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
3. 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.
4. ARCH AS FORM ACTIVE STRUCTURE SYSTEM
• 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.
5. 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”.
6. TYPES OF ARCHES:
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.
7. LOAD MECHANISM:
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,
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.
8. 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.
9. The strength of an arch can be demonstrated by using a stiff card :-
The secret of strength of the arch lies in the way it transfers its loads,
which is also called the arch action.
10. 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
11.
12. 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.
13. CABLE STRUCTURES
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
14. 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.
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.
15. 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
16. # 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
17. 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.
18.
19. THE TENT STRUCTURE SYSTEM”
Introduction
• 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 mesh
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).
20. 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.
22. 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
23. 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.
24. 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.
