Shell structures are seldom applied despite the fact that they use materials very eco-nomically.Thetimber used must have good strength. It is also an advantage ifthetimber is light and elastic. Fir, spruce, larch, oak, ash, elm and hazel are best suited.
Shell roofs made of timber have existed for thousands of years, particularly intent structures.There are two main types: 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 layers of crossed ton-gued and grooved boarding. The thickness of the boarding is approximately 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 13.51, depending upon the position of the columns.
A lighter version, well suited for small permanent buildings, consists of a rectangulargrid of battens.The battens are fastened togetherat all the intersections with small bolts.The shell can thus easily be put together 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. They have often been constructed in metal or plastics, but timber is also used. The method consists of a simple prefabricated system based on triangles, always assembled into the shape of a hemisphere or sphere. In this way a minimal, stable structure is produced that tolerates heavy loading. The spaces between the grid can be filled with thermal insulation.These domes are used as dwellings inthe northern parts of Canada and have been built in many countries. The most common use of them in Europe is for radar stations, though there are reports that their waterproofing is questionable.
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