7 | Rib structure of the roof in plan.
7 | Rib structure of the roof in plan.
6 | Tree column, scale i:ioo and 1:50. i Glulam segment. 2 300 x 160 x 32 mm scantling. 3 Concrete plate, grout 4 Pressure chamber for height adjustment. 5 Tree ring. 6 Hardwood dowel.
closely spaced trees form a hall-like room 5 to 7 m high, which opens out towards the swimming-pool. The glass facade skirts the curved boundary beams, which form the limits of the roof from one support to the next. At their lowest points these beams are carried by three-legged concrete supports. In the direction of the park the roof opens out with a wide-span arch 7.50 m above the ground.
Structure | The roof resembles a membrane stretched from five peaks to a central valley and to the open edges. The extremely complicated geometrical shape of the roof was calculated at the Institute for the Application of Geodesy in Building Construction of Stuttgart University using computer model testing. In the case of a very thin membrane the geometrical form results directly from its own internal forces, as with the film of a soap bubble. Here, in order to create a design that could be realized with a minimum of material, the relations between form and force were thus modelled in a way analagous to the calculation of rope networks.
The main timbers supporting the roof are the meridian ribs, which, as the main suspension members, stretch from the tree rings to the boundary beams and, in the "saddles", from tree ring to tree ring. They have a cross-section of 200 x 205 mm and are connected one to the other by thinner annular timbers of 80 x 80 to 120 x 140 mm. The surface of the roof is formed by a double layer of boards, which run diagonally in opposite directions, each covering two spaces, and are butted to the meridian ribs in staggered arrangement. The roof boarding is of high shearing strength, ensures excellent form retention and, in the event of heavy uneven loading, prevents any bending of the trees.
The annular members are glulam beams. Their cross-section is so inclined towards the horizontal that they form the surface shell of a truncated cone. The rings are of two parts with cross-sections of 85 x 800 and 120 x 800 mm at 12 cm centres. The 120x120 mm intermediate timbers are glued to the lower beam half. After installation of the meridian ribs the upper beam half was connected to the lower beam half by twenty 270 mm hex head screws and 20 mm hardwood dowels. Of similar design to the annular rings, the boundary beams are 1300 mm in width. Due to the varying height and slope of the roof edge they, like the ribs, are doubly curved and twisted. Their weight is transmitted to the ground via the facade columns. The meridian ribs unload their forces in the 12 cm spaces between the tree rings and edge beams and are held in position here with hot-dip galvanized dowels of 20 mm diameter. Most of the roof load in the boundary beams is transmitted at their lowest points to the concrete supports. Here the boundary beams abut and form a massive 170-mm-high seating element of dow-elled beech plywood. Let into each of these is a pot-type support of cast steel GS 52, which absorbs the bearing pressure from the roof shell and transmits it via a cast steel shoe to the reinforced concrete abutments.
The annular rings are supported by the "branches" of the trees, which are composed of several identical glued wood members running from tree ring to tree foot. Over the lower part they are bundled to form a trunk, the top part spreading like branches that carry the tree rings. The use of metal fasteners has been kept to a minimum throughout the whole of the wood structure. The metal dowels are protected from the aggressive salty air by knothole dowels, the heads of the hot-dip galvanized wood screws and nails by a coating of bitumen.
The roof is a non-ventilated roof, comprising a vapour barrier with aluminium coating, three layers of treadproof mineral wool board of 120 mm total thickness as heat insulation and a fabric-reinforced dark green pvc sheeting as roof skin. The sheeting is mechanically anchored with 3 to 4 plate anchors per square metre by means of thrust blocks of hard foam in screwed-on wood dowels.
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