjörn Adamson, its aluminium-clad, structural steel structure, dotted with portholes, seems to reference not just Étienne-Louis Boullée but the 1970s Japanese Metabolists.

Renamed Avicii Arena in 2021 after the late DJ, and in response to commercial and performance needs, it reopened in February after extensive transformation. Most notably, a vast new internal retractable roof of folding acoustic panels enables the space’s scale to be radically modulated according to event. 

The novel cable-stay design is by architect HOK with C.F. Møller Architects and engineer Schlaich Bergermann Partner (SBP), which worked on the 1972 Munich Olympics’ pioneering lightweight structures. Its sustainability attributes go well beyond minimal material use.

Set 30-43m above the field of play,  the new curved roof, with its acoustic and spatial intimacy, enables the city’s ice hockey teams, and fans, to adopt the arena. This has freed up the  adjacent Hovet Arena site for demolition as part of a massive urban regeneration of south Stockholm’s former meatpacking district. 

As John Rhodes, HOK design principal and director of sport and entertainment points out, the design – which includes new seating and event viewing configurations – in effect obviated the need for an alternative sports venue, and the embodied carbon and economic cost it would represent. It also creates a state-of-the-art facility for live performers who previously would have had to battle with the challenging acoustic and technical limitations of a cavernous 108m-high space.

‘There’s been real changes to the event ecosystem, with buildings needing to be more flexible in what they can host – and higher performing,’ adds Rhodes. ‘The existing arena could have supported 60 tonnes of kit, but with showmanship demands ever-increasing, it can be now be more than 200 tonnes. There is also a need to meet turnaround times, now measured in hours, for revenue reasons.’ These concerns, and the existing structure’s heritage-listing, led to the bespoke tensile net roof design.

Nuno Guerreiro, HOK project designer, described the found conditions of a lower ‘bowl’ of 48 huge steel rib sections on concrete shear walls with four ‘hero’ cores supporting numerous concrete audience tiers. A massive hemispherical Mero spaceframe meets this at the structure’s ‘equator’, and initial studies looked at using this as the support for the new roof. 

‘Accurate point cloud studies allowed us to conduct deep analysis of the structure down to the foundations but hopes for high levels of structural redundancy instead revealed very efficient engineering,’ explains Guereirro. ‘We realised quickly that we would need a lightweight structural solution that would not only support its own and imposed loads, but work with an MEP strategy that could deliver coolth and lighting to the venue floor.’

This led to the proposition of a hyperbolic paraboloid surface of pairs of 45mm diameter steel post-tensioned cables, attached back to a wave-shaped steel tension ring; its curvature generating a self-sustaining and stable cable net structure. But as SBP managing director Knut Stockhusen outlines, things weren’t as simple as the form. 

A massive yet delicate structure

‘It wasn’t possible to support this ring directly downwards as concrete tiers and arena structure got in the way, so we had to suspend it within an inclined truss connecting back to a larger ‘top’ ring running round the perimeter,’ he says. ‘This is a compression ring as all the forces acting here are tensile pulling against it, putting it into compression. It shortcuts all forces acting in the suspended cable net and truss, so that only vertical load transmits into the existing building.’ 

Stockhusen adds that some concrete columns were encased in steel sleeves, to take extra load and transfer it into the existing steel ribs.

The resulting 12m deep tri-chord beam is notable for two reasons. First, with 40mm-thick steel tension and compression ring sections of 1.1m x 0.5m and 0.6m x 0.5m respectively, linked by 320mm diameter steel trusses, it’s massive. Secondly, running around a nearly 350m perimeter, with its gentle curve supporting the ‘Pringle’-shaped suspended roof, it nonetheless appears a thing of great delicacy. 

Its design economy was complicated by a need for the roof to take account of a central ‘jumbotron’ display, which is lowered over the ice rink on match days, requiring a 9m x 9m hole in the centre of this highly interdependent cable mesh. ‘Hefty beams were needed on all sides of the opening to divert the 1,000kN forces running through the cables,’ notes Stockhusen, acknowledging the efforts of the roof steel and specialist subcontractor who built it, ‘given the demands of the construction tolerances.'

Rhodes says the idea for the 80m x 75m retractable roof system came from a previous proposal HOK had investigated, along with SBP’s realised Heart of Doha moving solar shading project; both were shown to the client to argue its viability. The solution adopted was a steel rail system running below and held up by the double cable tensile mesh, at 4.5m centres. 

HOK’s Guerreiro says the aim was simplicity. ‘Everything we opted for – rails, winches, chains, hangers, steel clamps and hinges – were off-the-shelf items; to cut weight, we ran with simple wire connections on lower panel edges instead of hinges,’ he says. The retraction system was tested as a prototype to ensure it worked, even on just one motor, since in fire scenarios, when the building must to be cleared in less than eight minutes, the roof needs to failsafe open for smoke ventilation purposes.

Each 8.7m² panel is made of a 3.84m x 2.26m aluminum frame section, with a 100mm core of porous, fire-resistant, sound absorbing material. ‘The panel’s “top” side is of acoustically transparent fabric to absorb low frequencies bouncing down off the dome, while the “bottom” is of stretched polymer material to reflect frequencies above 500Hz,’ explains Rhodes. 

Acoustic advantages of an unusual roof form

The ‘Pringle’ form had other benefits too. ‘When engineers began testing it, the shape generated much better acoustics than a horizontal ceiling – even the angled position of panels in the “deployed” position improves the general acoustics,’ adds Guerreiro. In terms of access and maintenance, HOK’s decision to run steel mesh over openings between cable net runs also means the whole roof is a walkable surface. ‘Operatives who want to hang tech for shows have a tartan grid of hatchways beneath which steels and cable trays run, so they can “bridle-in” and work at height safely,’ says Rhodes.

The venue’s roof is part of a larger reconfiguration. New bleacher seating layouts improve proximity to the ice rink and, retracted in concert mode, create an acoustically attenuated wall facing the large ‘mosh pit’. There’s also a pair of high-level structural interventions. 

New seating ‘gondolas’ bring 800 people closer to the action, freeing-up an underused corporate boxes level, Rhodes thinks, for ‘conferencing, a luminarium or sky garden perhaps’. Formed of four steeply raked tiers of 12mm thick folded steel plate, each is supported by steel struts running back to the perimeter structure, and serve to hide a large new MEP ring of servicing, which provides powerful air cooling via high-velocity nozzles, and field-of-play lighting.

Their vertiginous addition not only maintains the venue’s 16,000 capacity but further serves to spatially modulate this vast but formerly problematic arena, creating a spectacular vantage point from which to enjoy the game or concert. But with the elegant roof able to retract in just 10 minutes, it also allows for that complex intimacy to be exchanged for the spherical original’s simple shock.