A suspended clerestory
The four-storey extension contains a new entrance hall, galleries and a restaurant on the top floor, with a lecture theatre in the basement. The extension has been inserted into a long, narrow slot between the external walls of the National Gallery and National Portrait Gallery. The entrance hall, containing stairs and escalator, is a three-storey high space lit by a continuous band of 1.7m deep clerestory glazing with 180 x 35mm steel mullions at 2875mm centres.
Because the original walls and foundations could not accommodate additional loads, the extension has a steel frame structure. The top floor rests on universal beams which cantilever from a central steel truss. The clerestorey and the mullions are suspended from the beams. The base of each glazed panel sits on a 120 x 120mm steel angle fixed to the mullions and the head is restrained by a similar angle. The mullions are fire-protected with 8mm intumescent epoxy casing; the angles are clad with one-hour fire-resistant insulated board.
The glazing consists of an inner sheet of 21mm laminated glass, a 12mm cavity and an outer sheet of 6.5mm lamimated glass incorporating a uv filter. The head and sill frames are concealed by aluminium cover panels and the vertical edges are butt-jointed with silicone.
The clerestory is fitted with an aluminium sill which fits over the parapet coping of the National Gallery wall. It provides weather protection and insulation but is designed to move independently. A fire blanket on the inside of the parapet acts as a fire stop yet allows movement of the sill
The sill is fitted with external blinds which are remotely operated to reduce daylight for conservation purposes, and external light fittings which shine through the glass.