Feature

Strictly Business

Stanton Williams has created a calm, controlled extension to John Outram’s extrovert Judge Business School. But its cool appearance disguises some mighty structural gymnastics, finds Tony Whitehead

Adding an extension is always a delicate business. Do you ape the existing building? Go for contrast? Or attempt a bit of both? For Stanton Williams, architect of the latest extension to the Judge Business School in Cambridge, the situation was even trickier.

For the past 25 years the school has occupied the grade II-listed Old Addenbrooke’s Hospital in the historic centre of Cambridge. The main building’s defining feature is a long and imposing Victorian facade built in 1860, but in the 1990s it was dramatically extended in a flamboyant multi-coloured style by the radical postmodernist John Outram.

So how to add to architecture that already embraces two highly distinctive identities? Tasked with providing an additional 5,000m2 of teaching and office space, Stanton Williams has risen to the challenge in characteristic fashion – by producing a calmly authoritative, almost understated building. With its muted brick and precast concrete facades, and an interior featuring oak, marble and exposed concrete, it is, appropriately enough, rather business-like.

Of course Stanton Williams has good grounds to assert its own style with quiet confidence. Its design for the Sainsbury Laboratory – just half a mile down the road – won the 2012 Stirling prize. The judges praised that building for the way it blended with its historic surroundings, describing it as “beautiful and timeless”. No surprise then that the new Simon Sainsbury Centre at the Judge Business School, which reunites much of the same project team, employs a similar structure and aesthetic – including a concrete frame and large expanses of beautifully finished visual concrete.

Smooth performance

Despite the smoothness of finish required, and the sometimes complex structures, concrete contractor Whelan & Grant decided not to use self-compacting concrete for the Simon Sainsbury Centre.

“It wasn’t necessary,” explains contracts manager Steve Wood. “Some of the more complex features, like the ribbed ceilings and the terrace were poured in layered sections – rib then slab – so that made it easier to prevent voids forming. And even for the high, 5m walls, it was sufficient to pour them in short, full-height sections and vibrate them from the top.”

The mix included 50% ground granulated blast-furnace slag, which slowed curing times, but W&G was fortunate in that its contract started in the spring. “It was ideal timing,“ says Wood. “The warmer temperatures meant curing times did not cause programme issues.”

The main challenge was in creating formwork and finishes of unusually high standards. “For example, we had to mitre the joints that formed the ribs and hide the joint lines in the slab. The formwork had to be joinery standard throughout.”

W&G managed to complete the contract with only one small section of basement needing to be re-poured. The vast majority of concrete was able to be left with little or no making good, and simply rubbed down to create its distinctive smoothness.

The one exception to this was the sloping wall that forms the side and handrail to the atrium staircase. “We poured that in four sections, from top to bottom starting at the highest point,” says Wood, “as obviously the concrete has a tendency to slide down to the bottom of the staircase. We used top shutters with designated compaction points. Inevitably there were some air holes at the top so the smooth slope did require a fair bit of finishing and making good.”

This is not to say that the new design has arisen without regard to the older buildings, as Stanton Williams’ project associate Stephen Hadley explains: “The concrete and brick reflects the masonry construction of the old hospital – so the precast concrete on the facades, for example, is a nod to the stone dressing used on the Victorian building. Also, the brief was for a robust, long-lasting building. We wanted to avoid anything that was going to look too new or flimsy. Concrete construction suited that.”

In a counterpoint to Outram’s more outlandishly decorative style, Stanton Williams liked the idea of the structure being visible, with no obvious separation between structure and architecture. “That’s hard to do if you use a steel frame, which you then tend to clad and so disguise the structure,” says Hadley. “Here we wanted a more monolithic quality. Concrete was always going to be the way to go, and to be honest we didn’t seriously consider other options.”

Hadley adds that the original hospital also informed the massing and scale of the new building: “So we have four storeys, and particularly significant was the decision to carry through the 5m floor-to-ceiling height of the ground floor.”
Stanton Williams realised that what the Victorians favoured for a high-ceilinged hospital ward was also ideal for creating airy circulation areas and teaching spaces: “So we have put the two lecture theatres on the ground floor,” says Hadley. “And we worked hard with the structural engineer, AKT II, to get spans long enough for those spaces to be column-free.”

The solution here was to add deep ribs, or downstand beams, to give strength to the slab. These were typically 475mm deep beneath a 125mm slab and set at 750mm centres. “This not only helped to reduce the amount of concrete we needed to span the 9m across the lecture theatre, it also helped the slab to support loads from above,” says Hadley.

“Because the upper floors had different layouts, it meant loads were landing in awkward places above the lecture theatres. Concrete became a key material to transfer those loads.”
The ribbed ceiling perfectly illustrates Stanton William’s penchant for making structure work architecturally, particularly the way that the recesses between the ribs have been used to house bespoke lighting and colour-matched acoustic panels. “These would normally be hung from an exposed soffit,” says Hadley. “But pushing them up into the ribs gives a less cluttered aesthetic. In fact, the ribs themselves also help the acoustics by breaking up the surface and reducing reverberation.”

A long stretch

Creating the cantilevered canopy that extends over the Simon Sainsbury Centre’s reception area (shown on page 5), and stretches towards, but does not actually reach, the brickwork of the Victorian Addenbrookes Hospital, was one of the project’s key design challenges.

It looks elegant, even simple but, as AKT II structural engineer Steve Toon explains, its construction was anything but. “There’s a lot going on beneath the surface,” he says. “Essentially the canopy cantilevers from a first-floor, open-air terrace situated above and behind the reception. But whereas in the theatre area we used slabs with downstand ribs for extra strength, these do not work particularly well in a cantilever as you just have the bottom of the ‘T’ in compression.”

The solution was first to invert the ribs – enabling the smooth concrete soffit of the canopy – and then to place a second slab on top. “So in places we have slab, rib, slab – almost like a monocoque construction,” says Toon. “In others we are also using the depth of the tree planters on the terrace to accommodate upstand beams.”

In some areas this meant constructing the terrace in three phases (slab, rib, slab), which presented special construction challenges. “It took a lot of propping,” says Toon. “Because the bottom slab did not have the strength to support itself.”

This situation was further complicated by the use of 50% ground granulated blast-furnace slag – a mix that is prone to darkening or bruising if left in formwork too long. “It meant that when the time came to remove the formwork, we had to get it out of the way quickly and immediately prop the slab to prevent it sagging,” says Toon. “We had to do this in sections, sequentially, and then keep the slab propped while the ribs and top slab were cast.”

In some parts of the reception area AKT II insisted that the props remained in place even when all the concrete in the monocoque construction had reached full strength. “The Young’s modulus curve [the indicator of stiffness] comes up slower than the strength gain curve,” says Toon. “If you are not careful you can get excessive early non-elastic deflections not accounted for in the non-linear analysis. So, in deflection-critical points, the props remained even after the concrete had hit its 28-day strength.”

The ribs also contribute to the environmental performance of the building, which is expected to receive a BREEAM rating of Excellent. “Because of the high levels of thermal comfort required by the client, a degree of mechanical cooling was always going to be needed – especially in the highly populated lecture theatres,” says Hadley. “But the aim was for active cooling to be kept to a minimum, and so the thermal mass of the exposed concrete is used to passively absorb heat during the day. The fact that the ceiling is ribbed means it acts like the fins of a radiator – increasing the surface area and boosting the ability of the concrete to absorb heat during the day and release it overnight.” The result is that active cooling can be kept off for longer.

More structural ingenuity can be found in the reception area, but unlike in the lecture theatres, it is not on display. Above the reception desk a smooth concrete canopy, with a flat soffit, stretches towards the older building. It does not reach it, however, the gap between the two being bridged by a glazed roof. This allows plenty of natural light into the reception area and manages gently to connect the old and new buildings in a way that subtly acknowledges the architectural differences between the two.

“Because it reaches out but does not touch the old building, the concrete has to cantilever,” says Hadley. “That meant AKT II coming up with quite a complex, deep structure (see “A long stretch”, right). We wanted a smooth [unribbed] soffit here so the slab is strengthened with upstands this time. These have been incorporated into an open terrace above, where tree planters sit between them. The resulting structure is strong enough to take the weight of the soil, paving slabs and the live load of people enjoying the terrace.”

Throughout the centre the walls and soffits have been formed from non-phenolic plywood boards, which give a matt, stone-like finish. The formwork was drawn in some detail in order to ensure joints and tie holes aligned and were regularly spaced. Tie bolt holes were filled with site-cast plugs, to a 10mm recess. “We were fortunate that [concrete contractor] Whelan & Grant understood our requirements very well and had a team that could provide the kind of craftsmanship we needed,” says Hadley.

Outside, while brickwork dominates, precast concrete elements play a significant role in the aesthetic. Windows are formed from precast lintels, sills and vertical fins. These elements have a more yellow complexion than the in-situ concrete inside – the better, says Hadley, to blend with the brickwork and the tone of the neighbouring Victorian terraces. “The other reason we went for precast was because the site was very tight – especially where the building sits right up against the street pavement. Using precast meant those elements could be placed relatively quickly.”

The exterior fins align with the interior ceiling ribs, and fit well with the slotted appearance of the interior oak panelling. It is a solid, careful use of materials and forms, and one that exerts a calming influence on the extravagant style of the 1990s extension.

Photos: Hufton + Crow

PROJECT TEAM

Architect: Stanton Williams
Structural engineer: AKT II
Main contractor: SDC Builders
Concrete contractor: Whelan & Grant
Precast concrete supplier: Minsterstone