It divided the building in two parts: an arena on the upper level, with entrances away from nearby homes to avoid noise disturbance; and a sports centre on the lower level, with easy access for local residents.

Built above the filled-in workings of old iron ore mines, the sports centre was designed “as a rock” with precast concrete panels textured and coloured to resemble the area’s distinctive grey limestone. These, says ACXT provided “the key to design and helped resolve the complex functional and circulation programme.”

A great deal was required of this 30,800 square metre municipal facility in terms of its flexibility and functionality - including separate entrances and changing areas for players and the public, integral car parking and the provision of space for a restaurant.

The sports centre features a 26m-long swimming-pool, together with a smaller pool for young children. Other facilities include a 520 square metre multi-functional gymnasium, which can also be subdivided into smaller training areas.

In contrast, the playful-looking arena overlying the sports centre sits on slim but extremely strong, tree trunk-inspired steel pillars. Its “arboreal mass-influenced” façade is clad in a permeable envelope of 1500x750mm diamond-shaped leaves made from 7mm-thick steel sheeting from ArcelorMittal. The leaves, now dubbed ACXT’s  “Alligator Scales” by ArcelorMittal, comprise leaves in six standard RAL colours, including orange and brown as well as several shade of green.

The façade is permeable, says ACXT, in a bid to comply with low-budget and low-energy constraints. A galvanised steel mesh, attached to the structural frame by steel cables - the type used for retaining road embankments - gives the corridors encircling the arena an outdoor environment that requires no ventilation or air conditioning. It also ensures safe evacuation of the building in the event of fire.

An indoor cistern stores rainwater for irrigation while old pool water is recycled for street cleaning.

The Bilbao Arena and Sports Centre has been well received by the city’s inhabitants and was vote Building of the Year in 2011 by the architecture website ArchDaily.

Images courtesy of Jorge Allende and Aitor Ortiz