So many commercial developers crank out the same old generic office space. But here’s one that dares to be different. Stephen Kennett visits Southwark council’s low-energy headquarters in London – and, no, there’s not a false ceiling anywhere
In today’s economic climate commercial developers don’t have much appetite for experimentation. Yet before the current downturn, one decided to take a leap of faith. The result is 160 Tooley Street.
According to Simon Allford, of architect Allford Hall Monaghan Morris, it breaks the mould for commercial office buildings. “We are finding that tenants want something different, rather than just the generic office space cranked out by so many developers.”
Just a stone’s throw from More London, the Great Portland Estates development shuns the steel frame and ubiquitous glass cladding of its near neighbours, opting instead for what the architects call a lean office approach, which uses existing materials and technologies in more innovative ways.
The most visible consequence of this is the exposed concrete interior. There are no false ceilings, and support columns remain in their exposed condition. This isn’t simply for effect; it also helped meet one of the aspirations for the building – low energy use. When this was being considered, the ventilation system was an immediate target for attention. Rather than going for the default four-pipe fan-coil air-conditioning, the design team opted for displacement ventilation, which involves fresh air being delivered through a raised floor. “This meant that higher floor-to-soffit heights were needed, with exposed thermal mass to moderate the indoor air temperatures,” says Robert Romanis, associate director at AHMM.
This in itself might not sound radical, but combined with other aspirations such as offsite fabrication and lowering waste, it drove the way the construction team approached the structural design. According to Allford it is the most integrated building the practice has been involved with. Looking under the skin you begin to see why.
The quality of the exposed concrete was paramount. Precast concrete soffit panels, were used to achieve the high-quality finish of the exposed ceilings and eliminate the need for wet trades. Measuring 3,000 x 3,000 x 50mm, these panels also provided the formwork for the 300mm-deep, post-tensioned floor slabs, which enable floor spans of up to 10.5m. It was a first for Arup.
Main contractor Laing O’Rourke was brought in at stage C of the design and its precast division, Mallings, was responsible for creating the panels that were cast in purpose-made steel moulds using self-compacting concrete. Steel lattices were cast into the top surfaces to tie them into the structure.
On site, reusable table forms were used to support the panels, which were placed on bubble packs to prevent them marking. “To get a contractor to work on site with the finishes and not damage them requires a different sort of mentality,” says Allford.
A laser cut steel block – like a large tile spacer – was used to align the panels to make sure the gap between them remained uniform over the 80m run. To help, their edges were radiused to disguise any slight misalignment. Factory-applied butyl tape and cover strips were then used to seal the joints before the post-tensioned slabs – designed to class two standard to limit tensile stresses and prevent cracks – were poured, a quarter of a floor at a time.
It’s an economic solution, says James McLean, structural engineer with Arup.
“The speed of construction is faster than a fairfaced slab cast in situ. A post-tensioned slab with a sprayed plaster finish would have been cheaper but it would have needed painting, which in terms of aesthetics and maintenance wasn’t what we wanted.” The cost of the exposed soffits is around £100/m2, but with a new facility coming on line, Mallings believes this can be halved. “We think it has a future,” says McLean.
To make the floorplates flexible and adaptable, a spaghetti of ventilation ductwork was needed within the raised floor to supply the perimeter zone where cooling loads were highest. With the air delivered from plantrooms at the top of the building, McLean says they considered dropping ducts down at the perimeter but this would have been difficult with the barrel vault roof. “So we thought why not drop them down the structural columns”.
Arup had done this before with steel columns, but this was the first time it had tried it with concrete. Casting the hollow columns was a challenge. Like the ceiling panels, they are exposed and so needed a consistent, high-quality finish. However, although the external diameter remains the same, the internal diameter of the columns increases in three steps as they go up through the building. This accommodates the higher air flow needed at the top of the building, but also neatly packages the increased structural support needed at the lower levels.
Mallings fabricated a steel mould to cast the columns and used a Rapidobat cardboard tube lined with insulation to form the internal shutter to create the void. These tubes are usually used as an external shutter to create a solid column and as such are designed to be used in tension. For the hollow columns, Mallings created a removable internal support to prevent them collapsing. Once the concrete was set, the tube was left in place to prevent the air being warmed as it passed down the column.
According to McLean the first trials left air bubbles at the tops of the columns. Mallings reversed the process, pumping the concrete into the moulds from below. This overcame the lower hydrostatic pressures at the top of the moulds, eliminating the blemishes.
To make the installation simpler, the Rapidobat tubes were left to protrude from the top and bottom of the columns. As each column was assembled on site this formed the shutter around which the floor slab could be cast, doing away with complicated formwork on site. At 1.05m diameter, the columns are chunky – traditional columns would have been between 600mm and 800mm. “We persuaded the client they weren’t inappropriate for floor slabs of this size and once they understand the dual function they realise why,” says McLean. Allford describes them as “majestic”.
The hanging truss
A new entrance to the offices has been created by punching through the refurbished Victorian buildings on Tooley street. Walking in, visitors pass under the historic building into an atrium and are drawn to the entrance ahead. What’s interesting here is what isn’t here,” says Allford. The support columns have been designed out to give a clear, unobstructed view of where you are heading.
Arup has employed a bridge building solution to achieve this. A giant steel truss has been installed at roof level from which the cantilevered sections of the five floors below are hung using Macalloy rods.
This sounds simple enough, but the truss could only be installed after the lower storeys were in place. A temporary way of supporting the floors was needed. At the first storey this was straightforward – the floor was simply propped off the ground-floor slab. For the other floors the 100mm diameter Macalloy rods were employed.
These are designed to take a load in tension. However, during construction they would be in compression. In order to do this without them buckling, additional diagonal struts were used to support each rod until the roof truss was lowered into place and the load could be transferred. It was a tricky operation, recalls McLean, as the cantilevered floors had to remain static when the load was transferred to the truss.
Great Portland Estates sold the development to UBS Global Asset Management before the piling was completed and it in turn has let the entire building to Southwark council. The building has been awarded a B on its energy performance certificate, with carbon emissions of 50kg CO2/m2. Sometimes, it seems, it pays to be a bit different.
Specifier 17 October 2008
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Lean machine – Southwark council HQ
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Lean machine – Southwark council HQ