Structural detailing
With cable structures, it is essential that the details respect the system lines and system points of he cables and their intersections, as well as the likely movements of the cables at the connections. If the radial cables were continuous through the node points, the flexing at those points would cause the cables to fail prematurely in fatigue. At every hangar, therefore, the radial stringer cables are terminated and connected with barrel-shaped pins to the nodes - an arrangement that allows the cable ends to rotate in any direction.
Large numbers of cable terminations greatly increase the cost of making them up, but this pays off on site, because it means that the dimensions are fixed. The radial cables were attached at intermediate points to the 90 mm boundary cables. Friction clamps were used to avoid cutting and terminating the cables. One of the most complicated 3-d problems was the detailing of the huge mastheads, where no fewer than 23 separate cables had to be connected at a common intersection point. In the course of detailing, it proved necessary to extend many of the connector plates outwards to allow the individual cable ends to connect without clashing with each other. The masts themselves were relatively simple to detail with all the joints, including the site joints, since they are fully welded. All 12 masts are identical. At the bottom of each mast is a rubber pot bearing with a single locating bolt. The bearing allows slight rotation of the mast at the connection point with the pyramid.
Design verification
The calculations to verify the safety of the design were based on our "Tensyl" program. Analysis of the cable structure has shown that the behaviour of the structure is very sensitive to cable stiffness. Each cable has been dimensioned , but also for axial stiffness, in order to ensure that the cable system does not "go soft" when under load. Wind loads were taken initially from published data and then confirmed by wind-tunnel tests. Snow loads were derived from an analysis of snowfall data and observation of the extent of drifting in various situations.
Steelwork construction stage
The steel contractor, Watson steel, had to develop the engineers' design rawings into shop drawings for the production of the components. This process involved an element of detailed design of the components and connections. The cable work was subcontracted to Bridon Ropes of Doncaster. The cables have to be wound from wires that have been previously drawn and galvanized. For the dome project, class A galvanizing - the lightest - was specified for the cables beneath the roof; and Galfan, a more durable mixture of aluminium and zinc, for galvanizing the external cables. The cables had to be prestretched to eliminate the construction stretch, and then marked to the correct lengths under the specified load. Most of the cables were dead length without any provision for adjustment.
When the net was completely assembled and all the cable lengths had been checked, each of the 72 pairs of radial cables had to be tensioned. This was undertaken in a number of steps, using a 55-tonne-capacity "Enerpac" pull jack in the predesigned jacking points at the front of the perimeter masts. Because of the flexibility of the central ring and the boundary cables, the tensioning of the radial cables had to be executed to specified dimensions rather than to specified loads, with final adjustments made at the end.