EP1706553B1 - Method for the production of double-bent shells - Google Patents

Abstract

The aim of the invention is to be able to produce a shell that is curved twice in a three-dimensional manner without using formwork and without joining together arc-shaped elements. Said aim is achieved by a method that is characterized by the following steps: - a basal area (12) exceeding the area of the double-bent shell (10) is initially measured on a working area (15), preferably a plane; - a subarea of said basal area (12), which corresponds approximately to the difference between the basal area (12) and the area of the shell (10), is coated with at least one soft first material (20) having a low elastic modulus while a remaining subarea of the basal area (12) is coated with at least one second material (22) whose elastic modulus substantially exceeds the elastic modulus of the first material (20) and whose resistance to pressure is greater than that of the first material, at least one subarea coated with the first material (20) being located between two subareas of the basal area (12) that are coated with the second material (21); - a plate (14) having orthotropic properties, such as stiffness, is formed; - the basal area (12) of the plate (14) is then reduced such that the double-bent shell (10) is formed.

Description

The invention relates to a method for producing a doubly spatially curved shell and a shell produced thereafter.

Particularly suitable for the production of doubly spatially curved trays are materials which can be cast, such as e.g. Reinforced concrete, plastics and water or ice.

Shells made of the materials mentioned are spatially curved surface structures, which can be used, for example, as a roof for exhibition halls or event halls.

In addition to the above-mentioned field of use of shells for covering large areas, double-curved wooden shells can also be used as formwork for forming cavities in solid concrete structures, for example in dams.

Shell structures are characterized by the fact that, with suitable form and storage, they carry off loads predominantly by membrane forces. This leads to an extremely low material utilization and low material consumption. However, the savings in material consumption are offset by increased labor costs for the production of spatially curved formwork. Executed shell structures, such as in " Spatial Roof Structures - Construction and Design "by Hermann Rühle, Volume 1, VEB Verlag für Bauwesen, Berlin, 1969, p. 177, 248, 256 and " Heinz Isler - Schalen "by Ekkehard Ramm and Eberhard Schung (ed.), Karl Krämer Verlag, Stuttgart, 1986, p. 51, 68, 70, 77 are described, usually have complicated, spatially curved formwork made of wood and / or steel.

In order to save the costs for the construction of spatially curved formwork, pneumatic formwork has become known. Shells in spherical or cylindrical shape and shells with more or less slight modifications of these basic shapes can be made in this way, see eg " Dome construction with pneumatic formwork "by Franz Derflinger, in" Concrete and reinforced concrete construction ", born 1983, No. 11, pages 299 to 302 ,

Because the manufacture and use of molds and pneumatic formworks is very expensive, double-spaced sheet structures have been constructed from rods. at Half-timbered domes replace the continuous surface of the shell with rods of steel or wood which polygonal approach the curved shell surface.

Spatially curved surface structures are referred to as shells, if the surface is a continuum and unlike the above-mentioned half-timbered domes is not formed by individual rods.

The surface of a doubly spatially curved shell can also consist of a non-pourable material, such as wood. In the AT 410 342 a composite material of rigid, flat wood panels is proposed, which have the cut form of a triangle or quadrilateral and are hinged along adjacent edges. This succeeds in the production of arcuate shell strips, which are joined together and connected to each other, so that a spatially curved wooden shell is created. Producing a spatially curved shell according to this method requires the lifting and moving of arcuate elements and is therefore limited to small spans. In addition, a production of the large number of individual wood panels and their joints is labor-intensive and costly.

The invention has for its object to provide a method for producing a doubly spatially curved shell without the construction of a spatially curved formwork and the associated scaffold or without the use of a pneumatic formwork and without lifting, moving and joining of arcuate elements that not is limited to small spans.

• zunächst auf einer Arbeitsfläche, vorzugsweise einer Ebene, eine flächenmäßig die Fläche der zweifach gekrümmten Schale überschreitende Grundfläche eingemessen wird, und
• ein Flächenteil dieser Grundfläche, der etwa der Differenz zwischen der Grundfläche und der Fläche der Schale entspricht, mit mindestens einem weichen ersten Material mit einem niedrigen Elastizitätsmodul und ein übriger Flächenteil der Grundfläche mit mindestens einem zweiten Material belegt wird, dessen Elastizitätsmodul wesentlich über dem Elastizitätsmodul des ersten Materials liegt und dessen Druckfestigkeit höher ist als die Druckfestigkeit des ersten Materials,
• wobei mindestens ein mit dem ersten Material belegter Flächenteil zwischen zwei mit dem zweiten Material belegten Flächenteilen der Grundfläche liegt, und eine Platte mit orthotropen Materialeigenschaften, wie Steifigkeit, gebildet wird, worauf
• die Grundfläche der Platte unter Bildung der zweifach gekrümmten Schale verkleinert wird.

This object is achieved in that

• first on a work surface, preferably a plane, a surface area of the surface of the double-curved shell bordering base area is measured, and
• a surface part of this base area, which corresponds approximately to the difference between the base surface and the surface of the shell, is covered with at least one soft first material with a low modulus of elasticity and a remaining surface part of the base surface with at least one second material whose modulus of elasticity is significantly higher than the elastic modulus of the first material and whose compressive strength is higher than the compressive strength of the first material,
• wherein at least one surface part occupied by the first material lies between two surface parts of the base surface occupied by the second material, and a plate with orthotropic material properties, such as rigidity, is formed, whereupon
• The base of the plate is reduced to form the doubly curved shell.

By exploiting the orthotropic material properties of the flat plate, it is possible to form a doubly spatially curved shell, which would not be unwound in a plane in the case of linear elastic material behavior, from the plate. The advantageous load-bearing behavior of doubly curved shells in comparison to shells with simple curvature or to bends, already arises during the shaping process. Depending on the geometric shape and the ratio of the moduli of elasticity and the surface portions of the at least two different building materials creates a biaxial membrane stress state in the shell, which has decisive advantages in terms of stability failure compared to structures with simple curvature. Bending stresses in the shell during the molding process must be limited to values that can be absorbed by the shell by proper design of the shell shape and careful design.

Advantageous developments of the invention are defined in the subclaims.

A shell produced by the method according to the invention is characterized in that it is alternately formed over the surface by at least one soft first material with a low modulus of elasticity and at least one second material whose modulus of elasticity is substantially above the elastic modulus of the first material and whose compressive strength is higher as the compressive strength of the first material.

Further expedient embodiments of the shell are defined in subclaims.

Fig. 1

eine Draufsicht auf eine Platte und die Arbeitsfläche;

Fig. 2

einen Schnitt längs der Linie II-II der Fig. 1;

Fig. 3

einen Schnitt längs der Linie III-III der Fig. 1;

Fig. 4

einen Schnitt längs der Linie IV-IV der Fig. 1;

Fig. 5

eine Fig. 1 entsprechende Draufsicht auf die Schale während des Formgebungsprozesses;

Fig. 6

einen Schnitt längs der Linie VI-VI der Fig. 5;

Fig. 7 bis 10

jeweils einen gemäß Fig. 6 geführten Schnitt weiterer Ausführungsformen;

Fig. 11

eine Draufsicht analog zu Fig. 5 auf eine geänderte Ausführungsform;

Fig. 12

einen Schnitt gemäß der Linie XII-XII der Fig. 11;

Fig. 13

ein Spannungs-Dehnungs-Diagramm für ein weiches Material;

Fig. 14

eine Draufsicht analog zu Fig. 1 einer weiteren Ausführungsform;

Fig. 15

einen Schnitt nach der Linie XV-XV der Fig. 14;

Fig. 16 und Fig. 17

jeweils einen Schnitt gemäß den Linien XVI-XVI und XVII-XVII der Fig. 14, jedoch im vergrößerten Maßstab;

Fig. 18

eine Draufsicht auf die Platte einer anderen Ausführungsform;

Fig. 19

eine Draufsicht auf die Schale der anderen Ausführungsform nach Abschluss des Formgebungsprozesses;

Fig. 20

eine perspektivische Ansicht von der Schale der anderen Ausführungsform nach Abschluss des Formgebungsprozesses;

Fig. 21

eine Draufsicht auf die Platte einer weiteren Ausführungsform;

Fig. 22

eine Draufsicht auf die Schale der weiteren Ausführungsform nach Abschluss des Formgebungsprozesses;

Fig. 23

eine perspektivische Ansicht von der Schale der weiteren Ausführungsform nach Abschluss des Formgebungsprozesses.

The invention is explained in more detail with reference to embodiments illustrated in the drawings. Show it:

Fig. 1

a plan view of a plate and the work surface;

Fig. 2

a section along the line II-II of Fig. 1 ;

Fig. 3

a section along the line III-III of Fig. 1 ;

Fig. 4

a section along the line IV-IV of Fig. 1 ;

Fig. 5

a Fig. 1 corresponding plan view of the shell during the molding process;

Fig. 6

a section along the line VI-VI of Fig. 5 ;

Fig. 7 to 10

one according to Fig. 6 guided section of further embodiments;

Fig. 11

a plan view analogous to Fig. 5 to a modified embodiment;

Fig. 12

a section according to the line XII-XII of Fig. 11 ;

Fig. 13

a stress-strain diagram for a soft material;

Fig. 14

a plan view analogous to Fig. 1 a further embodiment;

Fig. 15

a section along the line XV-XV of Fig. 14 ;

FIGS. 16 and 17

each a section according to the lines XVI-XVI and XVII-XVII of Fig. 14 but on an enlarged scale;

Fig. 18

a plan view of the plate of another embodiment;

Fig. 19

a plan view of the shell of the other embodiment after completion of the molding process;

Fig. 20

a perspective view of the shell of the other embodiment after completion of the molding process;

Fig. 21

a plan view of the plate of a further embodiment;

Fig. 22

a plan view of the shell of the further embodiment after completion of the molding process;

Fig. 23

a perspective view of the shell of the other embodiment after completion of the molding process.

The method is preferably used for the production of reinforced concrete shells 10, but also for the production of shells 10 made of plastics, ice or wood. The first example illustrates the manufacture of an exhibition hall designed as a reinforced concrete shell 10.

As a first step, a plate edge 16 is measured on a work surface 15 and its circumference marked. Thus, a base 12 is set, the surface area exceeds the surface of the shell 10 to be formed.

As a second step, 12 strips of a soft first material 20 are designed on the base. A suitable material for the soft material 20 would, for example extruded polystyrene foam having a Young's modulus of 300 MP _a. The area fraction occupied by the first material 20 on the base surface 12 is greater than the difference between the base surface 12 and the finished shell 10. As the surface of the shell 10, if the shell 10 is very thin compared to its size, then whose surface meant. It could also be - for example, with larger wall thicknesses - an approximately centered thickness of the surface.

In a third step, the remaining part of the base 12 is filled with a castable material using a suitable, along the plate edge 16 mounted Randabschalung, so that a plate of constant thickness is formed. By hardening the castable material into a pressure-resistant second material 22, which in this example consists of reinforced concrete with a modulus of elasticity of 30,000 MPa _, a plate 14 having orthotropic material properties is produced. The stiffnesses are not determined locally in the pressure-resistant material 22 or in the soft material 20, but smeared over a larger area comprising a plurality of strips of soft material 20 and regions of pressure-resistant material 22 therebetween. For example, the tangential stiffness of the plate 14 in the vicinity of the plate edge 16 is lower because of the high proportion of soft material 20 than in the closer to the middle plate regions. The rigidity of the plate 14 in the radial direction is greater than the rigidity in the tangential direction.

An advantageous type of reinforcement is, for example, by approximately radially arranged steel rods 13, which in Fig. 1 However, only one point are shown, realized.

A section through the plate 14 and the base 12 is in Fig. 2 shown. In the vicinity of the plate edge 16, a clamping member 30 is disposed in the plate 14, and cast in concrete. The work surface 15 must be such that a sliding of the plate edge 16 on the surface 15 is possible. In the Fig. 2 shown plate 14 of soft material 20 and pressure-resistant material 22 is produced on a flat work surface 15.

The strips of soft material 20 can, as in Fig. 3 shown, have contact surfaces between soft material 20 and pressure-resistant material 22, which are normal to the central surface of the plate 14. Alternatively, the contact surfaces between soft material 20 and pressure-resistant material 22 may also include an angle other than 90 ° with the central surface of the plate ( Fig. 4 ) to make the molding process easier.

In a fourth step, as in Fig. 5 represented by the shortening of the periphery 18 of the plate edge 16 and the consequent compression of the softer first material 20 in the tangential direction reduces the base 12 and thus deformed the plate 14 to a shell 10 with positive Gaussian curvature. The diameter and the plan area of the shell 10 in the plan view according to Fig. 5 are therefore smaller than the diameter and the base 12 of the plate 14 in the plan view according to Fig.1 , The of The area occupied by the strips of the first material 20 is shaped in accordance with the expected tangential shortening of the plate 14 during the molding process, bearing in mind that the compressed first material 20 still forms a small portion of the surface of the shell 10.

Fig. 6 shows the shell 10 during the molding process, wherein the shortening of the periphery 18 by the tensioning of the clamping member 30 takes place. When the shell 10 in the center has reached the planned height, the edge 16 of the shell 10 can be connected to the foundation. An in-situ concrete layer 37 could then be applied to the surface of the shell 10 in order to achieve a greater rigidity of the shell 10 or to ensure a sealing of the surface against precipitation by the second concrete layer (cf. FIGS. 11 and 12 ).

Notwithstanding the example shown, the forming process could be assisted by lifting one or more locations of the plate 14 at the beginning of the forming process with a suitable hoist. Such support is in Fig. 9 illustrated. The working surface 15 is in this case covered with two superimposed membranes or foils 38 which exceed the base area 12. By creating a pneumatic pressure between the membranes or foils 38, buckling occurs as in FIG Fig. 9 shown. The sealing of the space between the membranes or films 38 takes place by the weight of the shell 10. It may be advantageous to provide a soft material 46 between the membranes or foils 38 in the region of the plate edge 16 for better sealing, in particular a thin layer of a Material having the same strength properties as the first material 20.

Another way of supporting the bulge of plate 14 is in FIG Fig. 8 shown, according to the one type Pneu 39 is applied at the beginning of the reduction of the base 12 with compressed air.

Fig. 7 shows a slightly curved, ie provided in the center region of the base 12 with an elevation 40 working surface 15, which elevation, however, - in relation to the extension of the base 12 - has only a small amount. The manufacture of the plate 14 with an elevation of the base area, a center area of, for example, one-hundredth of the plate diameter, has a favorable effect on the internal forces arising during the shaping process and reduces the risk of buckling of the shell 10 during the shaping process; this shows Fig. 7 , but distorted to scale.

Fig. 10 shows one of the Fig. 6 Corresponding section through a shell 10 with a modified arrangement of the tendons 32 and 36. The shortening of the circumference 18 or reduction of the base 12 via tendons 36 which are connected only at two points with the edge 16 of the shell 10 and during be tense of the shaping process. To increase the flexural rigidity of the shell 10 during the shaping process, clamping members 36 are arranged outside the shell 10, which is formed of soft material 20 and pressure-resistant material 22, in a distance predetermined by means of spacers 34. These external tendons 36 are each connected at the edge 16 with the shell 10 and increase the flexural rigidity of the shell 10 in the radial direction.

Further embodiments are based on the Fig. 14 20 for the production of a doubly curved shell 10, in which ice is used as pressure-resistant second material 22.

According to the in the Fig. 1 to 6 explained example is first according to Fig. 14 the base 12 eingemesssen, and it is mounted along the plate edge 16 a composite of individual parts edge formwork 41. These parts of the edge formwork 41 should be rigid and can be made of concrete ( Fig. 16 ) with a reinforcement 42, which projects into the second material 22, or of wood ( Fig. 17 ) be formed.

In the second process step, a part of the base area 12 is covered with a soft first material 20 having a lower elastic modulus compared to ice. The dimensions and arrangement of the strips of soft material 20 largely determine the shape of the shell 10 that sets up after the molding process.

How out Fig. 14 can be seen, the first material 20 extends to between the edge formwork 41 forming items. The arrangement of the tendons 43 is preferably carried out for an edge formwork 41 made of concrete in the concrete itself, for an edge formwork 41 made of wood outside the items.

In the third process step, the remaining part of the base 12 is filled with water, which is advantageously carried out in layers with solidification of the previously introduced layers. Preferably, a Glasfasergelege 44 is placed on the claimed on train side of the plate 14. It is assumed in this embodiment that the production of the shell takes place at outside temperatures below 0 ° Celsius. After freezing of the water to ice with a modulus of elasticity of 1000 MP _a , the pressure-resistant second material 22 of the plate 14 is thus produced.

In the fourth step, the base 12 of the plate 14 is reduced by means of bias, so that the strips of soft material 20 largely close and from the plate 14, a doubly curved shell 10 is formed. FIG. 19 shows a variant of a shell 10 with a base surface 12 shown in FIG. 18 in plan view and in FIG. 20 in an oblique view.

After connecting the shell edge 16 with a foundation, a part of the biasing elements not shown in FIGS. 18 to 20 can be relaxed. The shell 10 will have predominantly membrane stresses due to its own weight. In a span of 20 m, a stitch of 5 m and a shell thickness of 0.15 m, the stresses due to weight only about 0.15 MP _a will be. Creep deformations will therefore remain small, and the ice tray 10 shown in Fig. 20 may be used as an event hall for a few months during the cold season.

In a further embodiment, the production of a shell 10 with a negative Gaussian curvature is explained with reference to FIGS. 21 to 23, in which wood is used as pressure-resistant material 22.

In the first method step, according to FIG. 21, first the base area 12 is measured.

In the second step, a portion of the base 12 is covered with plates, which consist of pressure-resistant second material 22, for example, wood material.

In the third step, a further part of the base 12 is covered with a soft first material 20 so that some recesses 24 arise in the base 12, which are occupied neither with pressure-resistant second material 22 nor with soft first material 20. The strips of the first material 20 have a smaller width in the vicinity of the plate edge 16 in this example than in the central regions of the plate 14, because from the plate 14, a shell 10 is to be formed with negative Gaussian curvature.

In the fourth step, the base 12 of the plate 14 is reduced by means of bias, so that the strips of soft material 20 largely close and from the plate 14, a shell 10 with negative Gaussian curvature arises, which in Fig. 22 in Floor plan is shown. The bowl 10 shown in perspective in FIG. 23 can be used as an exhibition hall.

With the method according to the invention, the production of doubly spatially curved trays 10 of any shape over any plan is possible. For the production of trays 10 by the method according to the invention, it may be useful to use soft first materials 20 with different strength behavior and / or pressure-resistant second materials 22 with different strength behavior in a plate 14 or to produce the plate 14 with variable thickness, as this Fig. 15 shows.

The ratio of the area occupied by the first material 20 on the base 12 to the area occupied by the second material 22 depends on the desired curvature of the shell. Preferably, the area occupied by the first material 20 on the base area occupies 2 to 30% of the total area of the base area, in particular 4 to 10%.

The preferred material for the first material 20 is polystyrene or polyurethane with flow limit voltages for polystyrene between 0.1 to 1.0 MP _a and polyurethane between 0.3 to 1.2 MP _a. For the second material 22 is concrete, wood or ice in question, for concrete, the yield stress is between 20 and 100 MP _a under pressure. Wood has flow limit voltages between 10 and 40 MP _a, to be precise under tensile and compressive stress, ice cream from 0.5 to 1.0 MP _a compression load. This results in a yield point ratio of the yield point of the second material 22 to the yield point of the first material 20 between 2.5 and 200. Preferably, this is between 5 and 100.

The elastic moduli are for concrete 2000-80000 MP _a for ice between 500 and 2,000 MP _a and for wood 8000-16000 MP _a. Is modulus of elasticity between 50 and 300 give _a MP for the preferably used second materials. It follows that the ratio of the moduli of elasticity of the second material 22 to the first material 20 is between 10 and 300, preferably between 50 and 200.

The determination of the surface parts 12 to be occupied by the first and second material 20, 22 for a shell 10 with a hemispherical shape is explained below, wherein the hemisphere has the radius R, a surface O = 2 R ² π and a half circumference of b = R π.

ergibt.The diameter of the base 12 must be the size of b, resulting in a footprint of $A={\left(\frac{b}{2}\right)}^2\cdot \mathit{\pi }=\frac{R^2{\mathit{\pi }}^2}{4}\cdot \mathit{\pi }$

results.

From this and from the surface of the hemisphere results a surface difference: $$\frac{R^2{\mathit{\pi }}^3}{4}-2\mathit{\pi }{R}^2=R^2\mathit{\pi }\left(\frac{{\mathit{\pi }}^2}{4}-2\right)$$

From this, a ratio of the area difference to area of the base area is calculated: $$\frac{R^2\mathit{\pi }\left(\frac{{\mathit{\pi }}^2}{4}-2\right)}{\frac{1}{4}{R}^2\cdot {\mathit{\pi }}^2\cdot \mathit{\pi }}=\frac{{\mathit{\pi }}^2-\mathrm{8th}}{{\mathit{\pi }}^2}\cong 18.9\%$$

der Ausgangsplatte mit weichem ersten Material 20 zu belegen sind.Assuming now a ratio of the width of the first material 20 after upsetting to the initial width of 0.2, it follows that $\frac{18.9}{0.\mathrm{8th}}=23.7\%$

the starting plate are to be covered with soft first material 20.

Claims (27)

1. A process for the production of a shell (10) that is curved twice in a three-dimensional manner, characterized in that

   - a basal area (12) exceeding the area of the double-bent shell (10) is initially measured on a working surface (15), preferably a plane, and

   - a subarea of said basal area (12), which corresponds approximately to the difference between the basal area (12) and the area of the shell (10), is coated with at least one soft first material (20) having a low elastic modulus while a remaining subarea of the basal area (12) is coated with at least one second material (22) whose elastic modulus substantially exceeds the elastic modulus of the first material (20) and whose resistance to pressure is greater than the resistance to pressure of the first material,

   - wherein at least one subarea coated with the first material (20) is located between two subareas of the basal area (12) that are coated with the second material (21), and a plate (14) having orthotropic material properties, such as stiffness, is formed, whereupon

   - the basal area (12) of the plate (14) is reduced such that the double-bent shell (10) is formed.
2. A process according to claim 1, characterized in that the subarea of the basal area (12) that is coated with the first material (20) is dimensioned larger than the difference between the basal area (12) and the area of the shell (10).
3. A process according to claim 1 or 2, characterized in that, simultaneously with the reduction of the basal area (12) of the plate (14), at least one point of the plate (14) is lifted.
4. A process according to claim 2 or 3, characterized in that the reduction of the basal area (12) of the plate (14) is effected by shortening the circumference (18) of the plate rim (16).
5. A process according to claims 1 to 4, characterized in that a material displaying a nonlinear material behaviour is used as the first material (20) and permanent upsetting deformations are formed in the first material (20) during the forming process.
6. A process according to any of claims 1 to 5, characterized in that not the entire basal area (12) is coated with the first and the second materials (20, 22) so that the plate (14) exhibits one or several cavities (24) forming one or several predetermined recesses in the shell (10).
7. A process according to any of claims 1 to 6, characterized in that at least one of the contact areas between the soft material (20) and the second material (22) is oriented with an angle, which deviates from 90°, toward the central area of the plate (14).
8. A process according to any of claims 1 to 7, characterized in that spacers (34) are anchored in the second material (22) and that prestressing members (32) are placed on the spacers (34) and anchored on the plate rim (16), wherein said prestressing members (32) are tightened or slackened, if required, during the forming process.
9. A process according to any of claims 1 to 8, characterized in that prestressing members (30) are arranged in the plate (14), preferably in the circumferential area, and are tightened or slackened, if required, during the forming process.
10. A process according to any of claims 1 to 9, characterized in that the basal area (12) of the plate (14) is reduced by tightening prestressing members (36) each connected to the second material (22) on the plate rim (16) in two points.
11. A process according to any of claims 1 to 10, characterized in that a working surface (15) is provided with a camber (40) of a small height in order to assist in bulging.
12. A process according to any of claims 1 to 11, characterized in that a layer of the second material (22) or a layer having approximately the same elastic modulus is applied to the double-bent shell (10) upon completion of the forming process, which layer is connected to the double-bent shell (10) in a shear stable manner.
13. A process according to any of claims 1 to 12, characterized in that a portion of the basal area (12) is first covered with the first material (20) and the second material (22) is cast onto the working surface (15).
14. A process according to any of claims 1 to 13, characterized in that a portion of the basal area (12) is first covered with the second material (22) and the first material (20) is cast onto the working surface (15).
15. A process according to any of claims 1 to 14, characterized in that a material having an elastic modulus of between 1.000 MP_a and 80.000 MP_a is used as the second material (22).
16. A process according to any of claims 1 to 15, characterized in that the ratio of the elastic moduli of the second material (22) to the first material (20) is between 10 and 300, preferably between 50 and 200.
17. A process according to any of claims 11 to 16, characterized in that the height of the camber of the working surface (15) is changed when the basal area (12) is reduced, preferably by inflating bellows, a tyre (39) or the like.
18. A process according to any of claims 1 to 17, characterized in that the yield point ratio of the yield point of the second material (22) to the yield point of the first material (20) is between 2.5 and 200, preferably between 5 and 100.
19. A process according to any of claims 1 to 18, characterized in that the area taken up by the first material (20) on the basal area (12) is between 2 and 30%, preferably from 4 to 10%, of the total area of the basal area (12).
20. A process according to any of claims 1 to 19, characterized in that, when covering the basal area (12), the materials (20, 22) close to the centre of the basal area (12) are kept thinner than those in the rim area, with the ratio of the thickness in the rim area to the thickness close to the centre being adjusted to between 1.1 and 6, preferably to between 1.1 and 3.
21. A shell curved twice in a three-dimensional manner and manufactured according to a process according to one or several of claims 1 to 20, characterized in that, across its surface, it is formed alternately from at least one soft first material (20) having a low elastic modulus and at least one second material (22) whose elastic modulus substantially exceeds the elastic modulus of the first material (20) and whose resistance to pressure is greater than the resistance to pressure of the first material (20).
22. A shell according to claim 21, characterized in that it is designed as a shell (10) with a negative Gaussian curvature, i.e., as a saddle surface.
23. A shell according to claim 21 or 22, characterized in that it exhibits an essentially plane circumferential edge (18).
24. A shell according to any of claims 21 to 23, characterized in that it is provided with recesses (24), such as, for example, with recesses merging into the circumferential edge.
25. A shell according to any of claims 21 to 24, characterized in that the second material (22) is provided with a reinforcement (13, 44) such as a steel or glass-fibre reinforcement.
26. A shell according to any of claims 21 to 25, characterized in that it is covered, across its entire surface, by a layer (37) of the second material (22) or of a material corresponding to the former with regard to the mechanical-technological values and that said layer (37) is connected in a shear stable manner to the shell (10) formed alternately from the first material and the second material (20, 22).
27. A shell according to any of claims 21 to 26, characterized in that a stiffening boundary element (41), preferably comprising a reinforcement (42), is provided on the circumferential, edge (18), with a circumferential prestressing member (30) being provided in or on said boundary element.

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