The design of the 730,000 ft2 One Bank office tower at Canary Wharf is unusual in two ways. Developer Canary Wharf Group’s base-build architects, KPF, produced a receding convex main elevation which sits on a cantilevered lower segment. This bold architectural geometry takes an even more dramatic turn – several turns, to be precise – inside the building. Here, tp bennett, Adamson Architects and EeStairs collaborated to produce a remarkable asymmetrical ‘ribbon’ staircase linking three floorplate balconies.

tp bennett, working for One Bank’s financial sector tenant, designed the overall interior fit-out scheme, having carried out a due diligence appraisal of the building’s suitability for 3000 financial sector staff. Andrew McLean, a tp bennett director, explained that the key moves, developed at an early design stage with KPF, were to slightly rotate the core and then design the fit-out to generate a dynamic and creative workplace atmosphere.

The three large trading floors were designed to encourage staff interactions. The generally open-plan floorplates feature breakout areas designed to create alternative working and relaxation spaces. tp bennett used its own financial space design tools to maximise this flexible working environment, in collaboration with management and staff representatives.

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As a result, the interiors include a central hub, touchdown desks, informal meeting areas, and presentation areas. And tp bennett’s greening of the interiors with 9000 plants has contributed to One Bank’s BREEAM Outstanding environmental rating.

A key factor in the building’s creative and social atmosphere is the bravura feature staircase in the four-storey atrium. The staircase is asymmetrical in both plan and section, and rises between the slanting glass main façade and the radiused balcony-edges to link the trading floors.

tp bennett produced the initial design concepts for the staircase. “We originally worked up the design as a spiral,” explained Andrew McLean. “But we have a designer who’s very interested in the geometry of stairs and he produced a more interesting ‘ribbon’ design.” The refinement of that SketchUp concept was carried out by Adamson Architects who developed the detailed design in close collaboration with EeStairs’ Eastbourne team, led by director Steve Bray and project manager-design engineer Paul Barton. tp bennett were involved in some of the staircase design workshops.

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“I wanted to create a staircase that had a pure form,” said Simon Groves, a senior associate at Adamson. “Simplifying the ribbons of the inner and outer balustrade enclosures, and raising them to 1900mm, with no unnecessary undulation at the landings between the stair flights was key. This meant there were no stop-start points in the balustrades, just a pure form.”

And the gaps between the outer balustrades and the floorplates were minimised so that bridging was virtually non-existent – “which means that at levels six and seven, the ribbon is extremely close to the floorplate spandrels.” Engineers Arup examined and approved the structural integrity of the staircase in relation to the floorplates.

“To get these stairs to work took an awful lot of modelling,” said Andrew McLean. “We’ve worked with EeStairs before, and the tolerances they were working to were just exceptional.” And Simon Groves added: “Once we’d resolved with EeStairs how to support the installation of the stairs, from the top down, they flew with it. They just got on with it at their factory.”

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One important move, proposed by EeStairs, was to fabricate the staircase as a monocoque steel structure with a cast GRG soffit, and higher than usual balustrade heights to conceal the rest-landings. This means the staircase is effectively a self-supporting structure – a remarkable outcome, given that the its ‘ribbon’ form begins with a relatively tight rotation at its base, followed by three progressively wider rotations as the stairs rise.

Steve Bray, of EeStairs, explained that the stair’s structural performance was ensured by the box-section steel balustrades, locked together by the folded steel treads and risers.

But he emphasised that the installation of the staircase was as complicated as its design. The staircase, which weighs 29 tonnes, was fabricated by EeStairs in three complete sections, which they then cut very precisely into sub-sections small enough to be transported and tower-craned into the building via the terrace on level five.

This meant the staircase could be reassembled and fitted by EeStairs installation team without compromising loading and weight restrictions for the floor slabs. “The installation was a really serious piece of design in itself,” Bray added. “We solved it by installing the stair sections, starting from the top balcony, which required a tandem lift with two spider cranes, and several 12m props tied to the building’s primary structure.”