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Sadly, the stadium is still without a new roof, and the fate of the project lies in the hands of the financial wizards who must try to make the project stack up. Perhaps, by the time it comes back to life, Oasys’ winners will be in a position to be part of a real, commercial bid project team?

Students Ruud van Kippenburg, graduating in Innovative Structural Design, and Simon Cox, similar for Concrete Structures, both from Eindhoven University of Technology, prepared a paper describing the study of a new roof design for the stadium, maintaining the same philosophy as the original stadium, comprising a lightweight cable structure, instead of a superstructure with large rigid elements.

They have come up with a unique design where, in contrast to most closable stadiums, the roof is closed by sliding the membrane from the outside - a permanent roof structure - to the inside, obviating the need for large elements in the middle of the stadium. The closing membrane with six segments rather resembles the iris aperture of a camera; certainly an innovative feature for a roof.

There are two main elements to the slightly oval roof: an outer, permanent roof and an inner retractable one. The outer consists of a lightweight stressed cable structure, with two inner tension rings and an outer compression ring. The inner roof uses the two tension rings to support six radial pre-stressed cables trusses, which in turn support the movable roof membrane.

LOAD CASES
To maintain the same elegant and smooth design of the Kuip Stadium, the roof had to be similarly lightweight. A number of analyses and considerations had to be taken into account through time constraints ruling out the assessment of more than one snow and two wind load cases.

Ruud and Simon were aiming to keep the maximum deflection below one hundredth of an elements length and, to prevent vertically instability, no relaxation was permitted in any of the cables. The form-finding calculations for the cable roof structure were started with one secondary inner cable, increasing the complexity of the structural model with each study. Cable sections were determined for 68m secondary inner cable trusses by initially splitting the upper cable into 10 elements, freezing the deformed cable loaded by a triangular pattern after it had been pinned on both ends, and then mirroring the node co-ordinates to create the geometry of the lower cable.

To satisfy structural requirements, force densities (pre-stress) and snow loads were applied. This provided force distribution due to self-weight pre-stress affects. Cable sections could then be determined, based on the maximum force present.

THE MAIN CABLE TRUSSES
The six main inner cables were first modelled independently to get their vertical curved form. Pinning the end supports at this stage allowed the vertical geometry of the main cable to be determined, while ignoring complications from the movement of the supporting outer roof. The pre-stress in the main cable is directly related to the pre-stress of the secondary cables and on the structural requirements of the model, and the cable sections determined likewise.

The second study established that no relaxation occurred in the governing load case and that the largest relative deformation occurs with wind under pressure. The model was then extended to include three main cables and 6 x 9 secondary inner cables.

SECONDARY CABLE TRUSSES
The radial cable trusses were then form-found with the lateral secondary cables to get the curve on plan. Once this was resolved, the outer roof’s tension rings and radial cables were added to the model simultaneously, taking 2 factors into consideration - where the radial cables shape and stress the tension rings, and where the strong radial cables transfer the load of the main and secondary cables to the pinned supports.

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