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Filigree writ large in the urban web

BUILDING STUDY

A fine lattice of reconstituted stone plays a vital part in the integration of Foster's new Sainsbury's headquarters building into a key site at Holborn Circus in central London

The old Mirror building used to squat squarely on the south-west corner of Holborn Circus in central London. Now a great convex glass wall from the Foster office has replaced it, its lower two storeys intriguingly wrapped by a lattice grid. The whole facade of this new Sainsbury's headquarters building is enigmatic because the glass of the upper floors encases a stone-clad structural grid which, as you can make out through the glass, breaks back in the middle to form a wedge-shaped atrium with a glass roof. The groundlevel lattice grid breaks through the glass skin here and follows the inside walls of the atrium before emerging into the street again. Round on the west elevation, and the south as well, the same orthogonal filigree becomes the whole wall skin above second-floor level, which is currently occupied by a Sainsbury's store.

Holborn Circus is a kind of urban node: it is an unexpectedly big and quite friendly space, although its air is sharp with pollution from traffic bound in seven different directions. Foster project architect Angus Campbell says: 'We had the notion that the building should be in two halves so that it allowed views from different directions across the atrium to the Circus, which is a really nice space.' As a response to the pollution, he says: 'We have tried to give the idea of a stone building whose structural grid you can see through the hard protective glass superimposed on top. On the same facade where people come in contact with it - at ground level - we wanted a more human scale and so here is a brise soleil or, since there is no sun on these elevations, more accurately a lattice - which also adds a certain amount of privacy to the offices at street level.'

Fitting in

'On the west and south facades there was a problem of solar shading, 'Campbell adds, 'but the scale of these streets is a lot smaller and we knew that in the future they might be pedestrianised. So we wanted something more in tune with the age of the buildings adjoining it.

'Many of these buildings are faced with Portland stone and when we got planning it was on the basis of our using Portland stone. In the end we didn't want to use it because when we tried it out the minimum sizes of the members of the lattice, even with metal cores, turned out to be far too large - twice the size of members in reconstituted stone.

So we met with the planners and showed them the effect of the proportions we would achieve with reconstituted stone.'

The planners agreed to go with the constituent elements of the lattices in reconstituted stone with visible parts of the structure in a flamed Portuguese granite.

It's a wrap

The building's structural columns are located outside the perimeter of the floor slab. The curving north/east facade has unitised glazing in 1.5m-wide and storey-height panels which are bolted on outside the face of the Portuguese granite-faced columns. The two-storey-high lattice at ground level (and inside around the perimeter of the atrium) has glass units bolted on the back. The same glass-behind-lattice arrangement occurs on the south and west elevations. 'We looked at having the glazing bolted on in the factory, ' explains Campbell, 'but it made the units too heavy to transport on site.'

Although the lattice at the front and that on the rear elevations look the same - and have identically sized members - the front lattice was assembled on site. It is two storeys high and could not have been transported as single units. The lattices on the two rear elevations are part of storey-height precast units which incorporate the granite facing for columns and slabedge spandrels. The lattice on the west facade is formed by 3.8m-high storey-height precast panels, 9m wide; on the south facade they are 6m wide - aligning with the dimensions of the building grid.

The precast panels fit between the columns. Instead of giving the columns the granite facing they have on the north/east facade, the facing is incorporated in the vertical edges of the precast panels.

There is a 600mm granite-faced angle detail which wraps halfway around the adjacent columns where it meets the handed detail on the edge of the next precast panel. This leaves a 20mm shadow gap down the centreline of all the columns, a detail which is repeated wherever two pieces of stone join.

Slender elements The horizontal elements must be some of the thinnest legal concrete elements seen in recent years. They are 150mm deep, 75mm at the front sloping up to 85mm at the back. The minimum dimensions are less than this because there is a 20mm wide, 10mm deep rebate all around the profile at each end to provide a shadowgap between the horizontal and vertical concrete elements. You would normally expect a minimum thickness for concrete of around 120mm - the price of having to have adequate mechanical and fire cover for the reinforcement.

But this is not structural work, it is cladding and so, providing the reinforcement is unlikely to corrode, which it will not if stainless steel and dense concrete are used, fire cover is not an issue.

The dimensions of the mullions are equally surprising. They are storey-height on the precast units, and twice that, at 7.6m high, on the site-assembled lattice around the front ground floor. One wonders at the engineering of their extraordinarily slim dimensions of only 120mm wide by 300 mm deep. Former Techrete technical director Peter Flynn, now at the Arup office in Dublin, says modestly: 'There was a serious amount of structural analysis involved, not least so as to avoid movement, especially during erection.'

Campbell recalls of the louvres: 'Techrete produced a louvre with stainless steel reinforcement and brought it into the office. We put bricks under it at each end and jumped up and down on it and it failed. So we were faced with making its section a lot bigger. But Techrete came up with the idea of keeping the slim dimensions and prestressing it. So they did another one. It deflected when we jumped on it - but it returned to its original shape.' The engineering calculations checked out and it was agreed to go ahead with the slim, pre-stressed louvres.

Campbell points out: 'There are two different types of louvre.

One we put into the big storeyheight mould and cast into the panel. The other condition is at ground level around the north and east elevation and in the atrium, where it was all fixed on site. Here we put a stainless steel shoe on each end and cast a kind of saddle into the mullion.

Instead of reducing the profile [for the shadowgap rebate] at each end, it's got a stainless steel plate coming out which sits on the saddle, and there's a clamping block to fix the location and allow movement.' The louvres of the lattices are 3m long but are up to 4m long where they are used to face the glazing of the two stair-lift zones at each end of the curving north/east facade.

The reconstituted stone is a concrete made up of a white cement which has a Cornish grey aggregate, with a maximum size of 10mm. After being cast it was acid-etched to expose some of the aggregate. This also provides a final surface treatment consonant with the flame texturing used to enhance the sparkling mica content of the Portuguese granite.

Shadowgap detail

Campbell established the principle of a 20mm shadowgap which was to be used for all the stone detailing. He says: 'When you are designing stone you tend to keep the gap between pieces very small. But we decided to maintain a 20mm gap so it was more in tune with aluminium detailing than stone. It means the water drains down the rebates so that staining is controlled - there is a little integrated gutter detail between the stone elements so there's no water cascading down the face of the stone.'

MAKING FILIGREE

The very long mullions around the ground floor were made in a steel mould with the billets to hold the louvres in place cast in at the same time. John Stothard, production director at Techrete's UK factory at Brigg, where the precast work was carried out, says the precision needed for these was a major headache. He had to use steel moulds instead of timber. And for the louvres for this section (which had to be assembled on site) there was a tension bed with the stainless steel tendons running down the length of the steel bed. The company made 24 louvres at a time - half a dozen rows of four - which were cut once the concrete had set.

The day following pouring, Stothard tested the concrete strength. The louvres were taken out carefully and the next batch poured. Stothard says: 'It was a very complex mould which we disassembled every day.

And we had to fix lifting points at the back of the bigger moulds which were in the region of nine tonnes. It was a bit of a pig lifting them up from horizontal - we had to develop a new system for doing it. But once they were vertical the panels were relatively easy to handle.

'It was a very complex and difficult job. But it was made interesting because it was very intricate and very precise - and because of what you see at Holborn. It works, it looks good and everybody was impressed with it. I have done bigger precast panels but nothing as intricate.'

THE ENGINEER

Peter Flynn, now an associate with Arup's Dublin office, was Techrete's technical director. He says: 'This was a very fine filigree of stone and precast. The combination of very thin precast louvres incorporated into a large precast element was very novel.

'The thing you can do with precast products is that you can really tightly control both the position of elements and the quality, which is to say durability, of the concrete. You can do things in a mould which would be very difficult to do in an in-situ environment. There is so much quality and dimensional control in the factory. It took an awful lot of dialogue between the team and the works, and there was a lot of feedback - down to how the joints would be formed in the mould. There are a lot of elements in the panels which are actually driven by the manufacturing process - including the way the mould is broken down each time. And there was a lot of interface with Felix, the window company. It was a difficult but very enjoyable job to be associated with.'

Flynn pays special tribute to Foster's project architect Angus Campbell: 'He is one of the best architects I've worked with in 17 years as an engineer. Specialist subcontractors do come across architects who interact and who can discuss and really extract the best from the subcontractor's designer, who explain the constraints and the opportunities without being aggressive. Campbell did all this very quickly merely by questions and answers, sketches, drawings and coaxing. This means you end up talking with somebody who is trying to use all the strong aspects of a system as opposed to trying to make precast do things it's not really suited for, using the material incorrectly.'

CREDITS

ARCHITECT (SHELL AND CORE) Foster and Partners

M&E CONSULTANT Hilson Moran Partnership

STRUCTURAL ENGINEER Yolles Partnership

MAIN CONTRACTOR Bovis Lend Lease

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