Feature

Collar Instinct

Have Derwent London and AHMM created the perfect concrete workplace?

With its Silicon Roundabout “Factory”, AHMM takes the trend for sustainable, flexible, industrial-style office spaces to its logical conclusion, writes Tony Whitehead

Developer Derwent has done more than most to popularise the factory aesthetic for London’s commercial buildings. Characterised by high ceilings, an exposed frame and visible services, among the earliest examples was Derwent’s 2003 refurbishment of the Tea Building in Shoreditch, followed by the Johnson Building in Hatton Garden in 2006. Of the latter, Derwent’s Matt Massey recalls: “Letting agents would come round and ask when it was going to be finished – when was the plasterboard going in? How things have changed …”

Delighted with the growing popularity of their projects, Derwent attempted to analyse what had gone right. As Massey, senior project manager, puts it: “Why did these old industrial buildings made such great office spaces long after they were designed for another purpose entirely? What they had in common was a big, robust, often concrete frame. They were future-proof because the frame could take a lot of knocking about as the building was put to different uses.”

White Collar Factory, a 17-storey new-build office building situated on Old Street’s “Silicon Roundabout” on the edge of the City, is the latest and perhaps purest expression of the style pioneered by Derwent and its partner, architect Alfred Hall Monaghan Morris.

Much of its in-situ concrete construction – including columns, walls, cores and ceilings – is in clear sight, and the concrete finishes, with timber markings and tie holes visible, tell the story of its construction. Particularly striking is the floor-to-ceiling height. “This is one of the features of industrial-style buildings that tenants really love,” says Stephen Taylor, associate director with AHMM. “Here we have a 4m floor-to-floor height, incorporating a 350mm reinforced concrete flat slab and a 150mm raised floor. That leaves 3.5m floor-to-ceiling – considerably more headroom than a standard office, which has more like 2.7m.”

With so much visible concrete the mix was of paramount importance. The construction team aimed for (and achieved) an Excellent BREEAM rating so 50% of the cement was replaced with ground granulated blast furnace slag (GGBS) to reduce the carbon content. But, says Taylor, the GGBS resulted in a “whitish” finish: “We wanted a warmer, more traditional concrete look, so 200kg/m3 of PFA (pulverised fuel ash, now known as fly ash) was added to darken the mix.”

The final recipe, which also included recycled aggregate, was only arrived at after casting a dozen 1m2 medallions of various mixes, and then constructing a pair of 3m-high mock-ups of the two preferred options. “These also allowed us to try out working joints and corner details as well as test the effects of different species of timber formwork,” says Taylor.

Central to how the building works is the flat slab. Its thickness, says Taylor, is principally to enable it to cope with spans of up to 12m (see box). Slimmer ribbed slabs or post-tensioned solutions were  considered, but rejected for several reasons. “The 350mm helps give the building the thermal mass we wanted to help it stay cool in summer and warm in winter. We also wanted a clear, flat soffit to make it easy to fit – and then refit in the future – services like lighting and sprinklers.” 

The passive cooling offered by the slabs is boosted by miles of plastic water pipes embedded within it. To incorporate the pipework, the first layer of reinforcement was laid on tables over a plywood deck. The pipes were then tied to the reinforcement and a second layer of reinforcement placed above that. Before the slab was poured, the pipes were pressurised with air to check they weren’t leaking, and also to stop them being crushed by the concrete. Taylor admits that the idea of a pipe-cooled slab is hardly new – it was employed in the early 20th century by the American architect and concrete enthusiast Frank Lloyd Wright. “Unfortunately his pipes leaked because they were made of metal. Ours don’t.”

And unlike Wright’s water pipes, those at White Collar Factory also benefit from an advanced building management system (BMS) to control the water temperature. Water can be heated if necessary in winter and the coolness varied in summer for slight or vigorous chilling. Five sensors on every floor monitor slab temperature and air humidity to prevent condensation, and the BMS incorporates weather forecasts to allow for the time lag inherent in any thermal mass-based temperature-control system.

High space, high value

White Collar Factory’s 3.5m floor-to-ceiling height is rare for a reason: it reduces the number of floors. So why would a developer interested in net lettable space choose to build the White Collar Factory way?

“It’s true we could have fitted in another two floors had we gone for standard heights,” agrees Derwent’s Matt Massey. “But the height, like the exposed concrete, is part of the factory aesthetic, and we find it works for our tenants and for us.”

The bottom line, says Massey, is that quality space pays well: “We can charge higher rent, the buildings let so much quicker and we have fewer voids. At White Collar Factory, practical  completion was 28 February, and on 1 March we had six lots of fit-out contractors queuing up outside waiting to start work. If you are hitting completion and triggering leases the following day, that’s good business.”

Much the same can be said of the exposed concrete frame and soffits. Not only have they become fashionable, they also work practically. “The concrete is structure, cooling system and finish all in one – that‘s very efficient,” says Massey. “And it’s designed to be straightforward to build and simple to install services. It helps with our risk too, because the simpler it is, the more likely contractors are to finish on time.”

And even after completion, the strippeddown style continues to work for developer and tenant alike: “We find occupiers really embrace the ethos of the building. There are not many cellular-type layouts. One of our tenants is delighted he can make eye contact with his whole company from his desk. Moving here was a no-brainer for him.”

Obviously it is too early to predict the demise of the traditional plasterboard office – White Collar Factories are not for everyone. But this simpler style of commercial building certainly looks set to claim a larger market share for some time to come.

Little of this sophistication is apparent to the visitor, however. The soffits have a simple finish revealing the standard plywood boards used for the decking. Similarly raw, or apparently so, is the horizontal timber-marked plank pattern visible in the concrete around both cores and the ground floor external walls. But while this looks like timber marking, it is in fact the result of a rubber form liner inside standard plywood shuttering. “We did this because the cores are jump-formed,” explains Taylor. “It would not be practical to have chippies creating beautiful timber formwork every few weeks each time the core rose. So, having chosen our species of timber, and the way we wanted the boards to look in terms of variation and edging, we then had rubber moulds of our perfect wall made by Reckli in Germany.”

The largest wall in the project is some 9m x 6m in the reception area, and so this became the basis for the mould and therefore the repeat pattern – “like wallpaper“ according to Taylor, who adds that, unlike timber boards, the moulds can be reused many times and retain a consistent finish. “So it’s much quicker to simply reuse the latex liner every time the core jumps. Of course, we also used it wherever we wanted that board-marked finish.” 

This included the ground-floor perimeter walls, the only part of the building to be constructed using self-compacting concrete. While this is commonly specified where there are concerns about the finish, or the complexity of formwork or reinforcement, at White Collar Factory the choice resulted from a programming decision. “It took the walls off the critical path,” explains Taylor. “We wanted to build the first-floor slab early, so of course we could not then pour the walls from above because the slab would be in the way. Instead we
pumped into the forms for the walls from below, using self-compacting concrete.”

Whether the visitor is looking at the perimeter walls or those on the cores, the result is a remarkably well-defined board-marked concrete finish. Variations in board thickness, along with the grain of the timber, show through beautifully, and it is clear that using the latex moulds has enabled the construction team to attain that hard-to-achieve paradoxical ideal: consistent inconsistency.

Making the Factory fit

In 2008, Derwent asked AHMM, along with structural engineer AKT II and service engineer Arup, to take its back-to-basics aesthetic to its logical conclusion. What would the perfect strippeddown office space look like? The result of this concept study was a 9m x 9m grid concrete-framed building with high ceilings, a high thermal mass to minimise heating and cooling requirements – all combined with passive ventilation in the form of openable windows, with the end-user in control.

It meant that when Derwent acquired the Old Street site, it had the essence of the perfect design ready and waiting. The only trouble was that the concept study, naturally enough, was for a rectangular building. Bordered by the roundabout at Old Street, the new site, more a rhombus with chamfered corners, was anything but. 

“We had to work out how to apply the principles we had developed to an irregular-shaped site,” says the project’s structural engineer, Rob Partridge of AKT II. “It also came with all sorts of local constraints – principally two underground and two mainline railway tunnels.” 

Piles sunk from the southern perimeter of the site would have hit the railways, so AKT II developed a solution whereby large, 1.2-1.5m diameter bored concrete piles were sunk some 5m away from the tunnels. Massive 1m-thick full-height reinforced concrete walls in the basements were then used as transfer structures, cantilevering over the top of the tunnels to support the building above.

“This way all the complications happened in the basements,” says Partridge. “From ground floor up everything stacks, with no structural gymnastics. It’s predicated on standardisation, very repetitive, simple and easy to build. It means the building is inherently more flexible for the future because it will be easy to retrofit.”

Because of the basement complexities, the main core to the south was constructed bottom up. “We did this to minimise risk,” says Partridge. “It’s safer to be working in a blue-sky environment when constructing these very large structural elements.”

The less complex, smaller north core was built topdown to save time but both were jump-formed, with the form rising every four to six weeks. “Slip forming wasn’t an option,” explains Partridge. “The drag can leave a messy finish – not what we wanted for the exposed concrete finish around the cores.”

Having dealt with the foundation design, AKT II then started to adjust its generic 9m x 9m grid, looking all the time for structural efficiency as well as simplicity. “Of course, none of the dimensions divided by nine,” says Partridge. “We played around with column positions and ended up with spans of 9-12m – which is a lot for an ordinary-strength reinforced flat concrete slab.” 

AKT II could do this, says Partridge, because of extra analysis of the stresses involved, “mapping out slab stresses of several different column configurations in forensic detail”. This gave the engineers the confidence to be less conservative than usual about slab deflection predictions. “We are not scared of doing that – and the result is that as well as reducing the number of columns, those longer spans have created some really nice spaces.”

Partridge is in a unique position to check the longterm accuracy of his computer analysis: AKT II has now relocated to White Collar Factory. “We decided to move in after construction began. It certainly makes it very easy for us to continue collecting information about how the concrete is ageing, both structurally and aesthetically.”

PROJECT TEAM

Architect AHMM
Main contractor Brookfield Multiplex
Concrete contractor Dunne Group
Structural engineer AKT II
M&E engineer Arup