By Susan Dawson
16 January 1997
The glass walls of a new City pizza restaurant, which straddles a busy street, are supported by an innovative cast-glass system. By Susan Dawson.
The Pizza Express restaurant is directly over London Wall, on the first-floor public walkway of Alban Gate, Terry Farrell’s 20 storey ‘air-rights’ office block which straddles the street. The distinctive blue neon logo of the restaurant chain looks down on traffic coming from both directions.
The restaurant is divided into two self-contained units facing each other on each side of the walkway. One was originally intended to be a bar and the other a restaurant, but the demand for tables at lunch-times forced both into restaurant mode. Generally only one stays open in the evenings, which are less busy.
The restaurants, designed by Cantos Architects, have elegant interiors with circular staircases leading to mezzanine dining decks looking out over London Wall. The practice commissioned bere:architects to design the facades, comprising revolving glass doors, stacked-glass columns and glass walls which face each other across the walkway.
The glass walls, 19m long and 6m high, are mirror images of each other. Each runs out at an angle to the walkway to enclose a triangular dining area; the entrance, a glass drum containing revolving doors, is on the short side of the triangle. Uplighters - 12V dichroic luminaires - set in the floor turn the stacked-glass columns into sparkling masts of light, creating a series of almost magical reflections between the two dining areas.
The walls are of 3m x 2m x 12mm toughened clear single-glazes glass sheets, stacked two-high. the top sheets are hung from connections which take up movement in the Alban Gate concrete structure (+/- 20mm vertical movement and +11 to -15mm horizontal movement). Each top sheet hangs from a flange with a rotating steel arm which is fixed by a pivot joint to the concrete floor slab.
To achieve torsional stiffness, all the glass sheets are secured at four points by the columns. Each column is made of two 20mm diameter high-tensile stainless-steel rods, set 350mm apart and braced diagonally by a series of paired stainless-steel ‘dog-bone’ shaped struts. The connections are unusual: the struts, rising in a double zig-zag between the two rods, are bolted to the ends of cylindrical nodes slotted on to the rods. Stacked between the nodes are cast-glass isolators, resin-bonded to the rods. The isolators are a standard component in the electricity industry, used to isolate high-voltage cables on electricity pylons. Shaped like a large translucent cotton reel, each isolator can withstand a load of 18 tonnes.
The sheets of glass are secured to the columns by cast-aluminium ‘wippletree’ arms (so called because the original timber prototype resembled the attachment between a horse and plough, which used to be known in Somerset as a wippletree). The arms are 950mm wide and rebated at the centre to fit the cylindrical nodes, to which they are connected. The glass is bolted through holes in the ends of the arms with a complex series of washers and clamps to allow the sheets to be aligned accurately during construction. Sunburst-shaped cast-bronze washers eliminate the need to counter-sink the glass holes. At ground level the glass is restrained in a narrow steel channel and the columns terminate in a turned steel foot.
Although the walkway is covered, it is exposed to the wind funnelled between the two restaurants, Every column therefore had to be tested to resist a wind pressure of 1.2kN/m2.