Working Detail: Olympic Velodrome roof, by Hopkins Architects
[WORKING DETAIL 24.02.11] Velodrome roof structure node
‘The hardest detail of the whole project for us was getting the timber cassettes to sit on the cable net,’ says Hopkins Architects senior partner Mike Taylor. ‘We wanted it to be possible to just pick up the wooden panels and drop them straight into place.’
The node connections supporting the cassettes had to allow the cable net to move within the otherwise solid roof construction, as well as accommodating manufacturing and installation tolerances.
These nodes have 10 different types of connection plate, which control the locations of movement joints. Each connection plate type has a different permutation of holes, slots and oversized holes for fixings to the cassette bracket housings.
The brackets are also grouped with different permutations of holes and slots. The configuration of the connections allows for movement at the edges of the rooflights, which are vulnerable to expansion of the roof structure, and at the perimeter.
There are 17 cassette sizes, with a range of standardised and bespoke prefabricated units and perimeter make-up pieces fabricated on site.
The cable-net roof comprises paired 36mm diameter galvanised steel cables, clamped together at 3.6 metre centres. Each clamp was positioned at ground level using marks made when the cables were pre-stressed and cut to size in the factory.
Connection brackets were then secured to each clamp before the cable net was lifted into position and stretched to connect to the ring beam.
Each connection bracket has four receiver brackets that support the roof panels via steel shoes built into the corners of each panel.
Every roof panel is secured with one fixed, one slotted and two oversized receiver brackets, allowing movement but keeping panels orthogonal to the cable net.
Fixed, slotted and oversized connections are arranged to constrain the panels, confining movement to predetermined locations.
Movement in the north-south direction, across the Kalzip, is concealed in rooflight upstands. Where there are no rooflights, MJs are at 7.2m centres.
Movement in the east-west direction is evenly distributed across the roof, and hence accommodated in the normal range of the Kalzip fixings.
Chris Bannister, partner, Hopkins Architects