The Salvation Army has always maintained a firm focus on frontline charity, holding fast to the “Soup, Soap and Salvation” mantra of its founder, William Booth. So when the organisation decided that a new headquarters was essential, it did so almost reluctantly, and with a keen appreciation that its supporters do not drop coins into collecting boxes in the expectation of funding a grand new building or vanity project.
“The client was very clear from the start,” says Jonathan Pinfield, associate with architect TateHindle. “They wanted a quality building that would last, but absolutely they wanted cost efficiency.” The result is the SA’s new 5,100m2 HQ in Denmark Hill, south London. Rated BREEAM Excellent, it has been constructed next to the existing William Booth College, built in 1929 and designed by Giles Gilbert Scott. The new building comprises six floors of mainly open- plan office accommodation, wrapped around a central atrium.
“The massing and proportions were determined partly by the fairly constrained site, but are also designed to pick up on those of the existing college,” explains Pinfield. “One of our key challenges was to make those dimensions work in a way that delivered the right user experience, but at a price that would be acceptable. The choice of frame was central to this.”
The building has longish, 9m spans all round – the natural distance from the exterior facades to the central atrium which runs like a nave along the length of the building. “We could have shortened these with extra supports perhaps, but we didn’t want a forest of columns obstructing the natural light from the external glazing and the atrium. We looked in some detail into how we could achieve those spans while bearing in mind the SA’s concerns about cost.”
They examined both concrete and steel solutions, he adds. “But the client wanted the frame to have a 120-year design life and we found that was more easily achievable with concrete. The steel option had issues with finishes and headroom and, having checked with our QS, concrete also came out cheaper. This was partly because leaving concrete soffits and columns largely exposed saved money on materials like plasterboard that would be needed to encase a steel frame.”
Having chosen concrete, the challenge for the team was to come up with a design for the spans that was both sustainable and thrifty. “It helped that we were working with the owner-occupier from day one,” says Jess Davies, associate at structural engineer Davies Maguire. “The SA were operating from a number of different premises in London which were coming to the end of their usefulness, but when exactly they left them was up to them. It meant the programme was not particularly tight.”
That gave Davies some leeway with the slab: “We came up with the final ribbed design after considering steel and precast concrete solutions,” she says. “It was a little slower, because constructing the ribs with reinforced concrete poured in situ takes time compared with slotting in steel or constructing thicker, simpler slabs. But it’s worth it because the design delivers a number of important benefits.”
To span 9m, a flat slab would have been unusually deep and heavy: “The self-weight of the slab starts to get a bit much – you end up designing to support the slab, rather than any other loads. Using ribs releases the weight, uses less concrete, lowers the embodied carbon involved, but keeps the span.”
The design is repeated throughout the building, so while the ground-floor slab is 350mm deep, those above are just 100mm, supported by ribs 150mm wide but 350mm deep, and typically spaced 530mm apart. Ribs and slab are poured as one element, with the gaps between ribs created using removable polystyrene forms.
This comparatively lightweight design reduced the loadings for the whole building and this, in turn, means that the foundations are not as extensive as they might have been. As it is, these involve a contiguous piled concrete retaining wall (to cope with the Denmark Hill site’s 5m drop) and 137 bearing piles up to 600mm in diameter. “But a heavier frame would have needed longer piles, adding to the total concrete used,” says Davies.
The mix itself was also chosen with the carbon cost of the building in mind. “It’s fairly common to use 20-30% cement substitute such as GGBS,” says Davies. “But here the concrete in the columns and upper floor slabs had 50% of the cement replaced with GGBS. Again, it helped that the programme was not too tight, as the more GGBS, the longer the strike times.”
Despite the generous programme, the longer curing times did prompt contractor McLaren to split each floor into six pours rather than the planned four, to help maintain progress on site. The efforts to minimise embodied carbon have paid off, however: “We compared our solution with that of an equivalent steel frame with composite concrete and metal deck slabs, and found we were achieving a 30% saving on CO2 per square metre of floor space,” says Davies. “The ribbed design accounted for most of that saving.”
Davies Maguire also looked in detail at the impact of using GGBS as cement replacement: “Our final figure for the building was 297kg of embodied CO per m2 – which is 70kg less than if we had built the final ribbed design with a standard CEM I mix – that is, with no GGBS,” says Davies. The engineers worked out the CEM I values using their own in-house carbon calculator, based on the IStructE carbon calculator and ICE v3.0 Module A1-A3 data. The values for the GGBS mix, meanwhile, were from the actual project data during construction. “We included the full mix design values for the stated concrete mixes provided by the piling subcontractor, waterproofing subcontractor, and concrete frame sub-contractor – calculations which also included distances to plants.”
Davies adds: “The savings might have been even more, but some of the foundation works contained less GGBS. The ground-floor slab, for example, included a waterproofing admixture, and so the mix was to a specialist’s design.” The slabs are supported by 40 reinforced-concrete columns constructed using ply forms. “Most of these are 500 x 500mm,” says Davies. “But there are some 250mm x 1m rectangular columns where the architecture requires it – for example, where the perimeter steps in a little around the entrance.”
The rectangular columns have also been deployed where the upper floors step back from the street – a feature designed to echo the style of the existing college buildings. “Moving columns is always interesting,” says Davies, explaining that the upper columns land a meter or so back from the front facade columns of the first four storeys. “The ribbed slabs have a 1m-wide concrete band beam running the whole way around every bay, so where the columns step back on the fifth floor, they land on the beam. There is added reinforcement in the beam at those points.”
Despite the dominance of exposed concrete in the design, the interior does not feel in any way industrial, partly due to the colour of the concrete. “The high GGBS content gives the concrete a pale grey colour,” says Pinfield. “It helps maximise the natural light. At the top of the atrium we have deep glulam [timber laminate] beams which provide some solar shading, but in daylight they also have a warm glow about them which is taken up by the light-coloured concrete.”
There is a great deal of timber used throughout the building. It part-panels some interior columns and, on each floor, timber slatting part-covers the concrete ribbed slab soffits where they meet the atrium space. “The timber works hard acoustically,” says Pinfield. “The atrium is completely open to help provide connectivity between different departments, and people can easily see colleagues on different floors (see box). But the risk is noise transmission, so the ribbed slabs are useful here as they help disrupt sound reflections.”
In addition, the timber slats have acoustic backing, and the spaces between the ribs also contain acoustic panels. The effect is remarkable: despite the hundreds of occupants and open-plan scheme, the whole building enjoys a library-like calm. “The ribs also help to conceal lighting and some services,” adds Pinfield. “The client didn’t want the full, exposed services look – so the ribs are a way of largely concealing these while still leaving the soffit exposed. And because there is no false ceiling, the building can benefit from the thermal mass of the concrete.”
By absorbing excess heat during the day, the exposed concrete frame reduces the need for air conditioning in summer. Alternatively, by not venting the building overnight in winter, the stored heat can be used to reduce heating requirements. The thermal mass effect works in conjunction with air-source heat pumps to heat and cool the building, which is part-powered by 90 photovoltaic panels on the roof.
Designing the staircases
Standing in the central atrium of The Salvation Army’s new headquarters, there is much to admire: its cathedral-like proportions, the stylish timber bulwarks and the glulam top beams. But one of the most striking and important features are the elegant concrete staircases that rise from floor to floor, zig-zagging their way to the upper storeys.
They are meant to stand out, and to foster connectivity by enticing building occupants to explore other floors and other departments. Constructing them, however, was not straightforward.
“The original plan was to have them precast,” says Jess Davies, associate at structural engineer Davies Maguire. “They are very visible features, and precast would have guaranteed a good finish. But at more than 7m, we realised they were too long for most precasters’ facilities, and would not have fitted on a lorry because of the length and angle.”
She then looked at constructing the 1.2m-wide staircases from two precast sections, roughly 3.5m-long, but this solution was also rejected: “The stairs are unsupported by any columns, and to construct in two pieces would have involved having a moment connection cast in to the half-landing, which would have been quite complicated,” she says. “The stairs also have quite a slim, 275mm, waist (the minimum thickness from soffit to step) so there was not much room for complex connections.”
In the end, the stairs were cast in-situ. “This was quite a challenge for the contractor, as they had to start with the most visible staircase on the ground floor, so they had no time to practise the finish.” Each one was constructed in two pours using traditional ply formwork supported on scaffolding. “Mitchellsons worked hard to get a smooth, clean finish, and hide the joints between the pours,” says Davies. The steps are topped with terrazzo tiling to match the ground floor atrium.
Despite appearances, the stylish brickwork facades of the building feature yet more concrete: the brickwork is constructed using 150mm-thick precast panels set with 50mm-thick brick slips. Pinfield explains that the precast option suited the design, which features deep reveals and brick piers. “It means that when you look at the facade from an angle, you read it as a heavy brick building – in tune with the original college buildings. But from the front you see there is plenty of glazing to provide the interior with lots of natural light.”
The precast brick-faced panels offered a number of advantages over traditional brickwork: “You have the quality control of off-site manufacture, and its quick to build on site with less scaffolding and fewer people,” says Pinfield. “But, in any case, we would have struggled to create some of the tall slim piers – just one brick wide – traditionally. They would not have been stable. The precast panels, however, are structurally self supporting. They sit on a ground beam and this has the added benefit of not adding any loading to the upper frame.”
In all, 296 precast panels were used, the largest weighing seven tonnes and measuring 7.7m x 2.7m. The facade brickwork has been combined with creamy coloured glass-reinforced concrete to echo the brick and Portland Stone pallet of the original college. The overall effect is an elegant blend of modern and traditional, with the new HQ respecting the old buildings, while asserting its own identity. William Booth would surely approve.