Several structural options were researched for the roof with the preferred solution being a lamella structure in which the timber members form a pattern of lozenges creating a grid-shell of timber. This works by having simple, repetitive short lengths of timber glu-lam beams, made of fir and joined together to form the dome. The pattern changes at the crown where the structure terminates in a circular ring beam, defining a 30 metre-diameter oculus constructed from a simplified pattern of glu-lam members. The primary members of the dome are connected invisibly, so that even though they are bolted together, all of the metal is hidden within the wood so that observers beneath the dome will perceive only a continuous timber structure. The laminated beams are topped with two layers of plywood – 'Kerto' panels which aid structural stiffness – and a layer of insulation, topped with a seamless liquid application roofing system for a weather-proof finish.
The entire roof sits on a three metre high 'skirt' to overcome the possibility of unusable low space at the perimeter of the dome. The skirt is comprised of 20 boomerang-shaped columns supporting the domes perimeter beam. Inclined struts spring from these columns to directly support the ring beam where it meets the timber grid-shell, while providing stability to the entire structure. This approach not only provides a visual contrast with the steel used to construct the dish, but has the environmental advantage of being a sustainable and renewable natural resource. The choice of timber also meant that the structure could be exposed, to dramatic visual effect, as fire performance is achieved by sacrificing charring layers.
All the constituent parts – the facade, the roof-level spaces, the four internal segments and the adjacent Eforum are structurally independent, allowing for future flexibility and change to encourage a wide variety and changing rotation of activities to take place, including sports events, fashion shows and exhibitions.
Large external landscaped areas surrounding the bullring provide high-quality public open space that also serves to connect the building to the metro station and the adjacent park. The roof-top piazza and the viewing platforms provide additional public space with spectacular views over the city.
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