Client: Municipal Authority, Prien
Architects: Zeller & Romstätter, Traunstein
Structural engineers: Haumann-Fuchs, Traunstein & Peter Zeller, Ruhpolding

At certain seasons the Chiemgau region of Bavaria experiences a cold alpine wind. It was decided, therefore, to redevelop the outdated Prien outdoor swimming pool into a new all-year-round indoor leisure pool.

Curved laminated timber ‘roof trees’ and a shell-shaped plan have resulted in a dramatic building which achieves excellent natural lighting through its 1,800m2 transparent roof, has a low mass to help overcome problems with difficult foundation conditions, and is resistant to the chemicals and humidity associated with indoor pools.

The shell-like appearance is achieved using relatively simple, curved glulam ribs. These linear elements are mounted above the roof trees, so structurally speaking this is not a true grid shell or skin shell. There are eighteen principal radiating ribs, each of which is designed as a curved continuous beam in three segments.

These segments are neatly connected to one another with flitched-in true pin connections, similar to those used in glulam arches. Between the timber ribs are draped transparent roof coverings, triple-layered air-filled membranes maintained at an air pressure between 250 and 350 Pa. The pneumatics pass via light alloy valves and connectors that are associated with condensate drainage channels mounted on the ribs. The transparent membrane system is resistant to surface spread of flame and was also assessed for long-term behaviour in a pool atmosphere. At the rear, or thin end of the fan-shaped plan, the radial timber structure is anchored to a reinforced concrete construction. The tapered rib tips protrude slightly from the front of the envelope and have corrosionprotected metallic coverings. At this outer façade the slight droop and inwards curvature of the transparent roof membrane is displayed, giving the architectural appearance of a scalloped shell.

There are 65 different shapes making up the glulam roof trees and ribs, with the greatest radius of a single principal element being 47.2m. The roof tree forms vary, some having six ‘branches’ and others only four. At their bases they are connected rigidly to low pillars of reinforced concrete. Hence there are both radial and transverse encastré conditions that provide lateral stability to the structures.