Civil Engineering Reference
In-Depth Information
Figure 13.45: Possible combinations of double curved shells. Source: Schjödt 1959
Shell structures must cope with all weather conditions and penetrating damp, which
really tests materials. Fir, spruce, larch, oak, ash, elm and hazel are best suited for
this.
Shell roofs made of timber have existed for thousands of years, particularly in tent
structures. They are very light and economical in material use, which has been a neces-
sity for migrating nomads. There are two main types of shell roof: double curved shells
and geodesic domes.
Double curved shells (hyperbolic paraboloid)
A compact version of the double curved shell started to appear in Europe at the beginning
of the 1950s in buildings such as schools and industrial premises. Its span varies from
5-100 m. The shell is built in situ over a light scaffolding, and consists of two to three lay-
ers of crossed tongued and grooved boarding. The thickness of the boarding is approxi-
mately 15 mm. The shells are characterized by the fact that two straight lines can go
through any point on the surface of the roof. The boarding is not straight, but the curving
is so small that it can bend without difficulty. The shells are put together as shown in
Figure 13.45, depending upon the position of the columns.
A lighter version, well suited for small permanent buildings, consists of a rectangular grid of
battens. The battens are screwed together at all the intersections with small bolts. The shell
can be put together in this way for transport. When erecting the structure permanently, the grid
is fixed to a solid timber frame and the bolts are tightened. This structure can be used for small
pavilions or bus shelters, for example.
Geodesic domes
The first geodesic dome was erected using steel in Jena, Germany, in 1922. Timber is a
possible alternative. The method is a simple prefabricated system based on triangles,
always constructed in the shape of a sphere. In this way a stable structure is produced
which tolerates heavy loading. The spaces between the grid can be filled with thermal
insulation. These domes are used as houses in the northern parts of Canada. The most
common use of them in Europe is for radar stations, although there are reports that their
waterproofing is questionable.
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