Water wings

The sweeping curves of the Aquatics Centre roof are now getting a slick but simple aluminium covering. The result will be the most spectacular structure on the Olympic Park. Martin Spring reports.

In the instant Olympics city under construction in east London, the most dramatic structure by far is the Aquatics Centre. Its roof is a great curving pair of wings that floats over thin air and is balanced on just three pins 120m apart – further than a football pitch. The three pools inside will stage the Olympic swimming and diving events, so the sweeping curve of the roof enclosing them was designed to “reflect the fluidity of water” by architect-diva Zaha Hadid as the centrepiece of her competition-winning scheme.

On Construction Manager’s visit last month, silvery metal was being laid out in orderly geometric patterns across the 11,000m² roof. Along its central spine, profiled aluminium decking ran longitudinally in continuous furrows. At either side, where the roof bulges outwards by 27m, the same profiled decking was laid crosswise. And between these two areas of profiled decking was flat aluminium sheeting with slender metal upstands running diagonally.

The profiled decking and slender upstands were installed by subcontractor Lakesmere of Winchester as underlay components of the Kalzip system of standing-seam aluminium roofing.

“Which way the profiled decking runs is immaterial to the direction the standing-seam finish is laid on top,” explains Stuart Fraser, project director of design-and-build contractor Balfour Beatty. “We’ve used that flexibility of the decking to drain the roof. Along the central spine, it acts as a temporary gutter to get rid of water in heavy rain.”

The slender metal upstands running diagonally will have thermal insulation packed between them to a depth of 200mm and support the top surface of standing-seam aluminium. The top Kalzip surface will be laid in May and run longitudinally in parallel strips.

“The beauty of the Kalzip solution is its continuity,” continues Fraser. “We can roll it out along the entire 160m length of the roof without any joints, just the standing seams on either side of each strip. But because of the irregular shape of the roof, we can’t cover it entirely in parallel strips, so we have to have special purpose-made sheets in two areas.
These are also manufactured by Kalzip using the same system. In total, it’s a maintenance-free, solid roof with no penetrations at all.”

All around the perimeter of the roof runs an aluminium gutter 600-900mm wide, which was purpose-made and installed by Lakesmere. The rainwater discharges into downpipes located at the three structural supports to the roof, two of which drain into a branch of the River Lea that runs conveniently along one boundary of the site.

If the roof surface is reassuringly regular with standard off-the-shelf products, the roof structure directly below it is the opposite. Here, a full 3,175 tonnes of steel has been configured into a dense maze of curving trusses and cross beams up to 11m deep.

The 120m clear span between supports makes the roof structure more akin to a bridge than a building, points out Mike King of structural engineering consultant Arup. It was no coincidence that the contract was awarded to bridge specialist Rowecord of Newport, south Wales.

How to erect this massive steel roof was, in Fraser’s words, “the result of a tremendous amount of collaboration between ourselves, Rowecord and Arup” that entailed at least two radical switches of strategy. Arup’s initial idea of building the roof out of two sets of steel arches running lengthwise and crossways was rejected as too complicated to build. The solution was to use 10 trussed arches running longitudinally and fanning on either side in the central bulge, with simple steel struts providing cross bracing.

The steel roof structure was entirely fabricated out of basic steel plate up to 120mm thick and procured from Britain and Italy. “There’s no standard steel off-the-peg stuff up there,” says Fraser. “All the steel has been rolled specifically for this project, profile-cut by machine and continuous welded along each length to create top flange, web and bottom flange.”

The steel was fabricated by Rowecord in Newport into sections up to 15m long, the maximum that could be transported on an articulated lorry without the expense and complications of an escort.

Then came the second change of plan. Fraser’s first idea was to preassemble the whole roof on the ground and strand-jack it as one piece up 18.5m into its final position. “But the roof is 11m deep, so even if you make it all up on the ground, people would still be working at a height, and that’s our main risk. So we decided to make up small pieces on the ground and raise them up piece by piece.” Each of the 10 trussed arches was preassembled in four sub-sections up to 35m in length.

As for providing temporary support for the arch sub-sections, Rowecord solved this problem by erecting four trestles of scaffolding across the site, which were supported on the building’s permanent piled foundations.

The steel sub-sections were then lifted into place by three large mobile cranes, and they were spliced together into rigid arches with 1,300 special non-slip bolts manufactured and patented by Tension Control Bolts. A further 1,300 bolts were used elsewhere in the roof, and then the steelwork was protected from the corrosive pool environment with three coats of epoxy-based paint.

Once the whole roof assembly had been fully bolted together and tested, it was time for the tricky manoeuvre of removing the three temporary scaffolding supports. This involved strand-jacking one end up by 850mm. As the whole assembly had been rigidly bolted together, the opposite end dipped down slightly. The lifting operation was carried out by Italian heavy lifting specialist Fagioli over eight hours in October last year.

As for the greatest achievement of the whole construction project, Fraser has no doubts. “Technically the roof structure has performed exactly as predicted by both Arup and Rowecord. We had tolerance parameters for deflection of just +/-10mm across the whole 120m span of the roof, and we’ve met them without exception. We’ve had no remedial work at all associated with the roof after it was strand-jacked into position. It’s all about stiffness, and that means that every single connection out there is performing as expected.”

Since the three scaffolding trestles were removed last November, work has proceeded on the three concrete pool tanks at ground level. Once these are cast, Finn Forest UK can start installing the slatted ceiling in Red Louro timber. Scaffolding will again have to be erected, and this time it will be an extensive birdcage of scaffolding to gain access the whole soffit of the roof structure.

“We looked at moving gantries [slung under the roof] to give access,” says Fraser. “But you’ve got the immense span over the width of the pool below and the curved shape of the roof and frankly it’s more straightforward to put up scaffolding.”

Complications such as these, plus the bureaucracy of working in the congested Olympic Park, have all added to costs. The total cost of the project has multiplied more than threefold from an estimated £74m at the time of the Olympic bid in 2005 to £248m currently.

That said, the biggest challenge that constantly faces Fraser is the health and safety of his workforce. When work starts on the ceiling, people will be working simultaneously at five levels: roof surface, roof void, ceiling, pools at ground floor level and water filtration plant in the basements. “First of all, we must not have people falling off the roof. So we have robust arrest netting underneath the roof and surrounding the walkways within the roof. And the netting must be fine enough to stop even screws falling down.”

Ian Crockford, project manager for the client, the Olympic Delivery Authority, adds: “All the tools and equipment, even helmets, are attached by lanyards to any person working up there. There’s also a specific safety induction course for anybody working on or within the roof.”

Not for nothing can Crockford proudly call the Aquatics Centre roof “one of the greatest achievements of the whole Olympic Park”. In engineering terms, delicately perched on just three fingertips, it is a feat of structural acrobatics – and an achievement of Olympic proportions.

It’s a wrap – and a green innovation too

The most innovative envelope material in the Aquatics Centre will be an environmentally friendly plastic wrap on the two temporary seating wings. 

The wrap is a PVC fabric that is non-toxic and, like the temporary wings themselves, is intended to be reused on other sites.

To save costs, the Aquatics Centre design was drastically slashed in 2006, when 15,000 spectator seats were hived off from the main building and placed in two ungainly temporary wings. It was argued that 15,000 of the 17,500 seats needed for the Olympic fortnight would become redundant immediately after, so the main building that would remain as a permanent “legacy” could have a reduced seating capacity.

The plan is to wrap the temporary seating blocks in PVC fabric similar to the decorated temporary facades that are increasingly used to conceal buildings under construction. The wraps will be supplied and fitted by tensile structures specialist Architen Landrell of Chepstow, south Wales.

But PVC packs a hidden nasty in that it is plasticized with phthalates containing carcinogenic dioxins, which are ultimately released into the atmosphere when the material is junked by incineration.

To overcome this health problem, the Olympic Delivery Authority, together with the other public authorities organising the Games, have developed an ecological, non-toxic PVC that is plasticized without phthalates.

“We’ve worked with environmentalists like Greenpeace and WWF to find a non-phthalate plasticizer, and we’ve worked with chemical companies like BASF and DuPont to produce it,” says Ian Crockford, the ODA’s project manager for the Aquatics Centre. “It is part of
an investment we’ve put into making the industry better for the future. This ecological PVC will be applied in the Aquatics Centre first, where 17,500m² will be needed. After that it will be used extensively in temporary venues across the Olympic Park.”

The water polo centre, the basketball arena and temporary components within the main Olympic Stadium have all been earmarked for such reusable, non-toxic PVC wraps.

Why we specified...

Weber external wall insulation system

Cross Keys Homes social housing project, Peterborough

Paul Webb, operations manager, Retrofit UK

We’re working to upgrade properties built in the1950s and 60s, including flats, bungalows and townhouses, to improve thermal performance and bring them in line with today’s building regulations.

External insulation was considered a priority as most of the properties did not have cavity walls. They were also occupied, which meant avoiding access to properties during renovation work.

Perhaps the project’s biggest challenge was insulating 80 prefabricated steel shell bungalows built in the 1950s, the external walls of which were made of corrugated steel sheeting with no insulation other than an interior layer of plasterboard. Using Weber EWI we slashed U-values from 2.1W/m²k to 0.27.

Maintaining the external appearance of the buildings was vital and Weber provided various render finishes, including brick and stone-dash effects for the houses and silicone-based through-coloured render finishes for flats.

The weber.rend RB brick-effect render comprises a 8-10mm thick mortar coat, covered by a 3mm brick-coloured face coat. The top coat is then cut away by hand to reveal the mortar beneath, to create the effect of brickwork courses.

Weber EWI products are virtually maintenance free and cheaper than doing internal work. It’s also a good option for properties that have narrow cavity walls with little room for insulation.

There are some drawbacks to this method, however. Installing scaffolding around the building, for instance, creates disturbance and drilling the insulation to the structure is quite noisy. There’s also a fair amount of prep work needed, as pipes and services have to be removed from walls to create a clean surface.

We considered using Envirowall, which is very similar, but after cost analysis Weber came out on top. 

Product news

Two new colours have been added to Redland’s Heathland range of imitation hand-made clay tiles, Wealden Red and Elizabethan. The tiles feature a through-colour tile body with a surface finished in fine, coloured sand.

The Heathland range is primarily aimed at housebuilders and homeowners in the south of
England, where vernacular architecture and plain tiles predominate. The firm says the tiles are half the price of hand-made clay tiles and contain almost half the embodied carbon.

www.monier.co.uk

Kingspan Insulated Panels’ Optimo, Architectural Wall Panels and Longspan ranges are now available in widths of 600-1,000mm, and in increments as low as 1mm depending on the profile of the panel. Panels are suitable for new build and refurbishment projects and finishes include micro-rib, euro-box, flat, mini-micro and wave profiles.

All Kingspan products have an ECOsafe insulation core that contains no CFCs or HCFCs, achieving an A or A+ rating according to the BRE’s Green Guide to Specification.

www.kingspanpanels.com

Iko’s new hot melt waterproofing system for extensive, intensive and biodiverse green roofs includes a built-in root protection layer.

PermaTEC Anti-Root is a 6mm-thick monolithic root barrier that eliminates the need for a separate anti-root membrane, simplifying specification and installation, says Iko.

The firm has also produced new literature to complement the product, including a design guide and technical data sheets.

www.ikogroup.co.uk

Tips of the Trade

Specifying a roof-mounted natural ventilation system

01 Suitability

Almost every building is suitable for roof-mounted ventilation, but where the systems are fitted needs consideration. This information needs to be conveyed to the manufacturer to ensure, for instance, that the correct duct lengths are included in the ventilation calculations.

02 Position

Natural ventilation systems need to be positioned so the louvre profiles are exposed to the wind. It may be necessary to mount the systems on an upstand if located near a parapet wall, for example. Also consider the distance from mechanical extract points to ensure that cross contamination does not occur.

03 Assess internal heat gains

Building simulation modelling allows predicted internal temperatures to be determined against weather data over a complete year. Any building can be optimised, however, for instance by rationalising the density of occupation or considering other aspects such as the specification of the glazing.

04 Night-time cooling

An important part of any natural ventilation/passive design is the use of night-time cooling to reduce the build up of internal temperatures and keep the temperature of the overall building fabric lower.

05 Control and commissioning

Fully automatic control systems should ideally be specified to continually adjust ventilation openings, depending on temperature and CO₂ quality. Control systems should be weather-compensating and always allow the users to override the automatic settings, reverting to automatic control after a certain time.

By Nick Hopper, technical director, Monodraught