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‘Floating’ Stage Rotates, Features Submerged Track
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Aerial view rendering of the new temporary stage for the Bregenzer Festspiele
The new temporary stage for the Bregenzer Festspiele not only rotates, it also boasts a submerged train track and three 27 m tall steel sculptures attached to one another with footbridges. © Bregenzer Festspiele/ Simon Wimmer

The new temporary ‘floating’ stage for the Bregenzer Festspiele not only rotates, it also boasts a submerged train track and three steel sculptures attached to one another with footbridges.

July 23, 2013—Every two years the Bregenzer Festspiele, a festival centered on a theatrical performance staged on Lake Constance, Austria, unveils a new temporary stage in water that reaches an approximate depth of 4 m. A number of seemingly opposing requirements guide the design of the temporary stage. It must be fantastical in appearance and up to 2/3 larger than a typical stage so that it isn’t dwarfed by its natural surroundings, but it must also be as lightweight as possible because it is founded on steel and wood piles that extend into the lake bed and a permanent concrete core with a load capacity of only 10 kN/m². Once dismantled, the stage must be fully recyclable or reusable, but it must perform—and withstand—extremes in weather and temperature for two full years, including thunderstorms, gales of up to 135 km/h, snow loads of up to 2.10 kN/m², and temperatures that drop to -20 degrees Celsius.

This year’s stage opened just in time for the summer season and this year’s production of The Magic Flute. The stage is dominated by three 27 m tall steel sculptural elements fashioned into individual “dragon dogs”—known as Wisdom, Reason, and Nature—that are connected at their tops with rigid footbridges. The rotating, domed circular stage reaches a height of 7.5 m above the average water level and is fronted by a functioning, submerged train track located 2.5 m below the water level that encircles the stage.

Aerial view rendering of the two footbridges

Each of the two footbridges has a floor beam deck connected by
vertical struts to load-bearing handrails to form a
vierendeel girder.
© Bregenzer Festspiele/Anja Köhler

Three-dimensional modeling ensured that the structural steel supporting the dragon dogs could be continuously modified during the design process so that the structural components for each remained completely hidden underneath its polystyrene block- and glass-fiber-reinforced plaster body, according to Andreas Gaisberger, Dipl.Ing., an engineering consultant from Dornbirn, Austria, who designed the dragon dogs. Gaisberger wrote in response to written questions submitted by Civil Engineering online. 

The design team used RSTAB software (developed by the German software company Dlubal) with add-on design modules that complied with European codes for the spatial modeling, according to Gaisberger.

The dragon dogs stand upright on three rigid, interconnected supports—their two hind legs and tail—that are approximately 8 m long. Platforms that begin at the dragon dogs’ pelvic height each hold a 12 m tall octagonal tower that tapers from 3.5 m to approximately 2 m in diameter to form the bodies of the dragon dogs, according to Gaisberger. The towers that support the dragon dogs’ bodies are formed from slender girders with a double T profile; the arrangement of steel trusses and shear-resistant wood planking offers torsional rigidity against the gale-force winds that can occur. The dragon dogs’ heads were shaped around square central bars measuring 4 m to 5 m in length.

Two of the dogs, Wisdom and Nature, are founded on wooden piles that extend 4.2 m to the lake bed and an additional 6 m into the ground. The third dragon dog, Reason, is founded on a 24 m by 24 m permanent concrete core that houses the stage production’s costume and dressings rooms, mechanical room, and toilets. (The concrete core is the only permanent portion of the stage.)

Rendering of the stage which is a revolving dome

The stage is a revolving dome, the top of which is covered in
air-inflated blades of “grass.”
Glue-laminated timber beams
arranged in a star shape comprise the dome’s ribs.
© Bregenzer
Festspiele/Ralph Larmann

Preventing torsional movement of the dragon dogs was particularly important because they are linked at their heads by two footbridges that are used by the actors during productions. In order to avoid high tensile loads on the bearings, each of the two bend-resistant footbridges have a floor beam deck connected by vertical struts to load-bearing handrails to form vierendeel girders, Gaisberger said. “In the design of the footbridges, attention was paid to the fact that the handrail should not be subjected to any pressure load on top of its own weight,” he noted. To prevent this, the handrails were welded to the bridges after all the potential dead loads had been mounted on the decks. The sagging of the deck under its own weight—by approximately 50 percent—was deducted from the radius of the roll, he said.

“Installation of the footbridges made the dogs into fixed bearings, ensuring that the dogs are rigidly coupled, which in turn considerably improves the swaying characteristics of the footbridges connected to them,” Gaisberger explained. “The chosen design means that the handrails can carry some of the service load in the manner of vierendeel girders and provide additional reinforcement for the footbridge structure.”

The stage itself is a 360 degree, 22 m diameter, and 3.4 m high revolving dome that is founded upon 36 steel and wood piles that extend 6 m into the lake bed at its front, and on the permanent concrete core at its back. “As a construction project, the dome represents a combination of a geometrically complex civil engineering structure and a machine,” said Gerhard Lener, Dipl.-Ing., Dr.techn., an engineering consultant based in Feldkirch, Austria, who served as the structural and mechanical engineer for the dome. Lener wrote in response to written questions posed by Civil Engineering online. An approximately 2.3 m height differential in the water height had to be taken into consideration when the stage was designed.

Rendering of the bridge's dragon dogs, each stand upright on three rigid, interconnected supports

The dragon dogs each stand upright on three rigid, interconnected
supports. Platforms hold a 12 m tall steel octagonal tower that
tapers in diameter to form the bodies, while their heads are
shaped around square central bars. © Bregenzer Festspiele
/Karl Forster

The central column and round peripheral beam for the dome were fabricated from steel, for ease of construction and movement on the stage. “This made standard constructive solutions possible at the interfaces with the machine components, for instance [at] the large-diameter antifriction slewing ring in the central column, and the wire drive [used] to convert the rotary motion on the round peripheral beam,” Lener said. The dome’s rotation needed to occur as quietly as possible because of the microphones mounted on the stage that amplify the music.

Glue-laminated timber beams arranged in a star shape comprise the ribs of the dome’s framework. “This material weighs little, yet offers high strength and stiffness,” Lener said. Additionally, “a considerable advantage of wood construction is convenience when it comes to mounting scenery elements,” he noted. Specially developed steel components were used at the beams’ joints because of the limited dimensional tolerances and the need for a quick and easy assembly.

The 190 m long underwater track that surrounds the stage consists of two steel girders set 3 m apart that are joined by a series of transverse braces. The track is located atop 56 wooden piles measuring 30 cm in diameter and 7 m long. Two carriages with adjustable 3 by 4 m platforms—created to ease the loading of scenery backstage and so that the elements can always be transported at water level—move along the tracks, around the circumference of the stage. “The underside of the upper flange has been fitted with an anti-roll system, as the horizontal forces from the wind and waves also result in a lifting force on the carriages. The longitudinal joint of the rail track has been designed as a bend-resistant joint,” noted Martin Hübscher, a civil engineer with Hüttwilen-based exent AG, the civil and structural engineering firm for the circular rail track, in his written response to questions posed by Civil Engineering online. Because of the variable height of the water in the lake, the maximum wind and wave loading occurs at multiple heights.

The Bregenzer Festspiele operates as a nonprofit organization with an annual budget of €20 million (U.S.$26.3 million). The stage will remain in place for two years, at which time a new stage will replace it.


 

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