Ice as construction material
Ice is a fascinating and natural material. Most people associate ice with images of glaciers, icebergs, frozen waterfalls or lakes, icicles or ice crystals.
As a construction material, ice is suitable for temporary structures, but the low mechanical properties compared to other construction material have to be considered. Because of the low mechanical properties and the strong creep behaviour, ice is normally used as a decorative material on a supporting carrying structure or of constructions with small material stresses in the serviceability limit state. One example for an application is the yearly rebuilt ice hotel in Jukkasjärvi Sweden where blocks of ice are placed on curved profiled sheetings.
Using ice as a carrying construction material for the first time, cupolas are load bearing systems which are especially suitable because only very small stresses appear in the structure. Igloos and Japanese ice buildings for the storage of vegetables and sake in winter time can be given as examples for cupolas made of ice.
Shells in nature
From a geometric point of view, copulas are doubly curved shells. Nature has always been "using" these structures. Examples are egg shells, nuts, beaks and orange peels (Figure 1). Considering all these examples, the shell serves two purposes: On the one hand the shell is a protective cover against destructive environmental influences and on the other hand it is a statically efficient load carrying system. Additionally, for the load transfer of a uniformly distributed load such as dead load, the shell structure is the perfect choice because mainly normal forces appear in the cross section.
In today's architecture shells are very popular because of their elegant and spectacular design and the possibility to roof large areas with a minimum of material. Traditionally, shells are produced by means of complex formwork. In order to achieve a more economic production of shell structures, new production methods have to be developed.
One characteristic of doubly curved shells is that the surface is not developable which means that the surface can not be flattened onto a plane without distortion. If one tries flattern the hemispherical half of an orange peel into a plane surface, wedge-shaped gaps will open. (see Figure 1). Referring to the example of the orange peel, a shorting of the circumference of 36% is necessary to transform a circular disc into a hemisphere.
Ice shell project 2005
In order to enable the compressing of the midplane possible, the two materials ice and styrofoam were combined.
Based on the figure of the flattened orange peel, a circular plate consisting of segments of stiff material (ice) and soft deformable material (Styrofoam) between these segments was made.
This construction method for doubly curved shells made of a plane plate was tested in the laboratory of the Institute for Structural Engineering. A 4cm thick ice plate with a diameter of 5,2m was transformed into an ice cupola with a height of 90cm by stressing a tendon which was placed along the circumference of the plate.
In December 2005 a field test was carried out in Obergurgl, Tyrol. An ice dome with a diameter of 13m and a thickness of 16cm originating from one flat plate was constructed. With this experiment the functional capability of this construction method for ice shells could be proven on a large scale.
Ice shell project 2008/09
Based on the experiences on ice shells, the construction method has been advanced. For this new construction method the shell consists of individual plane elements. The shape of these elements has to be chosen according to the final shape of the shell. The elements are placed on a planar working surface and are assembled by means of tendons. In order to transform the plate into a shell a pneumatic formwork is used. While air is inflating the pneumatic formwork, the plane plate is transformed into a shell.
Contrary to the previously described construction method, it is now possible to build shells with large curvatures, such as hemispheres. Moreover, between the segments of ice, no styrofoam, which would have to be compressed during the shaping process is necessary, therefore the transformation of the planar plate into the shell needs significantly less force.
To demonstrate this new construction method, a small wooden model was created in the laboratory. In Figure 4 the pneumatic formwork, which is placed under the wooden elements was slowly inflated with air. The elements lifted and formed a shell.
In Winter 2008/09 this advanced construction method was tested on two ice shells in Obergurgl, Tyrol.
Initially 2 circular ice plates with a diameter of 6m, respectively 13m were built, then these plates were cut in 96 individual elements (Figure 5), held together by means of tendons.
Due to the inflating of the pneumatic formwork, the ice plate with the diameter of 6m became a hemisphere with a diameter of 4m and a height of 2m (Figure 6).