The Image Bank is a world-wide image resource organisation. The uk headquarters on Conway Street, Central London, has an extensive library of transparencies and film footage used by advertising, design and corporate clients. A recent acquisition of archive films has created a need to find space for this extensive film library.
The ground and the lower-ground floor of the building are divided at the front by an entrance foyer and a curved central staircase, and at the rear by the courtyard which acts as a lightwell to the lower-ground- floor rooms.
The new scheme fills in the formerly redundant courtyard and unifies the separate ground and lower-ground floor spaces by inserting a new floor at ground level with a glazed umbrella-like roof above; the floor is fitted with glass blocks to transmit light to the lower-ground floor.
The new structure has facilitated several modifications: the research department and the video library have been moved to the lower-ground floor while the ground floor has been replanned to incorporate a video-viewing suite with curved walls - this too designed by the architect.
The new space formed by the courtyard on the ground floor has become a room for meetings and a waiting/hospitality area for clients. Sliding doors open from the existing rooms on to both sides of the courtyard so one can see right through the building. A new circular staircase leads from it to the lower floor. A cantilevered planted trough along the rear wall is filled with bamboo and climbing plants.
The structure within the courtyard had to be independent of the walls - door and window openings on opposite sides of the courtyard do not align so it was not possible to span across the space. Column positions were limited by the circular brick stairwell which projects into the courtyard and by footings to existing walls.
To overcome these problems - in particular to avoid the clash of old and new floors - the structure was conceived as an independent two-storey steel portal frame. It comprises four 150 x 100mm rhs columns which rise from the lower-ground floor; they are placed in the centre of each side of the courtyard and support a rectangular slab at ground floor level placed at the diagonal to the walls. The corners of the slab are cantilevered from the columns.
The cast-in-situ concrete floor slab is fitted with Luxcrete circular glass blocks and supported by 152 x 152mm I-section beams; the corner cantilevers are supported by 150 x 90mm parallel flange channels. The structure is unbraced - moment connections between columns and beams give stability. The triangular strips to each side of the courtyard are filled with steel grilles which let light into the room below.
At the roof the four rhs columns bend to form a pitched portal and are welded at the apex to a central node, formed of a cruciform of 20mm steel spigot plates. (The columns have two bolted connections, one at the apex and one just above the floor slab, which allowed them to be taken into the building in pieces and bolted together on site).
The roof structure - four tapering arms of paired back-to-back 150 x 75mm steel angles - rise from the central node to support a roof of double- glazed rooflights framed with aluminium box-sections. The arms are set at 45degrees from the steel columns below them, so the roof looks poised in space like the rotor blades of a helicopter. The spaces between the glazed roof and the original walls are lined with a solid timber-framed roof deck.