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Don't roll away the stone

Buildings Structural stone

The dying art of stonemasonry is being revived to create a remarkable building. Mark Hansford investigates.

At first glance, the £25M eight storey office block being constructed in Finsbury Square - the heart of London's business district - is unlikely to grab the attention of even the most enthusiastic structural engineer.

Just another stone and glass clad building? Not this one.

With more than a nod to a bygone era, load bearing Portland Stone is being used in a blend of traditional, high quality stonemasonry and modern precast concrete beams to encase complex steel frames.

Not surprisingly, the decision by developer Scottish Widows to resurrect stone as a structural material for a relatively straightforward office development did not come about overnight. It owes much to the consultant and the conservation department of Islington Borough Council.

The plan was simple enough.

Demolish two disused office blocks and replace them with one modern facility maximising usable floor space while remaining within constraints set by sight lines to St Paul's Cathedral, rights of light for neighbouring buildings, and the proximity of the Bank of Nova Scotia next door.

However, with one of the existing buildings a historically important 1920s stone building, conservationists had to be appeased and during the four year planning process Portland Stone emerged.

Although it was initially intended to be merely aesthetic, structural engineer Whitby Bird & Partners soon began to question whether the stone could be used in a structural role. 'We took the contemporary approach to office development, ' explains Whitby Bird project manager Matthew Holden.

'We asked ourselves if we're going to be using it, why not use it as load bearing?'

While the idea was music to the ears of project development manager Jones Lang LaSalle and to Eric Parry Architects, Whitby Bird still had to demonstrate that its idea could work.

With no code of practice for the design of load bearing stone available, and little information on structural capacity in the British Standards, the question was only answered after extensive testing by Whitby Bird and the Building Research Establishment (BRE). 'Stone is traditionally selected on the basis of is aesthetic rather than structural properties, so cube tests on stone are rare, ' explains Holden.

After several hundred tests on cubes and mini-piers, the BRE declared the design strength of Portland Stone to be an acceptable 30N/mm 2. The final seal of approval came from a full-scale test on a column subjected to the maximum ultimate load derived from a computer model of the structure. The column resisted the 2,000kN axial load with ease.

There is no evidence of traditional arts or structural stone in the building's core, however.

Faced with a site formed of the basements of the two demolished buildings, HBG had to use every trick in the book to create a stable base for the basement, lower ground floor, ground floor and seven storeys above.

First, a combination of sheet piled walls and raking shores were used to support the perimeter of the site to allow the existing basement slabs to be removed. Of particular concern was the Bank of Nova Scotia, which shares a party wall with the new building. The wall was underpinned and restrained.

A total of 211 flight auger piles were then bored 23m into the London Clay, stopping short of the underlying saturated Woolwich and Reading beds. A suspended basement slab was placed on top of the piles.

From the basement, three slipform concrete cores, designed to provide lateral stability, were erected to the full height of the building. Each core took just seven days to set up and 10 days to pour. This was vital as it freed up the site's two tower cranes for critical path work later in the project.

The cores are positioned centrally along the north, south and east (rear) faces of the structure.

Four 400mm square columns also climb the full height of the building, set in a rectangular arrangement to provide support for a central atrium adjacent to the east-central core.

At each floor level, pairs of 550mm deep fabricated steel cell beams span between the north and south cores and atrium columns. These beams accept half the floor loading via 16m long floor cell beams at 3m centres. The other half of the floor loading - and the load from the external glazing - is transferred directly to precast concrete edge beams, doing away with unnecessary and unwanted internal columns. And this is where the stonemasonry comes in.

The precast concrete edge beams transfer load to a number of storey-height perimeter columns. To the front (west) and north face of the building, these perimeter columns are of structural stone.

Each stone column consists of four individual blocks, preassembled and tied together with two vertical Macalloy bars.

To the rear (east face) the stone is replaced by steel columns. The south face is the party wall shared with the Bank of Nova Scotia.

However, the interaction between edge beam and column is far from simple. The length of the floor cell beams means that mid-span deflection is significant (the beams are precambered to allow for 20mm deflection when the lightweight composite floor is laid). This translates to significant moment and deflection at the connection with the precast concrete edge beam.

A further moment is applied to the edge beam from the full length aluminium and glass cladding which, in a moment of true architectural whimsy, is set back 400mm from the stone face.

Furthermore, the geometry of the structure is such that while the cell beams are regularly spaced at 3m centres at every floor level, the stone columns are not.

As a result the edge beams are subject to quite severe bending forces. This is resolved by casting a 254 by 254 by 167 UC into the 900 by 400mm beam, with a welded stub connection meeting the cell beams via a moment connection.

However, without preventative measures, this would in turn transfer these bending forces onto the stone columns. The stone, while proven in compression, is far less resilient in bending, so elastomeric bearings are incorporated into each column.

Perhaps the most significant consequence of using stone as a load bearing structure is on the construction sequence. Unlike conventional steel-framed structures, each floor has to be constructed completely before work on the next level can begin.

Construction started in October last year and is progressing well.

'Although there are no examples of this kind of construction, all the construction methods are individually well understood, ' says HBG managing surveyor Mark Ramessa.

'There was an initial learning curve, but we're now completing a floor every two weeks.' Handover to the fit out contractor planned for spring 2002. 'We're both on target and on budget, ' he adds proudly.

So only one question remains: cost. 'It is not just a question of cost, but a question of value, ' argues Ramessa. 'With concrete you have the cost of finishing and cladding.'

Whitby Bird's Matthew Holden agrees: 'You have to look at the consistency and quality of the finish. You would spend years trying to perfect a concrete system that gives a finish as good as this.'

Who's who

Client: Scottish Widows Fund & Life Assurance Society

Development manager: Jones Lang LaSalle

Architect: Eric Parry Architects

Structural engineer: Whitby Bird

Main contractor: HBG Construction

Frame contractor: O'Rourke

Quarry: Albion Stone

Stonemason: Szerelmey

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