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Faith Zone

ZONES

As a tensile fabric structure the Faith Zone is a breed apart from the Dome itself. Where the giant Dome masts are clearly under compression and cables anchor the roof membrane to the ground, the fabric on the Faith Zone is stretched like skin over a self-supporting skeleton. Six spidery arced trusses make up the skeleton, with no other form of anchorage.

Zone structural engineer Techniker director Doug Pow believes this lack of supplementary anchorage makes the structure 'quite unique'.

He says design grew out of budget constraints and the peculiarities of the Zone's location, directly above the Blackwall Tunnel.

The design team of Techniker and architect Eva Jiricna had to ensure no extra loads were transferred to the tunnel, which was surrounded by a 10m wide piling exclusion zone. Ground beams could have been constructed to transfer forces to piles outside the exclusion zone, but, notes Pow, this would have been expensive.

Creating a very lightweight, long span structure was economical and gave the designers soaring interior spaces that evoked Gothic architecture.

Six Vierendeel truss 'legs' are arranged asymmetrically, creating a largest clear span of 46m. The whole structure rises 17m above ground.

To manage variations in geometry arising from differing lengths and radii, the trusses meet at a central, conical hub. They are articulated directly with the hub's lower ring; a tetrahedron of struts attached to the hub's upper ring lock the trusses in position.

The trusses themselves are triangular in section, with main corner members of 114mm diameter tubes braced by steel plates cut to an elegant flaring profile. Each plate is unique, corresponding to the spacing and geometry in a particular part of the truss.

Plates were cut using information direct from CAD models and required no extra preparation prior to welding. Welding itself is relatively easy because of the Vierendeel trusses' simple geometry. 'Compared to a conventional truss there is very little distortion during fabrication,' Pow observes.

Steelwork was fabricated by Czech subcontractor Seele using floor-mounted jigs. Each truss is composed of three radii, and was cut into three sections for transport. Main contractor Watson Steel re-joined the trusses on site. Welds were left rough to 'explain how the structure's put together,' says Pow.

Exact tolerances and a stiff structure were achieved with only 50mm deflection measured when the trusses were erected. However, the structure is still liable to move slightly under 'wind load' - gusts blowing through the Dome's open doors. To accommodate movement, universal joints have been used at the junction between trusses and the ground.

Design of the Faith Zone's acrylic canvas skin was carried out by tensile structure specialist Tensis using 3-D computer software. The fabric panels were tailored to act as an integral, stabilising structural element.

'Designing the panels was an iterative process,' recalls Pow. Finite element analysis was used to hone performance.

To maximise the Zone's covered footprint, steel 'push outs' - light trusses rigged via cables like a sailing boat's spinnaker pole, mid-way between main trusses - stretch the form-hugging fabric membrane outward. Visitors to the Zone enter it through portals over-reaching the space outside.

Zone facts

Sponsors: Hinduja Foundation, Laing Family Trusts, the Jerusalem Trust

Structural engineer: Techniker

Architect: Eva Jiricna Architects

Steelwork main contractor: Watson Steel

Steelwork fabricator: Seele, Czech Republic

Tensile cladding main contractor: Architen

Design of cladding: Tensis

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