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The Marshall Building at the London School of Economics is a nine-storey, mixed-use facility designed by Grafton Architects with structural engineer AKT II.

Grafton won last year’s Stirling Prize for Town House (see CQ 272) – another large multi-use building and “front door” for Kingston University. Like that project, the Marshall Building has been configured to accommodate a wide range of spaces. Both buildings are concreteframed, broadly brutalist in spirit and with large expanses of exposed concrete inside and out, and each achieves a BREEAM rating of Excellent. There the similarities end, however.

The concrete cladding of the 18,000m2 Marshall Building is smoother, milkier and sits above two lower storeys that are clad, like many of its WC2 neighbours, in genuine Portland stone. In contrast to Town House’s stark external framework, the tall rows of vertical concrete fins nod to the classical stone columns of the 19th-century Royal College of Surgeons next door.

But easily the most unusual and fascinating thing about the Marshall Building is its structure. Enter the large, open, ground-floor foyer, and you are at once aware that this building is put together in a less-than-straightforward way. In the centre there is a substantial concrete pillar from which huge beams branch, sloping up through a gap in the ceiling towards the underside of the third floor. Around the perimeter are massive vaultlike concrete structures, while a stunning curved, in-situ concrete feature staircase winds up to the first floor.

Climb the steps and you discover more sloping beams, more vaults, more concrete. Wherever you are in the lower half of the building, the structure is very evident, all around you, everywhere. Critics have already acclaimed the sense of wonder this induces. But this perhaps belies the fact that the building’s extraordinary structure is designed not just for aesthetic effect but in response to a very demanding brief. There was the very large column-free space required by the professional-sized sports hall, the dimensions of which are effectively set by Sports England. Then slightly smaller, but still large, spaces needed for the lecture theatres, and finally smaller spaces for offices. From a purely structural point of view, it would have made sense to have the larger spaces higher up, where the longer  spans would not have so much weight to support.

But this would not have satisfied the client’s aspirations, as AKT II design director Marta Galinanes Garcia explains. “LSE wanted a welcoming ground-floor reception, open to the public and multi-use – it is also used as a banqueting hall and for other events. That couldn’t work if the ground floor had been crowded with columns. And though the sports hall could have been placed high up, that would have brought the offices down into the basement floors with limited views and natural light.” That meant the column-free sports hall was put in the two-storey basement, below a largely column-free area of the ground floor, with lectures theatres above and offices on top – a structural challenge that has informed the shape and character of the entire building.

Fins at the Inn

The complex facades of the Marshall Building comprise 1,170 precast concrete units covering a total of 12,124m2. The north face of the building is perhaps the most striking, looking out onto the green of Lincoln’s Inn Fields, and featuring two rows of tall concrete fins, one on top of the other.

These echo the vertical columns and classical frontage of the neighbouring Royal College of Surgeons, furnish the defining aesthetic of the building, and provide solar shading.

The 38 fins in the upper row are 11.5m high and have a “hammerhead” 2.4m return at the top which links them to the top of the inner facade proper. Cast in one piece, the fins are 500mm deep and present a 300mm face to the street.

These 5.5-tonne units proved among the most challenging to manufacture, as Techrete project manager Sean Sharkey explains: “The architect, Grafton, wanted a consistent finish front and back – so we had to make these in a mould that was vertically oriented. Though a horizontal mould would have been much easier from a compaction point of view, it would have resulted in one side with a float finish with the other reflecting the lower face of the mould.”

Techrete made a prototype vertical mould in timber but found that it began to “belly” at the bottom after about four fins had been made. “This was caused by the pressure of the tall column of concrete and the amount of vibrating that was necessary to ensure even compaction along the height of such a tall element,” says Sharkey.

“Remaking the timber mould after producing only a few units would have become uneconomic, so we switched to a reusable single steel mould from which we could make three units a week. Being steel, it also helped ensure a smooth, consistent finish.” The 20, slightly shorter 10.6m fins in the lower row are arranged at an angle to the line of the main facade, and have no hammerhead projection. As they were less visible from all sides than the upper row,

Techrete was able to cast these horizontally from timber moulds. The other facades feature a further 19 hammerhead fins, but comprise mainly flat panels, window panels, benches, soffits and copings. Most of these were cast horizontally on table moulds made from steel due to the size of the elements – some of the double-height window panels were 7.5m high and about 7 tonnes.

As well as contributing to the uncomplicated look of the fin-free facades, the generous panel dimensions also decreased the number of lifts needed on this constrained central London site. “The installation of all the units was carefully sequenced,” says Sharkey. “The larger fins, for example, all interlocked and had to be assembled in order from left to right across the front the of the building.”

Unusually, Techrete used two slightly different mixes to create the facades – one with slightly more pigment than the other. “So the main fins are slightly darker than the panels behind them,” explains Sharkey. “This was because Grafton wanted the concrete to get a little lighter as you move from the edge in towards the building. We added to this effect by grit blasting the outer fins and the window panels behind them with the same level of pressure, while applying only a light grit blast to the recessed areas within the window panels.

The lighter grit leaves the finish less textured and so it appears lighter.” The basic mix for the cladding at the Marshall Building was originally developed for Grafton’s 2021 Stirling Prize-winning Town House project, for which Techrete also provided precast concrete.

“Apart from the pigmentation we added on this occasion, the main difference is that this concrete was more rigorously vibrated and compacted to further reduce blowholes and provide the smoother, more refined finish.”

The basement sports hall is surrounded by chunky 1.5m2 reinforced-concrete columns rising from the concrete raft foundation up to the underside of level two. The sports hall ceiling is a column-free, post-tensioned concrete slab. Supporting its perimeter, and spanning just over 15m between the columns, are cantilevering post-tensioned reinforced-concrete beams. These are extraordinary, being 3.75m deep where they meet the columns but tapering to 1.65m deep in the centre. AKT II refers to these beam-column arrangements as transfer “trees”.

The shallow triangles of space created where the sloping “branch” from one beam meets another give the perimeter structure its vaulted appearance. “Grafton took inspiration from a nearby chapel in Lincoln’s Inn,” says Galinanes Garcia. “That too is vaulted and we have used this idea of achieving a space you can walk into, framed by the structure – even if ours is a little less ornate.”

The same structural arrangement applies for the open ground floor, where the vaulted tree beam effect is accentuated by a central concrete “feature tree”. This appears to reach upwards to help support the centre of the upper floors – but the look is deceptive. “Below this feature tree is the centre of the sports hall,” reveals Galinanes Garcia, “so there can be no supporting column beneath it and it transfers no load down to the basement raft. In fact this tree is constructed from steel beams encased in reinforced concrete and  effectively hangs from above with a movement joint separating it from the post-tensioned ground-floor beam.”

This tree is structurally activated only when a live load, such as might occur during a heavily populated event on the ground floor, starts to deflect its centre – at which point the movement joint closes and the tree acts like a hook, supporting the ground floor from above. Another feature tree, similarly constructed from steel and concrete, rises from the first floor through ceiling gaps to the underside of the fourth floor – the two together producing a “forested” effect.

For its next trick, AKT II has rotated the entire structural grid through 45° to create the lecture theatre spaces. This is achieved by using a slightly slimmer version of the transfer trees, a 10.8m grid, and beams that are 3.5m deep where they join the 900mm2 columns, tapering to 1.5m at the top of the vaults they create. The more central of these columns land in the centre of the transfer beams below – potentially their weakest point. Although the geometry suggests otherwise, the transfer beams are in fact designed as post-tensioned cantilevers to support the columns above.

Not content with doing this once, AKT II has then shrunk the whole grid again, and rotated it a further 45 degrees to configure the office space in the upper storeys. Here the structure adopts a more standard configuration, arranged on a 7.6m grid with standard flat beams and post-tensioned floor slabs. Some of the 500mm2 columns also land in the centre of vaulted beams below, where they again derive support from the fact that the beams cantilever.

It is undeniably a clever structure, and one that provides aesthetic interest, with the very visible structure inside and angled facades outside. But it is also a logical response to a specific context, says Galinanes Garcia. “We used it to fit very particular internal space requirements into the size and shape of this site. For example, the sports hall could only fit in one place which is quite orthogonal on the page – and rotating the lecture theatre grid allowed us to create some dual aspects. We spent a lot of time experimenting with various spans before we got everything to fit perfectly.”

The “transfer on a transfer” structure added further complexity, she adds. “Concrete naturally deflects and creeps, which is normally easy enough to calculate and allow for. Here, though, we were looking at predicting deflection, on deflection, on raft settlement – so even though we love concrete at AKT II, our first instinct was to simplify the calculations by using steel.” This, however, would not have satisfied the architect’s intentions. “Grafton wanted an honest concrete aesthetic,” says Galinanes Garcia. “It would not have been ‘truthful’ to have a steel structure clad with concrete as a dead weight. Grafton also wanted to use the thermal mass of the exposed concrete to retain heat and reduce heating costs in the winter, while absorbing heat and reducing the need for cooling in summer.” With hindsight, she adds, concrete was definitely the right decision – not least from a carbon point of view. “Although we have used a lot of concrete, a steel structure would have needed tobe bespoke and plated, which has a much higher carbon footprint than standard hot-rolled components.”

Other measures, designed primarily to speed the project and cut costs, also had the effect of reducing the embodied carbon associated with material use. “We reused the existing basement retaining walls,” says Galinanes Garcia. “They weren’t quite deep enough, so instead of requiring an additional piled wall, we underpinned the existing piles by a couple of metres to give us the depth we needed to get our raft in.” The raft, too, was adapted to save time and material: “We originally envisaged a piled solution, but when we plotted the pile caps they overlapped to such an extent that they resembled a raft anyway. We looked into doing without piles and, after reducing the load of the building by changing the standard floor slabs to post-tensioned slabs, we were able to demonstrate that a raft, 1m thick but 2m thick where the columns landed, would be sufficient to keep settlement to 62mm long-term – within acceptable limits.”

This strategy resulted in saving all the time and material associated with piling, and actually freed up more of the site for use: “We are next door to the Old Curiosity Shop here. It is an old and delicate building, and if we had used a piled solution we would have had to keep our distance from it to avoid it being disturbed by the work.” Further material savings have been made by leaving the interior concrete exposed, and so avoiding the need for plasterboard linings. It meant, however, that attention had to be paid to getting the finish to an acceptable standard.

A first mock-up of one of the trees was too rough, with too many blowholes, so concrete contractor Getjar improved the finish by changing the release agent, paying extra attention to keep the reinforcement free of debris, and ensuring good compaction through careful vibrating. The concrete mix itself contains 25% or 50% GGBS to further reduce the carbon content, which also gives the finished concrete an attractive pale tone. Galinanes Garcia explains that since GGBS slows the curing process, the proportions of cement replacement were determined by factors such as programme and weather. “But we kept the percentages consistent from floor to floor to ensure there was no noticeable colour difference.”

The end result is a fine, light, consistent finish. It is naturally textured, but not so much that it distracts from the real star of the Marshall Building show – that extraordinary structure, and the unique shapes and spaces it has created. 

 

WE SPENT A LOT OF TIME EXPERIMENTING WITH VARIOUS SPANS BEFORE WE GOT EVERYTHING TO FIT PERFECTLY

Project Team

Architect

Grafton Architects

Structural engineer

AKT II

Contractor​

Mace

Concrete contractor

Getjar

Precast concrete

Techrete

Photos

Mark Harrington / AKT II