Grafenegg, Austria

Cloud Tower

The site for the open-air pavilion is a park based on the model of an English landscape garden. In the 250 years of its existence, successive periods have left their traces, and this stylistic variety is one of the reasons for the park’s appeal. The area, with a castle and moat at its centre, has a size of approx. 31 ha. Parallel to the pavilion’s construction, works for a subtle contemporary replanting and restructuring of the historical park have been undertaken.

The open-air pavilion is accessible year-round – as an attraction for flaneurs and excursionists and as stage during festival season in summer – changing its appearance with the atmospheric conditions and the seasons.

The pavilion is inserted into the existing relational field of entrance gate, riding school, castle, ‘Black Gate’ and the ‘Große Senke’ (lit. transl.: large depression) and in the process re-contextualizes the network of views in the landscape. Through its topographical configuration it reinterprets formal elements of the landscape garden – the play with perspective and visual relations, with contraction and expansion, with enclosure and opening.

The basic rule of acoustics for open-air stages, ‘what you see is what you hear’ serves as a cue to explore affinities between perspective and acoustic space. The topography of the existing depression – the ‘Große Senke’ – was emphasized by modeling the terrain. Artificial hillocks are created by further excavating the depression and subsequently redistributing the soil at its perimeter, thus creating the auditorium tiers. Clearly distinguished from the natural terrain by their geometry, stage and auditorium nonetheless merge fluidly with the topography of the site.

As a spatial figure the pavilion is generated from elements of the landscape – the depression, the slope, the incision, the hillock – and the architectural folding of the roof. The ‘Schneise’ (loosely translated: an incision in the landscape) creates a vista linking the riding school to the ‘Black Gate’, and serves as an entrance to and a passage through the auditorium area. The staging of views and spatial sequences, the framing and hiding of points of attraction, often achieved by the meandering layout of paths in the traditional landscape garden, is a theme taken up by varying the elevation of the incision to achieve these effects.

Coming from the main entrance, the visitor is enticed to proceed by the silhouette of the stage roof, visible behind an artificial mound. Immersing himself into the incision, he tunnels through the hill and – after passing this deep and narrow space – enters the wide arena of the auditorium and the stage, the stage roof suspended above it.

TECHNICAL / STRUCTURAL

Acoustics Conceptually the pavilion is based on the idea of a structure that is not bound by any particular purpose and that is perceived as a stage only during performances. How can such a free structure serve as an acoustical instrument, able to focus sound energy and direct it to 1700 seats for an ideal music experience?

Open-air stages are special acoustic cases, since strictly speaking they do not provide a spatial enclosure. Without reflection, direct sound prevails and music in these spaces is characterized by high transparency and clarity, coupled to extremely low spatiality.

Topography was used to place the stage at a lower level relative to the surrounding terrain and to form the basis for the amphitheater-like tier arrangement of the auditorium, thus creating ideal conditions for an outdoor music venue. Additionally the structured geometry of the stage-roof and enclosing walls frame the orchestra and are crucial to build and direct the necessary sound energy. Convex metal panels suspended from the roof’s ceiling enhance the musicians’ reciprocal hearing. To ensure ideal hearing conditions for the audience, the average distance between stage and auditorium seat is limited to 30 meters, with the most distant seats positioned at a distance of 40 meters.

Structurally the concrete lower part of the stage emerges from the topography of the terrain to support the metallic stage roof, suspended above the ground at the level of the surrounding treetops. The complex geometry and plasticity in both concrete and steel are structurally advantageous. The spatial distribution of loads in the folded structure activates membrane-tensions and reduces flexural stress, resulting in the use of slim, resource-saving elements. At the same time these characteristics required an exceptionally high degree of detailing and a precise preparation of the construction process.

The folded concrete structure consists of slabs arranged at different angles around a 13 meter high concrete slab. All exposed concrete elements serve as climatic envelope, without additional weather-proofing.

The folded surfaces of the 8 meter high steel roof are designed as orthotropic slabs, usually employed in bridge engineering because of their elevated structural capacity and economic use of material. The roof’s skin is made from welded 10-12 mm thick steel sheet, with steel sheet bracers attached to the underside, resulting in a monolithic construction that serves both as waterproof cladding and as structure.

Special attention was devoted to the connection between concrete base and steel roof, since materials with different thermal conductivity meet. In summer, temperature differences of up to 40 K are to be expected, with the steel roof heating up to a maximum of 80°C. In order to avoid temperature-induced tensions at the interface between the two materials, the roof is placed on bearings.

MATERIAL

Conceptually, materials were chosen to underscore the open-air character of the site and to strengthen the bond between built structure and landscape. The stage is a monolithic structure of fair-faced concrete, embedded into an enveloping hillock.

The stage roof is designed as an autonomous, sculptured object. Suspended above the landscape on a level with the tree canopies it is placed among the groups of trees as if it were one more of them. The shiny metal surface on the outside reflects the sky and the trees, turning the roof into a cloud-tower. The audience tiers were designed using prefabricated concrete elements, compacted gravel surfaces and lawn-covered, geometrically precise hillocks.

The hillocks take up a tradition for contrasting flowing, natural landscape formations with geometrical lawn-pyramids and exactly defined slopes that reaches back to the early years of the landscape garden, such as Prince Pückler-Muskau’s lawn pyramids at Branitz.

The play of light on the surfaces gains in atmospheric density through the plasticity of the stage geometry. The materials chosen for enveloping the stage – steel and reinforced concrete – have the added advantage of being highly reflective and serve to build the necessary acoustic strength. The selection of colors – warm reds and browns used for some of the enclosing surfaces reflects the color spectrum of string instruments, and introduce an intimate element usually associated with indoor spaces to the stage area.

Europaconcorsi Beta

Cloud Tower