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A glass bridge and a precast concrete staircase

working details Office, Wicklow Street, King's Cross Squire and Partners

A 6m x 6m section of the ground floor slab of the '50s office building has been removed to create a 'singlevolume space', revealing the lower ground floor and making it seem more spacious. A glass-floored bridge crosses the space to link the reception and cafe; a precast concrete staircase alongside the bridge rises from the lower ground floor.

The staircase consists of a precast concrete spine beam in the shape of a tapered ovoid with a stepped upper face, on which rest precast treads. They are bolted together through a series of holes cast through the spine and fitted with 42.2mm diameter sleeves.A stainless steel M16 bolt is slotted through each sleeve and threaded into a socket cast in to the tread soffit. The bolts terminate on the underside of the spine with pig-nosed ends which are recessed into cast countersunk holes. The treads are fixed asymmetrically to the beam and taper towards their ends.Each front edge is fitted with an 8mm stainless steel bar which projects 1mm to prevent slipping.

The staircase balustrades comprise 40 x 25mm solid rectangular mild steel bars;

the balusters are slotted through precast holes in the upper tread and bolted to M8 threaded bars cast into the back of the tread below with M8 countersunk screws.

The bridge is supported by two 200 x 100mm RHS beams pinned at their ends and bolted to the existing floor slab, braced at the ends with solid rods. The floor is formed of five Cellbond doubleglazed panels of 4mm toughened glass with a central core of 17mm aluminium honeycomb, framed with aluminium box sections. The top edges of the panels are sandblasted to provide a non-slip surface.

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