Working with Foster & Partners,Techrete has developed a cladding panel which is more void than solid,creating a delicate grillage which is superimposed on a glazed facade.The building is the headquarters of Andersen Consulting and replaces the former Daily Mirror offices at Holborn Circus,central London.Two eight-storey wings of office accommodation fan out from a central core;the space between the wings forms a glass-roofed atrium opening on to the Circus.
The main structure is a steel frame.The glazed facade from ground
to second floor is clad with a skeleton-like grillage of precast concrete, set in front of the glazing.It consists of a series of granite-clad precast mullions at 9m centres,with pairs of smaller precast mullions with a reconstructed Portland stone finish between them,3m apart.The mullions measure only 120mm wide x 330mm deep and are up to 6m high.Precast reconstructed Portland stone louvre blades are set between the mullions at 600mm intervals.Only 75mm deep and spanning 3m,the louvres were prestressed in the factory to achieve the necessary rigidity;they were secured in position on site with stainless steel fixings - with a 2mm tolerance - set in the sides of the mullions.
The facade to the upper floors is composed of precast window panels of mullions and spandrels; ofa slenderness and composition which push forward the boundaries of concrete technology.The main mullions are clad with granite;screens of precast louvres set between them are fixed to intermediate reconstructed Portland stone mullions.
The ratio of solid to void is such that,if the solid material were spread evenly over the panel,it would be just over 90mm thick.
To speed construction,and to avoid fixing on site,9m x 4m floorheight window reveals were cast in a single massive unit.The prestressed louvres were cast separately and stitched into the mould before casting;the reconstructed stone and granite-faced units were cast together in the same mould.