At Bristol’s new Cabot Circus retail centre, a ‘floating’ glazed gridshell roof cleverly maintains shoppers’ illusion of being outside.

A symbol of capitalist consumerism and a staple location for smart-ass American teen movies, the shopping centre, or ‘mall’ remains an object of cultural fascination in the West. But these hermetically sealed, environmentally controlled spaces hardly blend well with the existing cityscape, and allow little room for architectural flair.

A new agenda in retail development is emerging, however, one that moves away from the fully enclosed shopping mall and back towards the retail street. In Bristol, the designers of the £500m Cabot Circus mixed-use retail and leisure development have managed to free up the scheme’s three main shopping streets and central square by incorporating a series of 10 ‘floating’ gridshell roofs that hang from the buildings without columns for support.

While they engage with, or duck under and over building profiles and provide an element of protection from wind and rain, the roofs never completely seal the street. Freed from the constraints of having to fully connect their buildings to the roof, the architect and structural engineers were able to develop bolder designs, while natural ventilation meant mechanical air-conditioning systems were no longer necessary.

‘The whole point is that it’s an urban streetscape, an extension of Bristol city centre, so the roofs were intended to provide an element of cover, but never to enclose,’ said Simon Scott, director at architect Chapman Taylor, the scheme’s masterplanner. ‘Any other roofs like this will have a column somewhere to support the structure – these don’t. They genuinely seem to float between the buildings.’

The roofs started out as a piece of public art, a condition of the planning application, but soon developed into a main element of the design. For several months, artist Nayan Kulkarni worked alongside Chapman Taylor and structural engineers Schlaich Bergermann and Waterman Partnership to develop his ideas into a fully functioning structure and what has subsequently become the UK’s largest art glass roof.

‘Our main concern was that this should be seen first as a piece of art rather than something functional, but that’s a difficult balance to achieve when you’re also having to calculate engineering parameters like steel member thicknesses and loading,’ admits Robin Dobson, project delivery director for developer Bristol Alliance, a 50/50 joint venture between Hammerson and Land Securities.

Maintaining the illusion that shoppers were in an external space meant doing away with columns and reducing the size and weight of structural members in the roof to increase the amount of light and connection to the sky – a particularly tough task on the central glazed dome, which at about 77m x 55m is approaching the size of a football pitch. A number of alternative designs for the dome were considered, including a monopitch roof and one involving lots of cross-wire cabling, but the initial plan for a gridshell won through.

The relatively smooth curvature of the gridshell form meant the team could utilise rectilinear panes of glass instead of triangular ones, enabling larger panels and therefore fewer structural elements.

The glass itself, manufactured by UK-based Romag, has a low iron content to remove green colouration. This means natural colours appear unchanged when viewed through the glass, enhancing the impression of being outside and preventing the building façades from changing colour when viewed through sections of roof that sweep down in front of them.

The 10 roofs include a total of over 2,800 individually sized, 17.53mm-thick panes of laminated glass and around 300 tonnes of steelwork. Resolving the huge forces generated by these loads without the use of columns was achieved via edge beam gutter details along the sides of the roof panels, which in turn transfer loads on to armatures fastened into the building façades.

The light flooding in looked amazing after all that darkness

Graham Lill, Sir Robert McAlpine

‘There had to be a high level of co-ordination between the structural engineering teams to determine how it would be done, what load there would be on a specific point and how much movement it would mean,’ says Chapman Taylor’s Scott. ‘It resulted in very clever details for the armature and pin connection to deal with load transfer and movement. There are only around 20 points where the roofs are resolved against the buildings.’

Space between the roof edges and the buildings emphasises the external nature of the scheme, but it was important that in certain areas the rain couldn’t get through, such as outside the cinema so that audiences wouldn’t arrive damp. In these locations, building facades were either altered to shield the rain, or elements were added to the façade to create an overlap with the roof, or the angle of the roof itself was altered to meet the building.

Installation was on the critical path, which meant that for about nine months of the build programme (which lasted two years and nine-months) the central space was chock full of birdcage scaffolding.

‘It effectively sterilised work on the rest of the scheme for that time and until it was completed we couldn’t start work on the floors or public areas,’ admits Graham Lill, project manager for Sir Robert McAlpine, the main contractor on Cabot Circus.

To speed things up and reduce quality control issues, a great deal of the structure was manufactured off-site. ‘As much effort went into planning it as actually installing it,’ adds Lill. ‘Some prefab panels were as much as 100ft long, with awkward shapes and radii.’

The sections of roof over the three streets were lifted in whole by crane on to temporary supports and attached to the gutters. For the central dome, prefabricated steel frames comprising spaces for four panes of glass were lifted by crane and lowered into special U-shaped heads on top of scaffolding.

A screw jack was then adjusted to raise the panels into the precise position for welding to adjacent panels. Once the entire frame was supported on the scaffolding, the glass was installed on site by glazing specialist Portal, which had worked with structural engineer SKM Anthony Hunt on the detailing. It was then a case of gradually releasing the dome from the scaffolding and transferring the load to the supporting armature and through into the buildings.

‘This had to be done in a strict sequence,’ says Lill. ‘If it was released incorrectly the frame would move too much and the glass would crack. Once we saw that the roof was behaving as expected and we could strip the scaffolding out, it was a great relief. The space and light flooding in looked amazing after all that darkness.’

The roof was completed in May this year and Cabot Circus was opened to the public on September 25. The scheme’s natural ventilation and unconditioned streetscape have won it a 2008 BREEAM Award for outstanding achievement in environmental design.

‘We sometimes lack finesse in modern building and architects are as responsible for that as anybody. The master craftsman attitude is sometimes forgotten, so to bring together the artist, engineer, architect, contractor and specialist supplier to create that roof was quite special,’ concludes Scott. cm