5. INTRODUCTION
ļ A cable is a flexible structural component that offers no resistance when
compressed or bent in a curved shape. Technically we can say cable has zero
bending rigidity.
ļ It can only support tensile loading.
ļ Cables are often used in engineering structures for support and to transmit load
from one point to another when used to support suspension roofs, bridges and
trolley wheels, cables form the main load carrying element in the structure.
ļ In analysis of cables the weight of itself cable is rejected . We assume that cable is
flexible and inextensible. Due to its flexibility cables offers no resistance to shear or
bending.
6. Continuedā¦ā¦
ļ Being inextensible the cable has constant length before and after
the load is applied. As a result once the load is applied the
geometry of cable remains fixed.
ļ The easiest structure type to think is a tension structure to resist
only tensile force and of these , the simplest are those which
sustain only unidirectional tension as represented by a cable or
thin rod.
ļ A cable is the main component of cable supported bridge or
suspended roof structures that are classified as follows.
7. Types of Cables
There are generally two types of cables structures.
1- Suspension type Cables.
2- Stayed type Cables.
11. Suspension Bridge:
ļµ A suspension bridge is a type of bridge in which the deck (the load-bearing
portion) is hung below suspension cables on vertical suspenders.
ļµ This type of bridge has cables suspended between towers, plus vertical suspender
cables that carry the weight of the deck below, upon which traffic crosses. This
arrangement allows the deck to be level or to arc upward for additional
clearance.
ļµ The main type of force in a suspension bridge are tension in cables and
compression in the pillars.
12. ļ The suspension cables must be anchored at each end of the bridge, since any load applied
to the bridge is transformed into a tension in these main cables.
ļ The main cables continue beyond the pillars to deck-level supports, and further continue
to connections with anchors in the ground.
ļ The roadway is supported by vertical suspender cables or rods, called hangers.
ļ The bridge will usually have two smaller spans, running between either pair of pillars
and the highway, which may be supported by suspender cables or may use a truss bridge
to make this connection. In the latter case there will be very little arc in the outboard
main cables.
13. Assumptions
ļµ Cables are pure tension members.
ļµ Used as
ļµ Supports to suspension roofs
ļµ Suspension bridges
ļµ Trolley wheels
ļµ Self weight of cable is neglected in analysis of above structures
ļµ When used as cables for antennas or transmission lines, weight is considered.
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20. Cable-Stayed Bridges
ļµ A cable-stayed bridge has one or more towers (or pylons), from which cables support
the bridge deck.
ļµ There are two major classes of cable-stayed bridges: harp and fan.
ļµ In the harp or parallel design, the cables are nearly parallel so that the height of their
attachment to the tower is proportional to the distance from the tower to their
mounting on the deck.
ļµ In the fan design, the cables all connect to or pass over the top of the towers. The fan
design is structurally superior with minimum moment applied to the towers but for
practical reasons the modified fan is preferred especially where many cables are
necessary. In the modified fan arrangement the cables terminate near to the top of
the tower but are spaced from each other sufficiently to allow better termination,
improved environmental protection, and good access to individual cables for
maintenance
21. Load Bearing Mechanism Of Cable-Stayed Bridges
ļµ In the cable-stayed bridge, the towers are the primary load-bearing structures which
transmit the bridge loads to the ground.
ļµ A cantilever approach is often used to support the bridge deck near the towers, but
lengths further from them are supported by cables running directly to the towers.
ļµ This has the disadvantage, compared to the suspension bridge, that the cables pull to the
sides as opposed to directly up, requiring the bridge deck to be stronger to resist the
resulting horizontal compression loads; but has the advantage of not requiring firm
anchorages to resist the horizontal pull of the main cables of the suspension bridge.
ļµ By design all static horizontal forces of the cable-stayed bridge are balanced so that the
supporting towers do not tend to tilt or slide, needing only to resist horizontal forces from
the live loads.
25. Advantages Of Suspension Bridges
ļ Suspension bridges have a high strength to weight ratio.
ļ They are flexible (can also be disadvantage) and can span long distances
with no piers therefore good on very high places, across water etc. and
they require little access from below aiding construction.
ļ They can be very thin and therefore less visible.
ļ They have an elegant look.
ļ The area spanned by a suspension bridge is very long in proportion to the
amount of materials required to construct bridges.
26. Disadvantages of Suspension Bridges
ļµ Flexibility Disadvantages
Suspension bridges are flexible, which is an advantage until conditions become severe.
Instability in extremely turbulent conditions or during strong earthquakes may require
temporary closure. In 1940, high winds caused the Tacoma Narrows bridge, near Seattle,
Washington, to collapse.
ļµ Foundation Disadvantages
When built in soft ground, suspension bridges require extensive and expensive foundation
work to combat the effects of the heavy load on foundation towers.
ļµ Heavy Loads
Flexibility also becomes a disadvantage when heavy, concentrated loads are involved.
Suspension bridges are not generally used for regional rail crossings that carry maximum
weight loads, which adds dangerous stress to the structure.
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28. Advantages of Cable-Stayed Bridge
ļµ The cable-stayed deck is in compression, pulled towards the towers, and has
to be stiff at all stages of construction and use.
ļµ A great advantage of the cable-stayed bridge is that it is essentially made of
cantilevers, and can be constructed by building out from the towers.
ļµ cable-stayed bridges possess higher stiffness and display smaller deflections
when compared with suspension bridges
ļµ Construction time is less for cable stayed bridges.
ļµ Cable Stayed Bridges require less cables
29. Comparison
Suspension Bridge
ļµ Suspension bridges is normally limited to
two towers.
ļµ Suspension bridges require more cables
ļµ Construction time is longer for suspension
bridges.
ļµ Suspension Bridges possess less stiffness
and display larger deflections when
compared with cable stayed bridges
Cable Stayed Bridge
ļµ Cable-stayed bridges lies in the fact that it
can be built with any number of towers
ļµ Cable Stayed Bridges require less cables
ļµ Construction time is less for cable stayed
bridges.
ļµ cable-stayed bridges possess higher stiffness
and display smaller deflections when
compared with suspension bridges
30. Suspension Bridge
ļµ The deck of a suspension bridge is usually
suspended by vertical hangers, though But
the structure is essentially flexible, and
great effort must be made to withstand the
effects of traffic and wind
ļµ Suspension Bridge is not made of
cantilevers
Cable-Stayed Bridge
ļµ The greater inherent rigidity of the
triangulated cable-stayed bridges,
compared with the suspension type, makes
life easier for their designers and builders.
ļµ A great advantage of the cable-stayed
bridge is that it is essentially made of
cantilevers, and can be constructed by
building out from the towers.
