Type of Shell Roofs

Based on the geometry of the middle surface, shells may be classified as:

1. Domes
2. Shell Barrel vaults
3. Translation shells
4. Ruled Surface shells

1. Domes

A dome is a type of roof of semi-spherical or semi-elliptical shape. The materials used for construction of domes are stone, brick or concrete. They are supported on circular or polygon shaped walls. Domes are preferred for covering large areas and of architectural importance, such as assembly halls, gymnasiums, field houses and other monumental structures.

The domes can be either (i) smooth shell domes or (ii) ribbed domes. Smooth shell domes may have varied or constant thickness. A lantern may or may not be provided. The dome surface is subdivided into number of triangles by ribs.

Here a tension ring constructed at the perimeter of the structure is a thrust resisting member. The ring is usually supported on columns spaced around the perimeter and braced to provide lateral stability for the structure. In order to support bearing walls are also constructed. In order to span the space between the ribs and to support the roof deck purlins are provided. Figure 25.14 shows three types of domes (Arora and Bindra, 2005).

2. Shell Barrel Vault

Elements of a shell barrel vaults consists of edge beams, end frame and curved membrane. Different parts are shown in Fig. 25.15.

Figure 25.14 Three types of domes

Figure 25.15 Elements of a shell barrel vault

Different types of shell barrel vaults are, short-span barrel vault, Multi-barrel double cantilever barrel vault, North-light barrel vault, Cant barrel vault, Cantilever barrel vault and Multiple barrel vault. Two of the barrel vaults are shown in Fig. 25.16.

3. Ruled Surface Shells

This type of shell is developed by moving a straight line so that it ends lie on two fixed vertical curves. These vertical curves may be different types or of similar type.

If two of these curves are of similar type, then the resulting shell is a cylindrical shell. Instead, if one of the curves is circular, elliptical, etc., then the resulting shell is a conoid.

Such shells also have two opposite curvatures, and have saddle surfaces. A cone is a special form of conoid in which the horizontal line is a point and the other curve is circular. Hyperbolic paraboloid is a ruled surface. Elements of a cylindrical shell are shown in Fig. 25.17.

Figure 25.16 Two types of barrel vault

Figure 25.17 Elements of a cylindrical shell