Leeds was traditionally known for its textiles. How fitting then that folded fabric was the inspiration for a spectacular concrete diagrid facade that wraps around the city’s new John Lewis department store, part of the recently completed Victoria Gate shopping centre.
Designed by architect ACME in collaboration with engineer Waterman Group and concrete contractor Techrete, the acid-etched facade is a tour de force of geometry and texture in keeping with the architect’s desire to create a shopping development of rich character. Physical retail experiences “need to be more seductive” in the age of internet shopping, according to ACME associate director Catherine Hennessy. With its enjoyment of pattern and visual delight, Victoria Gate is certainly that.
The £165m project – designed for Hammerson in the Victoria Quarter shopping district – consists of a row of three elements: a shopping arcade, the five-storey department store and a car park. Leeds’ architectural heritage was a strong inspiration, and the architects carried out extensive analysis of the many surviving Victorian arcades and the area’s grand civic architecture.
The facade concept emerged from a desire to create an articulated frontage with a sense of depth, and with a modularity that could accommodate the store’s need for transparent and non-glazed areas. The concrete diagrid did all this through the incorporation of either glazed or solid panels within the composition, as well as nodding to the city’s textile heritage in its pleats and folds.
“We felt concrete was an obvious choice for something that repeats and can provide depth,” says Stefano Dal Piva, director at ACME, adding that the white concrete gives a quality of permanence while referencing nearby polished-granite buildings and stone civic architecture such as the Corn Exchange and Leeds Town Hall. “Concrete is a fantastic material to get sharpness and to play with weight so that it looks as light as cloth.”
The store has in-situ reinforced concrete cores, slabs and columns, while the facade is formed from precast modules fixed to a diagrid steel frame behind the rib lines of the lattice. This accommodates the maximum facade depth of 400mm specified by John Lewis.
The big challenge was to avoid joints that would disrupt the harmony of the pattern. The solution was 326 trapezoidal modules configured so that joints coincided with a fold or crank in the diagrid, which starts from the first-floor level. Module size was governed by the 4.2m maximum height for fitting on a low-loader lorry, with the largest rhombi measuring approximately 7m x 2m. The diagrid cladding panels decreased in size up the building to give the illusion of a curving facade.
A huge amount of work went into rationalising the geometry of the diagrid structure. Extensive structural analysis, including time history and thermal analysis methods, was carried out to establish critical movement for the 500 nodal intersections. Then Waterman Group set out a series of nodes on a 4.2m horizontal grid using six different repeating types with variations at special areas such as the entrance and the stair cores. In total, 50 node variants were used.
There are 16 panel types, cast using four different modules. The formwork for each was adapted to give four thicknesses ranging from 150mm to 400mm in order to create the sense of depth the architects wanted, with the depths becoming smaller towards the centre of each diamond.
ACME worked with Techcrete in Ireland to specify the aggregate for the concrete after testing eight samples of varying stone type, size and cement using sand sourced from Ballylusk, County Wicklow. Rather than a pure white, the practice was looking for a textured appearance akin to Portland stone, in reference to buildings along nearby Eastgate.
In order to emphasise the main diagonals in comparison with the rest of the diagrid, different finishes were specified, with a polished finish used for the main diagonals and acid-etched for the infills. To ensure that the joints were unobtrusive, ACME specified “dusted joints” using crushed concrete bonded onto silicone mastic.
The precast modules, backed with insulation, were installed onto the steel diagrid between September 2015 and January 2016. At upper levels, the panels are propped off the diagrid using corbels at restraint points. While there are other concrete structural gymnastics at play such as the creation of the store’s 21.6m x 19.2m column-free central atrium, the facade is definitely the star attraction.
For the adjacent arcade building, ACME also used precast panels, but this time faced with brick. These are combined with a black precast plinth – another reference to the polished granite ofnearby buildings.
Realising this development, and in particular the concrete diagrid, has been an undertaking that was only possible with the team’s shared use of BIM. The result is a distinctive contemporary addition to Leeds’ rich built heritage.
Structural engineer Waterman Group
Contractor Sir Robert McAlpine
Concrete diagrid panels Techcrete
Brick-faced concrete panels Thorp Precast
Photos: Jack Hobhouse