Seminar report

On

CABLE SUSPENSION BRIDGE

To be submitted in Partial Fulfilment of the requirement for the Degree of

Bachelor of Technology

in

Civil Engineering

Under the guidance of

Dr. JYOTI PRASAD

Professor

Department of Civil Engineering, College of Technology

Submitted by:

GAURAV RAWAT

ID. NO.45339

COLLEGE OF TECHNOLOGY

G.B.PANT UNIVERSITY OF AGRICULTURE & TECHNOLOGY

Dr. JYOTI PRASAD

Professor

Department of Civil Engineering

College of Technology

G. B. Pant University of Agriculture & Technology

Pantnagar

District: U. S. Nagar

Uttarakhand

APPROVAL

This project report entitled “CABLE SUSPENSION BRIDGE” submitted by

GAURAV RAWAT (Id. No.45339) is hereby approved as credible work of

engineering subject carried out & presented in a satisfactory manner. The

undersigned warrant its acceptance as pre-requisite to the degree for which it has

been submitted.

Dr. JYOTI PRASAD

(Seminar Guide)

Acknowledgement

I would like to articulate deep gratitude and veneration to Dr. Jyoti Prasad,

professor, Department of Civil Engineering for his sincere exhortation, inspiring

guidance, constant encouragement and constructive criticism throughout the

seminar work. We express deep sense of obligation to him as she made it possible

for us to submit the report in the present form.

This journey would not have been possible without the support of my family,

professors and mentors, and friends. To my family, thank you for encouraging me

in all my pursuits and inspiring me to follow my dreams. I am especially grateful

to my parents, who supported me emotionally and financially. I always knew that

you believed in me and wanted the best for me.

I must thank the Department of Civil Engineering, College of Technology and G

B Pant University of Agriculture and Technology for the best four years of my

life.

GAURAV RAWAT

ID. NO.45339

LIST OF CONTENT

S. No. Title

1 Introduction

2 Types of bridges

3 Suspension bridge and its hostory

4 Structural Elements — Elevators, Spire,

And More

5 Compnents of suspension bridges

6 Types of suspension bridges

7 Manufacturing process and design

8 Loads on suspension bridge and its

working

9 Failures of suspension bridge

10 Pros and cons of suspension bridge

11 Conclusion

12 References

BRIDGE

Bridge is a structure that provides passage over obstacles such as valleys, rough terrain or bodies

of water by spanning those obstacles with natural or manmade materials. They first begun be

used in ancient times when first modern civilizations started rising in the Mesopotamia. From

that point on, knowledge, engineering, and manufacture of new bridge building materials spread

beyond their borders, enabling slow but steady adoption of bridges all across the world.

In the beginning bridges were very simple structures that were built from easily accessible

natural resources- wooden logs, stone and dirt. Because of that, they had ability only to span very

close distances, and their structural integrity was not high because mortar was not yet invented

and rain slowly but constantly dissolved dirt fillings of the bridge. Revolution in the bridge

construction came in Ancient Rome whose engineers found that grinded out volcanic rocks can

serve as an excellent material for making mortar. This invention enabled them to build much

more sturdier, powerful and larger structures than any civilization before them. Seeing the power

of roads and connections to distant lands, Roman architects soon spread across the Europe,

Africa and Asia, building bridges and roads of very high quality.

Types of Bridges

Bridges by Structure

Arch bridges – These bridges uses arch as a main structural component (arch is always located

below the bridge, never above it). They are made with one or more hinges, depending of what

kind of load and stress forces they must endure. Examples of arch bridge are “Old Bridge” in

Mostar, Bosnia and Herzegovina and The Hell Gate Bridge in New York.

Beam bridges – Very basic type of bridges that are supported by several beams of various shapes

and sizes. They can be inclined or V shaped. Example of beam bridge is Lake Pontchartrain

Causeway in southern Louisiana.

Truss bridges – Very popular bridge designs that uses diagonal mesh of posts above the bridge.

The two most common designs are the king posts (two diagonal posts supported by single vertical

post in the center) and queen posts (two diagonal posts, two vertical pots and horizontal post that

connect two vertical posts at the top).

Cantilever bridges – Similar in appearance to arch bridges, but they support their load not trough

vertical bracing but trough diagonal bracing. They often use truss formation both below and

above the bridge. Example of cantilever bridge is Queensboro Bridge in New York City.

Tied arch bridges – Similar to arch bridges, but they transfer weight of the bridge and traffic

load to the top chord that is connected to the bottom cords in bridge foundation. They are often

called bowstring arches or bowstring bridges.

Suspension bridges – Bridges that use ropes or cables from the vertical suspender to hold the

weight of bridge deck and traffic. Example of suspension bridge is Golden Gate Bridge in San

Francisco.

Cable-stayed bridges – Bridge that uses deck cables that are directly connected to one or more

vertical columns. Cables are usually connected to columns in two ways – harp design (each cable

is attached to the different point of the column, creating harp like design of “strings” and fan

design (all cables connect to one point at the top of the column).

Fixed or moveable types

Fixed – Majority of bridges are fixed, with no moveable parts to provide higher clearance for

river/sea transport that is flowing below them. They are designed to stay where they are made to

the point they are deemed unusable or demolished.

Temporary bridges – Bridges made from modular basic components that can be moved by

medium or light machinery. They are usually used in military engineering or in circumstances

when fixed bridges are repaired.

Moveable bridges – They have moveable decks, most often powered by electricity.

Types by use

Car Traffic – The most common type of bridge, with two or more lanes designed to carry car

and truck traffic of various intensities.

Pedestrian bridges – Usually made in urban environments, or in terrain where car transport is

inaccessible (rough mountainous terrain, forests, etc.).

Double-decked bridges – Built to provide best possible flow of traffic across bodies of water or

rough terrain. Most offen they have large amount of car lanes, and sometimes have dedicated area

for train tracks.

Train bridges – Bridges made specifically to carry one or multiple lane of train tracks.

Pipelines – Bridges made to carry pipelines across water or inaccessible terrains. Pipelines can

carry water, air, gas and communication cables.

Viaducts – Ancient structures created to carry water from water rich areas to dry cities.

Commercial bridges – Modern bridges that host commercial buildings such as restaurants and

shops.

Types by materials

Natural materials

Wood (Wooden bridges)

Stone

Concrete and Steel

Advanced materials

SUSPENSION BRIDGES

A suspension bridge is a type of bridge in which the deck (the load-bearing portion) is hung

below suspension cables on vertical suspenders. The first modern examples of this type of bridge

were built in the early 19th century. Simple suspension bridges, which lack vertical suspenders,

have a long history in many mountainous parts of the world.

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. Like other suspension bridge types, this

type often is constructed without falsework.

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. In some

circumstances, the towers may sit on a bluff or canyon edge where the road may proceed directly

to the main span, otherwise 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.

History

Precursor

The Tibetan saint and bridge-builder Thangtong Gyalpo originated the use of iron chains in his version of

simple suspension bridges. In 1433, Gyalpo built eight bridges in eastern Bhutan. The last surviving

chain-linked bridge of Gyalpo's was the Thangtong Gyalpo Bridge in Duksum en route to Trashi Yangtse,

which was finally washed away in 2004. Gyalpo's iron chain bridges did not include a suspended deck

bridge which is the standard on all modern suspension bridges today. Instead, both the railing and the

walking layer of Gyalpo's bridges used wires. The stress points that carried the screed were reinforced by

the iron chains. Before the use of iron chains it is thought that Gyalpo used ropes from twisted willows or

yak skins. He may have also used tightly bound cloth.

Chain bridges

The first American iron chain suspension bridge was the Jacob's Creek Bridge (1801) in

Westmoreland County, Pennsylvania, designed by inventor James Finley. Finley's bridge was the

first to incorporate all of the necessary components of a modern suspension bridge, including a

suspended deck which hung by trusses. Finley patented his design in 1808, and published it in

the Philadelphia journal, The Port Folio, in 1810.

The diagram of the chain bridge over the Menai constructed near Bangor, Wales, in 1820

Early British chain bridges included the Dryburgh Abbey Bridge (1817) and 137 m Union

Bridge (1820), with spans rapidly increasing to 176 m with the Menai Bridge (1826), "the first

important modern suspension bridge". The first chain bridge on the European continent was the

Chain Bridge in Nuremberg Germany. The Clifton Suspension Bridge (designed in 1831,

completed in 1864 with a 214 m central span) is one of the longest of the parabolic arc chain

type. The current Marlow suspension bridge was designed by William Tierney Clark and was

built between 1829 and 1832, replacing a wooden bridge further downstream which collapsed in

1828. It is the only suspension bridge across the non-tidal Thames. The Széchenyi Chain Bridge,

spanning the River Danube in Budapest, was also designed by William Clark and it is a larger

scale version of Marlow bridge.

An interesting variation is Thornewill & Warham's Ferry Bridge in Burton-on-Trent,

Staffordshire (1889), where the chains are not attached to abutments as is usual, but instead are

attached to the main girders, which are thus in compression. Here, the chains are made from flat

wrought iron plates, eight inches (203mm) wide by an inch and a half (38mm) thick, rivetted

together.

Wire-cable

The first wire-cable suspension bridge was the Spider Bridge at Falls of Schuylkill (1816), a

modest and temporary footbridge built following the collapse of James Finley's nearby Chain

Bridge at Falls of Schuylkill (1808). The footbridge's span was 124 m, although its deck was

only 0.45 m wide.

Development of wire-cable suspension bridges dates to the temporary simple suspension bridge

at Annonay built by Marc Seguin and his brothers in 1822. It spanned only 18 m.The first

permanent wire cable suspension bridge was Guillaume Henri Dufour Saint Antoine Bridge in

Geneva of 1823, with two 40 m spans.]The first with cables assembled in mid-air in the modern

method was Joseph Chaley's Grand Pont Suspendu in Fribourg, in 1834.]

In the United States, the first major wire-cable suspension bridge was the Wire Bridge at

Fairmount in Philadelphia, Pennsylvania. Designed by Charles Ellet, Jr and completed in 1842, it

had a span of 109 m. Ellet's Niagara Falls Suspension Bridge (1847–48) was abandoned before

completion. It was used as scaffolding for John A. Roebling double decker railroad and carriage

bridge (1855).

The Otto Beit Bridge (1938–39) was the first modern suspension bridge outside the United States

built with parallel wire cables.

Types of Suspension Bridges

Suspension bridges are bridges whose deck is held in place by suspender cable which hang

vertically from suspension cables. But they are not all the same. They use different techniques

and materials to achieve the same thing – span distances that could not be crossed differently.

“Simple suspension bridge” is a bridge that has no towers nor piers and is suspended on

the cables that are anchored at their ends and nothing else. It is also known as a rope

bridge, swing bridge, suspended bridge, hanging bridge and catenary bridge and is the

oldest variant of the suspended bridge. The deck of this bridge follows is arched

downwards and upwards and has additional ropes at a higher level which form the

handrail. It is a pedestrian bridge and cannot carry modern roads and railroads. "Simple-

suspension bridge" can also be name for a suspended deck bridge that has a deck which is

not stiffened, hence - "simple".

“Suspension bridge” is a name for a modernly designed suspension bridge – a

suspended-deck suspension bridge. It has towers and, from them, cables that hold up the

road deck. These cables transfer the weight of the deck, by tension, to the towers and then

to the ground by cables whose ends are anchored. This type can carry heavy vehicles and

light rail. The first designs of this type of bridge appeared in 16th century but they were not

built until 18th century when more materials appeared which allowed for this type of

bridge to be made. Longest suspension bridges of today are of this design.

“Underspanned suspension bridge” is a type of bridge that was developed in the early

19th century and which has deck that is raised on posts above the main cables which are,

at their ends, anchored. It is a very rare design in practice because its deck is not too

stable. Some of the bridges built like this are Pont des Bergues (built in 1834), are James

Smith’s Micklewood Bridge at Doune in Scotland (it was probably the first one built like

this and had chains instead of cables which also makes it chain bridge). Hammersmith

Bridge has parts of the roadway built in this manner.

“Stressed ribbon bridge” is a modern, improved variant of a classical simple suspension

bridge. It has a rigid deck which lays on suspension cables which are in turn embedded in

the deck. Deck follows a catenary arc between supports and is stressed in traction, which

adds to its stiffness and prevents swaying and bouncing like at simple suspended bridge.

This bridge is usually made of concrete reinforced by steel tensioned cables and can carry

vehicle traffic. Concrete plates are premade and placed to form the initial structure.

Sandbags are place upon the tiles to prestress cables that hold the tiles and gaps between

the tiles are filled with concrete. When the concrete dries, sandbags are removed and

cables compress, stiffening the bridge and making it more durable.

"Self-anchored suspension bridge" has its main cables attached to the ends of the deck

rather than ground like standard suspended bridge which allows for construction on

elevated piers, or in areas of unstable soils where anchors would be loosen over time.

This method of building appeared in mi-19th century and was designed by Austrian

engineer Josef Langer in 1859. American engineer Charles Bender patented this method

in United States in 1867. Earliest bridges built with this method in United States were

Three Sisters Bridges of Pittsburgh, built between 1924 and 1928. Suspension cables

cannot be anchored until the deck is finished with this design so a false-work is used to

hold them until then.

Differences Between Cable Stayed Bridges and

Suspension Bridges

The main difference between cable stayed bridges and suspension bridges is in the way

that they transfer loads from deck to pylon. As depicted in Figure 1, in cable stayed

bridges straight cables transfer deck loads directly to the pylon (Walther et al., 2003, P19-

39). But as shown in Figure 2, in suspension bridges, there are main cables (suspension

cables) that carry vertical cables. These vertical cables behave as restraints for the deck

and transfer deck loads to the main cables.

Figure 1: Cable Stay Bridge (Credit by: Wikipedia)

Figure 2: Suspension Bridge (Credit by: Wikipedia)

Usually main spans of suspension bridges are longer than cable stay bridges; therefore, decks of

the suspension bridges have less stiffness in comparison with cable stay bridges. As a result,

suspension bridges have more vibration concerns. In addition, design and construction of

suspension bridges are more complicated rather than cable stay bridges; and that's the reason

why most of the failures of the cable bridges happened in suspension bridges.

Components of a Suspension Bridge

Construction of various parts-

A suspension bridge should consist of the components shown on the diagram, other elements are

added for aesthetic purposes and design.

Deck: The deck on a suspension bridge is also referred to as a roadway, where vehicles are

allowed to pass to and from points A and B. They can carry motorists, pedestrians, rail traffic

etc. They are made out of steel reinforced concrete and each deck is of a large span.

Steel Cables: The decking or the roadway is suspended by steel cables. They can be as thick as a

tall male human, and are made up of many smaller steel cables; steel is used instead of iron

because it is an alloy, which makes it superior in tension and compression and it is stronger. The

smaller cables are fastened to one another forming one huge cable enough to hold up to 150,000

tonnes.

Suspenders: The suspenders connect the decking to the steel cables and help shape the bridge.

Without the suspenders, the roadway would sway out of control; they help reinforce the decking

even more as well as having steel cables.

Towers: The heavy weight of the steel cables are transferred onto the towers that help the bridge

stay standing; the weight that is now supported by the towers is focused onto the ground,

reinforcing the tower feet into the ground and keeping the bridge upright.

Anchorage Block: These weigh more than the amount of cables that is holding up the deck; this

is because it has to withstand a huge proportion of the roadway. Not only this, but it must be

strong enough to endure the amount of road traffic and vehicles crossing the bridge at any time.

They are often made out of concrete as it is extremely heavy and strong. They appear at both

ends of the bridge and preserve the tension from the steel cables.

Foundation of Tower: because of the weight pushing down on the towers, they must have a

secure foundation. The foundations are pushed far below the soil to keep the towers from tilting

and to make sure that they are vertical and strong enough to withstand the weight from the

cables. Depending on the softness of the soil, depends on how far down the foundations go; if the

soil is soft, then the foundation would be pushed further down.

Truss: The truss if found to be underneath the roadway/ decking to support it. Not only this, but

it helps stiffen the decking which reduces the probability of it swaying vertically just like it did

in the Tacoma Narrows Bridge example.

Raw Materials

Many of the components of a suspension bridge are made of steel. The girders used to make the

deck rigid are one example. Steel is also used for the saddles, or open channels, on which the

cables rest atop a suspension bridge's towers.

When steel is drawn (stretched) into wires, its strength increases; consequently, a relatively

flexible bundle of steel wires is stronger than a solid steel bar of the same diameter. This is the

reason steel cable is used to support suspension bridges. For the Akashi Kaikyo Bridge, a new

low-alloy steel strengthened with silicon was developed; its tensile strength (resistance against

pulling forces) is 12% greater than any previous steel wire formulation. On some suspension

bridges, the steel wires forming the cables have been galvanized (coated with zinc).

The towers of most suspension bridges are made of steel, although a few have been built of steel-

reinforced concrete.

The Manufacturing Process

Construction of a suspension bridge involves sequential construction of the three

Tower constructions that will stand in water begin with caissons (a steel and concrete cylinder that acts as

a circular dam) that are lowered to the ground beneath the water, emptied of water, and filled with

concrete in preparation for the actual towers.

major components: the towers and cable anchorages, the support cable itself, and the deck structure.

Tower construction

1 Tower foundations are prepared by digging down to a sufficiently firm rock formation. Some

bridges are designed so that their towers are built on dry land, which makes construction easier. If

a tower will stand in water, its construction begins with lowering a caisson (a steel and concrete

cylinder that acts as a circular damn) to the ground beneath the water; removing the water from

the caisson's interior allows workers to excavate a foundation without actually working in water.

When the excavation is complete, a concrete tower foundation is formed and poured.

2 Construction details vary with each unique bridge. As an example, consider the Akashi Kaikyo

Bridge. Each of its two steel towers consists of two columns. Each column is composed of 30

vertical blocks (or layers), each of which is 33 ft (10 m)

Anchorages—structures that support the bridge's cables—are massive concrete blocks securely

attached to strong rock formations. When the towers and anchorages have been completed, a pilot

line must be strung along the cable's eventual path, from one anchorage across the towers to the

other anchorage.

tall; each of these blocks, in turn, consists of three horizontal sections. A crane positioned

between the columns lifted three sections into place on each column, completing a layer. After

completing a block on each column, the"bootstrapping" crane was jacked up to the next level,

where it lifted the sections of the next layer into place. At appropriate intervals, diagonal bracing

was added between the columns.

Anchorage construction

3 Anchorages are the structures to which the ends of the bridge's cables are secured. They are

massive concrete blocks securely attached to strong rock formations. During construction of the

anchorages, strong eyebars (steel bars with a circular hole at one end) are embedded in the

concrete. Mounted in front of the anchorage is a spray saddle, which will support the cable at the

point where its individual wire bundles (see Step 5) fan out—each wire bundle will be secured to

one of the anchorage's eyebars.

Cable construction

4 When the towers and anchorages have been completed, a pilot line must be strung along the

cable's eventual path, from one anchorage across the towers to the other anchorage. Various

methods can been used to position the pilot line. For the Niagra River bridge, for example,

Roebling offered a reward of $10 to the first youngster who could fly a kite with a pilot line

attached across the gorge to make the connection. Today, a helicopter might be used. Or the line

might be taken across the expanse by boat and then lifted into position. When the pilot line is in

place, a catwalk is constructed for the bridge's entire length, about 3 ft (1 m) below the pilot line,

so workers can attend to the cable formation.

5 To begin spinning the cable, a large spool of wire is positioned at the anchorage. The free end

of the wire is looped around a strand shoe (a steel channel anchored to an eyebar). Between the

spool and the strand shoe, the wire is looped around a spinning wheel that is mounted on the pilot

line. This wheel carries the wire across the bridge's path, and the wire is looped around a strand

shoe at the other anchorage; the wheel then returns to the first anchorage, laying another strand in

place. The process is repeated until a bundle of the desired number of wire strands is formed (this

varies from about 125 strands to more than 400). During the spinning, workers standing on the

catwalk make sure the wire unwinds smoothly, freeing any kinks. As spools are exhausted, the

end of the wire is spliced to the wire from a new spool, forming a continuous strand. When the

bundle is thick enough, tape or wire straps are applied at intervals

Once the vertical cables are attached to the main support cable, the deck structure must be built in

both directions from the support towers at the correct rate in order to keep the forces on the

towers balanced at all times. A moving crane lifts deck sections into place, where workers attach

them to previously placed sections and to the vertical cables that hang from the main suspension

cables.

to keep the wires together. The wire coming off the spool is cut and secured to the

anchorage. Then the process begins again for the next bundle.

The number of bundles needed for a complete cable varies; on the Golden Gate Bridge it

is 61, and on the Akashi Kaikyo Bridge it is 290. When the proper number have been

spun, a special arrangement of radially positioned jacks is used to compress the bundles

into a compact cable, and steel wire is wrapped around it. Steel clamps are mounted

around the cable at predetermined intervals to serve as anchoring points for the vertical

cables that will connect the decking to the support cable.

Deck construction

6 After vertical cables are attached to the main support cable, the deck structure can be started.

The structure must be built in both directions from the support towers at the correct rate in order

to keep the forces on the towers balanced at all times. In one technique, a moving crane that rolls

atop the main suspension cable lifts deck sections into place, where workers attach them to

previously placed sections and to the vertical cables that hang from the main suspension cables,

extending the completed length. Alternatively, the crane may rest directly on the deck and move

forward as each section is placed.

Finishing

7 When the deck structure is complete, it is covered with a base layer (e.g., steel plates) and

paved over. Painting the steel surfaces and installing electric lines for lighting are examples of

other finishing steps. In addition, ongoing maintenance procedures begin. For example, a

permanent staff of 17 ironworkers and 38 painters continue to work daily on the Golden Gate

Bridge, replacing corroding rivets and other steel components and touching up the paint that

protects the bridge.

Design of suspension bridges-

The Road Deck The road deck of a suspension bridge is very important.

Most deck designs are made from open trusses that allow

wind to pass through. It is important to build the deck

aerodynamically or else it will twist and could snap. One of

Tacoma Narrows Bridge.

The truss work of the deck was

too flexible and it snapped in strong winds.

The Building Steps

1.First huge concrete caissons are sunk into the bedrock to provide a solid

base for the towers.

2.Next the towers are constructed on top of the caissons.

3.Giant anchor points are created on both ends of the bridge to keep

tension in the cables.

4.Then the main cables are strung across the span of the bridge

.

5.A temporary walkway is constructed beneath the main cables so that

construction can begin on the road deck.

6.Suspender cables are put into place as the road deck is built to provide

strength.

7.When the road deck is finished, a layer of concrete is poured over the

steel, followed by a layer of asphalt.

LOADS ON SUSPENSION BRIDGES-

The diagram below shows the tension in the cables of a suspension bridge. These cables are

capable of withstanding tension but offer no resistance to compression. These types of bridges

work in a completely different way to the arch bridge.

Compression The force of compression pushes down on the suspension bridge's deck, but because it is a

suspended roadway, the cables transfer the compression to the towers, which dissipate the

compression directly into the earth where they are firmly entrenched.

Tension

The supporting cables, running between the two anchorages, are the lucky recipients of the tension forces.

The cables are literally stretched from the weight of the bridge and its traffic as they run from anchorage

to anchorage. The anchorages are also under tension, but since they, like the towers, are held firmly to the

earth, the tension they experience is dissipated.

Almost all suspension bridges have, in addition to the cables, a supporting truss system beneath

the bridge deck (a deck truss). This helps to stiffen the deck and reduce the tendency of the

roadway to sway and ripple.

They come in two different designs: the suspension bridge, recognized by the elongated 'M'

shape, and the less-common cable-stayed design, which has more of an 'A' shape.

The cable-stayed bridge does not require two towers and four anchorages as does the suspension

bridge. Instead, the cables are run from the roadway up to a single tower where they are secured.

HOW DO SUSPENSION BRIDGES WORK?

On the Severn Bridge, the two main cables act a bit like a washing line. The tension in a

washing line supports the weight of the clothes that are pegged to it. In the same way, the

tension in the main cables supports the weight of the deck and traffic. The bridge deck is

hung from the main cables using wire hangers (rather than clothes pegs). And because the

main cables are held up by the towers, the weight of the whole bridge is carried down

through the towers, on to the underlying foundations.

If you put something heavy on a washing line, it will sag at that point. With a suspension bridge,

the road is supported by a stiffening girder, which spreads out the weight of the traffic, so

avoiding excessive sag under an exceptional load. If you hang something on a washing line

away from the centre, the point will not only sag but it will also move towards the nearest end

(try it!). Similarly, as a heavy load travels over a suspension bridge, it will not only dip

downwards at the point of the load, it will also move longitudinally towards the nearest tower.

If you stand on the walkway of the Severn Bridge, you can feel it moving as the traffic travels

over it. If you stand by one of the towers and watch the expansion joint, you can sometimes see

the whole bridge moving as the weight of the traffic travels across. We should not worry that the

bridge moves. It is meant to do this. This is how it absorbs the weight of the traffic and transfers

it into the main cables.

Diagram showing the main loads in a suspension bridge

The tension in the main cables carries the whole weight of the bridge deck and the traffic. This

tension is resisted by the anchorages at each end, just as the tension in a washing line is resisted

by whatever it is tied to at each end. And because the main cables are held up by the towers, the

weight of the whole bridge is transferred through the towers to the ground.

Why do bridges collapse?

Bridges don't fail very often, but when they do, the results are spectacular and unforgettable.

Once you've seen the footage of the Tacoma Narrows bridge resonating in a gale bucking back

and forth before the deck breaks up and crashes to the river below, you'll never forget it. Imagine

how terrifying it would have been if you'd been on the bridge at the time!

Bridges always collapse for exactly the same reason: something happens that makes them unable

to balance the forces acting on them. A force becomes too great for one of the components in the

bridge (maybe something as simple as a single rivet or tie-bar), which immediately fails. That

means the load on the bridge suddenly has to be shared by fewer components, so any one of

them might also be pushed beyond its limit. Sooner or later, another component fails, then

another—and so the bridge collapses in a kind of domino effect of failing materials.

Photo: This is the remains of the I-35W Mississippi River bridge a steel-trussed arch bridge that used to

carry a very busy highway over the river. It collapsed unexpectedly in 2007, killing 13 people and

injuring 145 more. A report into the disaster found that a metal plate had ripped along a line of rivets,

causing a catastrophic failure. Ironically, the bridge was carrying a massive extra load of construction

equipment for repairs and reinforcement at the time. Riddled with fatigue cracks and corrosion, it had

been deemed "structurally deficient" as far back as 1990. Photo by Joshua Adam courtesy of US Navy.

There are two different ways in which a bridge component can fail catastrophically: weakness and

fatigue. First, and simplest, it might be too weak to cope with a sudden transient load. If a bridge is

designed to carry no more than 100 cars, but 200 heavy trucks drive onto it instead, that creates a

dangerous, transient load. Or if hurricane-force winds buffet the bridge, twisting the deck much more than

it's designed to cope with, that can be catastrophic too. So a bridge can fail through weakness because a

force exceeds what's called the ultimate tensile strength (the most you can pull) or compressive

strength (the most you can push) of the materials from which it's constructed.

But a bridge can also fail even if the forces on it are relatively modest and well within these limits.

Everyday materials usually have to undergo repeated stresses and strains—for example, a bridge deck is

loaded (when a truck drives across) and then unloaded again immediately afterward, and that can happen

hundreds or thousands of times a day, hundreds of days a year. Just as a paperclip snaps when you

repeatedly bend it back and forth, the endless cycles of stress and strain, flexing and relaxing, can cause

materials to weaken over time through a process known as fatigue. Eventually, something like a metal

cable or tie in a bridge will snap even though it's not experiencing a particularly high stress at that

moment. Fatigue is often compounded by gradual corrosion (rusting) of metal components or what's

informally known as concrete cancer (such as when reinforced concrete cracks after the metal

reinforcing bars inside it start to rust).

Engineers try to protect against bridge failures in two main ways. If we learn to see bridges as "living

structures," constantly aging and being degraded by weather and the environment. it's easy to understand

that they need regular maintenance, just like our homes and bodies. Periodic inspections and preventative

maintenance helps us spot problems and correct them before it's too late. Engineers can also protect

against bridge failure by building in a factor of safety—designing them so they can cope with forces

several times larger than they're ever likely to encounter. That might include extra "redundant"

components or reinforcements so that even if one part of the structure fails, others can safely share the

load until the bridge can be reinforced or repaired.

FAILURES OF SUSPENSION BRIDGE-

Deck Vibration

Due to the low stiffness, light weight, and long spans of cable bridges, the lateral and torsional stiffness of

these bridges are low in comparison with regular non-cable bridges . Usually, cable bridges are built in

vast areas such as rivers, coasts, and valleys. Therefore, they are exposed to wind loads. The speed of the

wind through a bridges varies frequently; in some moments it decreases and in some moments it

increases. If the wind speed variations follows a regular pattern, then the time distance between tow

adjacent peeks of wind speed graph can be called as period of the wind loads. Usually wind loads have

long periods. Because of high stiffness and short natural vibration period of the regular bridges (non-cable

bridges), wind resonance usually cannot happen on them. But, cable bridges have long periods in both

lateral and torsional vibration; therefore, resonance is a design concern in this type of bridge (Miyata,

2003, P1407 and Plaut, 2008, P613-5). Due to the above mentioned difference between cable bridges and

regular bridges (non-cable bridges), a lot of collapses and performance failures have happened since the

cable bridge invention. Tacoma Narrows Bridge (Wiki Failures) and Silver Bridge over Ohio River are

two examples of cable bridge failures which led to complete collapse of structures. In Figure 3 and 4 the

Tacoma Narrows bridge is shown during the fluttering and at the moment of collapse, respectively.

Figure 3: Tacoma Narrows Bridge with severe viberation (Credit by: YouTube)

Figure : Tacoma Narrows Bridge after collapse due to wind fluttering (Credit by: Wikipedia)

At the earlier time of use of cable bridge systems, due to the lack of information about the behavior of this

type of bridge, designers didn't pay enough attention to the deck stiffness of the bridges. As a result, a lot

of bridges were faced with wind-fluttering problems. In bridges with this problem, when the period of

wind impact loads becomes close to the period of deck vibration, they begin to oscillate. This oscillation

can continue to the point of damage or collapse of the bridges.

There are two major solutions for fluttering problems of cable bridges (Miyata, 2003, P1403-5).

a) Modification of the Deck: In this solution the shape and configuration of the bridge deck become

modified to create a more stiff deck (with respect to the length of the span). In addition, the shape of the

deck can be modified to a more aerodynamic shape resulting in less wind load on the deck.

b) Use of Dampers: By use of damper the induced energy to the deck and cables can be damped. In

addition, dampers limit deformation of the deck and by this means keep the deck away from large

deformation.

Low lateral, torsional ,and vertical stiffness of the cable bridges can cause performance failure of the

bridges in some cases. A good example of this type of failure is Millennium Pedestrian Bridge in London

which became closed a few days after opening for fixing its problem. The problem was side vibration of

the bridge deck due to pedestrian walking synchronous lateral excitation. Two main reasons of this

vibration was low lateral stiffness of the deck and low damping potential of the bridge for lateral

movements and deformations. The problem of the Millennium bridge was solved by installation of lateral

dampers (Newland, Cambridge University website). In Figure 5 a view of this bridge is shown. In the

following YouTube link a movie of vibration of this bridge is shown:

Figure : Millennium Bridge in London

Ship Collision with Pylon

Generally, cable bridges have long-spanned deck and high-rise pylons which results in a high distance of

the free-board. With these characteristics, passing of huge vessels is possible through the spans.

Unfortunately, the redundancy of this type of bridge is low; as a result, if one of the pylons fails, all parts

of the bridge will fail. Therefore, soundness of the pylons is an essential factor in stability of the cable

bridges. As we can see in the history of bridges, collision of vessels with piers is one of the most common

reasons of failures. Fortunately, pier collocation evidences before and after the invention of cable bridges

made designers aware to prevent collision with pylons (Svensson, 2009, P21-31). There are two main

solutions to prevent collisions with pylons which are listed in the following:

a) Pier barriers: In this solution, some barriers, which are in fact short columns, become installed around

the pylon. By this means, if a ship mislead through the pylon, it will collide with the barrier instead of

pylon. In Figure 6, the Sunshine Skyway Bridge is shown which is protected by barriers. The old

Sunshine Skyway Bridge collapsed in 1980 due to collision of a vessel with one of the piers of the bridge

(Sayers, 2007, P1) and it cost $244 million to reconstruct a new bridge.

b) Decreasing the number of collision-exposed pylons: Characteristic of cable bridges makes it

possible to have longe spans. Therefore, designers prefer to place pylons out of water, have longer spans,

and by this means prevent collision of vessels with pylons

Tendon System Problems

Like each relatively new material, a tendon has its unknown problems. In addition, long tendons have

different behavior in comparison with short tendons. Fortunately, due to use of multiple-tendon cables,

and high redundancy cable systems, failure of tendons often only results in temporary performance issues

with the bridge. Therefore, by regular control and maintenance of the cables, we can prevent failures,

when without a frequent maintenance procedure we can expect the collapse of the bridge.

One of these failures is rain/wind-induced vibration of cables. If some of the cables become loose or if the

pretension loads in the cables be non-compatible with the dead load distribution, then under the

combination of light wind and rain loads, they began to vibrate like strings (FHWA, 2007, P13-14).

There is three solution for this type of failure (FHWA, 2007, P14-36):

a) Special surface shape: Specially roughed surface of the cables ducts efficiently increases aerodynamic

stability of the cables. In Figure 7 different common shapes of the ducts are depicted.

b) Use of Dampers: By use of dampers, the movement of the cables become limited and the vibration

energy of the cables become damped. Usually, these dampers are install between deck and cables

(perpendicular to the cables direction). In figure 8, use of dampers in Ravenel Bridge is shown.

c) Tie of the cables together: By installing the cross-tie on the cables, internal restraints for the cables

become established, and as a result, transverse stiffness of the cables increases. In Figure 9, a sample

cross-tie is shown.

Figure : Common types of cable duct surface in cable stayed bridges

Figure : Dampers on Ravenel Bridge cables (Credit by: Wikipedia)

Figure: Cross-tie on cables

Another type of failure in the cable systems is cable anchorage failure. Transfer and distribution of

concentrated load in the cables are two main duties of cable anchorages. Due to the changes in the

magnitude of the cable load, fatigue can easily happens in anchorages. Therefore, a regular inspection is

necessary to avoid fracture in in the anchorages. In Figure 10, tearing out of one of the of Sabo Bridge

anchorages is shown (Wiss, 2012, P35).

Figure : Fracture in anchorage of Sabo Bridge

Failure During Construction

Due to the different job site conditions, varying construction procedures, and heavy construction loads, the

construction of cable bridges is very complicated. Many failures occur during the construction phase and the design

group must maintain constant communication with contractors during in this period. The reasons of the failures

during construction can be categorize in the following three major groups:

a) Mistake in evaluation of load of construction: Due to very complex load path and presence of very

concentrated loads in different part of the structure, construction methods of this type of bridges are very

complicated and should be studied by the design group. Some times, design group makes mistake on

evaluation of the construction loads or select a risky method for construction, and these can lead to

collapse of the whole structure.

b) Mistake in selection of suitable construction machinery and mistake of workers: Generally, due to

special construction situations such as high elevation job site, heavy weight of bridge parts, and using

gigantic machinery like cranes, workers feel high pressure on themselves and are stressful during

construction of cable bridges, In these situations, workmen mistakes are more likely to happen. This type

of mistake can cause small failures or total collapse of structures.

c) Natural disasters during construction: Construction period of the cable bridges are long in

comparison with regular bridges. Before installation of all parts of the bridge, the bridge has not too much

redundancy and most of the times decks are hanged like cantilever beams; therefore, if a sever load such

as high wind pressure or earthquake load, which is not considered in the design of the stages of

construction, is induced on the bridge, the structure may have not enough reserved resistance for the

additional load and in this situation the bridge is very susceptible to collapse.

Collapse of Kukar Bridge in Indonesia is one of the most recent cable bridge collapses during

construction. As depicted in Figure 11, the deck of this bridge totally collapsed. The main reason of this

failure was detachment of hanger cables from main cable which was due to mistake in design phase and

overloading of deck during construction (Matsuno, 2007, P3-5).

Figure : Kukar Bridge in Indonesia

On October 1970, West Gate Bridge at Melbourne, with 112 meter main span, collapsed during

construction and killed 35 people. Many factors are mentioned as the causes of the collapse of

this bridge. Most important of them were the unusual erection method used by the contractor,

insufficient study on construction loads and temporary structures, and high work pressure on

construction team due to tight time schedule which increased the risk of human mistakes

Another example of the failures in construction period happened when an earthquake occurred

during construction of Akashi-Kaikyo Bridge in Japan. The main span of this bridge which is

depicted in Figure 12 was 1990 meter. At the time of the earthquake the pylons were built; and

due to earth movement the main span (distance between pylons) increased about 1.3 meter; but,

as the result of good cooperation between design and construction groups, construction

continued to the end without any problem (Nasu et al., 1999, P312).

List of Pros of Suspension Bridges

1.Low Construction Costs

What makes suspension bridges practical is the inexpensiveness of these bridges due to required

materials needed for construction. With three basic necessities such as cables, anchorages and

roadways, suspension bridges are possible to construct. Having said, this, suspension bridges are

great solutions to provide communities with functioning and useful bridges without much need

for funding. These are beneficial in areas that lack infrastructure funds. And in the case of

allotting budget for projects, the inexpensive costs in building these types of bridges can allow

for other projects to be financed.

2. Long Span

Another advantage of suspension bridges is the possibility to construct them at different lengths,

from 2,000 to 13,000 feet and is lengthier than other types of bridges. This makes it possible to

build suspension bridges to connect very long distance locations. Depending on the demand and

possibility given, these bridges can be underspanned like the Pont des Bergues and the

Micklewood Bridge. On the other hand, three long suspension bridges are in Denmark, Japan

and China.

3. Ease of Maintenance

Apart from inexpensive construction costs, suspension bridges are known for their minimal

maintenance requirements. Once construction is completed, there are no immediate needs for

additional materials like cables. What is called for is simply regular maintenance. Moreover, it is

known for durability and longevity, making major repairs not needed as often. Consequently,

maintenance costs are also not that high.

4. Versatility

Suspension bridges do not only cost less to build, they can also be built practically anywhere so

long as there are places for building support towers and anchorages. This is also because of the

design which is suspended in the air, no inflow restrictors are needed to be placed underneath.

They can also bear the beatings of earthquakes.

5. Attractive

Tourists, local and foreign in America love to cross the Brooklyn Bridge and visit the Golden

Gate Bridge in San Francisco. Compared to truss and beam bridges, suspension bridges are more

aesthetically pleasing because of the different shapes of these bridges. The curved and linear

features of these bridges make them structurally beautiful. This is on top of the cables giving

support to these bridges, making them versatile bridges.

4. Has Flexibility

One common reason that the choice to build a suspension bridge is reached is if it is being built

in a high earthquake zone, like California. This is because suspension bridges are flexible due to

the cable system they are held up by. The bridge can “move” with the wind and during natural

disasters such as an earthquake.

5. Simple Construction

No access is needed from below the bridge while it is being constructed, making it a great choice

for areas that ships and waterways need to stay clear.

2. Can Be Built High Up

Suspension bridges can be built very high up over waterways. This is essential for any area that

needs to be able to allow passing ships to come through.

List of Cons of Suspension Bridges

1. Loss of Income

Despite the low costs of constructing suspension bridges and the job opportunities they offer, the

length of time needed to finish building these bridges are long. What happens is that the

businesses that are within the vicinity will be affected since business operations will be

hampered. Consequently, there will be loss of sales and profit. This can have a negative impact

on the economy of the city or town. Also, bridges built to connect locations between bodies of

water can affect the course of ships carrying supplies since they need to divert their routes. This

can also result to loss of money since deliveries of goods can take longer.

2. Weak in Winds

Despite flexibility and strength to withstand earthquakes, these bridges are not too strong when it

comes to powerful winds caused by hurricanes. Too much strong winds can result to damages to

suspension bridges. A classic example is the Tacoma Narrows Bridge which collapsed on

November 7, 1940 in winds of at only 40 miles per hour. Although the disaster was blamed on

design and construction, what happened that time presented risks associated with suspension

bridges.

3. Load Limitations

Another disadvantage of suspension bridges is the material used which are the cables. These

cables have limitations when it comes to bearing the weight of loads. Although it can allow a

minimal weight with regard to vehicles passing through, too much weight can lead to the

breaking of cables

4. Limited Applications

Suspension bridges, despite their cost-effectiveness in construction and flexibility when it comes

to site location, have limitations when it comes to its use. This is because they can be destroyed

by strong winds and not durable enough to hold limitless weight, careful consideration should be

taken before construction. That said, they can only be used by general traffic.

4. Soft Ground Issues

If the suspension bridge needs to be built in an area that has soft ground, like over water, very

extensive foundation work in order to make it safe for heavy loads.

2. Too Flexible

Flexibility of the suspension bridge design is a major advantage, until conditions become severe.

Underneath extreme winds or very heavy the load the bridge can move so much that the bridge

would need to be closed.

3. Cannot Support High Traffic

Since very little support is needed underneath the deck of the bridge, and the cables are light, the

bridges are very light weight. This restricts the amount of traffic and the weight of heavy loads

that can use it.

CONCLUSION

A suspension bridge can be made out of simple materials such as wood and common wire rope.

Longer main spans are achievable than with any other type of bridge

Less material may be required than other bridge types, even at spans they can achieve,

leading to a reduced construction cost

Except for installation of the initial temporary cables, little or no access from below is

required during construction, for example allowing a waterway to remain open while the

bridge is built above

May be better to withstand earthquake movements than heavier and more rigid bridges

Bridge decks can have deck sections replaced in order to widen traffic lanes for larger

vehicles or add additions width for separated cycling/pedestrian paths. Considerable stiffness or aerodynamic profiling may be required to prevent the bridge deck

vibrating under high winds

The relatively low deck stiffness compared to other (non-suspension) types of bridges makes it

more difficult to carry heavy rail traffic where high concentrated live loads occur

Some access below may be required during construction, to lift the initial cables or to lift deck

units. This access can often be avoided in cable-stayed bridge construction.

Due to relatively short age of the cable bridges and the use of high tech materials and machinery

in the construction phase, we should do more study to have thorough understanding of the cable

bridges behaviors .Construction of cable stayed bridges are very expensive, and due to

dimensions of the structure, failure of them can endanger too many people's lives; therefore,

existing bridges are very valuable investments in our hand for more study. In addition, as

discussed in different parts of this article, a lot of failures of the bridges are due to a lack of

regular inspection and maintenance; therefore, by doing of more study on existing bridges, we can

learn more about the characteristics of the cable bridges and prevent failures of the under-study

bridges.

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