Technical

Steel’s green signal at Moorgate

Occupying a previously vacant City of London plot, 21 Moorfields spans an important underground station with an architecturally detailed exposed steel frame

The eastern facade (along Moorgate) cantilevers out over the Elizabeth Line ticket office. Image: Ben Bisek — The eastern façade (along Moorgate) cantilevers out over the Elizabeth Line ticket office. Image: Ben Bisek

Accommodating Deutsche Bank’s London headquarters, the 17-storey 21 Moorfields project required some complex engineering to create a structural steel frame that spans the full width of Moorgate station, a distance which is equivalent in length to the wingspan of a jumbo jet.

Maximising the project’s footprint, the eastern façade (along Moorgate) cantilevers out over the Elizabeth Line ticket office with a seven-storey, fully exposed perimeter truss creating a signpost to the building’s main entrance.

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The cantilever is created by a combination of tripod supports and a series of bowstring trusses, measuring 25m-long and each weighing 70t.

Using the capacity of an existing slab, which is also the station roof, the structural design features six 7m-deep ‘launching trusses’, which are up to 55m in length.

Award: 21 Moorfields, London

Architect: WilkinsonEyre
Structural engineer: Robert Bird Group
Steelwork contractor: William Hare Limited
Main contractor: Sir Robert McAlpine
Client: Landsec
Consultant: Gleeds

Built above the rail assets, the trusses created a temporary support for the floors during the construction programme, while in the completed scheme, they accommodate the building’s first floor and main entrance, as well as a mezzanine (second floor) within their depth. Below the trusses, the existing slab supports a basement, at ground floor level, for the building’s back-of-house and plant equipment areas.

During construction, each truss facilitated the build-up of a 10-storey steel box section mega arch, which in turn enabled the installation of the concrete floor slabs and the remaining steel frame.

The mega arches are integrated into the building’s cores, only exiting on to the floorplates at levels 7 to 10, thereby minimising the number of columns to only six within each of the 100m-long x 60m-wide floorplates.

Detail shot of the eastern façade cantilever structure. Image: Ben Bisek

Above the mega arches, the structure continues upwards in a more traditional beam and column design to level 17. Incorporated into these floors are terraces and set-backs, which satisfy a number of rights to light issues, and historical sight paths to St Paul’s Cathedral.

Before any steelwork could be erected on site, a piling conundrum had to be solved as there are a number of assets below ground, including six London Underground lines, two Elizabeth Line tunnels and a ticket hall, as well as a major sewer. This meant locations for any new piles were extremely limited and their installation required a significant amount of temporary steelwork.

A total of 16 piles, each 2.4m or 1.8m in diameter and 60m-long were threaded between the numerous under-site constraints.

Once the piling had been completed, the project team were able to reuse much of the temporary steelwork to support the launching truss installation, which contributed to cutting the carbon footprint of the building.

“A project of global significance with an immensely challenging site above the operational rail interchange and new station box. The elegant solution underplays the complexity involved in the design and sets a new benchmark.”

SSDA judges

Each truss/arch system is connected and founded on a pile at each end but, because of the limited locations, the spacing between each truss and the shape of each arch varies. The piles and their locations have consequently dictated the column lines for the entire superstructure.

“Structural steelwork was selected for its ability to meet the complex demands of the site. It allowed for lightweight and stiff long-span solutions that minimised the number of columns and foundation points required to achieve the development’s potential,” explains Chris Papanastasiou, UK structures division lead for Robert Bird Group.

“Steel’s versatility enabled significant architectural expression without compromising on size or weight, ideal for integrating bespoke structural elements like transfer structures and launching trusses.”