2. • Shell is a type of building enclosures.
• Shells belong to the family of arches . They can be defined as curved or
angled structures capable of transmitting loads in more than two
directions to supports.
• A shell with one curved surface is known as a vault (single curvature ).
• A shell with doubly curved surface is known as a dome (double
curvature).
SHELLS
3. Classification of shells
• There are many different ways to classify shell structures but
two ways are common:
1. The material which the shell is made of: like reinforced
concrete, plywood or steel, because each one has different
properties that can determine the shape of the building
and therefore, these characteristics have to be considered
in the design.
2. The shell thickness: shells can be thick or thin.
4. Thin Concrete Shells
The thin concrete shell structures are a lightweight construction composed of a
relatively thin shell made of reinforced concrete, usually without the use of
internal supports giving an open unobstructed interior. The shells are most
commonly domes and flat plates, but may also take the form of ellipsoids or
cylindrical sections, or some combination thereof. Most concrete shell
structures are commercial and sports buildings or storage facilities.
There are two important factors in the development of the thin concrete
shell structures:
• The first factor is the shape which was was developed along the history of
these constructions. Some shapes were resistant and can be erected easily.
However, the designer’s incessant desire for more ambitious structures did
not stop and new shapes were designed.
• The second factor to be considered in the thin concrete shell structures is the
thickness, which is usually less than 10 centimeters. For example, the
thickness of the Hayden planetarium was 7.6 centimeters.
5. Types of Thin Concrete Shells
1. Barrels shells
The cylindrical thin shells, also called
barrels, should not be confused with the
vaults even with the huge similarity in
the shape of both structures, because
each of these structures has a different
structural behavior as well as different
requirements in the minimum thickness
and the shape.
6. • On one hand, the structural behavior of the vault is
based on connected parallel arches, which transmit
the same effort to the supports . Therefore, the
materials used in these structures have to be able to
resists compressions (e.g. stone) and the thickness is
usually higher. Furthermore, the shape of the vaults
must be as similar as possible to the arch in order to
achieve the optimum structural behavior.
• On the other hand, the structural behavior of the
barrels shell is that it carries load longitudinally as a
beam and transversally as an arch. and therefore, the
materials have to resist both compression and tension
stresses. This factor takes advantage of the bars of the
reinforced concrete, because these elements can be
placed where tension forces are needed and
therefore, the span to thickness Ratios can be
increased. Furthermore, the shape has fewer
requirements than the vaults and therefore, new
curves like the ellipse or the parabola can be used
improving the aesthetic quality of the structure.
7.
8. Types of Thin Concrete Shells
Advantages of Folded Plate Roofs over Shell Roofs are:
(a) Movable form work can be employed.
(b) Form work required is relatively simpler.
(c) Design involves simpler calculations.
Disadvantages of Folded Plate Roofs over Shell Roofs are:
(a) Folded plate consumes more material than shells.
(b) Form work may be removed after 7 days whereas in
case of shells it can be little earlier.
2. Folded plate
A thin-walled building structure of
the shell type.
18. Types of Thin Concrete Shells
3. Hyperbolic Paraboloid (Hypar)
A Hypar is a surface curved in two
directions that can be designed as a shell
or warped lattice.
A hypar is triangular, rectangular or
rhomboidal in plan, with corners raised to
the elevation desired for use and/or
appearance. The edges of Hypars are
typically restrained by stiff hollow beams
that collect & transfer roof loads to the
foundations. Rhomboid
23. Types of Thin Concrete Shells
5. Dome
A rounded roof, with a circular base,
shaped like an arch in all directions..
First used in much of the Middle
East and North Africa whence it
spread to other parts of the Islamic
world, because of its distinctive
form the dome has, like the
minaret, become a symbol of
Islamic architecture.
Dome has double curvature and the
resulting structure is much stiffer
and stronger than a single curved
surface, such as a barrel shell.
24.
25.
26.
27.
28. Types of Thin Concrete Shells
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.
29. • Advantages of Concrete Shells:
The curved shapes often used for concrete shells are
naturally strong structures.
Shell allowing wide areas to be spanned without the use
of internal supports, giving an open, unobstructed
interior.
The use of concrete as a building material reduces both
materials cost and the construction cost.
As concrete is relatively inexpensive and easily cast into
compound curves.
• Disadvantages of Concrete Shells
Since concrete is porous material, concrete domes often
have issues with sealing. If not treated, rainwater can
seep through the roof and leak into the interior of the
building. On the other hand, the seamless construction
of concrete domes prevents air from escaping, and can
lead to buildup of condensation on the inside of the
shell. Shingling or sealants are common solutions to the
problem of exterior moisture, and ventilation can
address condensation.
