The mighty folding structure of the Mülimatt Sports Training Centre is more than the sum of the load-bearing components: it forms the façade into a sculpture, serves as a weather skin and at the same time testifies to a structural design that goes far beyond the usual and requires close cooperation between architect and engineer.
The Mülimatt sports training centre in the canton of Aargau unites all interior spaces under an impressive concrete folding structure. The atmospheric surroundings on the edge of a residential area in Windisch, between the railway embankment and the banks of the Aare, were the starting point for the design: According to the architects, in a place that radiates such serenity, all they wanted was a "shelter against rain and the blazing sun". The resulting idea of erecting a large, cantilevered roof was implemented with a prestressed concrete folding structure. It spans a field of 80 x 55 m in the transverse direction. However, the view from above from the moving train, from where the roof becomes the fifth façade, was also a decisive factor in the choice of the supporting structure. For the engineers, the challenge was to bridge such a large area without columns with a structure that was as filigree as possible. For this reason, they deliberately explored the limits of what was feasible when dimensioning the concrete elements and, by combining the folding structure, prestressing and self-compacting fine concrete, came up with extremely small load-bearing cross-sections: the roof panes are only 16 cm thick and the stem panes 20 cm thick. This is where the post-tensioning cables, which are small units of one to six strands, and their anchors are placed. The thin structural cross-sections made the use of self-compacting fine concrete necessary. This minimally covers the tension cables and the anchors and leads to a compact, homogeneous surface, which was also architecturally desirable.
Pure supporting structure
The monolithically connected folding structure is composed of 27 frame elements, which in turn are joined from the stems on both sides and three roof elements that are joined together. Due to the sloping terrain, the stems on the river side are longer than those on the railway embankment. The two legs of the 2.59 m deep stems spread upwards in such a way that they connect exactly to the V-shaped roof elements, so that the façade merges seamlessly into the roof. At the corners of the first and last frames, a triangular disc made of in-situ concrete deflects the forces of the roof edge.
Although the high-density fine concrete ensures a surface that would be tight even without surface protection, the roof cladding is provided with liquid plastic – this increases the durability of the supporting structure and guarantees cost-effective maintenance, while effectively sealing the joints between the elements poured in in-situ concrete on the other. The folding structure is thus both a weather skin and takes care of its own drainage, neither a roof edge nor a rain gutter or downpipes disturb the appearance: The rainwater is collected in the wave trough of the roof and from there is led openly on the inside of the legs of the stems to a drainage channel in the floor slab.
Unity and transparency
Inside the seemingly closed folding structure, glass facades and a coffered ceiling suspended from the folding truss girders at the top limit the interior spaces. Unfortunately, this means that the soffit of the supporting structure remains hidden. A gap between the stems and the façade shows the independence of the folding, whose oblique lines contrast with the mullion-transom construction of the glass skin. Only the tension members at the level of the floor slab of the sports halls connect the sticks to the in-situ concrete construction of the interiors and indicate the formation of the hall floor as a tension disc.
[Source: www.db-bauzeitung.de] (https://www.db-bauzeitung.de/db-themen/db-archiv/betonharmonika/)
Photos: René Rötheli - rrphoto.ch Baden, CH
