Towering more than 20m above the floor of the Dome, the cardboard columns of the Shared Ground Zone serve as a graphic illustration of the underlying theme of sustainability and recyclability. Columns and cladding alike are almost entirely constructed of recycled paper, forming what is claimed to be the largest structure of its type in the world to date. Even the steel and plywood mezzanine floor at the elliptical heart of the Zone's two-storey spiral is designed to be easy to recycle, says structural designer Buro Happold project engineer Helen Gribbon.
'In particular the floor is made up of plywood over steel Holorib panels without any concrete in them. But it was the 100, 500mm diameter columns that were the real challenge,' she says.
Tests at the University of Bath had shown cardboard failed at a compressive stress of less than 9N/mm2. Unfortunately, creep was high, and there was only partial recovery after loading was removed. 'So we decided a safe working stress would be 0.8N/mm2, less than 10% of failure loading,' Gribbon reports.
Cardboard specialist Sonocco used high quality recycled paper bonded with PVA adhesive to make up columns with 14.5mm thick walls. Aluminium foil was bonded to the outside of the columns to protect against moisture ingress. Cardboard collected after an appeal on BBC TV's Blue Peter programme went into the horizontal bracing tubes and into the cladding, basically two sheets of card glued onto a central cardboard honeycomb.
The columns range in height from 11.5m around the core to 22m at the end of the dramatic wing wall, and are set up at 2m centres. Despite being protected by the Dome, the Zone still had to be designed to resist significant air movements.
The tops of the central columns are braced with a steel truss and 'wind' loads are taken by crossbracing between four pairs of columns with cables joined at a central steel flitch plate.
More cables tie the 22m tall raking end column back to the roof level truss. A clear intumescent coating applied to all cardboard elements provides the necessary Class O spread of flame performance. Where needed, the cladding panels hide steel sheets or fibre fill, for enhanced fire resistance or acoustic performance respectively.
Architect: Gumuchdjian + Spence with Shigeru Ban
Structural and M&E engineer: Buro Happold
Main contractor: Mivan