Farringdon Station, London

Project team

Client:Crossrail

Architect:Aedas (for Crossrail), Atkins (for BFK)

Structural Engineer:Scott Wilson (for Crossrail), Atkins (for BFK)

M&E Engineer:Scott Wilson (for Crossrail)

Concrete Contractor:Evans Concrete

Photos:Morley von Sternberg

Passengers at the new Farringdon Crossrail station will be greeted by a sequence of stunning concrete ceilings, writes Tony Whitehead

In March, Farringdon became the first Crossrail station to be handed over to Transport for London. It is notable for a number of reasons, not least its BREEAM Excellent environmental rating. But for passengers departing from this fashionable quarter of the capital, the first thing that will strike them is a stunning cathedral-like concrete ceiling. In fact, this is the first of three such spaces, each a showcase of what different types of concrete construction can achieve.

The station has two entrances, some 400m apart, and as you enter the westernmost of these, at Cowcross Street, the first of the spectacular ceilings is soon visible. Known as the upper apse, it comprises a lattice of concrete beams arranged in a diamond pattern. These support the sloping concrete panel soffit of a wide stairway with escalators leading down to the station’s intermediate level. The beams are massive – up to 12m long. Yet they are also smooth and elegant.

Crossrail says that the diamonds reflect the fact that this entrance is adjacent to the gem-dealing quarter of Hatton Garden – and indeed the X-shaped concrete nodes at the beam intersections even feature cast-in spaces for powerful downlights that shine like enormous solitaires. But the design also has a subtler genesis. “The design evolved under the influence of many factors,” says Soji Abass, Crossrail’s lead architect for the station.

“For example, we wanted to avoid metal ceilings, which require ongoing maintenance. Metal panels get opened and closed to access hidden services and begin to look tired after a while, and there are safety issues with working at height. In contrast, these concrete ceilings are low maintenance.”

Hatton Garden certainly influenced the diamond shape, he adds. But the angle at which the beams are set is also taken from the meeting of the two railway lines at this point. “Once we had the angle, the diamonds naturally followed, and we’ve then taken advantage of the column-free space to exploit the perspective offered by the sloping soffit.

The idea is that the pattern acts as a sign or wayfinder, so people intuitively know where to find the route to the platform. But above all, we just wanted a delightful way for passengers to get from the street to the lower level.” This use of perspective is very effective, though passengers will have little clue how difficult the 25m-wide ceiling was to build. The whole 360-tonne ceiling is suspended from an invisible steel structure above – so the overriding challenge for the contractor was how to safely construct such a ceiling, high above a sloping stair ramp, and beneath the structure that would support it.

A number of potential solutions were examined, as Duarte Seixas, project manager with BFK (the Bam, Ferrovial, Kier joint venture that built the station) explains: “We looked at making this ceiling from in-situ concrete, but it would have been very hard to achieve the finish we wanted on an incline. It would also have been difficult to crane material and reinforcement through a heavy steel structure above. We looked at making just the beams from in-situ and putting precast panels above, but dismissed this for the same reason, so in the end we agreed that the whole ceiling should be precast.”

There remained, however, the problem of how to thread 102 large concrete elements through the supporting structure above the ceiling. “It was not doable,” says Seixas, “so the solution was to construct a large, inclined, temporary steel structure on which the concrete ceiling could be assembled. The permanent steel structure was built on top, connected to the concrete, and then the temporary steel structure below was removed.”

To check that this would work, a large mock-up – with steel supports and an entire diamond’s worth of beams and panels – was constructed at the factory of the precast supplier, Evans Concrete Products in Alfreton, Derbyshire. “This helped us get a lot of answers,” says Seixas. “We tested the finish, the joints, the cast-in recesses for the lights, and even how to lift the sections. Remember, we had to lower these large elements onto an inclined structure, so the lifting eyes had to be oriented to accommodate the angle and positioned to ensure the weight would be balanced.”

Once the beams and nodes had been assembled on the temporary structure, the diamonds were then “filled in” with 63 precast slabs placed on ledges running along the sides of the beams. Interestingly, these slabs are not fixed to the beams, but lie loose on the ledges. The reason for this is that, in the event of a bomb blast, the force of the explosion will briefly lift the panels from the beams. This will allow the blast pressure to escape into the voids above, and so prevent the force of any explosion remaining concentrated in the highly populated areas below.

“Mitigating the effects of bomb blast is something that affected quite a bit of the design,” says Seixas. “To minimise flying debris, for example, cladding panels needed firm fixing to a resilient substrate. Concrete or blockwork walls offered the structural robustness we needed for this.”

With the ceiling fully assembled, the permanent steel structure could be constructed above, and fixed to the ceiling via large steel connecting plates cast into the beams. Then came the nerve-racking process of removing the temporary steelwork below. “We checked and triplechecked everything before doing this,” says Seixas. “We carried out a finite element analysis to determine how the ceiling was going to behave – how it would deflect – once the supporting structure was removed.

If we got it wrong, the ceiling could crack, so we worked out a sequence of releasing the support in phases so that at no stage of its removal would any part become too stressed. After each area of support was removed, the deflection was checked before moving onto the next one.” The result is worth the effort – the intersecting beams creating a bold pattern which, Abass says, changes depending on where you view it from. Ironically, while the concrete appears to express the structure, it is not itself structural, being suspended from the nowinvisible steelwork above.

By comparison, the ceiling of the area at the bottom of the escalators, known as lower apse, was simpler to make, though still unusual in its construction. This comprises 225 flat diamond shaped precast panels, each set slightly proud of an in-situ concrete slab. This ceiling supports itself, so there was no issue this time with steelwork above. Nevertheless, various methodologies for its construction were discussed. “Again we considered doing this with insitu,” says Seixas, “but thought we would struggle to match the finish of the precast upper apse ceiling.

Having them both precast helps them talk to each other visually.” So after a temporary deck had been put in place, each of the unique, 100mm-thick precast diamond panels was arranged in the desired pattern, and the 100mm gap between each filled by silicon-based formers. These were only 50mm deep, so when the slab was cast on top, 50mm of the gap between each panel was filled with poured concrete. “The silicon is very smooth and comes away from the set concrete easily,” says Seixas. “So the small amounts of visible in-situ soffit are also smooth.”

Again, the result is stunning, with the pale precast diamonds shown to great effect against the shadowed in-situ spaces between. Above the panels, the substantial 850mm-deep in-situ slab is further reinforced with steel I-beams. To save concrete and reduce the weight of the ceiling, 550mm-deep polystyrene formers were placed between the steel beams. Farringdon’s third spectacular ceiling – at the eastern ticket hall exit onto Long Lane – is different again, being constructed entirely from in-situ concrete. This time the look is more classically brutalist – a series of large square concrete coffers spanning just over 20m.

“This entrance is nearer the Barbican, which was definitely an influence,” says Abass. “But also this quarter was less about gems and more about heavier crafts like blacksmiths. So I like the way you can see the natural stratification of the concrete in the coffers – how it too, if you like, has been made by craftsmen.” This ceiling is formed from a 1.4m-deep reinforced in-situ concrete slab, with 1.1m-deep coffers formed by timber void formers, and with the beams in between further strengthened by steel I-beams.

Like the other concrete in the station, the coffers are a simple grey: “We thought about adding a sparkly aggregate to lighten the look,” says Abass, “but in the end we thought, no, this is London, not Barcelona.” The temporary deck on which the slab was laid was arranged so that the board joints would run along the centre of each beam, he explains. “The joints were primarily introduced to allow each coffer, or waffle, to be independently struck. We carried this joint through into the concrete though, to express the construction.”

Designers sometimes flaunt such “marks of making” for their own sake, but here the lines that grace the centres of the 600mm-wide concrete beams tell the story of the ceiling’s construction while simultaneously making it better-looking. The coffers are more than just structural and aesthetic features, however: they also house acoustic boards, and downlights that highlight the ceiling’s impressive concrete as well as illuminating the ticket hall. There will be plenty to admire, then, for the 82,000 passengers that are expected to use the station each day.

To paraphrase Le Corbusier, light becomes a significant architectural element when it enters into dialogue with the forms it reveals. If you want to see what he meant, just look up when you visit Farringdon station.

Featured in CQ Summer 2021 (pg 14)