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Glass - from planes to tubes

A Pilkington PlanarTM structurally bolted glass facade takes an organic curved form in a new headquarters building

Richard Hywel Evans envisaged the shape of his new building, the headquarters of Cellular Operations, as a transparent yet softly curved, organic form tapering at one end to a bull-nose. He has designed a delicate glass vault which swells out at the base and rises to form both wall and roof. The vault is 9m high but looks less; like the Millennium Dome, the curve reduces the apparent size of the building to a more user-friendly scale.

The glass is supported on a series of paired CHS tubes which curve gently, in a structure very like a rib-cage, over two floors of open-plan office space; the paired tubes are set one below the other and braced together for stiffness with flat steel spacers. These also act as the fixing points for the glass facade.

The complex and changing curve of the roof is created by glass panels which are faceted at different angles, some differing in angle at all four of their edges.To add to the complexity, few of the panels are true rectangles - most are parallelograms. A CAD programme created a grid of glass panels over the surface of the enclosure which allowed them to be sized.

The problem was finding a connector which would accommodate the range of glass bolt-hole positions.

The contractor who built the curved rib structure, Baco Contracts, had some experience of large vaults in working on the Millennium Dome. Cast aluminium arms, welded to stubs of aluminium tubes, are bolted to the sides of the flat spacers between the tubular ribs. The arms rise above the structure to support the structural glass facade. Each arm terminates in a flange which has a stainless steel casting bolted at each side of it. Each casting consists of a base with slotted holes and two fingers which slope upwards from the base and terminate in a bolt-hole. The corners of four glass panels are bolted to the ends of the fingers with a specially modified Pilkington PlanarTM bolt.

The casting was designed by Pilkington Architectural to allow a range of adjustment: coarse adjustment (about + or - 10mm) could be made at the connection between the cast aluminium arms and the steel structure finer adjustment (of + or - 10mm) could be made at the bolt between the arms and the fingers. Slotted holes in the base of the cast fingers match opposed slotted holes in the flange to give tolerance in two directions. Packer shims could be slotted between the bolt holes to allow adjustments in plane the hole at the ends of each finger had to line up with drilled holes in the corners of the glass panels, which, being faceted, would lie at different angles to each other.

This could only be achieved by separating the four fingers into two castings with two fingers each a Pilkington PlanarTM bolt, designed to accommodate faceted panels, was used. It is fitted with two conical washers which allow the bolt to be swivelled to the same plane as the facet.

The castings were produced using a lost wax and metal die method.

Leading edge design

The glass vault is not a consistent curve; from ground level it swells out to a height of about 2m before arching back over the first floor as a curved roof.To imagine this as volume, think of a deep saucer with a pudding basin inverted over it. The horizontal line where the saucer and the basin meet was a particularly difficult junction to design. On the inside a special casting had to be made to connect the two rows of glass panels which met at an angle of 57 degrees .

The opposite problem occurred on the outside; the two rows of double-glazed panels met in a horizontal 'bird's-mouth' edge which was too wide to seal with silicone in the usual way. Pilkington Architectural designed a stainless steel trim to give a clean line round the perimeter.

Joints between panels at other positions on the facade are sealed with a 12mm silicone joint.

Glazing the nose

At the entrance end the building tapers to a bull-nose of faceted glass panels supported by a structure of curved CHS tubes.

At the apex the tubes are braced by curved ring beams.The steep and complex angles at which four faceted panels of glass met had to be accommodated by a special cast fixing. Richard Hywel Evans describes the process. 'At the nose we needed a 'finger' component with the ability to take up much greater tolerances than in the rest of the building; we wanted something which would give us flexibility in the region of 80mm.'

The architect sketched a possible solution - each casting has four fingers slotted into tubes to act telescopically; at the end of each finger is a bolt, connected to it by a threaded cylinder which allows further adjustment.

'I took the sketches to Wymondham Engineering in Norfolk, which creates facsimile replicas of vintage Bugatti and Maserati cars. The latter's experience of milling suspension components was ideal; we sent him our computer model, he worked out the tolerances and produced a prototype. We attended the testing of the protoype; a one tonne weight - actually a Citroen engine that was lying in the yard - was suspended from the casting.When it held, another engine was added. The casting failed at two tonnes, fulfilling our requirements. The casting was used on site with great success, every possible adjustment was used; you can see the glass today with some fixings at minimum adjustment and some at maximum - it's amazing!'

THE GLASS SPECIFICATION

The glass panels were all doubleglazed units with Pilkington K GlassTM (a low emissivity glass) used to improve thermal insulation.

Panels in roof-top positions had to include a laminated inner layer for safety during maintenance.They comprise an outer panel of 12mm toughened glass, heat-soaked to DIN standards, a 16mm cavity, and an inner panel which consists of two 6mm heat-strengthened panes of glass laminated together with a 2mm interlayer. They were heat-strengthened so that, if fractured, they would break as large pieces which would adhere to the interlayer (toughened glass was not used as it fractures into small particles). The upper parts of the roof were also screen-printed with a dot-matrix pattern to reduce solar radiation. The dots, fitted to the inside of the 12mm outer panel, are purple in colour, to match the company logo.

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