Form
The six Sun Valleys are funnel-shaped elements made of steel and glass with a polygonal mesh freeform frame structure. The diameter at their base is approximately 16 m and expands to widths from 70 to 90 m at their upper rim. They cover an area of approximately 5,000 m² each, optimized according to structural and design characteristics in numerous developmental steps. Their frame structure comprised of triangular shapes is comparable, in a similar way as the membrane roof, to natural structures – i.e. based on reduction of material resources.
Structure
The structure is the result of two basic considerations – for one, the geometry requires an increase in the number of triangular shapes from bottom to top. The alignment as well follows the main direction of force in order to create a polygon mesh structure of maximum efficiency. This is why the alignment is vertical at the structure’s base and ring-shaped at its upper rim. Therefore, instead of using 6 way nodes, special 5 and 7 way nodes are included at mid-height to produce a 30 degree rotation of the mesh structure. Location and number were defined in order to maintain the homogeneity of mesh sizes of approximately 2.20 m. The Sun Valleys need to bear dead loads, snow, wind, and earthquake impact, and also membrane loads of the PTFE coated fibre glass mesh. In order to determine the strength of the membrane, at first calculating the spring stiffness of load transmission points along the Sun Valley was necessary. The results of this calculation were then used for determining necessary steel profile sizes.
Optimisation
The strong loads originating in the local distribution points of the membrane are, thus, only recognizable in the increase of steel profile sizes within four segments surrounding the connecting points. Here, the general profile size of 180 by 65 mm is increased to 500 by 140 mm. Members were grouped according to their load bearing properties and profile sizes were optimised correspondingly. This resulted in the development of 48 different steel hollow profiles consisting of longitudinally welded flat steel elements. A precise adaptation to the individual load situation is achieved by gradually decreasing the gauge of flat steel elements. Thus, solid profiles also find use where required. The maximum elastic deformation was determined in cooperation with local experts.
Structural concept
The membrane roof covers an area of 65,000 m² and consists of a PTFE coated fibre glass mesh of the strongest type V with a tensile strength of 160 kN/m. The minimum level of prestressing is 2.5 kN/m. The structure is characterised by the exterior pylon high points as well as the low points at interior columns as connecting points for suspending the textile mesh. These are complemented by wind suction cables in the membrane plane, corresponding hangers that prevent the membrane from collapsing, and connecting cables to the exterior pylons.
Construction
The interior columns, 750 mm in diameter, bear the majority of vertical loads and are connected to the exterior pylons through wind suction cables. A compression ring connects main membrane and anchor cables to the interior columns. The main membrane loads are distributed into the pylon high points along the exterior. These are secured externally by two tie-back anchors with diameters of up to 160 mm. Gravity foundations on piles provide the necessary mass to bear tensile loads of, in part, far above 10,000 kN. The 2 by 2 m base plates of the return anchors permitted tension adjustment via threaded bars in the first months of spreading out the membrane. A special challenge was posed by establishing controlled load bearing of anchors and pylon bases, since every change in geometry also resulted in a change of load transmission within the membrane. Membrane or cable failure and resulting slacking can be ruled out, even in the case of heavy wind impact. The prestressing was optimized within a complex developmental process. The low points of the interior columns are the points where the greatest vertical loads are concentrated. Two layers of fibre glass mesh are necessary in these areas. Both layers are laminated and form a rosette shape. Numerous geometric optimisations ensure that radial forces don’t exceed peak limits and that a minimum level of tension is maintained within the ring in order to prevent membrane creasing. Along each Sun Valley, the membrane is connected in two to three points in order to fit it tightly along the funnel shapes. Double joint connectors prevent the transmission of unwanted moment forces resulting from changes in cable inclinations due to wind impact. The ball bearing mounted interior columns are equipped with individual pylon heads. Their shaft is comprised of a single component with up to 2 m long splicing plates. The large lengths produce significant local moments due to cable and axial deflection that needed to be determined for every single column head. Membrane drainage is integrated into the interior columns. The main membrane is connected to a compression ring at the top of the interior columns. Coping strips cover these connections and ensure seamless drainage. A further membrane of type II, significantly less stiff, comprises the connection between underside of compression ring and interior drain. Due to typhoon hazards typical for Shanghai, loads and deformations caused by wind impact were a crucial factor. In extensive wind channel experiments and via a more than 20 m long model of the roof construction, various loads and impacts were simulated at Tongji University. These complementary experiments are surprising and enriching for European engineers who, for the most part, rely only on computer calculations.
