Influenced by client priorities and the curving shape of the site, structural engineer Symmetrys opted for a concrete frame on a south coast retirement development.
Project: Horstley, Seaford
Contract value: £8.3m
Main contractor: Midas
Structural engineer: Symmetrys
Concrete frame subcontractor: O’Connell’s
Architect: RCKa Architects
Start date: April 2016
Completion date: August 2017
The retirement homes sector is booming.
An ageing population, cashed up with unmortgaged housing assets, is fuelling a development drive among retirement home builders such as PegasusLife. Dubbed the ‘Waitrose’ of the sector, it is building 800 high-end retirement homes over the next two years.
Given the high demand, it might be expected that developers like PegasusLife would favour fast building methods, such as light-gauge steel or cross-laminated timber. But not always.
Right frame of mind
One of its developments is on the south coast at Seaford, East Sussex, where a 38-apartment, six-storey scheme is being built with a flat-slab concrete frame. Russell Thomas, associate director at structural engineer Symmetrys, explains the choice of framing material.
“We considered several different options for the frame,” he says. “The original plan was either light-gauge steel frame or cross-laminated timber. They offered programme benefits compared to concrete, but they both rely on partitions being load-bearing. This presented flexibility issues, as Pegasus wasn’t sure how the market would develop in terms of demand for one, two or three-bed apartments.
“Currently its market is 50 per cent single women, 25 per cent single men and 25 per cent couples. They wanted the ability to amend the layouts during the design phase and, potentially, once the building had been constructed,” Mr Thomas adds.
“Concrete is more adaptable and was easier to fit into the curving shape of the building’s footprint”
Russell Thomas, Symmetrys
Another factor in favour of concrete was the geometry of the 4,200 sq m site. The apartment building is 55 m long by 16 m deep, and runs in an arc along Stafford Road from roughly north-east to south-west, sandwiched between two party walls (a library and another block of flats). The rear of the site will be landscaped into gardens.
“Concrete is more adaptable and was easier to fit into the curving shape of the building’s footprint,” Mr Thomas says.
“Additionally, PegasusLife liked the solid feel of concrete, and felt it was more suitable for a retirement community. Concrete also makes it easier to achieve the required insulation and sound values.”
Although the site is less than half a kilometre from the sea, it is far enough away for the salty air not to be a concern. “I’ve worked on harbour projects where the corrosive effect of salt in the rebar can be a real problem for a concrete frame,” Mr Thomas says. “But not here.”
However, there have been plenty of sea breezes to contend with. CN’s visit to the site came the day after Storm Doris, when the tower crane, sited in the rear garden, was out of action. On average, the project team has been ‘winded off’ a day every week, says Midas project manager Jason Cusack.
In such weather conditions, a concrete frame offered further advantages. “When there were high winds, we were able to get on with fixing the reinforcement – but obviously you can’t do that kind of preparation work with steel,” Mr Cusack explains. “Also, when you know wind is forecast, you can plan the installation of the shuttering in advance on days that aren’t windy. The concrete pump isn’t affected by winds.”
Pegasus Symmetrys Steel frame of rear gallery
All 1,200 cu m of concrete specified for the project is environmentally friendly, with ground-granulated blast-furnace slag used to the maximum permitted level.
The rebar is all made from recycled steel. “The GGBS requires slightly longer curing time, so we made provision in the programme for the propping to be co-ordinated with the fit-out of the lower floors,” Mr Thomas says.
“But in the event, the frame erection was quick, so there was no overlap. Concrete contractor O’Connell’s completed the seven-month frame programme seven weeks early.”
Planning permission for the scheme was granted in November 2015 with work starting on site the following spring. The building now taking shape comprises a basement car park and five storeys above, with two penthouses on the top level.
Frame affects foundation design
A knock-on effect of choosing concrete for the frame was the use of a different foundation design.
“We could have made a 10-15 per cent cost saving on the foundations with a lighter superstructure frame built from steel or timber,” Mr Thomas says. “The underlying chalk is quite stiff so we could have used shallow pad foundations.
“However, the party walls here are close to a public highway, which meant a contiguous piled wall was required for temporary works. So while we had the piling rig here, we thought we’d use gravity piles instead.”
Pegasus Symmetrys Curve of site at rear from ground level 3
Some 271 reinforced concrete CFA piles were cast in situ, each 450 mm in diameter and to a depth of 14 m.
The basement is more of an undercroft, rising half a storey above ground on the street elevation, and with a retaining wall decorated with gabion baskets to the rear on the garden side. At its floor, the basement is 1.8 m below street level and will contain 35 car parking spaces.
“The layout requirements of the car park are obviously different from the apartments above,” Mr Thomas says. “The structural grid is 8 m by 7.5 m, which allows space for three parking bays but bears no relation to the overlying columns.
“Therefore a transfer structure had to be designed into the frame at ground-floor level. The ground floor slab is 350 mm thick and supported by four 600 mm-deep by 800 mm-wide downstand beams, which curve around the full 55 m length of the building, supported by columns 350 mm by 800 mm thick.”
The structure is not all concrete. For the penthouse level, which commands a sea view, the design did not match the column layout of the underlying apartments, so Symmetrys decided to use a lighter framing material. “The intention was originally to use light-gauge steel frame, but we found it hard to source a supplier for such a small project, so the penthouse is being built using SIPs [structural insulated panels] instead,” Mr Thomas says.
At the rear, there is a glazed gallery on each level, intended as a circulation space. “This is predominantly a steel frame, but includes structural timber joists for the floors, which are supported by the steelwork,” Mr Thomas says. The steel frame spans are each about 7 m to tie in in with the concrete frame, and the gallery’s depth is about 3 m.
The garden elevation also includes the development’s signature architectural feature: a lattice-clad spiral staircase, which starts inside a retaining wall at basement level and extends up to the top floor. “We haven’t decided yet if this will be concrete or steel,” Mr Thomas adds.
The Stafford Road street frontage will be clad with imported bricks from Belgian firm Van Der Sanden, and includes inset balconies with timber decking for each one of the dual-aspect apartments. The roof will be galvanised sheeting.
The design engineering features two other transfer structures at each party wall end of the building.
Pegasus Symmetrys Curve of site from above at roof level
“The contiguous piled wall could not be tight against the party wall – there needed to be a 600 mm space – so the basement does not run the full length of the building,” Mr Thomas says.
“Obviously this means the perimeter columns of the upper storeys do not line up with the piles in the basement structure, hence the need for the transfer structure. Above ground level, the ends of the building are concrete columns, with concrete block infill, which abut the party walls.”
The superstructure is set out on roughly a 7 m grid, with the floor slabs 225 mm thick and the columns mostly 200 mm or 175 mm in thickness. “The spans will allow partitions to be fitted, should market demands require it,” Mr Thomas says.
Services are accommodated vertically in seven risers, spread across the floor plate, with a 75 mm-deep ceiling void for horizontal runs. The M&E plant is in the basement, and the building’s one lift is located at the west end in a concrete shaft.
“When there were high winds, we were able to get on with fixing the reinforcement – but obviously you can’t do that kind of preparation work with steel”
Jason Cusack, Midas
Logistically, the project has been tricky to manage for Midas. “The site is as tight as anything I’ve worked on in London,” Mr Cusack says.
One half of Stafford Road, running the whole length of the site, was taken over for the compound and for most deliveries. The exception was the steelwork, which came in through the other side of the site, on Sutton Park Road, which will serve as the main entrance for the completed development.
“We will also take the tower crane out that way once the landscaping is finished,” Mr Cusack says.
The remaining work on the scheme is mostly the envelope, landscaping and the high-specification internal fit-out. “There is a focus on quality and attention to detail, in line with the Pegasus brand,” Mr Thomas adds.
The project, which PegasusLife has called Horstley, is scheduled to be complete in August 2017.