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CABLE SYSTEMS

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Title: CABLE SYSTEMS


1
CABLE STRUCTURES
SUBMITTED TO AR.KARAMJIT S.
2
CABLE SYSTEMS
MAJOR SYSTEM ? FORM ACTIVE STRUCTURE
SYSTEMS. ? Non rigid,
flexible matter shaped in a certain way and
secured by fixed ends, an support itself span
space. The 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.
3

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. Form
active systems because of their dependence on
loading conditions are strictly governed by the
natural flow of forces and hence cannot become
subject to arbitrary free form design.
4
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 o
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.
5
CABLE SAG
  • 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.

6
OPTIMAL 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 45o
triangle cable configuration with thrust p/2.
7
GEOMETRIC FUNICULAR FORMS
If the load is shifted from midspan 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.
8
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.
9
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10
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
  • 1. Single Curvature Structures
  • 2. Double Curvature Structures
  • Double Cable Structures

11
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.
12
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.

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
14a). 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 14b).
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15
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.
16
STIFFENING TRUSSES
Stiffening trusses are usually rigid in the
direction of bridge axis, but less so in
transverse directions. Modern suspension bridges
are made sage against lateral wind displacements
by using stiffening GUY WIRES OR STAYS which have
the double role of supporting the truss
stabilizing it.
A cable truss system has a triangulated
structural form which increases stiffness,
particularly under non-symmetric loading.
17
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18
Double-layer prestressed cable-truss system
19
DESIGN OF SUPPORTING ELEMENTS
In addition to actual roof cables, other
structural elements egs. masts, guy cables are
needed to make a building structure. The elements
typically support the cable in space and provide
means of transferring its vertical horizontal
thrusts to the ground. The design of these
elements is as crucial as the cable design.
20
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.
21
Today the longest suspension bridge has a span of
1410 m. (4226 ft.) the longest suspension roof
the Burgo Paper Mill in Mantcia has a span of 163
m. (535 ft.). The roof was designed like a
suspension bridge. The cable flexibility is not
wholly advantageous as in bridge. Excessive
vibrations can not be tolerated in a building.
Water proofing of the roof is difficult. Most
suspension roofs are therefore prestressed to
reduce their flexibility some also have
concrete roofs.
22
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.
Suspension roof with parallel cables anchored to
reinforced conc. Structure supporting the banked
seats. The horizontal reaction is absorbed by
cables buried in the floor structure.

23
Raleigh Arena(span-99m)
Yale University-skating rink
24
Structures using suspended cables have a
functional advantage for arenas, because the
shape is better suited to an array of banked
seats than that of a dome. A suspension roof
requires a smaller volume of air than a dome.
This can produce imp. economics in
air-conditioning heating.
25
Roof over sports arena, Munich by Fvei Offo.
Approximate span of the structure is 130 m. (430
ft.). The tentlike simplicity of this prestressed
cable structure is deceptive. The roof-over the
entire sports arena cost about 48 million.
The design required a great deal of theory as
well as model analysis.
26

Memorial Auditorium in Litica, New York. Span
73 m (240 ft.). Two sets of cables, are separated
by struts that cause them to act in conjunction.
The amount of prestress for upper and lower
cables varied. Vibrations in one set of cables
are different or out of phase with the other and
the opposing forces damp the vibration of the
structure.
27
A double layer of cables covered with pre-cast
concrete slabs. These were loaded temporarily
with a large weight of building. Materials to
prestress the cables, and the joints between
concerete slabs were then filled with cement
mortar to auction the prestress. Rainwater was
pumped off the roof.
Cables can be used to increase the span of
cantilevers and is particularly useful for
aircraft hangars and other buildings than require
large entrances as well as unobstructed interior
span.
28
Cable-stiffened cantilever roof. The structure
is several 100 stronger than the cantilever on
its own. The cable provides the tensile component
of the resistant moment, so that the cantilever
becomes the compression member, and the distance
between the cantilever cable of the support
provides the lever arm of the resistance moment.
Other applications of cable structures can be
for exhibition pavilions, sports complexes, army
shelters etc.
29
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.

30
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 mans 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.
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