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It all stacks up

Its version of the white box on stilts presented Springett Mackay with a number of challenging engineering problems

Architect Springett Mackay has created a backland scheme in the former garden of a house in London's Regent Park Road. The new frontage is not the customary back alley but a street, one end of which was closed off by wartime bombing and never reopened.

Happily, the site faces north-south so solar control is easy to work out. There was the usual nimby stuff but the architect gained planning permission in early 2002.

The design belongs to that well-rehearsed genre of the white box on stilts, which is Miesian or early Corb depending on your allegiances, but certainly Modernist. Side walls, which are set in a bit from the boundaries, are sometimes of insulated render, sometimes of etched glass. First-floor north and south elevations are, as you might expect of the genre, walls of glass. The right-hand side of the front ground-floor elevation is in dark, stack-bonded Flemish engineering bricks. This wall is set back 200mm behind the leading edge of the box above and has a clerestory window strip that provides light for the spare bedroom behind and helps, with the setback, to emphasise the way the upper floor floats. The left-hand side actually does float: it is cantilevered over the open carport and entrance. At the back of the carport is the one storey-high glazed entrance lobby with a 3.7m wide stairway rising almost ceremonially up to a landing, which leads on to a raised rear courtyard and doubles back at the right as a conventional-width stair up to the first floor.

The plan is an inversion of the conventional arrangement, with the main spaces - living, dining, kitchen and study - at firstfloor level, and two bedrooms and en suite bathrooms below, next to the carport and ceremonial stairs. People don't spend a lot of time during the day in their bedrooms so this is a rational proposition, and of necessity front and back bedrooms have high-level windows.

The clerestory front bedroom windows are thus for privacy and to emphasise the hovering effect. The rear windows are much the same because the floor of the rear courtyard is half a storey above floor level.

Looking at the simple street elevation you might expect the plan to be a rectangle.

But the one storey-high entrance lobby, set back 5.5m from the front, complicates the issue both structurally and because it involves a separate roof.

Although the first-floor plan is open, its four zones are demarcated reasonably clearly.

The study and living room are across the front, with the dining area in the middle under a great openable skylight, and with marble and granite-based reconstituted stone tiles on the floor, which is otherwise oak strips. The kitchen zone overlooks the courtyard half a level below.

It is possible to separate the spaces physically, using 1,250mm by 2,800mm, ceilingheight etched glazed panels. They are hung from two connected ceiling tracks, one running the length of the building, the other running across the house between living and dining zones.

Environmental control

The glass of both front and back elevations consists of fixed argon-filled double-glazed low-E panels, which have a solar coating.All this gives the all-glass walls a U rating nicely within current Part L requirements.

Springett Mackay partner Matthew Springett says: 'The whole ground floor has under-floor heating coils set in the screed.

The first floor has trench heaters behind the front and back glass facades. There is no air conditioning but, because a 2.5m by 4m area of the first floor skylight can be opened, a stack effect is created when the ground floor windows are also open. We've had it open during that recent hot spell and it was excellent. It's a great strategy devised by the M&E people, Fulcrum.'

The main and side roofs are both warm roofs, using a Sarna single-ply polymeric membrane with the insulation in the depth of the roof structure. It's actually a very thin roof, no more than the depth of the front fascia. There is a second, fixed, 3m x 1m skylight over the side roof. Both roofs are totally flat and are not accessible because of overlooking issues.

Moment connections

The foundations are a series of piles on whose caps sit the primary square-section steel columns that support the steel structure of the first floor. This consists of seven interconnected 11m by 3m rectangular frames, brought on site as welded-up pairs.

Because of the configuration of the entrance lobby and its stairs, two frames towards the back have a more convoluted shape. The ground floor is an in-situ slab with the ground-floor brick-and-block walls sitting on its edge.

Because the design called for a slender structure, there had to be moment connections everywhere, though temporary bolts and lugs were used to locate the steel members precisely. Techniker engineer Franck Robert explains: 'We have these stiff frames because the structural depth had to be shallow.On a normal building you would have an aspect ratio of 20 to 30. Here it's 40.We have had to fully weld the frames to make sure the connection between the beams and columns is stiff to prevent deflection.' An aspect ratio of 40 is nearly impossible. Springett says:

'Techniker managed to get the sizes down quite remarkably.'

'In terms of dynamics, the structure is light, ' says Robert. 'But because of the large cantilever on the east side, there was a problem with dynamic response. In a bridge an acceptable factor for people walking over it is 10 or 20. But domestic acceptance of movement is much lower: two to four. So we tried to make the floorplate as light and stiff as possible. So there are timber floors - plywood screwed into the sub frame. All the joists were secured against timber blocking, running the full depth of the I-beam webs, using little timber wedges at the end so there was a direct transfer of load.'

Dead loads

Even the glass walls with their high shearstrength silicone joints were included in the load-transfer equation. But Robert says: 'We couldn't afford to put too much stress in the glass.' He managed to arrange the stresses so dead loads were more important than live loads. 'This had an influence on the building process.We propped the cantilever and made sure as much dead load was in the building as possible before we installed the glass.'

The cantilevered study was a major engineering problem. Robert says: 'Because the wall stops half way back, we had no structural continuity. And worse, at the point of rotation the main wall of the house stopped.

So here you lost all your stiffness. There were two challenges, one aesthetic, the other to prevent the building collapsing. The big problem was how to prop the cantilever.'

The way they did it, says Springett, was 'to use a large, 450mm deep beam, which is pivoted about a column on the side of the building at the back of the carport'. Robert elaborates: 'It's like a see-saw rotating about an axis except that the front of the building is permanently pushing down on the beam.

The axis is a true pin connection.'

The forward end of this deep beam supports the edge of the hovering study. The four front frames also cantilever laterally but the main load is carried by this beam. Its rear length is tied by what appear to be 100mm square mullions down into the concrete retaining side wall. They have links to the piles directly below so the load is transferred 20m into the ground. The beam that runs two-thirds of the way along the glazed side wall of the entrance lobby is hidden from view by a long section of etching.

At Christmas, the steel subcontractor went into liquidation, so the main contractor went to another steel fabricator, Brent Fabrications. Springett says: 'It is based in the Thames Estuary and usually makes oil rigs and tunnels. So though there was a delay, the steelwork was done without problems and Brent worked to incredibly high tolerances - higher than the British Standards, like five to10mm rather than 15 and 20mm. And they were very good. There was also a huge amount of site welding. I can't complain: the work was of very good quality.'

CREDITS

MAIN CONTRACTOR Harris Calman Construction

STRUCTURAL ENGINEER Techniker

QUANTITY SURVEYOR Roger Rawlinson Associates

M&E Fulcrum Consulting Services

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