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Frame of glory

Structural innovation

Precast perimeter panels forming Vierendeel trusses were a major contribution to cutting costs on a prestigious London office development. Colin Cleverley reports on the winner of this year's Construct Award.

An innovative approach to concrete frame construction has produced a landmark office development with what is reputed to be the cheapest building frame and envelope in the City or West End for the last ten years. It was for this approach plus the fact that the solutions proven on the project can be readily adopted for other developments that 40 Grosvenor Place won the Construct Award for Innovation and Best Practice 2002.

The award was established by Construct, the Concrete Structures Group, to recognise and reward innovation and best practice in the use of structural concrete. Submitted projects are judged for their beneficial impact on the concrete industry, buildability for repetition, simplicity, economy and value for money. This year's winner met all these requirements.

A high quality office development with a gross area of 23,140m 2, 40 Grosvenor Place, London SW1, was built on time and on programme. Originally, the frame was budgeted at £6M but, thanks to the innovative construction solution adopted, eventually came in at £4.2M. This cost saving allowed a stunning atrium to be afforded within the original cost plan.

The six-storey building has a column grid, generally 7,500mm x 7,500mm, which varies according to the shape of the floor plan.

It could have been constructed in a traditional manner with a conventional reinforced concrete frame, but to achieve the widely spaced column grid, and to provide greater value for client Grosvenor, structural engineer Whitby Bird & Partners proposed a radical approach to the structural form.

A well-proven, cost-effective concrete flat slab option had already been selected, with steel cruciform section shear connectors at beam/slab joints to reduce depth. However, Whitby Bird proposed that further cost savings could be achieved by removing the internal concrete shear walls and using the 200mm thick precast external backing panels to the stone cladding as vertical and horizontal loadbearing elements.

This was achieved by using the backing panels as precast perimeter columns when a whole window opening is placed in line with the next panel. A continuous row of panels when fixed together with an insitu infill forms a Vierendeel truss which provides resistance against horizontal forces acting on the building such as wind or notional horizontal forces. The dramatic atrium, an added bonus to the approach, is formed by four 40m long, stepped Vierendeel trusses and is glazed to suit the raking profile.

The individual panels that frame the window openings were produced by Histon Concrete Products. These were transported to site and lifted into position one after the other, abutting to form the composite Vierendeel. The speed of erection was such that half of the building envelope for each floor could be erected by contractor Byrne Bros in just one to two days. This further decreased the cost of the frame.

Structural action of the external frame was achieved by casting a 200mm x 25mm saw tooth profile on the edge of each of the floor units. When two adjacent units were dropped into position a small, 200mm void was left between. This was filled with insitu concrete to provide a key against the horizontal shear forces imparted within the Vierendeel. The joint between the precast panels and the floors above were achieved with projecting dowel bars. The panels for the floor above were simply dropped into position above the units below, the dowel bars securing them in place.

With such stiff perimeter bracing elements, the need for traditional reinforced concrete walls in the internal lift and stair cores was eliminated. This further simplified the construction approach and saved more time.

Precast perimeter panels have been used before in construction. However, using them for horizontal stability and to free internal space is an innovative approach and one that could be used for many other developments with dramatic cost and time-saving results.

Project team

Client: Grosvenor

Quantity surveyor: David Langdon & Everest

Structural engineer: Whitby Bird & Partners

Main contractor: Laing Eastern (now O'Rourke)

Services engineer: Arup

Concrete frame contractor: Byrne Bros

Precast supplier: Histon Concrete Products Ltd

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