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Do the Strand

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technical & practice

For the first time in the UK, an office building has been completed using structural insulated concrete panels

Just off Waterloo Bridge, on the corner of Savoy Street and the Strand, is a busy location to construct a sevenstorey office block. In order to reduce the construction time and possible disruption to traffic, the architect, Squire and Partners, opted for a structural insulated concrete panel system, which would negate the need for any support steel. External walls in this scheme are prefabricated, loadbearing facing units, which have been designed to carry the floor structure.

Construction on site took six months. Because the 3.6m-high x 1.5m-wide precast perimeter panels act as the floor to ceiling structure, the walls to each storey were erected and the concrete floor cast before moving on to the next floor. Each storey took only two weeks, and as the rest of the structure was being installed higher up, the fit-out work could be rolled out below.

The building was built without having to erect scaffolding on the outer face of the site area - effectively constructing the building from the inside.

Usually, structural concrete schemes require decorative finishes or rainscreen cladding to be added to the structural frame when the overall framework has been completed, but in this scheme, by incorporating the finished face within the precast unit as delivered, external scaffolding and separate on-site trades were further made unnecessary, resulting in significant savings.

The principal facades are made up from structural panels at 6m centres combined with infill panels of either window or non-loadbearing insulated panels. The main structural panels comprise 50mm Portland stone, cast onto a 70mm concrete 'backing' panel, on 40mm rigid extruded polyurethane foam with alu-polythene foil, on 225 structural concrete.

Precast infill panels are the same specification except the internal concrete is 120mm thick instead of 225mm.

'Have stone, will travel'

As a prototype in the UK, the architect learned that the drawback to bringing applied finishes to site is that there is a danger of damage during transit, although given the circuitous route of the materials, damage was relatively minor.

The Portland stone, which was necessary to maintain the aesthetic of the neighbouring buildings on the Strand, was sourced from Coombefield Whitbed quarry in Portland, Dorset, and blocks of it were taken to Stirling Stone in County Durham to be cut into 50mm-thick slabs. The cut stone was then taken to Mouscron in Belgium to be set into the panellised units, which were then shipped back to site in London.

The Belgium company DeComo had experience of this technique, and had worked with the client on sites in mainland Europe. Project architect Barnaby Johnston acknowledges that DeComo's assistance was invaluable.

In factory conditions, the 225mm concrete was poured over the reinforcement in the mould with 200mm steel reinforcement connectors protruding 125mm through the face of the concrete. After the insulation layer was applied, the 70mm concrete backing layer was cast onto these ties in order to hold it to the main concrete structure as a unitised whole.

The tolerances were just 3mm deviation over 3m.

Each panel has the insulation and facing stone layers projecting beyond the top of the structural concrete, and inset by about 40mm at the base, to take account of the floor dimensions.

The 350mm-thick floorslab is cast in situ to rest on top of the structural concrete, bedding into the protruding reinforcement lifting brackets.

When the slab is cured, the next panel is laid on top of the slab edge, such that the finished face of the panel runs through. Because of inevitable discrepancies, and in lieu of a mastic joint, a polyester powder-coated trim has been run around the perimeter of the panels to demarcate a clear shadow line.

Visual interest

Ten floor-slab reinforcement bars are bent up specifically to align with each structural concrete panel. As each panel is hoisted into place, these rebars slide into cast holes in the base of the slab, to hold it in place. Through the rear face of the concrete panels, the rebar slots are pressure-grouted to set them in place. This means that the building comprises a monolithic concrete structural frame of slabs and panels at 6m centres. Movement joints are included between structural panels and infill panels.

At the topmost storey, the walls have been set back from the main line of the elevation. Here, a steel frame has been constructed, fixed between concrete panels to take beams to support the fully glazed storey height.

To add visual interest to the Strand facade, an etched Portland stone facade strip has been built in featuring a design by Langlands & Bell. The design, replicating the street map of the building's location, uses 80mm Portland stone cut out to a depth of 30mm.

In order to maintain the flush internal and external lines, the structural concrete element of the panels are only 195mm thick (reduced by 30mm).

Furthermore, by maintaining a structural grid punctured by nonstructural panels, the architect has been able to play around with the elevations, introducing different planes which protrude from the main face of the building.

On the rear of the building, facing Savoy Gardens, in the tradition of less auspicious elevations being treated differently, the panels have been faced in stack-bonded Lincoln Red brick rather than Portland stone, although these bricks had to be manufactured to 1.5mm tolerances.

Given the savings in time and money, the architect is keen to experiment further with this technology.

Barnaby Johnson says: 'The whole process was a challenging one that involved investigating and developing construction processes and learning through site installation how well that process worked. This required a high degree of detailed coordination to produce a building with workable details that maintained the simplicity and elegance of the design. The end result both reduced the construction period and also maintained the high standard of finishes, creating a facade that is certainly worthy of its prominent location.'

CREDITS

GROSS INTERNAL AREA 4,410m 2

COST PER m 2£1,899

CONTRACT JCT 98 (Private without Quantities) The panel casting period was from March 2001 to November 2001.The panel installation period was September 2001 to January 2002

PHOTO CREDITS Morley von Sternberg

COMPUTER RENDERING Stephen Davis

CLIENT City & West End Developments: Patrick Despard, Martin Thomas, Dan Nicholson

ARCHITECT Squire and Partners: Michael Squire, Mark Way, William Jefferies, Barnaby Johnston, David Graham, Andrew Edmonds, Myles Taylor, Sam Cheeseman, Tony Ip

CONTRACTOR Kier London: John Michell, Richard Lines, Ian Tidey, Martin Smith, Stefan Rehli, John Bolton, Mary Mason, Antony Luxton

STRUCTURAL ENGINEER Campbell Reith Hill: Mark Kaminski, Theodore Tai, Elizabeth Brown

SERVICES ENGINEER Mecserve

QUANTITY SURVEYOR EC Harris

SERVICES QS Mott Green Wall

STONEWORK DeComo (Belgium)

ARTISTS Langlands & Bell

ART CONSULTANT Modus Operandi

SUB CONTRACTOR AND SUPPLIERS structural pre-cast facade DeComo; windows and curtain walling Contano; in situ concrete works Getjar; piling Stent Foundations; M&E IEI; lifts Kone; steelwork Dyer; metalwork Coulson; drylining/ceilingsWallrite Interiors; raised access floors Kingspan; louvres Interactive Ventilation; joinery Jaysam Contractors; internal stonework/tilingMarmi; entrance screen Rush Entrances; facade stone (Portland stone) Hanson Bath and Portland stone; bricks (Lincoln Red) Marshalls Brick; window sections Hueck; ceilings SAS; external lighting iGuzzini; waterproof concrete Caltite; ironmongery John Planck Costs (Excluding demolitions)

SUBSTRUCTURE £489,443

SUPERSTRUCTURE £3,404,186

INTERNAL FINISHES £565,818

FITTINGS £76,376 SERVICES £1,807,585

EXTERNAL WORKS £172,757

PRELIMINARIES £1,718,100

CONTINGENCIES £143,812

TOTAL £8,378,077

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