Technical

Blowing bubbles

Glulam beams spread out throughout the structure and are connected by cylindrical nodes made from galvanised steel

An ambitious, competition-winning sports centre in Scunthorpe challenged the contractor to build five pods each with a different roof covering. Martin Spring reports. Photographs: Ben Clarkson

An ambitious new £26m sports centre nearing completion in Scunthorpe puts several exciting new spins on geodesic dome construction. The concept design is by Lord Rogers protégé, Andrew Wright Associates, which beat 44 other entries to win an RIBA architectural competition in 2006. Its informal name, the Pods, refers to a cluster of five shallow domes that make up its single roof and flow into each other like soap bubbles floating on water.

Each dome is a different diameter and shape. Then, to cap it all, the domes are covered in a medley of materials, from traditional shingles in western red cedar to high-tech single-ply PVC membrane, polyester-powder-coated aluminium and clear glass. Finally, since the building aspires to “excellent” BREEAM rating, it has a living green roof planted in sedums.

It is only when you enter the building that you grasp its huge scale. The interior is dominated by a vast undulating roof comprising a network of timber triangles in the mutually supporting manner of a geodesic structure. What appear at first sight to be exposed timber rafters turn out to be chunky glulam beams up to 600mm deep. Where six triangles meet at their corners, they are connected by cylindrical nodes in galvanised steel that are the size of dustbin lids.

The building contains two swimming pools, a sports hall with a gymnasium, six badminton courts, and a dance studio (see diagram overleaf). Upstairs, on an open mezzanine, a cafe offers grandstand views of the pool. The oversailing roof, which requires no internal columns other than along a central spine of steel beams, offers freedom for the internal layout.

A roof of such complexity and variety is enough to bring dismay to any building contractor. Added to that, what looks like a standardised system of structural triangles in the roof is frustratingly deceptive. Each glulam beam is actually a different length, and each steel node has flanges projecting at different angles to connect to the beams.

In the event, a design-and-build contract was awarded to Derbyshire-based Bowmer & Kirkland Construction in 2009, working with novated design practice S&P Architects, a specialist in sports projects. Its sister company, B&K Timber Structures, was able to help in tackling the tricky and complex roof structure. Advanced Roofing, also from Derbyshire, was brought in as subcontractor for roof coverings.

“The non-standard structure was a big challenge,” admits B&K’s in-house quantity surveyor, Sean Larkin, who was involved in the two-stage tender process. “We suggested a raft of savings as part of a value engineering process, but none concerning the roof were accepted by the client, North Lincolnshire Council.”

Now, with four months to go before handover, Larkin maintains “the costing of the roof has not changed”, which suggests that he had priced it realistically at the start to cope with the complexity. “B&K has a [computer aided manufacture] software package which has addressed that well,” he adds. The CAM package also picks up neatly from the computer aided design package devised by the structural engineer, Buro Happold.

Roof erection actually started below ground level, where piles were bored up to 10m deep at 36 bases encircling the building. These restrain the considerable outward thrust of the shallow domes.

The thrust is carried to the ground by pairs of raking glulam beams that burst through the building envelope like flying buttresses and meet in dynamic v-formations at the base plates.

Next, the structural triangles that make up the domed roofs were erected in diagonal strips working across the building from one end to the other. “It went up just like an igloo,” comments B&K’s project manager, Simon Fearn. The trick, he says, was to set out the precise location of each steel node, not just on plan but in the vertical dimension as well. Evidently, the irregular shape of the building on plan and in section presented no obstacles. “As long as you’re given the right co-ordinates, you can set out anything with GPS instruments,” adds Fearn.

As part of the erection process, every second or third steel node had to be temporarily held in place by a standard scaffolding tower with an adjustable pyramid on top. The glulam beams were delivered with six steel rods resin-bonded into each end face, and each was slotted into a rectangular housing projecting from the node and bolted up from the inside.

After that, the roof decking was laid as a series of boxes, or cassettes, prefabricated to fit each roof triangle in plan and 225mm deep. Sides and tops of the cassettes were made of 18mm plywood, while the soffits were untreated larch ceiling boards. To muffle the noise and reverberations generated by a sports centre, the 80mm ceiling boards were set 20mm apart to expose a layer of acoustic insulation laid directly on top.

According to Ron Wallwork, associate at S&P Architects, the original intention had been to pack the thermal insulation into the prefabricated roof cassettes. But calculations by Buro Happold revealed that the moisture-laden air above the swimming pools might cause interstitial condensation within the cassettes. Instead, the thermal insulation was laid on site on top of the cassettes and a vapour barrier directly below the roof membrane.

Once all the cassettes were in place, the challenge for Advanced Roofing was to lay the multiple roof coverings. A problem posed by such a diversity of materials was that the continuous sealed PVC sheeting, metal and glass all shed rainwater in a radically different manner from traditional overlapping shingles and a sedum roof.

The solution was to wrap all the roofs, except for the transparent glazed entrance dome, in the same continuous, impervious membrane of single-ply PVC. The membrane serves both as the exposed roof finish covering the larger domes over the sports hall and a concealed underlay on all the others. In effect, this means that the western red cedar shingles, supplied by John Brash, simply act as a rainscreen covering.

In practice, however, wrapping all the roofs in a single membrane was not that straightforward. The shingles and planted sedums needed battens and upstands to be fixed on the outside of the impervious membrane, and this called for ingenious, if rather complicated, new techniques (see box story). Another problem was that shingles covering the tops of the domes at very shallow angles would be prone to wind uplift. Here the solution was more basic: the shingles were simply fixed with double the usual number of copper nails.

A more unpredictable misfortune was December’s freezing weather. Advanced Roofing’s managing director, Richard Clapp, recollects: “The snow was lying up to waist height across the site and the roofs. On a flat roof, you could have scraped it off, but there was no chance here. So we closed down our operation from 29 November to 4 January.”

The five-week delay has not dented Clapp’s confidence of meeting the project handover in May. “The complexity of the project meant that time and cost estimates could have gone horribly wrong, so I probably overestimated. We’re still ahead of programme.”

For Clapp, the project’s biggest challenge has been something completely different. “We’ve had to work off ropes, which we’ve never done before. You can’t stand unaided on single ply membrane on slopes of 20 degrees or steeper. So we had to lay on a lot of training.”

Anton Clifford, Clapp’s site manager, adds: “Holding on to a rope for seven or eight hours a day puts great strain on your arms and blisters on your hands, even if you’re wearing gloves. I just knew it wasn’t a job for middle-aged lads. Here the average age is 24.”

Despite, or perhaps because of, the complexities, severe weather and physical strains, the site has a palpable sense of excitement. “We’re all really proud to be working here,” says Clapp.

Andrew Wright Associates’ competition-winning scheme was rationalised into a simpler, five-dome design, due for completion in summer 2011

Getting out of a fix

Fixing traditional shingles of natural western red cedar to an advanced synthetic roof membrane poses a basic problem of incompatibility. With Scunthorpe’s Pods, the shingles are nailed to preservative-treated softwood laths in perfectly traditional manner. But how to fix the laths to the underlying roof structure without piercing the waterproof seal of single-ply PVC membrane?

Fortunately the PVC membrane maker, Renolit Alkorplan, had recently introduced an ingenious detail to fix photovoltaic panels. The company makes hollow tubes from the same PVC membrane as the roof along with square-sectioned aluminium battens. On site, the hollow PVC tubes are hot welded to the continuous roof membrane, and the aluminium battens threaded along their length. Then the softwood laths are screwed on to the embedded aluminium battens and the shingles nailed to the laths.

Richard Clapp, of roofing subcontractor Advanced Roofing, reckons the Scunthorpe Pods is the first instance of this solar panel fixing device being used for fixing shingles or tiles. But he was concerned that shingles were heavier than PV panels and that this extra weight could cause the embedded battens to slip down the steeper lower slopes of the Pods’ domes. So an extra 100mm wide metal strip was purpose-made, laid underneath the membrane directly below each support batten and screwed through the thermal insulation to the plywood cassette below.

For the green roof, a similar method of upstands was adopted to contain the soil on the roof slopes. But as these upstands were larger than Renolit’s solar support battens, they were purpose made and screwed directly to the plywood cassettes through the PVC roof membrane. The roof’s waterproof seal was then made good by covering each upstand in an extra layer of membrane that was hot-welded to the main roof membrane.