Client: Solemar Health Resort, Bad Dürrheim
Architects: Geier & Geier, Stuttgart
Structural engineers: Prof. Wenzel, Frese, Pörtner Haller & T. Barthel, Karlsruhe

The exciting roof of this Black Forest salt water health spa is a fine example of freedom of design and economy of material. The project pioneered a number of subsequent German spas with timber roofs. As timber grid-shells are now beginning to be used in the UK, it is only a matter of time before they appear in our sports and leisure structures.

This type of freely expressed suspended timber shell owes its origins to the architect-engineer Frei Otto, who began working in the 1950’s on structures with minimal mass and form. His theme is ‘Shape = Structure’, and requires the acceptance that catenary forces can be resisted in simple tension, rather than fighting the more unnatural forces that are created by formal, stiff shapes. Provided it is straight-grained and free from gross defects, timber has excellent tensile strength-to-mass ratios. Since Otto’s experiments, the timber industry has come a long way in developing accurate grade selection techniques, complex laminating methods and reliable end-jointing procedures, so that the advantages that he proposed are increasingly easily attained.

Timber’s natural resistance to highly alkaline and corrosive atmospheres makes it an ideal material for roofing swimming pools and salt water baths. In such conditions plain steel fasteners should be avoided completely. An appropriate grade of stainless steel is an option, but completely metal-free connections are much to be preferred. In this dramatic building, five ‘roof trees’ between 9.1m and 11.5m tall are placed around an inner court in an irregular plan shape, providing a total roofed area of 2,500m2. The vertical structures support a lattice shell over 6m to 8m diameter rings of hollow box section. The shell itself comprises suspended meridian and annular ribs. Two layers of diagonally offset sheathing are attached to these ribs via shear connections. The meridian elements hang on their natural catenary lines, traversing from ring to ring or from ring to perimeter arch. Following their primary stress trajectories, they are thus stressed mainly in pure tension. Each rib measures just 200mm x 205mm in cross section. Stepped into the meridians are thin annular laths of 80mm x 80mm or 120mm x 140mm section, spaced at 800mm centres. The ribs are glulam, and since many are in double curvature and twisted, they are built up from large numbers of individual laminates. These have to be carefully assessed for strength and for the reliability of their end joints. The manufacturing technique involves re-sawing elements that are already laminated, and are hence curved in one direction and flat in the other. These pieces are then bonded a second time, to create the complex curvature.

To connect the meridian ribs to the box sections of the tension rings and the perimeter arches, true pins are required, and adjustable pads are incorporated in the tree supports. These two measures ensure as far as possible a true membrane, with the lattice shell forces being kept to a minimum. For durability, special fixings have been used, for example the hardwood dowels that connect the tenons within the branching roof tree structures.

Despite their transparency, the façades incorporate a load-bearing structure, which provides the horizontal as well as the vertical reactions to the roof. There are simpler solutions, but suspended shells and grid-shells make an enormous aesthetic impact, and, as much of the structure is prefabricated, there are no erection time penalties for this level of sophistication. Through care, the volume of structural material, and hence the supported mass, is kept to an absolute minimum, making elements light to lift.

Timber shell technology is constantly improving; some of the laminating complications described above are now avoided, while the rapid progress in computing power facilitates three dimensional modeling at the design stage, linked to accurate laser monitoring of the true spatial dimensions during erection.