Hopkins’ double-curvature roof is elegant in its economical use of material as well as its appearance, writes Felix Mara
You need flair to design a flamboyant roof. But to design one which is also elegant calls for both talent and technical nous. The roof of Hopkins Architects’ Velodrome for the 2012 Olympic and Paralympic Games in London doesn’t just look elegant. It also achieves elegance in its economy of means and materials, its integration with the overall design, its speed of erection and its low cost.
The Velodrome has a double-curvature roof which helps to minimise volume and heating costs. Its structural members and deck are lightweight, comprising a tension cable net, timber cassette panels and OSB. The cables running north to south, which resist gravity, have their high points where they connect to the roof’s ring beam, while the cables running east to west, which resist wind uplift, have their low points at the ring beam. The cables are grouped in pairs, enabling their diameter to be limited to 36mm.
The vapour barrier and Part-L-beating 300mm-deep insulation sit on top of the roof deck (which also supports the fixings for the Kalzip roof cladding) in lengths of up to 130m, with standing seams on the east-west axis, which channel rainwater into a perimeter gutter with twin 300mm-diameter downpipes at its two low points. The outlets at the west end discharge into a rainwater-harvesting tank. Glass rooflights with two interlayers of white light-diffusing PVB, forming strips parallel with the cycle track’s main east-to-west axis, help to reduce lighting costs.
‘The roof has about 100 tonnes of steel in it and the structure is as light as we dare go,’ says Hopkins Architects senior partner Mike Taylor. The absence of roof-top kit such as photovoltaic panels (PVs), helps to minimise the loading on the roof; because of the Olympic Park’s district heating system, individual buildings do not need to generate renewable energy. Taylor questions the logic of the PVs on the roof of Herzog & de Meuron’s ‘Bird’s Nest’ stadium at the 2008 Olympics, because of the length of time it will take for these panels to pay off the carbon deficit generated by the steelwork needed to support them. ‘The Bird’s Nest was scandalously wasteful of steel,’ says Taylor.
‘In many ways the velodrome’s roof is low tech,’ says Hopkins Architects partner Chris Bannister. Nodes used to support the roof deck panels are clamped to the pre-stressed roof cables that were hoisted and pulled into position, using pressure-monitoring hydraulic jacks that enabled their ends to be attached to the ring beam, where there are non-adjustable connections.
The design team decided not to put insulation inside the timber roof cassettes because movement between the cassettes was a concern and, with a vapour barrier below the insulation, the cassettes would have been vulnerable to condensation at low temperatures. With insulation above the cassettes, it was possible to rationalise the scope and sequence of operations on site. After one subcontractor had installed the structure, with temporary waterproofing above the deck, another could put down the vapour barrier and install the insulation and cladding.
The project team challenged the design team’s proposed cable structure, thinking that it would cost more than a conventional steel frame and would carry greater risk. But when the Velodrome’s general contractor, ISG, said a cable-net structure would cost £2 million less and could be completed three months earlier, the original proposal was reinstated. The cable net went up in four weeks, using four tower cranes and a safety cable net, with no scaffolding.
It’s difficult to find fault with the design logic, although the economical roof structure carried one cost that might be hidden from the layman. ‘With this very light roof you need more concrete in the ground to stop overturning,’ says Taylor. The layman might also be forgiven for mistaking the large downstands, which carry electrical services, for beams, perhaps detracting from the elegance of the ultra-thin roof structure, although it would be no mean feat for such shallow beams to span the full length of the Velodrome. This level of electrical servicing is partially driven by the need for high lighting levels during the Games, and may be seen as a luxury after the camera crews depart.