History of the swimming facility
Headed by Jörg Schlaich, a team made up of the architects Behnisch + Partner and Frei Otto, and the engineers Leonhardt + Andrä, developed a wide range of original tensile structure solutions. The roof consists of a number of parts – the stadium roofs, sports hall, and the swimming facility – but from the ground they appear to be one continuous landscape (ill. A). Although the acrylic-glass skin was only expected to last twelve years, it was not necessary to replace the roof surface until 1997. The roofs of the sports hall and swimming facility also function – in contrast to the stadium’s room – as thermal envelope and thus require insulated suspended-membrane ceilings. In 1972, visitors to the new swimming facility were met with a space enclosed by a translucent ceiling and flooded with daylight. The ceiling, of PVC-coated polyester fabric, was suspended from an exterior cable net. In order to fulfil the interior space’s building-physics requirements, the membrane was composed of – from bottom to top – a moisture barrier, transparent thermal insulation (14 cm pleated PVC foil) and a sealing layer of PVC-coated polyester fabric. Solar gains between the suspended membrane ceiling and the outer layer of acrylic-glass sheets, however, led to unforeseen temperatures – upwards of 100 °C – which the original insulation could not withstand: At the opening ceremony the first dark splotches had already appeared, a sign that the insulating material had been scorched. Because the deterioration of the insulating material continued at a rapid pace, the roof assembly was refurbished in 1974. Additional thermal insulation – in the form of 8 cm thick mineral-wool panels, as well as an additional covering of PVC-coated polyester fabric – were added, to the detriment of the translucence, to the top of the existing multi-layer membrane assembly. Thirty years after this refurbishment, the membrane’s strength had become compromised, in particular due to the prevalence of moisture and chlorine. In 2003, the Stuttgart-based engineering firm Bergermann und Partner and Munich-based architecture firm Auer + Weber were commissioned with planning the refurbishment of the suspended ceiling and the lighting bridge.
Original ceiling structure
The suspended ceiling’s membrane – an 8250 m2 surface – was originally composed of seventeen elements, ranging in size from 80 m2 to 950 m2, which were secured with rope lacing. The cable net was suspended from 218 “clover leaves”, each consisting of four high-strength, spring-steel rods with an overall diameter of 1.20 m (ills. B, C). One of the two apexes is supported by an interior mast, the other suspended from an exterior tower. At the apexes, the membrane was connected via rope lacing to the rings, made of steel tubes with a diameter of 5.00 m or 4.00 m. The funnel-shaped nadirs were secured to the facade structure with rope lacing, as well. The membrane is secured at the circumferential edge connection via tension cables in the cable net. Lighting is located on seven transverse lighting bridges and a longitudinal connecting bridge, suspended from the cable net 16 metres above the basin.
The new suspended ceiling
The design for the new suspended ceiling seeks to restore the atmosphere of the original structure. In addition to aiming to reproduce the original translucence to as great a degree possible, we also sought to reduce the number of suspensions required. This involved, on the one hand, taking advantage of the improved properties of new membrane materials, and on the other, minimizing building physics problems related to the suspensions and the heat loss.
Design
The design for the new suspended ceiling calls attention to the different curvatures of the respective areas: at the extremities (apexes and nadirs) the arrangement is radial; in the flatter areas between them, it is parallel running perpendicular to the length of the pool (ill. D). The most attention, however, was given to mastering the roof assembly’s structural and building-physics requirements. The insulating material, in particular, had to be moisture resistant, translucent, flexible, and able to bear foot traffic. Regrettably, no product was available which could offer a reliable solution for these combined requirements. The roof assembly which was ultimately implemented is – again, from the bottom to the top – as follows: load-bearing membrane of PVC-coated polyester fabric, 7 cm polyester-fleece insulation, and transparent ETFE foil sealing layer.
On the advice of a subcontractor, a responsive ventilation system was installed in lieu of the lower moisture barrier. The measurable translucence of the roof assembly is approximately 1.5 % – under the given set of circumstances – the best result possible (ill. E). The new lighting bridge was adapted to meet the current codes. In order to continue using the ladders to the ventilators in the apexes, it runs at the same height and position as the old structure’s longitudinal bridge: from the ladder at the mast it runs – over 90 meters – parallel to the swimming pool (ills. E, Q). A transverse bridge has been installed to make it possible to reach the second apex (ills. H, S).
Roof assembly specifications
The roof assembly was determined by architectural, building-physics and functional considerations: it was to be as transparent as possible in deference to the original design, yet should sufficiently insulate the pool space. In order to prevent condensation, the lower layer had to be moisture-proof, the upper sealing layer, on the other hand, open to diffusion. Aside from the specified UV-resistance, it had to withstand the roof cavity’s temperatures of up to 100 °C. Functional requirements arose with respect to geometry, installation and maintenance of the roof assembly: the insulating material had to be capable of adapting to the biaxial curvature of the roof.
Load-bearing membrane: glass fibre or polyester fabric?
For the structural membrane of the suspended ceiling assembly, both PTFE-coated/glass-fibre textile and PVC-coated polyester fabric were taken into consideration. The PTFE /glass version had a higher standard – both visually and tactilely. Its resistance to soiling was also preferable to that of PVC / PES. But the PTFE / glass version was more susceptible to mechanical damage, especially during installation, and the membrane fades when exposed to UV radiation. Due to the roof surface’s different orientations and the corresponding varying radiation intensities, the suspended ceiling would have faded unevenly. Because pre-fading the fabric would have resulted in a strength loss of up to 30 %, the PTFE/glass version was ruled out. Because it will not fade, the version which was implemented, of PVC / PES, fulfils the requirement for a uniform appearance.
Ventilation pipes or moisture barrier?
The requirements transparency and weldability necessitated that a polyethylene (PE foil) moisture retarder be installed on the structural membrane. Primarily due to cost considerations, an ETFE foil moisture barrier – an alternative which was also under consideration – was out of the question. On top of that, welding PTFE foil is an elaborate procedure. A version which was favoured for a time – which involved equipping the structural membrane with the appropriate coating so that it could act as moisture barrier – was discarded because suitable products were not available. The version that was implemented has no moisture barrier toward the interior. Instead, an active ventilation system which is regulated by moisture sensors and responds when condensation forms was included to dispel moisture from the insulation.
Translucent insulation
Transparent honeycomb insulation of cellulose-acetate was investigated as insulation material. The material’s high degree of transparency was, however, accompanied by a number of disadvantages: It is not flexible enough, can not withstand foot traffic, and is not waterproof. Due to these disadvantages, waterproofed polyester fleece, a less translucent insulating material, was ultimately selected: It withstands temperatures above 100 °C, and is flexible and elastic (and as a result can withstand temporary loads).
Construction of the ceiling’s membrane structure
The membrane with the parallel arrangement is made of PVC-coated polyester fabric (type II) with a breaking strength of 84 kN/m in the warp direction, while the radial membrane is PVC-coated polyester fabric (type IV) with a breaking strength of 150 kN/m in the warp direction. In contrast to the original design with rope lacing, the membrane – consisting of two large segments – is secured with a clamping plate. The edge connection is continuous, as in the original design, via 1.50 m long rods in membrane pockets which are anchored in the cable net with open spiral cables (Ø 10 mm). The specified pre-tensioning of the membrane is attained by applying tension to the anchors with turnbuckles. The uniform distance to the cable net is achieved with struts made of rods which, via cross-members, are supported by the cable net (ill. M). At the apexes the membrane is attached to the steel rings via aluminium clamping plates. The rings are suspended from the roof structure’s ridge cables with open spiral cables (Ø 22 mm). They are stabilized horizontally via oblique tension cables and the circumferential membrane (ill. Q). The nadirs are also secured to the facade structure via aluminium clamping plates with a diameter of 5.00 m or 7.00 m. The existing foundations and the terraces’ reinforced steel structure bear the loads (ills. G–K). The execution of the clover leaves takes cues from the original design: to prevent stress peaks in the membrane, the suspensions must be highly flexible.
Dismantling the existing structure
First the roof assembly on the upper side of the structural membrane was removed. Because the pool was in operation during the entire construction phase, the 50 m basin was then temporarily completely enclosed. This protective encasement doubled as installation platform for the membrane and the lighting bridges. The diving basin was emptied and scaffolding was set up in it. That facilitated lowering the lighting bridge’s existing steel structure and subsequently removing the clover leaves as well as the structural membrane. The tension members at the edges and the frieze at the junction of the facade and the roof were then dismantled. Finally the new roof assembly (consisting of membrane, insulation and sealing layer), the active ventilation system and a 400 m long pneumatic frieze were installed.
