Post on 22-Nov-2014
S H E L L
S T R U C T U R E S
R A H U L S R I K R I S H N AR O L L N O : 3 9 0 6 0 9 0 1 0 6 6
INTRODUCTION
DEFINITION
Shells can be defined as curved structures capable of transmitting loads in more than two directions to supports.
Loads applied to shell surfaces are carried to the ground by the development of compressive, tensile, and shear stresses
acting in the in-plane direction of the surface.
Thin shell structures are uniquely suited to carrying distributed loads and find wide application as roof structures in building.
They are, however, unsuitable for carrying concentrated loads.
SHELLS IN NATURE
Eggshell - The most efficient example as lightness proportion
to its strength is amazingly high.
Bird’s nest is a good example of an inverted dome, results
from bird’s astonishing instinct to capitalize on the efficiency
of the domed geometric shape and on materials that are readily
available
TYPES & FORMS OF SHELL STRUCTURES
FOLDED
PLATES
BARREL
VAULTSDOMES
WARPED
SURFACES
SHORT
SHELLS
SHELL
ARCHES
INTERSECTION
SHELLS
FOLDED PLATE
DOMES
Folded plates
3-segment folded plate
Z-shellWalls
continuous with shell
Tapered folded plates
Edge supported
folded plates
Folded plates truss
Folded plates rigid frame
• Folded plate is the simplest of the shell structures.
• The distinguishing feature of the folded plate is the ease in
forming plane surfaces. Therefore, they are more adaptable to
smaller areas than curved surfaces which require multiple use of
forms for maximum economy.
• A folded plate may be formed for about the same cost as a
horizontal slab and has much less steel and concrete for the
same spans.
• The structure above may have a simple span, as shown, or
multiple spans of varying length, or the folded plate may
cantilever from the supports without a stiffener at the end.
FOLDED PLATESInclined
Plates
Edge Plates
– to stiffen
wide plates
Stiffeners – to carry
loads to support &
to hold plates in
line.
Columns to
support the
structure in air.
Walls are of tilt – up concrete,
cast on the floor & raised into
place by cranes.
Walls braced on top &
continuous with roof plates,
so columns not needed.
•The dimensions of the plates are dependent on both the width of the barrel and
on the span.
•The depth of the shell should be about 0.10 times the span and the maximum
slope of a plate should not be greater than 40 degrees.
•The slope of the plates is 6/8, which is about 37 degrees and is satisfactory. The
thickness of the plates could be about 3 ½ inches.
3-SEGMENT FOLDED PLATES
•The Z shell is not an efficient structural shape since it is
discontinuous and its effective depth is much less than the actual
vertical depth.
•The spans are limited in comparison to the plates having a large
number of units side by side.
Z-SHELL
3 segments
Large sloping
plateWindow ventilation –
reflected south light
Edge plates
Roof surface is painted
with Al so that sunlight
is reflected through
windows to the ceiling.
WALLS CONTINUOUS WITH SHELL
•A two segment structure
is not desirable because it
has very little torsional
resistance.
CANOPIES
Four segments
BARREL VAULTS
• Barrel vaults are perhaps the most efficient of the shell structures because they can span upto 150 feet with a minimum of material.
• If the span is large in comparison to the width, the form is called a long shell. The span/radius ratio is 5. If the length is short, it is called a
short shell. The span/radius ration less than 1 and shells between these limits are called intermediate shells.
• The thickness of the barrel shell is usually based on the minimum thickness required for covering the steel for fireproofing + the space
required for three layers of bars + some space for tolerance.
• If these bars are all half inch rounds - minimum of 3 1/4 inches. Near the supports the thickness may be greater for containing the larger
longitudinal bars.
• If more than one barrel is placed side by side, the structure is a multiple barrel structure and if more than one span, it is called a multiple
span structure.
• Provision should be made for expansion joints in a large structure.
Single barrel
vault
Edge Plates
Stiffeners – to carry loads
& to support the vault
Columns to support
the structure in air.
Deep concrete beams with
very thin web structures
Use of arch form reduces stress
& thickness in transverse
direction Transmission of forces
SHORT SHELLS
•This structure is a cylindrical shell having a large radius in comparison to the
length.
•The principle structural element is the stiffener, usually a reinforced concrete
arch, although steel arches or trusses have been used.
•Many structures built with short shells, such a large hangars and auditoriums, could
have been built with little more dead load by using a ribbed slab or other lightweight
concrete framing system rather than the shell.
•The curve of the shell is determined by
the proper shape of the arch and may be a
circle for small structures.
•The minimum shell thickness should be at
the top in the center of the span. At the
arch, the shell thickness is increased slightly
for local stresses.
Arch Structure
Shell spanning between arches
Edge beams (can be omitted if shell is thickened)
Edge beams act like folded plate structures
Transmission of forces
Long shells
Short shells
DOMES
INTERSECTION SHELLS
• The surfaces that produce the shell appear to meet at an
intersection.
• The structural efficiency depends on the angle of the
intersection of the surfaces. If the angle is small, then a
natural rib is formed by the adjacent elements of the basic
shells which is much stiffer than the adjacent shells on each
side. An intersection for which the angle is very large is
called here a shallow intersection.
• An intersection of 90 degrees is the optimum value
because it gives a stiff rib.
Dome formed by using triangular pieces
of a cylindrical shell arranged in the
form of a square. Angle between
the components
is small
Best type of dome to
cover square area.
Structural action
of a short shell
Loads are carried by
the cross beamsEdge beam –
stiffening element
Form is suitable for large
spans – nearly circular in planColumns are at the
center of each panel
not at the ribs.
Skylight can be placed at
the crown of the dome4 cylindrical barrels
Supported by 4 columns at the
intersection
Cantilevered part of the barrel – span should not be large to
take the bending forces.
Architectural advantage – structure appears to
float in air.
SHELL ARCHES
• Shell arches are somewhat in the same category as short shells in that the
shell action is subservient to the arch action. All the thicknesses can be made
quite small of an arch is used because the stresses will be principally
compression.
• The curve of the arch has to be generally a funicular form, that is, it should
fit the thrust line of the applied loads.
• Shells are not very efficient structures if the bending moments are high, as in
the folded plate rigid frame.
• There are types of shells that fit in several categories. The hyperbolic
paraboloidal dome is really a shell arch.
Long span arches are possible as bending moments are much less in an arch than in a beam
Shearing forces are not very large – larger holes than barrel vaults can be used
Skylights can be used for lightingSurface is more difficult to form –widths can be more than folded plates
Suitable for long spans
Each unit is self supporting
Continuity and Curvature
The essential ingredients of a shell structure in all of the foregoing
examples are continuity and curvature.Thus, a fiberglass hull of a boat
is continuous in away that the overlapping planks of clinker
construction are not. A pressure vessel must be obviously constructed
to hold a fluid at pressure, although the physical components may be
joined to each other by riveting, bolting or welding. On the other
hand, an ancient masonry dome or vault is not obviously continuous in
the sense that it may be composed of of separate stone subunits or
voussoirs not necessarily cemented to each other. But in general domes
are in a state of compression throughout, and the subunits are thus
held in compressive contact with each other. The important point here
is that shells are _structurally continuous in the sense that they can
transmit forces in a number of different directions in the surface of the
shell, as required. These structures have quite a different mode of
action from skeletal structures, ofwhich simple examples are trusses,
frameworks, and trees. These structures are only capable of
transmitting forces along their discrete structural members.The
fundamental effect of curvature and its effect on the stregth and
stiffness of a shell.
EXAMPLES
Santiago Calatrava
L’Hemisfèric (Planetarium)
Valencia, Spain
L’Hemisfèric, the distinctive eye-shaped
construction designed by Santiago
Calatrava, was the first element to be
opened to the public in the City of Arts
and Sciences, in April 1998.
The building’s unique architecture comes
alive as the lids of the colossal “Eye of
Knowledge” opens up to reveal the
fascinating setting.
The globe of L’Hemisfèric (the Planetarium), which also
houses the Omnimax theater, is roofed over by an elliptical
shell structure and placed within an elliptical pod that cradles
it like the pupil of an eye.The L’Hemisfèric is set slightly
below grade to avoid visual conflict with the Science Museum
and Palacio de las Artes
The concrete socket of the eye incorporates elongated
aluminium awnings that differ in length and fold
upwards collectively, or as individual units, to form a
brise-soleil roof that opens along the curved central axis
of the eye shape. The concrete encasement has been
extended upwards, and the brise-soleil narrowed and
replaced by a system of slats mounted on each side of
pivoting, to imitate the structure of a feather.
Conclusion
• The present technology makes it feasible to build long
span roof structures in wood, steel, or reinforced
concrete.
• Cost of wood is the highest + Span limitations.
• Concrete is cheaper than steel as it can be produced
anywhere at a lower cost.
• Steel is more expensive on a world wide basis.
• Therefore, reinforced concrete thin shell should be the
structural system to erect permanent roof structures
enclosing long span buildings.
• But the life span of reinforced concrete thin shells is
less than a century, which is comparatively lesser than the
other systems.
• Concrete domes are becoming increasingly popular.
British Museum –Norman Foster
Sydney Opera House – John Utzon
Swiss Re “Gherkin”
– Norman Foster
Hussain – Doshi Gufa
Venugopal Temple -Manipal
Sangath –B.V.Doshi
Model