form active structure system

1. 1
THEORY OF STRUCTURES
FORM ACTIVE
STRUCTURES
SUBMITTED TO:
ER. BEENA VASHISHT
SUBMITTED BY :
ANMOL DEEP KAUR 2015ARA007
GUNJAN SHARMA 2015ARA011
NAVNEET KAUR 2015ARA023
SEHAJDEEP KAUR 2015ARA036
ZARNAIN 2015ARA040

2. SINGLE STOREY LONG SPAN STRUCTURE
Classification of structural forms:
• Form active systems
• Vector active systems
• Section active systems
• Surface active systems
FORM ACTIVE STRUCTURAL SYSTEMS
. . . are systems of flexible, non-rigid matter, in
which the redirection of forces is effected by
particular form design and characteristic form
stabilization
Example of structures:
1. Arch structures
2. Tent structures
3. Pneumatic structures
4. Cable structures
5.Shelled structures
2

3. Parallel cable Radial cable Biaxial cable
Illustrated examples of parallel cable structures
3

4. 4

5. Examples of cable structures formed by arch
Examples of tent structures 5

6. Examples of pnuematic structures
6

7. 7

8. ARCH AS FORM ACTIVE STRUCTURE SYSTEM
An arch is a curved structure that spans an elevated space and may or may not
support the weight above it.
Terminology:
• KEYSTONE: the wedge shaped, often embellished voussoir at the crown of an
arch.
• VOUSSOIR: any of the wedge shaped units in masonry arch or vault, having side
cuts converging at one of the arch centers.
• SPRINGER: the first voussoir resting on the impost of an arch.
• EXTRADOS: the exterior curve, surface or boundary of the visible face of an
arch.
• INTRADOS: the inner curve or surface of an arch forming the concave underside.
• SPRING: the point at which an arch, vault or dome rises from its support.
• RISE: the height of an arch from the springing line to the highest point of the
intrados.
• LINE OF THRUST: the set of resultants of thrust and weight each part of an arch
imposes on the next lower one. For bending to be eliminated throughout an
arch, the line of thrust must coincide with the arch axis.
• ARCH AXIS: the median line of an arched structure.
• THRUST: the outward force or pressure exerted by one part of a structure
against another.
• DRIFT: the thrust of an arched structure on its abutment proportional to the
total load and span and inversely proportional to the rise.
8

9. TYPES OF ARCHES:
Arches have many forms, but all fall into three basic categories:
• circular
• pointed
• Parabolic
 Arches with a circular form were commonly employed by the builders of
ancient, heavy masonry arches. Ancient Roman builders relied heavily on the
rounded arch to span large, open areas. Several rounded arches placed in-line,
end-to-end, form an arcade, such as the ”Roman aqueduct.”
 Pointed arches were most often used by builders of Gothic-style
architecture. The advantage to using a pointed arch, rather than a circular one,
is that the arch action produces less thrust at the base. This innovation allowed
for taller and more closely spaced openings, typical of “Gothic architecture”.
 The parabolic arch employs the principle that when weight is uniformly applied
to an arch, the internal compression resulting from that weight will follow a
parabolic profile.
Of all arch types, the parabolic arch produces the most thrust at the base, but can
span the largest areas. It is commonly used in bridge design, where long spans are
needed.
LOAD MECHANISM:
 An arch is a pure compression form.
 It can span a large area by resolving forces into compressive stresses and, in
turn eliminating tensile stresses. This is sometimes referred to as ARCH
ACTION.
 As the forces in the arch are carried to the ground, the arch will push outward
at the base, called THRUST. As the rise, or height of the arch decreases, the
outward thrust increases.
 In order to maintain arch action and prevent the arch from collapsing, the
thrust needs to be restrained, either with internal ties or external bracing,
such as abutments.
Instead of pushing
straight down, load is
carried outward along
the curve of the arch
to the supports at
each ends.
Abutments carry load and safely transfer it to the ground
without spreading it. Since an action has a reaction,
ground is squeezed and pushes back on the abutments.
Ground pushes back on the abutment creates a resistance
which is passed from unit to unit, until it is eventually
pushing on the keystone which is supporting the load.
9

10.  Forces:
• Compression : arch is always under compression. The force of compression is
pushed outward along the curve of the arch towards the abutment.
• Tension: the tension in an arch is negligible. The natural curve of the arch and
its ability to dissipate the force outwards greatly reduces the effects of tension
on the underside (intrados) of the arch. The greater the degree of curvature
the greater the effects of tension on the underside (intrados).
• It is important to minimize the arch thrust so as to reduce the dimensions of
the tie rod, or to ensure that the soil will not move under the pressure of the
abutments. The thrust is proportional to the total load and to the span, and
inversely proportional to the rise of the arch.
• In arches RISE TO SPAN RATIO should not be less than 1/8. Riser minimum
should be 1/8 of the span and 2/3rd maximum. Lesser rise takes compression
but not tensile load.
 The strength of an arch can be demonstrated by using a stiff card :-
Model bridge constructed
like this is not capable of
supporting a wooden block.
But if we construct an arch
supported at its ends, now we
have a structure which can
easily support the same
wooden block.
The secret of strength of the arch lies in the way it transfers its loads, which is also
called the arch action.
The load is distributed equally
onto both the bases. The force is
resolved into vertical component
which goes to the ground
(compression) and the horizontal
component which is taken up by
the buttress (tension).
Removing the buttress and
attaching a strong rope across the
base of the arch is another way to
prevent the horizontal force from
spreading the arch. The arch
thrust is absorbed by a tie-rod
whenever the foundation
material is not suitable to resist it.
10

11. Classification of arches:
An arch may be classified according to their:
1)Material of construction and workmanship
2)Shape of curve formed by their soffit or intrados
1) BASED ON MATERIAL AND WORKMANSHIP
a) BRICK
ROUGH BRICK ARCHES GAUGED BRICK ARCHES AXED BRICK ARCHES
b) STONE
ASHLAR ARCHES RUBBLE ARCHES
c) GAUGE
PRECAST CONCRETE BLOCK ARCHES MONOLITHIC CONCRETE ARCHES
11

12. 2) BASED ON SHAPE OF INTRADOS OR SOFFIT
FLAT ARCH SEMI-CIRCULAR ARCH SEGMENTAL ARCH
POINTED ARCH DUTCH OR FRENCH ARCHES
Advantages of arch structure:
• Spanning Distance-Using an arch as the primary structure when there is a
need for spanning a long distance is highly beneficial. An arch can span
further (between two points of vertical support) than a straight beam. This is
due to the way an arch handles the forces, or vectors. Due to this
phenomenon, an arch can handle more loads than a straight horizontal
member.
• Carrying Loads-An arch works excellently in compression. A structural arch
can carry much more load than a flat beam or plank. The forces exerted by an
arch are tangential to the ends of the arch, and are called thrust.
• Materiality and Form- Since the arch form is so effective at spanning and
carrying loads, a variety of materials and forms can be used to construct an
arch. This gives the designer a lot of flexibility when it comes to the aesthetic
properties of the structure.
• Multitude of Uses- Arches have a variety of different uses. On a small scale,
arches work very well at holding up the roof structure of a house. On a much
larger scale, arches can be used to make a bridge across a wide river.
12

13. DESIGN OF ARCH STRUCTURES :
• The first important consideration when designing a brick arch is whether the
arch is structural or non-structural. That is, will the arch be required to
transfer vertical loads to abutments or will it be fully supported by a steel
angle.
• While this may seem obvious, confusion often develops because of the many
configurations of arch construction. To answer this question, one must
consider the two structural requirements necessary for a brick arch to
adequately carry vertical loads:
a)vertical loads must be carried by arch and transferred to the abutments.
b)vertical load and lateral thrust from arch must be resisted by the abutments.
 DESIGNING FOR LOAD VARIATIONS
One of the most significant aspect of the modern arch is that it can be designed
to sustain some amount of variation in load without changing shape nor
experiencing damage. The shape of an arch is initially determined as a response
to its primary loading condition (e.g.: parabolic for uniformly distributed loads). If
either the arch or the abutment is deficient, the arch must be considered as
non-structural and the arch and its tributary load must be fully supported by a
steel angle or plates. Alternately, reinforcement may be used to increase the
strength of either or both the arch and the abutments.
 SUPPORT ELEMENTS
A basic issue is that whether or not to absorb the horizontal thrusts by some
interior element (a tie rod or by the foundations). When it is functionally
possible the rods are frequently used. The rod is a tension element and highly
efficient to take up the outward arch thrusts. Usually there is less need to support
an arch on the top of vertical elements, the use of buttressing elements is
generally preferable as head room has to be maintained.
 CHOICE OF END CONDITIONS
There are 3 primary types of arches used that are normally described in terms of
end conditions :- Three hinged arch ,Two hinged arch ,Fixed end arch. Different
end conditions are preferable with respect to different phenomenon. The
presence of hinges is very important when supports, settlements and thermal
expansions are considered.
13

14. INTRODUCTION
• Pneumatic structure is a membrane which carries load
developed from the tensile stresses.
• Its stabilization is done by pre- stressing the membrane either by
a) Applying an external force which pulls the membrane taut
b) Internal pressurizing if the membrane is volume enclosing.
• The whole envelope has to be evenly pressurized for best
stability.
HISTORY
• The word pneumatic is derived from the greek word
“pneuma” (meaning breath of air), thus these are the structure
which are supported by air.
• The concept of pneumatic structures were developed during the
development of hot air balloons.
• A brazilian priest Gusmao conducted the first experiment in
1709.
• Although pneumatic structures have been used by mankind for
thousand of years; it was only introduced in the building
technology about 40 years ago.
PNEUMATIC SRUCTURES
14

15. PRINCIPLE
1. Its principle is the use of relatively thin membrane supported by a
pressure difference.
2. In pneumatic constructions, pressure differences between the
enclosed space and the exterior are responsible for giving the building
its shape and also for stabilizing the hull.
3. Through increasing the inside air pressure not only the dead weight of
the space envelope is balanced, but the membrane is stressed to a
point where it cannot be indented by asymmetrical loading.
4. Membrane can support both tension and compression and thus
withstand bending moment.
15

16. TYPES OF PNEUMATIC STRUCTURES
AIR SUPPORTED STRUCTURE
• It consist of a single membrane (enclosing a functionally useful
space) which is supported by a small internal pressure difference.
The internal volume of a building air is consequently at a pressure
higher than atmospheric.
• They have air higher than the atmospheric pressure supporting the
envelope.
• Air locks or revolving doors help to maintain the internal pressure.
• Air must be constantly provided.
• Life span of 20 – 25 years.
• Relatively low cost and they can be installed easily.
• They are either anchored to the ground or to a wall so that leakage
is prevented.
16

17. AIR INFLATED STRUCTURE
• It is supported by pressurized air contained within inflated
building element. The pressurized air in the pillow serves only to
stabilizing the load carrying membrane. The covered space is not
pressurized.
• Supporting frames consist of air under high pressure.
• Internal pressure of building remains at atmospheric pressure.
• There is no restrictions in number and size of openings.
• It has the ability to support itself.
• They have potential to support an attached structure.
17

18. GENERAL CHARACTERISTICS OF PNEUMATIC
STRUCTURES
LIGHT-WEIGHT
• The weight of the structure as
compared to the area it covers
is very less.
• The weight of the membrane
roof, even when it is stiffened
by cables, is very small.
• Low air pressure is sufficient to
balance it.
SPAN
• There is no theoretical
maximum span.
• To span a distance of 36 km for
a normal building is hard while
such spans are quite possible
for pneumatics.
SAFETY
• Pneumatic structures are safer
than any other structure.
Otherwise, a proper care
should be taken while
establishing. They are fire
resistance structures.
QUICK ERECTION & DISMANTLING
• Suitable for temporary
constructions.
• 1 km² area can be brought
down in 6 hours and can be
establish in less than 10 hours.
GOOD NATURAL LIGHTING
• If envelope is made up of
transparent material, good
natural light enter into the
structure. Around 50% – 80% of
sunlight can be obtained.
HUMAN HEALTH
• In most cases, pressure of not
more than 80-100mm and not
less than 60mm.
• Man can withstand pressures
between 0.20 atm to 3 atm.
Therefore no health hazard is
presented by continuous stay in
a pneumatic structure.
18

19. SYSTEM COMPONENTS
ENVELOPE
• They can be made up of different
materials.
• Cannot be used as one
continuous material.
• Materials are seamed together by
sealing, heat bonding or
mechanical jointing.
• The design of the envelope
depends on an evenly pressurized
environment.
CABLES
• They act as the supporting
system.
• They experience tension force
due to the upward force of the air.
• Can be placed in one or two
directions to create a network
and for better stability. They do
not fail since they are pulled tight
enough to absorb the external
loads.
PUMPING EQUIPMENT
• It is used to supply and maintain
internal pressure inside the
structure.
• Fans, blowers or compressors are
used for constant supply of air.
• The amount of air required
depends on the weight of the
material and the wind pressure.
ENTRANCE
• Doors can be ordinary doors or
airlocks.
• Airlock minimize the chances of
having an unevenly pressurized
environment.
19

20. FOUNDATION
• Pneumatic structures are secured to ground using heavy weights,
ground anchors or attached to a foundation.
• Weight of the material and the wind loads are used to
determine the most appropriate anchoring system.
• For bigger structures, reinforcing cables or nets are used.
• For a dependent pneumatic structure (roof only air supported
structure) the envelope is anchored to the main structure.
• When anchoring is done to soil, the cable is attached to the
anchor directly inserted and frictional forces of the soil to hold it
down.
• Soil anchoring systems include screw, disk, expanding duckbill
and arrowhead anchors.
20

21. CLASSIFICATION OF PNEUMATIC STRUCTURES
Pneumatic Structures can be further subdivided on the basis of:-
A. Type of Differential Pressure
B. Degree of Differential Pressure
C. Type of Surface Curvature
D. Proportions
PNEUMATIC
STRUCTURES
Type of
Differential
Pressure
positive
pressure
negative
pressure
Degree of
Differential
Pressure
Low pressure
systems
High pressure
systems
Type of Surface
Curvature
Single curved synclastics anticlastic
Proportions
21

22. ENVELOPE MATERIALS
FIBERGLASS
POLYESTER
ETFE
NYLON
22

23. MAJOR SYSTEM
 FORM ACTIVE STRUCTURES
 Non rigid, flexible matter shaped in a certain way and secured by fixed
ends,an support itself & span space. This transmit loads only through
simple normal
stresses; either tension or through compression.
Two cables with different points of suspension tied together form a
suspension system. A cable subject to external loads will deform in a way
depending upon
the magnitude and location of the external forces. The form
acquired by the cable is called the FUNICULAR SHAPE of the cable.
Form Active Structure Systems redirect external forces by simple normal
stresses : the arch by compression,
the suspension cable by tension.
The bearing mechanism of form active
systems vests essentially on the material
form.
CABLE STRUCTURES
23

24. LOADING MECHANISM :
# The high tensile strength of steel, combined with the efficiency of
simple tension, makes a steel cable the ideal structural element to
span large distances.
# Cables are flexible because of their large shall lateral dimensions
in relation to their lengths. As uneven stresses true to bending are
prevented by flexibility the tensile load is evenly divided among the
cable strands.
In order to understand the mechanism by means of which
a cable supports vertical loads, one may first consider a cable
suspended between two fixed points, located at the same level and
carrying a single load at mid span. Under the action of the load the
cable assumes a symmetrical triangular shape and half the load is
carried to each support by simple tension along he two halves of the
cable.
24

25. • Works by Tension AND
Compression
# The natural stress line of the form active
tension system in the funicular tension
line.
# Any change of loading or support
conditions changes the form of the
funicular curve.
25

26. The triangular shape acquired by the cable is
characterized by the SAG : the vertical distance between the
supports and the lowest point in the cable. Without the sag the
cable cannot carry the load, since the tensile forces in if would be
horizontal and horizontal forces cannot balance the vertical load.
The undivided pull of the sagging cable on each support may be
split into two components :
• a downward force equal to half the load
• a horizontal inward pull or thrust.
The thrust is inversely proportional to the sag; halving the sag
doubles the thrust. This raises an interesting question of economy
through.
CABLE SAG :
A large sag increases the cable length, but
reduces the tensile force & allows a reduction of cross-
section. A similar sag requires a larger cross-section. Hence
the total volume of cable (product of cross-section &
length), must be minimum for some optimal value of sag
Optimal sag equal half the span for a given horizontal
distance & corresponds to a symmetrical 45
o
– triangle
cable configuration with thrust = p/2.
OPTIMAL SAG :
26

27. GEOMETRIC FUNICULAR FORMS :
If the load is shifted from mid span position, the cable changes shape.
If two equal loads are set on the cable in symmetrical positions the cable
adapts itself by acquiring a new configuration with three straight video.
FUNICULAR POLYGONS :
# As the number of loads increases, the funicular polygon approaches a
geometrical curve – the PARABOLA large number of loads are evenly spaced
horizontally.
CATENARY :
• If the equal loads are distributed evenly along the length of the cable,
rather than horizontally, the funicular curve differs from a parabola,
through it has the same general configuration. It is a catenary.
• A cable carrying its own weight ad a loads evenly distributed horizontally,
acquires a shape that is intermediate between a parabola & catenary.
This is the shape of cables in the central span of suspension bridges.
27

28. SPECIAL DESIGN CONSIDERATIONS:
(And Corrective Measures)
Lightness of the flexible suspension cable is the demerit of
the system, which can be largely eliminated through pre-
stressing so that it can receive frictional forces that also may
be upward directed.
Cable structures are more correctly categorize into either
suspension structures or cable-stayed structured suspension
structures can be typically sub-classified into :
• Single Curvature Structures
• Double Curvature Structures
• Double Cable Structures
DYNAMIC EFFECTS OF WIND ON TYPICAL
FLEXIBLE ROOF STRUCTURE :
A critical problem in the design of any cable roof structure is
the dynamic effect of wind, which causes an undesirable
fluttering of the roof.
28

29. The principal methods of providing stability are the following:
(i) Additional permanent load supported on, or suspended from,
the roof, sufficient to neutralize the effects of asymmetrical
variable actions or uplift (Figure a).This arrangement has the
drawback that it eliminates the lightweight nature of the
structure, adding significant cost to the entire structure.
(ii) Rigid members acting as beams, where permanent load may
not be adequate to counteract uplift forces completely, but
where there is sufficient flexural rigidity to deal with the net
uplift forces, whilst availing of cables to help resist effects of
gravity loading (Figure b).
PREVENTIVE MEASURES :
There are only several fundamental ways to combat flutter.
• One is to simply increase the deal load on the roof.
• Another is to provide anchoring guy cables at periodic
points to tie the structure to the ground.
• To use some sort of crossed cable on double-cable system.
29

30. LIMITATIONS DUE TO VIBRATIONS &
CHANGING LOADS :
The limitations in the application of cables stem directly
from their adaptability to changing loads :
CABLES are unstable and stability is one of the basic
requirements of structural systems. The trusses hanging
from the cables of a suspension bridge not only support the
roadway but also stiffen the cables against motions due to
moving or changing loads.
30

31. 31

32. Because of their identity with the
natural flow of forces, the form
active structure system is a suitable
mechanism for achieving
long spans and forming large spaces.
Suspension cables are the
elementary idea for any bearing
mechanism and consequently the
very symbol of man’s technical
Seizure of space.
Before of their long span qualities, they have a particular
significance for mass civilization and its demand for large scale
spaces. They are potential structure forms for future building.
MATERIALS :
Steel, nylon ropes or plasticated cables may be used for
different structures.
• Steel Cables : The high tensile strength of steel combined
with the efficiency of simple tension, makes a steel cable the
ideal structural element to span large distances.
• Nylon and plastics are suitable only for temporary
structures, spanning small distances.
Other structural members like masts, compression rings,
arches or beams and compression struts may be of concrete or
steel preferably. Struts may also be of timber.
Suspension Cables, because of their being stressed only by
simple tension – with regard to weight/span are the most
economical system of spanning space.
32

33. APPLICATIONS OF CABLE SYSTEMS :
The earliest use of cables in buildings dates back to A.D.
70 to roof a Roman amphitheater by a rope cable structure.
Rope cables anchored to masts spanned in a radial fashion
across the open structure supported a movable sunshade that
could be drawn across to cover the arena. The span was 620 ft.
along major axis and 513 ft. along minor axis.
Raleigh Arena(span-
99m)
Yale University-
skating rink
The first modern roof was an Arena. Load bearing
cables are suspended from two intersecting arches, anchored
against one another. At night angles to the load bearing are
secondary cables prestressed to ensure tautness even on a hot
day. Corrugated sheets supported on the cable network.
33

34. SHELLED STRICTURES
INTRODUCTION
Thin-shell structures are also called plate and shell structures. They
are lightweight constructions using shell elements. These elements,
typically curved, are assembled to make large structures. Typical
applications include aircraft fuselages, boat hulls, and the roofs of
large buildings.
DEFINITION
A thin shell is defined as a shell with a thickness which is small
compared to its other dimensions and in which deformations are
not large compared to thickness. A primary difference between a
shell structure and a plate structure is that, in the unstressed state,
the shell structure has curvature as opposed to the plates structure
which is flat. Membrane action in a shell is primarily caused by in-
plane forces (plane stress), but there may be secondary forces
resulting from flexural deformations. Where a flat plate acts similar
to a beam with bending and shear stresses, shells are analogous to a
cable which resists loads through tensile stresses. The ideal thin
shell must be capable of developing both tension and compression.
The Forest Opera, an open-air
amphitheatre in Sopot, Poland,
with a membrane roof.
Great Court, with a lattice thin-
shell roof by Buro Happold with
Norman Foster, British
Museum, London

35. TYPES AND FORMS OF SHELLED STRUCURES
• Folded Plates
• Barrel Vaults
• Short Shells
• Domes of Revolution
• Folded Plate Domes
• Intersection Shells
• Warped Surfaces
• Combinations
• Shell Arches
Folded Plates
The elements of a folded plate structure are similar to those of a
barrel shell except that all elements are planar, and the moments in
the slab elements are affected by the differential movement of the
joints.
For the structure shown, the end supports and the side supports are
both complete walls

36. BARREL SHELL
The elements of a barrel shell are:
(1) The cylinder,
(2) The frame or ties at the ends, including the columns, and
(3) The side elements, which may be a cylindrical element, a folded plate
element, columns, or all combined.
For the shell shown in the sketch, the end frame is solid and the side
element is a vertical beam.
A barrel shell carries load longitudinally as a beam and transversally as an
arch. The arch, however, is supported by internal shears, and so may be
calculated.

37. The elements of a folded plate structure are similar to those of a barrel
shell except that all elements are planar, and the moments in the slab
elements are affected by the differential movement of the joints.
For the structure shown, the end supports and the side supports are both
complete walls
The elements of a short shell are the barrel, which is relatively short
compared to radius, the element at the base of the cylinder to pick up the
arch loads, and the arches or rigid frame to pick up the entire ensemble.
In this case it is a rigid frame arch. The size of the arch could have been
reduced by horizontal ties at the springings. There may be multiple spans.
The short shell carries loads in two ways:
(1) As an arch carrying load to the lower elements. and
(2) As a curved beam to the arches.
The thickness of the shell can be quite thin due to these properties.
Domes
Domes are membrane structures, the internal stresses are tension and
compression and are statically determinate if the proper edge conditions
are fulfilled. 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. If the dome is less than a full hemisphere, a ring is required at
the base of the dome to contain the forces

38. 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.
MOST SUITABLE MATERIAL
The material most suited for construction of shell structure is concrete
because it is a highly plastic material when first mixed with water that can
take up any shape on centering or inside formwork. small sections of
reinforcing bars can readily be bent to follow the curvature of shells. once
the cement has set and the concrete has hardened the R.C.C membrane
or slab acts as a strong, rigid shell which serves as both structure and
covering to the building.
CENTERING OF SHELLS
centering is the term used to describe the necessary temporary support
on which the curved R.C.C shell structure is cast. the centering of a barrel
vault, which is part of a cylinder with same curvature along its length; is
less complex. the centering of conoid, dome and hyperboloid of
revolution is more complex due to additional labour and wasteful cutting
of materials to form support for shapes that are not of uniform linear
curvature. the attraction of shell structures lies in the elegant simplicity of
curved shell forms that utilise the natural strength and stiffness of shell
forms with great economy in the use of materials. the disadvantage of
shell structure is their cost. the shell structure is more expensive due to
considerable labour required to construct the centering on which the
shell is cast.

39. ADVANTAGES OF CONCRETE SHELLS
Like the arch, the curved shapes often used for concrete shells are
naturally strong structures, 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 a construction cost, as concrete is relatively
inexpensive and easily cast into compound curves. The resulting
structure may be immensely strong and safe; modern monolithic
dome houses, for example, have resisted hurricanes and fires, and
are widely considered to be strong enough to withstand even F5
tornadoes.
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 build-up of condensation on the inside of the shell. Shingling
or sealants are common solutions to the problem of exterior
moisture, and dehumidifiers or ventilation can address condensation

40. Introduction
40
THE TENT STRUCTURE SYSTEM”
• A membrane is a thin, flexible surface that carries loads primarily
through the development of tension forces.
• Holding a stress tension force.
• Provide strong lighting features.
• Desert architecture identity, inspired from ten design and geometry.
• Net structures are conceptually similar; expect that their surface are
made from cable net meshes
There are several ways of stabilizing a membrane or net surface:
1. An inner rigid supporting framework.
2. Restressing surface by:
a. External forces (tents)
b. Internal pressurization
(pneumatic structure).

41. WHY TENSILE (TENTS) ARE THE SHAPE
THEY ARE ?
• Large flat pieces of fabric are very poor at resisting loads.
• Imagine four of you pulling on the strings laced through a tennice
ball. Fig. 1: A fifth person pushing down on the ball can deflect it
easily.
• Imagine a flappy marquee roof. Fig 2: Try lifting two opposite strings
and lowering the other two. The ball is now locked in space. Apply
this principle to fabric and you have created ‘anticlastic double
curvature.
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1. Saddle roof
2. Mast supported
3. Arch supported
4. Combinations
TYPES OF TENT FABRIC STRUCTURES

42. 1. Saddle roof
• Four or more point system when the fabric is stretched
between a set of alternating and low points.
• The roof plan, taken directly from the structural engineering
working drawings, illustrates the roof configuration and its
components.
• The saddle shaped roof of the stage cover nestles under the
auditorium roof of the project. The leaning A- farms and stay
cables which hold them back are clearly visible, along with
the radical cables which shape the tent units. The corner
tripods, each consisting of a vertical mast and two sloping
cables, are connected to concrete anchors rising from the
water
Section through the
project showing the
stage roof tucked under
the auditorium roof
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43. 2. Mast supported
• Tent- like in appearance, mast supported structures typically
have one or sometimes several peaks are supported by
either interior or perimeter masts.
• The fabric is attached to the interior mast by special
connections, usually a bale ring or cable loop
• Mast supported structures can also be supported by
adjacent buildings. The peaks of mast supported structure
are determined by the design and how the fabric is
attached.
• Openings are typically ovoid or elliptical. The fabric that
extends from the top of the opening is seamed and can
necessitate patterning.
• Mast supported systems are suitable for long span roofs.
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44. 3. Arch supported system
• Curved compression members are used as the main
supporting elements and cross arches are used for lateral
stability .
• In a plane arch, large differences between the thrust lines
and main geometry will produce large bending moments
that in turn produce large changes in shape and high
stresses in the arch chord section. One method to
significantly reduce these effects is to restrain points along
the arch chord to reduce the initial large deformations of the
chord.
• The buckling length of the arch chord can also be reduced by
discretely or continuously supporting the chord with tension
elements or systems comprised of cables or membranes.
• Typical arch shapes defined by physical and ergonomic
constraints.
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45. COMPONENTS AND MATERIALS
Base plate:
• Connection to concrete foundation pillar
Membranes:
• Forms the enclosure of the structure. Connections can be glued or
heat welded.
• PVC coated polyester (polyvinylchloride)
• Silicon coated glass
• Teflon coated glass P.T.F.E (poly tetra fluro ethylene)
Bale Ring / Membrane plate:
Provide a link between membrane and structural elements.
• Bale rings are used at the top of conical shapes.
• Membrane plates accept centenary cables and pin connection
hardware.
Types of fabric material
• PVC : less expensive { 15 to 20 years life span and easy to erect)
• Silicon glass : higher tensile brittle
( subject to damage from the flexing 30+ year life span)
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46. Specialized hardware:
• Tensioner
• Tripod head with centenary cables
• Centenary cables and side connection
• Extruded section with membrane plate and centenary cables
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47. CABLE CLAMPS:
Edge cables with clamps. Used mainly for PTFE- coated fiber
glass fabric, but also for PVC coated Polyester fabric when
edge spans are longer than 20 m.
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• Bale rings are a good way to control stresses in fabric roof at high or
low points. Used at high points they must be covered to make the
structure watertight. If used at low points. They can be used to
gather rainwater and snow for redistribution on site.
• Channel (with grommets) and lacing. Used with PVC coated
polyester fabric where the edge has grammets spaced or frequent
intervals.
• Rope is located through grammets and to a tie rod within the
channel.

48. Disadvantages
• Little to no rigidity
• Loss of tension is
dangerous for stability
• Thermal values limit use
• Load transfer on fabric
structures
• Attached weaknesses in
Mono cover fabric
structures
Advantages
• Longer life cycles of
materials.
• Materials can be reused in
form.
• Most materials are
completely recyclable
• Less impact on site.
• Less construction debris
after demolition
• Unique design
• Lightweight and flexible
• Environmentally sensitive.
• High strength weight ratio
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Tent system design shaped by aluminum
section