Towering achievement

Why settle for just a chimney when you could turn it into a piece of art? That was the thinking behind the strikingly faceted flue tower of the Greenwich Peninsula Energy Centre, which will be a familiar sight to anyone used to queuing on the adjacent approach road to the Blackwall Tunnel. 

Recently awarded a Commendation in the Structural Steel Design Awards, the project began 10 years ago when architect CF Møller won a competition for the design of a 3,000m² low carbon energy centre to serve the planned regeneration of Greenwich Peninsula in south east London. With a mission to demystify the energy generating process, the brief included a visitor centre.

Initially, the concept was for a plinth form to contain the energy centre with a thin blade tower for the flue stack according to architect Brian Cody of CF Møller.
‘It was intended to be a striking gateway building as you approach Greenwich Peninsula. Originally the concept was for a much flatter surfaced tower design exploring ideas about translucency and playing with light,’ he says.

The concept of a podium and blade was kept when the client decided to integrate artwork into the chimney and appointed artist Conrad Shawcross. The piece, entitled The Optic Clock, was realised after intense collaboration with CF Møller, engineer Price & Myers and steelwork contractor Billington Structures, who created the tower from 345 tonnes of galvanized steel and supplied 200 tonnes of steelwork for the structure of the energy centre.

‘It could have been quite a standard industrial energy centre but developer Knight Dragon chose to embrace an artistic contribution to the building,’ Cody says. ‘The idea was to create a very distinctive plinth building in a streamlined, satin-finished black box to contrast with the flue tower, which becomes a 3D sculptural form as it emerges from the plinth, setting up a tension between the two.’

Steel was the natural choice for the structure of both the 90m long x 25m wide plinth and the 49m high tower of the energy centre, enabling the creation of 20m spans.
‘A steel-framed building allows for flexibility. The number of combined heat and power (CHP) and gas boilers will grow over the years as the peninsula is developed. So we needed large column-free spaces in the plinth,’ Cody says.

 

‘The tower required a very thin structure so steel was an obvious choice that enabled us to achieve a thin edge, and allowed for the tower to be perforated to deal with the wind loading.’

On the south side of the building, the architects have included a large expanse of glazing. This not only facilitates views into the energy centre but will maximise natural light in anticipation of future adjacent developments which are planned to rise to the height of the chimney flue.

The tower gives the illusion of cantilevering out from the plinth at the 3m mark, although the structure actually descends to ground level where it is secured with metre-long bolts. But because its base is encased in the same black cladding as the rest of the centre it disappears into it when viewed from afar. 

For the structural design, a BRE wind tunnel study of a 1:200 scale replica was undertaken to measure the effects of a 1 in 50 year storm event, says structural engineer Amanda Constantinesco of Price & Myers.

‘The results of this analysis were used to inform the design, cladding porosity, and form to reduce the effects of vortex shedding and fatigue during its 50-year design life,’ she says. ‘Acting as a braced cantilever, the tower is formed from five 49m tall latticed girders, connected by a series of raking beams and bracing. The structure of the tower was designed not only to perform the function of a flue tower, but also provide support to the artist-designed cladding system.’

Artist Conrad Shawcross and his team worked with CF Møller’s brief for a 3.15m deep tower capable of accommodating 10 flues in a single row. The resulting artwork creates a distinctive moiré effect of faceted, anodised aluminium cladding panels over the complex structure of the steel tower. The angled facades are designed to catch the light so that the appearance varies in different light conditions. One of the trickiest elements was creating a hidden door within the artwork to provide maintenance access to the tower.

‘We relied on the expertise of Price & Myers to develop a solution in terms of sizing of the steel as well as Billington’s experience in producing ladder girders,’ Cody says.

‘We had anticipated the biggest challenge being how we maintained our design concept through to delivery with addition of the artistic intervention. But we found that there was a harmonious common ground between architect, artist, engineer and steelwork contractor towards creating a sustainable and well-crafted building.' 

Eventually, some 15,000 homes and workspaces will be powered by the Energy Centre, which is the largest new build residential heat network in Europe. ‘Over the next 10-15 years the area immediately around it will develop and it will be interesting to see how it sits within that,’ Cody says.  

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The flue tower
The 49m high tower is a self-supporting structure independent of that of the energy centre. This avoids any effects arising from fatigue or cyclic loading transferring to the main building. Containing four flues with capacity for up to 10, it is created from five ladder girders spaced 4.5m apart with diagonal bracing elements clad in aluminium. These were constructed in three sections varying from 13.2m to 20.1m.‘Our intention from the outset was to maximise their size to limit the work on site and more importantly under­take as much of the work as possible within our fact­ory where quality is much easier to control,’ says Billington principal engineer (design & build) Craig Clayton.

Weighing up to 23 tonnes each, these sections were galvanised at Worksop Galvanizing, and were according to the steelwork contractor the largest frames to be hot-dip galvanised in the country. One of the major challenges was to retain a sense of lightness by aligning the diagonal bracing with the joint lines in the triangular cladding panels to both support the cladding and minimise the visual impact of the supporting structure. At node positions up to eight diagonal members intersected with the vertical ladder column through connection details developed by Billington.‘This required many different, irregular and multi-member connections, all with unique combinations of connection forces derived from the dynamic structural and fatigue analyses based on wind loading assessed during the wind tunnel testing,’ says engineer Amanda Constantin­esco of Price & Myers. 

‘Close coordination with the cladding subcontractor was fundamental to achieving the correct setting out and detail for the hundreds of fixing brackets. Each was fabricated as part of the steel frame with sufficient tolerance to allow connection and adjustment of the cladding panels throughout the build.’

Each ladder girder was brought to site in three pieces and connected on site.