|Short description of the project
||The roof over a grand stand at Oldenburg covers 5000 seats, arranged in 21 rows, 130 meters long. Though this design goes back to an earlier competition entry submitted by Schlaich and Bergermann at Karlsruhe, careful consideration was made of several alternative solutions - particularly of cantilevered roofs decked with trapezoidal steel sheeting suspended from steel cables, as frequently built in recent years. More satisfying aesthetically and costing little more, the present solution was chosen, using a steel tube, cable and membrane structure. Pretensioned membranes have to be strongly curved in order to take up loads economically. The anticlastic surface curvatures should be similair in bot directions where, as here, roughly equal loadings from snow or wind suction can be expected. Under these boundary conditions the long form precluded a single membrane, thus there are 14 rectangular or trapezoidal elements, connected at upper horizontal level by their adjacent edges along radial struts and each tensioned downwards to a low point. The rectangles are 9,25 x 23 m in plan and their lower points are 4 m below the horizontal edges; roof projection is 17,6 m over seating area and 5,4 m behind. The horizontal struts are cable suspended from masts, 11,45 m high and held down by another set of cables. At each end of the whole roof a triangular cable truss in plan collects the horizontal forces to a point carried on steel trestle supports.
The assembly procedure was as follows:
- firstly the masts were erected with the cables exact in length, there being no adjustment devices (tolerances required for the reinforced concrete ±10 mm and mast construction ±5mm);
- then pre-assembly of the membrane near site with compression struts and tension cables;
- lifting of pre-fabricated segments into position and securing support cables to masts;
- connection of end cable trusses between roof and trestles and gradual tensioning of cables (ca. 150 kN per cable) with continual monitoring of overall geometry and of prestressing forces,
- graduate tensioning of membranes at low points;
- final stressing of whole structure via end trusses and trestles;
- connection of stainless steel rainwater tubes to the low points.
[The Art of Structural Engineering, The Work of Jörg Schlaich and his Team, Alan Holgate, p146,149]