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A steel structure with precast coffers

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The structure of a typical 15m office wing consists of a boxsection spine beam, formed by paired 430 x 100mm channels set toe-to-toe and supported on 180 x 180mm steel columns.

These divide the office space into a 9m and a 6m bay. A series of secondary 457 x 191mm universal beams runs at right angles to the spine beam. These beams are set 3m apart and extend to the north and south facades, where they rest on delicate 120 x 120mm SHS columns.

Rows of curved precast concrete coffers, each 750mm wide, span between the secondary beams and rest on the bottom flanges. The design is reminiscent of nineteenth-century 'jack-arch' construction, where brick arches span between wrought-iron beams. An in-situ concrete topping ties the coffer units together, which appear as a series of smooth white-painted vaults with dove-grey steel strips between - these being the visible bottom flanges of the supporting beams.

The vaults act as the path along which natural cross-ventilation from the high-level windows is channelled (allowing heat exchange between the air and the concrete for overnight cooling). To maintain this path, there are no downstand edge beams - the spacing of perimeter columns allows the last coffer unit to act purely as a tie.

This arrangement also enables the lightness of the structure to be expressed through the double-glazed facade, as the shape of each coffer frames one opening and one fixed window.

The construction of the spine beam from toe-to-toe channels allows services to be carried in the void. The beam is perforated with 200 x 200mm holes with radiused corners, to continue the path of natural cross-ventilation.

External solar shading to south and west elevations modulates and controls direct solar heat gains within the building.

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