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Concrete Thin Shell Structure Types and Forms

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What is concrete thin shell structure?

Concrete thin shell structure is a three-dimensional spatial structure that constructed from one or more curved slabs or folded plates. The thicknesses of curved slab and folded plates are small compared to their other dimensions. The outstanding features of concrete thin shells are their three-dimensional load-carrying behavior which is governed by number of factors. For example, nature of the applied load, the manner in which thin shells are supported, and the geometry of thin shell forms. In this article, different types and forms of concrete thin shell structure will be presented.

Types and Forms of Shell Structure

1. Folded Plates

It is a type of concrete shell structure that made up of thin flat slabs. These slabs are jointed along their edges to construct three-dimensional structure.

Fig.1: Folded plate structure

Folded plate spans up to 30m with only 60mm thickness. nonetheless, folded plate structure with greater spans can be constructed.  Added to that, it is aesthetically pleasing, reduce material and construction cost. However, difficult shuttering, requirement for extensive labor work and supervision, high cost, and difficulty in rising roofs are drawbacks of this type of plates.

Fig.2: Distribution of loads in folded plate structure

There are three types of folded plates which include:

Prismatic folded plates

It composites of planar thin slabs joining along their longitudinal edges to form a beam-like structure spanning between supports.

Fig.3: Prismatic folded plate

Non-prismatic folded plates

It composites of planar thin slabs joining along their longitudinal edges to form a beam-like structure spanning between supports.

Fig.4: Non-prismatic folded plate

Faceted folded plates

It consists of triangular or polygonal planar thin slabs joining along their edges to form thin plate structure.

Fig.5: Faceted folded plate structure

2. Cylindrical barrel Vaults

Basically, barrel vaults are deep concrete beams with considerably thin web member that can be designed using conventional reinforced concrete design. Moreover, this type of concrete thin shell composes of cylinder, frame or ties at the ends including columns, and side elements which include a cylindrical element; a folded plate element; columns; or combination thereof.

Fig.6: Cylindrical barrel vault

Furthermore, barrel vaults assumed to be very efficient due to the use the arch form to reduce stresses and thicknesses in the transverse direction. Added to that, it spans up to 45.72m that is why barrel vaults are the most useful shell structure. Finally, a barrel shell carries load longitudinally as a beam and transversely as an arch. The arch, however, is supported by internal shears, and so may be calculated.

Fig.7: Different types of cylindrical barrel vault structure

Fig.8: Different configuration and elements of barrel vault structure

3. Short shell

It is a cylindrical shell that has a considerably large radius in comparison with its length. Short shell consist of shell element spans between arches, arch structure, and edge beams provided at the lowest point of the shell. In small structure, it is permissible to omit edge beams in small structure.

Fig.9: Basic elements of short shell

Moreover, the short shell carries load in two ways; as an arch carrying load to the lower elements. and as a curved beam to the arches. Lastly, the thickness of the shell can be quite thin due to these properties.

Fig.10: Different types of short shell structure

4. Domes (surfaces of revolution)

They are structures that cover a more or less square or circular area. Domes are hemisphere in shape and used as a roof structure. Support element of domes include columns, circular or regular polygon shaped wall.

Fig.11: Dome structure

Moreover, they are membrane structures; the internal stresses are tension and compression and are statically determinate if the proper edge conditions are fulfilled. Furthermore, in a dome of uniform thickness, under its own weight, the ring stresses are compression until the angle to the vertical is about 57 degrees. Lastly, if the dome is less than a full hemisphere, a ring is required at the base of the dome to contain the forces.

Fig.12: Domes used in the construction of shopping center and sport center

5. Folded plate domes

It is a type of thin concrete shell structure that consists of plane slabs and plates. various configurations of folded plate domes are available and being constructed.

Fig.13: Folded plate domes

Domes may be constructed with small angles between the plates or with large angles between plates and the structural action may be considerably different for each type. folded plate dome surfaces is easier to construct since they are flat. Nonetheless, for slab spans over 16 ft, the shell wall is thicker than a curved surface because bending must be considered. the sound waves in areas covered with folded plate domes are not converge. Therefore, it is more desirable to the curved dome for use in an auditorium. Finally, the structural design of folded plate domes follows that of folded plate barrels.

Fig.14:Folded plate dome structure

6. Translation Shells

A translation shell is a dome set on four arches. The shape is different from a spherical dome and is generated by a vertical circle moving on another circle. All vertical slices have the same radius. It is easier to form than a spherical dome. The stresses in a translation shell are much like a dome at the top, but at the level of the arches, tension forces are offset by compression in the arch. However, there are high tension forces in the corner.

Fig.15: Translation thin shell structure

7. Wrapped surfaces

Warped surfaces have a great advantage for shell structures because they may be formed from straight form boards even though they are surfaces of double curvature. There are two types which are most useful namely Conoid and hyperbolic paraboloid shell.

Fig.16:Conoid shell structure

Fig.17: Hyperbolic paraboloid shell

8. Combinations

The above shell structure are basic types. So, it is possible to construct different and safer shell structure by combining portion of the basic shell structure. Intersection shells, barrel shell and folded plate, barrel shell and short shell, barrel shells and domes of revolution, and barrel shells and conoids are all concrete shell combinations. So, numerous combinations can be formed to reach the desired safety and capacity.

Fig.18: Combination of basic types of shell structures; Dome and Barrel vault

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