SSDA 2019: Retail centre buys into steel design

1 November 2019

Two ribbon trusses help create the undulating form of the roof system (©Hufton + Crow)

A curving steel structure, spanning between two restored Victorian buildings, forms the roof of London’s latest world-class retail destination.

The King’s Cross redevelopment programme, one of Europe’s largest regeneration schemes, has converted a run-down industrial site in north London into a vibrant neighbourhood.

Award: Coal Drops Yard, London
Architect: Heatherwick Studio
Structural engineer: Arup
Steelwork contractor: Severfield
Main contractor: BAM Construction
Client: King’s Cross Central Limited Partnership

One of the centrepieces is Coal Drops Yard, a recently opened high-end retail outlet housed in two Victorian buildings, built in the 1850s for receiving and sorting coal as it arrived in London by train.

The buildings, approximately 150m long and 120m long respectively, sit side-by-side while splaying outwards in a southerly direction. A new steel-framed roof straddles the area between the two structures, which is 30m wide at the northern end, creating an impressive piece de resistance. 

The roof structure is approximately 75m long on one side and 65m long on the other. It curves inwards, from the south and north ends, and then rises up in the middle to a maximum height of 25m.

Two “ribbon” trusses, sat atop each building, help form the undulating shape of the roof structure. The trusses are fabricated from 610mm circular hollow sections (CHS) with 508mm CHS verticals and 219mm CHS bracings.

Exposed steelwork emphasises the complex geometry of the roof structure (Photos ©John Sturrock; Paul Carstairs/Arup)

“To create the complex geometry of the sweeping roof structure, steel was the only choice and CHS sections were used as they could be bent to form the curved ribbon trusses,” says Arup senior engineer Simon Bateman. 

The trusses are each created from four individual segments (eight in total), each one bespoke, due to the curvature of the roof and the splay of the buildings.

Above the trusses the new roof is primarily supported by a compression-tension system, spanning the distance between the buildings.  

This is supported on new steelwork at each end within the two Coal Drops buildings. The compression aspect of the system is made up of four fabricated box “giraffe” girders – so-called as they look like giraffe necks in 2D elevation.  

The “giraffe” girders, which span 50m from building to building, are 1,000mm deep x 600mm wide, with 40mm flanges.

What the judges said:

“The new exposed steel is extremely well integrated and carefully detailed to be in keeping with the original structure, strengthening and extending it to suit its new purpose.”

The tension is taken through a single tie, made from a series of plated steel elements, that is connected to the bases of the “giraffe” girders.

At the middle point of the roof, there is a large kink where the two sides nearly meet: the “kissing point”. As there are huge bending moments generated in the steelwork in this area, a large 100 tonne steel node is positioned at this point.

Meanwhile, the roof steelwork is doing two jobs, as well as spanning the void between the existing buildings, it also supports a new column-free upper level of the development.

Produced by the BCSA and Steel for Life in association with Construction Manager

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