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Trading places, curving spaces

Gensler took its inspiration for the curved timber gridshells of a financial company's headquarters from a converted watermill, to create a stunning modern building that is structurally innovative Gensler has designed a roof of four doublecurved timber gridshells for the headquarters of a financial company. It satisfies the client's requirements for a modern-looking building and its affection for timber in buildings. The four light and elegant gridshells which curve over the main trading floor of the company's Home Counties headquarters are structurally innovative, environmentally sustainable and functional.

The architect, Gensler, had previously collaborated with the client on several occasions, including the adaptation of a converted historic watermill to its former headquarters, which became too small when the trading floor needed to accommodate an increase in staff numbers from 18 to 90.

Wishing to stay in the countryside, the client bought a site with a Grade II-listed Neo-Georgian house in 3.3ha of formal gardens, set on the crown of a small hill.

Planning permission had been granted for an additional two-storey office in NeoGeorgian style surrounded by surface parking. It soon became obvious that this would not meet the client's desire for a sustainable, low-energy modern building.

Gensler's brief included a trading floor that would be more interactive and allow groups of traders to work together with direct visual contact. Parking for all staff was required, plus leisure facilities.

The building is a long, low, single-storey steel-framed pavilion under a complex roof.

The four curved gridshells, each 13.2m long by 7.2m wide, are flanked by perimeter zones of flat roofing which are 5-6m deep.

The curved lunettes formed at the edges of the gridshells are filled with canted clerestories, so that the gridshells appear to float above the flat perimeter roof.

In effect, the roof structure defines the functions of the spaces below; the curved gridshells hover directly above the main trading floor, while cellular spaces - shared private offices, conference room with video tele-conferencing, fully serviced kitchen, computer room and administrative support - are contained in a flat-roofed zone to the east. On the west is a relaxation/reflective zone with glazed walls looking out over the main lawn. Car parking is hidden from view on two basement levels of cast in situ concrete below the trading floor.

The new pavilion lies to the north-east of the house, which was designed in 1909 by Ernest Gimson. A 25m pool, a gym and changing rooms, are housed in a pavilion on the south-west side. The main plant room is in a separate building, partly below ground to reduce its visual and environmental impact; its roof is being laid with an artificial grass sports deck with access from the main lawns.

The provision of the latest information technology on the trading floor was combined with a low-energy approach to the servicing of the building. Services are in the basement. Air is conditioned below ground and distributed via ducts, cast in the concrete floor beams, to a plenum formed by the raised access-floor. It rises naturally above the dealer desks and is dissipated through opening lights in the clerestories, which are operated by the building management system (BMS) when required.

'The client wanted a new, modern design, ' explains project architect Andrew McNeill, 'but liked the beamed timber atmosphere of the previous headquarters in the old watermill. The concept of the four grid shells developed from this.'

In form, each gridshell is a barrel vault with a complex double curvature - imagine the vault as a rectangular section cut from a bicycle-tyre tube, with the four corners pulled down to create the double curve, so that the radius varies across the span. It consists of an orthogonal grillage of kerto LVL (laminated veneer lumber) at one metre centres, bolted together in a lamella sequence. The grillage is overlaid with a deck of LVL boards.

John Westmuckett, the structural engineer, adds: 'The design allows for the gridshell and the deck to act together compositely, in that the deck provides in-plane shear action as well as transmitting part of the compressive and tensile forces to the supports.' A finite-element analysis was carried out to establish the distribution of the true shell stresses produced by the geometry, and several other models were analysed to approximate to the in-plane shear action of the deck. The final design was a compromise between these solutions to arrive at a 'best fit', taking into account the anisotropic nature of the timber and allowing the deck to contribute to the stiffness of the final composite form.

The result was a roof structure in which the timber element plays a structural role, including the canted window mullions which act as columns. Some discreet cable bracing in the plane of the windows provides lateral stability to resist wind loads.

The windows are canted to intersect with the grid at 90degrees.

ARCHITECT Gensler

STRUCTURAL ENGINEER Parkman

SERVICES ENGINEER Oscar Faber

CONTRACTOR Try Construction

TIMBER GRIDSHELL FABRICATION Cowley Structural Timberwork

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