Heliotrop Solar Eriergy House Freiburg

Rolf Disch

Subject | Architects who are actively involved in low-energy and environmentally compatible designs readily make use of timber because it is a replenishable and recyclable material. The architect Rolf Disch from Freiburg is very active in this field. Over a period often years he developed a prototype for a solar-energy house which, if the design is fully applied, not only requires no outside energy supplies but indeed can feed electricity back into the public network! A so-called "negative-energy house". He has attempted to reduce building costs by employing series production techniques and standardization. This is essential because the engineering input for maximum energy savings on this scale is very cost-intensive. The Freiburg solar-energy house described here has approx. 200 m2 of usable floor space. The architect built it for his own use as house, office and demonstration project. Interesting in the "Heliotrop" design is the practical use of timber to meet extreme functional demands, in this case due to the need for the whole house to rotate.

Design | Working with the idea of optimum passive-energy utilization, the architect created a house which on one side has large window areas consisting of solar-control glass which allow the passage of sunlight, and also permit the occupants to enjoy the view, and on the other side, walls with a high degree of thermal insulation and no openings. He then refined the age-old principle of aligning a house in the north-south direction by incorporating an electronic control on the window side which turns the building to follow the sun or, on hot summer days, turns the building away from the sun. This led to the concept of the "treehouse" which stands on a "trunk" and then widens out above for the accommodation spaces. This design achieves a far better utilization of solar energy than conventional, fixed houses.

The central "trunk" contains a spiral staircase. The bottom 3 m above the massive foundation stands alone and contains the entrance. The actual house itself starts above this and is 11 m in diameter, i.e. the rooms are 4 m wide arranged around the central staircase tower. However, the floors are not just flat discs. The, in total, 18 bays around the staircase are divided into groups of five bays at diffe-


Ziegelweg 28,79100 Freiburg/Breisgau, Germany

Client and Architect

Rolf Disch, Freiburg/Breis-

Structural Engineer Lignaplan/Blumer AG, Waldstatt, Switzerland, and Prof. E. Gehri, ETH Zurich

Project Engineer A. Wirth

Controlling Engineer Jürgen Ehlbeck, Karlsruhe

Timber Construction Préfabrication Blumer AG, Waldstatt Assembly

KERTOWood Panels Interpan Engineered Wood Products GmbH, Finnforest, Düsseldorf

Date of Completion 1994

6 | Vertical section through facade, scale 1:5. The 70 x 130 mm notch in the 155 x 200 mm beam accommodates a roller blind. The beam only serves to fix the facade and window elements. The floors are supported on the radial 120 x 240 mm beams which are fixed directly to the columns. The windows are fixed on the outside: wood frames covered with aluminium sections, solar-control glass, triple-glazed with a U-value of 0.4 W/m2K (filled with xenon gas).

7 | Connection in the joint between the KERTO-O boards: 1 KERTO-O board. 2 Swiss threaded and ribbed bars, 55 mm. 3 Threaded bar, 46 mm. 4 Joint for poured epoxy resin. 5 Tar sealing tape.

8 | Horizontal section through facade, scale 1:5. The enclosing windows and external walls are always fixed in front of the load-bearing beam-and-column construction (with 120 x 200 mm columns). The peripheral balconies in front of the facade are made from galvanized steel sections and are self-supporting.

rent levels. These levels are arranged like a giant spiral around the central tower, rising in steps of 900 mm. (There is one bay between each group of five which creates a sort of step, i.e. 2 x 450 mm, between the levels.) The cleverly arranged partitions lead to interesting room layouts. A "solar sail" is mounted on the roof. This can be rotated independently and is fitted with a photovoltaic unit which can deliver several times more electricity than the house needs. The massive foundation contains a basement with approx. 120 m2 of usable space reached via a separate entrance.

Structure | The principal loadbearing element is the "trunk", the central tower containing the staircase. It is fabricated in one piece like a giant tube, 3 m in diameter and 14.50 m high. It has 18 sides, each of which is formed by a 110-mm-thick by approx. 500-mm-wide Finnish KERTO-O board - a crossply-bonded veneer laminated wood panel which enables the "trunk" to meet the F90-B fire protection requirements. The vertical loads from the three full-height storeys and the roof, including the "solar sail" installation (170 tonnes including superimposed loads!), and the horizontal loads amounting to 12.5 tonnes, are transferred through the staircase tower into the pivot at the base. In addition there are the bending moments due to wind and a non-symmetrical superimposed load to be considered. The total bending moment of approx. 1850 kNm corresponds to a load of "400 people, all standing on one side of the tower and looking out of the windows", as the engineer checking the calculations so succinctly expressed it. The staircase located within the tower prevents the inclusion of the normal internal bracing. However, the floors and roof attached to the outside of this hollow core are plates forming external, spiral-shaped frames which prevent the walls of the core from deforming.The structural engineers designed a special connecting system of integral glued steel hoops with peripheral longitudinal steel elements finished off with poured epoxy resin for connecting the segments of the tower. The detail is reminiscent of the poured joints on precast concrete components. This connection can also accommodate bending stresses perpendicular to the surface.

The pivot itself consists of two solid steel rings, each nearly 3 m in diameter, which form a ball bearing. The inside ring is bolted to the staircase tower by means of a steel shoe, the connection to the KERTO boards being realized via 480 No. 14 mm steel dowels and the connection to the cast joints via vertical threaded steel bars. Only in this way can the enormous forces be transferred from the wooden tower into the steel ball bearing.

The external envelope of the building is formed by a timber framework (F30-B fire rating) supported on a triangular frame made up of 120 x 240 mm glulam members. The substantial axial stresses in the struts and cross-members are transferred by means of steel dowels (BSB system) to inside and outside steel rings which on the one hand balance the opposing horizontal forces and, on the other, transfer the residual differential forces into the central tower. The framework for the facade consists of 120 x 220 mm columns and 160 x 200 mm profiled cross-members. The solar-control glass, triple-glazed windows are placed in front of the framework so that good sealing against rain and draughts is possible. The other facade, with no openings and a high degree of thermal insulation, is provided with silver-grey, profiled aluminium cladding on the outside (with an air gap behind to allow for natural ventilation). The thermal insulation in the walls is 180 + 100 mm, i.e. 280 mm thick in total, and is covered internally with a vapour barrier and two layers of 12.5 mm plasterboard.

The loadbearing members in the floors are formed by 120 x 240 mm glulam beams which span radially between staircase tower and façade framework; the g | Section through facade, scale 1:20.

Wall construction: 120 x 200 mm loadbearing columns with 180 mm thermal insulation in between, vapour barrier and 2 layers of 12.5 mm plasterboard on inside; 100 mm thermal insulation batts between ioo x 60 mm battens on outside behind silver-grey, profiled aluminium clad-130 ding.

Floor construction: load-bearing 120 x 240 mm glulam beams, 50 mm tongue-and-groove boarding, 100 mm floating screed including footfall sound insulation. The horizontal facade beams are fixed at the level of the boarding and screed. Roof construction: 120 x 240 mm glulam beams, 60 mm tongue-and-groove boarding, vapour barrier, T35/38 mm footfall sound insulation mat, 21 mm Styrodur 3000S, waterproofing layer, 20 mm protective mat and loosely laid wooden open-grid flooring.

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