WORKING DETAILS A screen of concrete mullions and louvres
The new building consists of two nine-storey wings of office space fanning out from a central core; the space between forms a curved glass-roofed atrium opening on to Holborn Circus.
The glazed upper storeys step back to form terraces.
The main structure consists of composite concrete/steel floors on a steel frame. The curved facade facing Holborn Circus is glazed from ground to second floor, with a delicate screen of precast concrete mullions and louvres set in front of the glazing; upper floors are glazed with a curtain-wall system.On the adjacent facade to Fetter Lane, the ground- to secondfloor glazing has a series of precast clad columns running in front of it; upper floors are clad with a similar screen of precast concrete mullions and louvres set in front of the glazing.
The screen is divided into floor-height bays set between columns at 9m centres; the columns are clad with precast concrete panels faced with Portuguese granite. Each bay consists of two precast concrete mullions 3m apart. The mullions measure only 120mm wide by 330mm deep, yet stand up to 6m high; they have a reconstructed Portland stone finish.
A series of precast concrete louvres with a reconstructed Portland stone finish run between the mullions at 600mm intervals. Only 75mm deep and 150mm wide, they were prestressed in the factory to achieve the necessary rigidity to span 3m. They are secured on site with stainless steel plate and saddle fixings - with a 2mm tolerance - cast in the mullions.
To speed construction, and to avoid fixing on site,9m x 4m bays, including spandrel and lintel sections, were cast in a single massive unit. Both reconstructed stone and granitefaced units were cast together in the same mould.