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Purple prose

A huge, multi-angled wall is one of the technical challenges facing engineers at the new Lowry Centre in Salford.

Purple, orange and yellow walls; moveable seating tiers supported on air cushions; 'transparent' silver towers that glow in the dark. These are some of the characteristics of the new museum and arts centre building which will house the works of the artist LS Lowry when it opens in 2000.

Such architectural finery is a long way from the dour 1920s industrial landscapes of Northern factory gates peopled by Lowry's famous matchstick men. But with its two theatres and two art galleries - one housing over 350 Lowry canvasses - the Lowry Centre in Salford Quays is expected to be Britain's largest combined visual and performing arts complex. And with nearly half its £127M cost funded by Lottery grants, the centre will carry the prestigious label of being the nation's landmark millennium project for the arts.

But such boasts are for the commissioning ceremony in spring 2000. The current challenge for management contractor Bovis Construction is creating the multi-angled structure itself, and in particular a centrepiece sloping wall - as striking during construction as it will be when finished in bright purple cladding.

'This wall is the most complex structure I have ever been involved with and is an example of how we are pushing structural design to the limit to give the architect what he wants,' says Steve Gregson, director for consultant Buro Happold. 'No two sections have the same shape or slope, and at one level it is more holes than wall.'

The 28m high wall he is describing forms the seven sided perimeter of the centre's main theatre, the Lyric. Each section of the 600mm thick concrete structure slopes outward at different angles up to 17 and requires temporary support until it is tied back into the theatre's interior at ground floor, balcony and roof levels.

Now two thirds complete, the total 180m long wall sports large openings in its lower sections to allow access into or beneath the theatre. At higher levels, the normally solid structure contains some 40 cells cut into its inner face to accommodate service pipes, save concrete and reduce the weight of this out of plumb upper section.

The £400,000 structure was originally to be formed in steel until architect Michael Wilford & Partners added so many lower access box-outs that the remaining steel between the holes would have been too highly stressed. Concrete gives more flexibility to spread loads and heavy rebar helps transfer horizontal thrust from the steepest sloping front face back to side sections.

Equally heavy shuttering comes courtesy of Austrian formwork specialist Doka, leaving concreting subcontractor Heyrod Construction to deal with the wall's numerous connections to the steel-based theatre floor, cantilevered seating and roof.

In contrast to this surrounding wall, the 1,650 seat Lyric Theatre itself is relatively conventional in layout, with a standard rectangular shaped proscenium stage. Not so the arts centre's nearby second auditorium, appropriately called the Flexible Theatre.

Here every conceivable configuration between performers and audience is catered for, with central seats and stage sections moved around using air cushions or height adjustable spiral scaffolds. Theatre directors can request traverse, thrust, proscenium or 'in the round' performances, with the option of positioning scenery and actors behind the main tiered seating.

To allow such flexibility, the four levels of horseshoe-shaped balcony seating will hang from the theatre roof, supported by 20 Macalloy bars each 13m long. But the architect went further, demanding that steel frame balconies, and the 40mm diameter Macalloy bars, be left exposed to create a slender balcony support structure with clear views of the stage.

Such a requirement sits uncomfortably alongside the strict fire regulations for a 450 seat public theatre. Had the designer applied conventional one hour fire resistance rules, under the current 1991 Building Regulations, most steelwork, especially the exposed support bars, would have either had to be coated with thick protective paint or surrounded by cladding.

Fortunately, Buro Happold was able to avoid this requirement. It exploited a provision in the regulations which allows standard fire resistance requirements to be set aside if alternative designs are backed by a rig- ourous fire engineering analysis.

The consultant fielded its own fire engineering specialist Fedra, which used computers to work out every conceivable fire load scenario and then subjected the unprotected steelwork to the worst possible case - a fire immediately adjacent to a support hanger. Fedra was able to prove to Salford City Council's building control department that, even under the most severe temperature regime possible, this unprotected structure could still survive the required hour before any possibility of collapse.

The result is exposed steelwork everywhere, a satisfied architect and a happy client - the Lowry Trust - which has been saved the £100,000 additional cost of fire protection.

Construction now centres on further architectural gems, including multi- shaped towers and a hands on, interactive children's space. The relatively conventional rooms earmarked to house Lowry's paintings will provide a more sober backdrop for the matchstick men canvasses.

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