Cable strucutres(presentation)

CABLES STRUCUTRES

Group Members

SHOAIB SAFDAR (12-CE-12) M.FAIZAN (12-CE-76) JIBRAN BALOCH (12-CE-152)

Contents

Introduction Types Suspension Bridges Cable-Stayed Bridges Examples Advantages Disadvantages Comparison

INTRODUCTION

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.

It can only support tensile loading. 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.

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.

Continued……

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.

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.

A cable is the main component of cable supported bridge or suspended roof structures that are classified as follows.

Types of Cables

There are generally two types of cables structures.

1- Suspension type Cables. 2- Stayed type Cables.

Golden Gate Bridge San Francisco

Akashi Kaikyo Suspension Bridge:

Load Bearing Mechanism of Suspension Bridge

Suspension Bridge:

A suspension bridge is a type of bridge in which the deck (the load-bearing portion) is hung below suspension cables on vertical suspenders.

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. 

The main type of force in a suspension bridge are tension in cables and compression in the pillars.

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.

The main cables continue beyond the pillars to deck-level supports, and further continue to connections with anchors in the ground.

The roadway is supported by vertical suspender cables or rods, called hangers. 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.

Assumptions

Cables are pure tension members. Used as

Supports to suspension roofs Suspension bridges Trolley wheels

Self weight of cable is neglected in analysis of above structures When used as cables for antennas or transmission lines, weight is considered.

Cable-Stayed Bridges

A cable-stayed bridge has one or more towers (or pylons), from which cables support the bridge deck.

There are two major classes of cable-stayed bridges: harp and fan. 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.

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

Load Bearing Mechanism Of Cable-Stayed Bridges

In the cable-stayed bridge, the towers are the primary load-bearing structures which transmit the bridge loads to the ground.

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.

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.

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.

Kanchanaphisek Bridge, Bangkok.

Rion-Antirion Bridge

Advantages Of Suspension Bridges

Suspension bridges have a high strength to weight ratio. 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.

They can be very thin and therefore less visible. They have an elegant look. The area spanned by a suspension bridge is very long in proportion to the

amount of materials required to construct bridges.

Disadvantages of Suspension Bridges

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. 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. 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.

Advantages of Cable-Stayed Bridge

The cable-stayed deck is in compression, pulled towards the towers, and has to be stiff at all stages of construction and use.

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.

cable-stayed bridges possess higher stiffness and display smaller deflections when compared with suspension bridges

Construction time is less for cable stayed bridges. Cable Stayed Bridges require less cables

Comparison

Suspension Bridge Suspension bridges is normally limited to

two towers. Suspension bridges require more cables Construction time is longer for suspension

bridges. Suspension Bridges possess less stiffness

and display larger deflections when compared with cable stayed bridges

Cable Stayed Bridge Cable-stayed bridges lies in the fact that it

can be built with any number of towers Cable Stayed Bridges require less cables  Construction time is less for cable stayed

bridges. cable-stayed bridges possess higher stiffness

and display smaller deflections when compared with suspension bridges

Suspension Bridge 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

Suspension Bridge is not made of cantilevers

Cable-Stayed Bridge The greater inherent rigidity of the

triangulated cable-stayed bridges, compared with the suspension type, makes life easier for their designers and builders. 

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.

References

http://science.howstuffworks.com/engineering/civil/bridge7.htm http://www.madehow.com/Volume-5/Suspension-Bridge.html http://en.wikipedia.org/wiki/List_of_longest_cable-stayed_bridge_spans http://en.wikipedia.org/wiki/Russky_Bridge http://www.roadtraffic-technology.com/features/featurethe-worlds-longest-cable-stayed-bridges-4180

849/featurethe-worlds-longest-cable-stayed-bridges-4180849-2.html http://science.howstuffworks.com/engineering/civil/bridge6.htm http://en.wikipedia.org/wiki/List_of_longest_suspension_bridge_spans#mediaviewer/File:Akashi_brid

ge.jpg http://www.engineeringcivil.com/what-are-the-advantages-of-cable-stayed-bridges-over-suspension-br

idges-for-span-less-than-1000m.html

http://www.engineeringcivil.com/what-are-the-advantages-of-cable-stayed-bridges-over-suspension-bridges-for-span-less-than-1000m.html

http://www.brantacan.co.uk/cable_stayed.htm http://books.google.com.pk/books?id=8HFECTPDXu8C&pg=SA26-PA1&lpg=SA26-PA1&dq=

load+bearing+mechanism+of+cable+stayed+bridge&source=bl&ots=2LSVTRhbHG&sig=bjDYSHnufYjrEFg1Zz7eWkSZ5Jg&hl=en&sa=X&ei=Q8VxVLjTJsH9ygP3_ILQBw&ved=0CCYQ6AEwAg#v=onepage&q=load%20bearing%20mechanism%20of%20cable%20stayed%20bridge&f=false

http://en.wikipedia.org/wiki/Cable-stayed_bridge#Comparison_with_suspension_bridge