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Saturday30 August 2014

Solving the panel puzzle

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The Maggie’s Centre certainly provided a stern test of the capabilities of precast concrete supplier Thorp Precast. The job involved creating 56 precast panels, and although many of these were similar, very few were identical

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The Maggie’s Centre certainly provided a stern test of the capabilities of precast concrete supplier Thorp Precast. The job involved creating 56 precast panels, and although many of these were similar, very few were identical.

The walls of the central, elliptical drum-shaped area of the centre, for example, were made from just two large curving precast panels, one of which had a “letterbox” cut out to make the panoramic sea-view window on the first floor.

“These two large convex [from the exterior] elements were made from the same mould, but with a plug cast in one of them to form the letterbox,” explains Harvey Thorp, managing director of Thorp Precast.

The challenge then, he says, was to find the most efficient way of casting all the other wall panels when they differed from each other in at least four ways.

He says that the curving wall panels vary first in height - from over 4m to less than 1m high. “They also vary in that some are convex and some concave. In addition, the top of each panel features a kind of cornice that runs all the way along the top edge of the walls.”

Crucially, says Thorp, this cornice, or troughstein, changes in profile, becoming slimmer as the panels reduce in size towards the end of the building’s “dragons’ tails”.

Finally, the panels have irregularly positioned punched window openings, many of which span across two adjacent panels.

Instead of creating more than 50 bespoke moulds, which would have added to the cost of the project, Thorp set about creating two master moulds - one to produce the panels for the two concave walls, the other to produce panels for the two longer, convex walls such as the one to the left of the main entrance.

“The building’s symmetry meant that each panel, concave or convex, has a ‘twin’ on the opposite side,” says Thorp. “So each panel was cast twice.”

Achieving the different heights required to make the walls taper down was relatively straightforward: “We introduced a former at the base of the mould, and once two panels had been cast we would move this some 200mm or so to produce the next two, slightly shorter, panels.”

Creating the flaring cornice detail at the top of each panel required even more problem-solving. Thorp explains that the shape of the cornice had first to be cut from a plywood grid using a computer controlled (CNC) router. Thin strips of plywood were then attached to this framework to create the subtle curve of the cornice. This then formed a specialist “top” to the master moulds.

“We would adapt the top, move the former at the base, and then cast a pair of panels,” says Thorp. “We then had to remove the top end of
the mould, replace it with another specially made top in which the cornice shape would be a little different, shift the base former again, and then cast another two panels.”

The final challenge was the need to create the punched hole window openings. “Although these are not big windows, they would make gaps big enough to compromise the structural integrity of an individual panel,” says Thorp.

“Obviously we didn’t want any breaking, which could happen during demoulding, transport to site or installation. So to prevent this we cast in sacrificial jambs which were only removed when the panels were firmly fixed into position on site.”

This technique involves creating thinner, less reinforced shapes within the panel which can be easily cut out on site to create the windows. Thorp admits to some anxiety as to whether the window openings on adjacent panels would exactly match, and whether the cornice detail at the top would flow smoothly from panel to panel.

“Until you see it on site you can never be sure,” he says. “Achieving the result we did was really a nice combination of very traditional techniques - making the cornice moulds was a little bit like boat-building - married to the latest 3D computer modelling techniques. It takes a bit of extra effort, but it’s very satisfying when it works.”

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