Dr.

Babasaheb Ambedkar Technological University, Lonere

ABSTRACT

A bridge is a structure built to span across a valley, road, body of water, or other
physical resistance, to provide passage over an obstacle. Bridges are those marvels in civil
engineering tool kits that help in connecting the places located on another side of the bank.
Varieties of bridges have evolved throughout history. Of them, one is a suspension bridge. It
is constructed to span across a water body or valley. Nowadays these are the pioneers in
bridge technology. Of all the bridge types in use today, the suspension bridge allows for the
longest span ranging from 2,000 to 7,000 feet. Also, they have quite attractive views which
have added to the gloom of suspension bridges.

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Dr. Babasaheb Ambedkar Technological University, Lonere

INTRODUCTION
There are six main types of bridges:-
1. Arch Bridge
2. Beam Bridge
3. Cable-stayed Bridge
4. Cantilever Bridge
5. Truss Bridge
6. Suspension Bridge

Suspension bridge:
The suspension bridge is most commonly built to span across water bodies. It is
built by suspending the roadway from cables attached to a master cable that runs above the
length of the bridge. In addition to being strong and lightweight, suspension bridges are also
beautiful. The design of a suspension bridge is straightforward and takes advantage of several
techniques to distribute the weight of the bridge safely and evenly. The main forces in a
suspension bridge are tension in the main cables and compression in the pillars. Since almost
all the force on the pillars is vertically downwards and they are also stabilized by the main
cables, they can be made quite slender. In a suspended deck bridge, cables suspended via
towers hold up the road deck. The weight is transferred by the cables to the towers, which in
turn transfer the weight to the ground. Most of the weight or load of the bridge is transferred
by the cables to the anchorage systems. These are embedded in either solid rock or huge
concrete blocks. Inside the anchorages, the cables are spread over a large area to evenly
distribute the load and prevent the cables from breaking free.

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Dr. Babasaheb Ambedkar Technological University, Lonere

HISTORY
The basic design of a suspension bridge has been in use for centuries.
Thousands of years ago, people crossed waterways by swinging hand over hand on
suspended cables. Later, these rope cables were replaced by iron which carried more load.
Major bridges were still built using a truss design until 1808 when an American inventor
named James Finley filed a patent on an early version of a suspension bridge. Finley's design
involved stretching two strong chains over the top of several towers and anchoring them on
either side of the bridge. He hung lesser chains from the two master chains and used them to
suspend a rigid deck, and the modern incarnation of the suspension bridge was born. In 1830,
French engineers realized that strongly woven cables were safer than chains, and began to use
them in the construction of suspension bridges. Today all use this cabled design, but the basic
form of the suspension bridge has remained the same, and engineers continue to push the
limits of the spans that suspension bridges can cross. The construction of a bridge seems to be
simple, but engineers' quest for longer-span suspension bridges makes the construction a
challenge.

Famous golden gate bridge in San Francisco

The details are as follows:

Raw materials:
Many of the components of a suspension bridge are made of steel. 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. The towers of most suspension
bridges are made of steel, although a few have been built of RCC.

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Dr. Babasaheb Ambedkar Technological University, Lonere

Design:
Each suspension bridge must be designed individually to take into account many
factors. A thorough survey of topography must be carried out and all the factors affecting the
bridge must be considered. For example, the geology of the site provides a piece of
information about the foundation for the towers and cable types. Also, the seismic activities
in the region must be taken care of, to make the bridge not susceptible to earthquakes. The
depth and nature of the water being bridged (e.g., fresh or saltwater, and the strength of
currents) may affect both the physical design and the choice of materials like protective
coatings for the steel. In waters that are frequently used by ships, the height of the bride also
must be taken into account. In very long bridges, it may be necessary to take the earth's
curvature into account when designing the towers. The timing of the start of work must also
be taken to see that the weather conditions don't hamper the critical work when in progress.

The manufacturing process:

Construction of a suspension bridge involves sequential construction of the towers
that will stand in water. It begins with caissons (steel and concrete cylinder that acts as
circular dam) that are lowered to the ground beneath the water, emptied of water, and filled
with concrete in preparation for the actual towers.

Some of the major components of a bridge are:

1: The towers and cable anchorages

2: The support cable

3. The deck

A) The tower and cable anchorages

(a) The towers:

Tower foundations are prepared by digging down into the earth 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, caissons are sunk and any soft bottom is excavated for a foundation.
Water is removed from the caisson's interior which allows workers to excavate a foundation
without actually working in water. If the bedrock is too deep to be exposed by excavation or
the sinking of a caisson, pilings are driven to the bedrock or into overlying hard soil to
distribute the weight over less resistant soil may be constructed, first preparing the surface
with a bed of compacted gravel. The piers are then extended above water level, where they're
capped with pedestal bases for the towers. When the excavation is complete, a concrete
tower foundation is formed and poured. When the towers are founded on dry land, deep
foundation excavation or pilings are used. From the foundation, towers of single or multiple

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Dr. Babasaheb Ambedkar Technological University, Lonere

columns are erected using high-strength reinforced concrete, stonework, or steel. Concrete is
used most frequently n modern suspension bridge construction due to the high cost of steel.

(b) Cable anchorages (Anchorage construction):

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. Anchorages
are constructed, usually in tandem with the towers, to resist the tension of the cables and form
the main anchor system for the entire structure. These are usually anchored in good quality
rock but may consist of massive reinforced concrete dead weights within an excavation.
During the construction of the anchorages, strong eye bars (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 wire bundles fan out— each wire bundle
will be secured to one of the anchorage's eyes bars. Some of the anchorages are even made
airtight so that they don't come in contact with air, which often results in rust. 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.

Large devices called saddles, which will carry the main suspension cables, are
positioned atop the towers. Typically of cast steel, they can also be manufactured using
riveted forms, and are equipped with rollers to allow the main cables to shift under
construction and normal loads.

One of the prime points while constructing anchorage would be to see that it is strong
enough bears the load of entire cables. Sufficient space must be provided inside the
anchorage to carry out the maintenance work.

B) The support cable:

The main suspension cable in older bridges was often made from a chain or linked
bars, but modern bridge cables are made from multiple strands of wire. This contributes
greater redundancy; a few flawed strands in the hundreds used pose very little threat, whereas
a single bad link or eye bar can cause the failure of the entire bridge. Another reason is that as
spans increased, engineers were unable to lift larger chains into position, whereas wire strand
cables can be largely prepared in mid-air from a temporary walkway. The cables are made of
thousands of individual steel wires bound tightly together. Steel, which is very strong under
tension, is an ideal material for cables; a single steel wire, only 0.1 inches thick, can support
over half a ton without breaking. 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 be used to position the pilot line. 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, the temporarily suspended walkways, called
catwalks, are erected using a set of guide wires hoisted into place via winches positioned atop
the towers. 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. These catwalks follow the curve set
by bridge designers for the main cables, which are accordingly to the shape of a bridge.

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Dr. Babasaheb Ambedkar Technological University, Lonere

Typical catwalks are usually between eight and ten feet wide and are constructed using wire
grates and wood slats.

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 eye
bar). 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 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 numbers 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.

C) The deck:

Most suspension bridges have open truss structures to support the roadbed, particularly
owing to the unfavourable effects of using plate girders, discovered from the Tacoma
Narrows Bridge (1940) bridge collapse. Recent developments in bridge aerodynamics have
allowed the reintroduction of plate structures. This type of construction is to be used without
the danger of vortex shedding and consequent aero elastic effects, such as those that
destroyed the original Tacoma Narrows Bridge 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 to keep the forces on the towers balanced at all
times. The wire used in suspension bridge construction is a galvanized steel wire that has
been coated with corrosion inhibitors.

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 to keep the forces on the towers balanced at all times. In one technique, a moving crane
rolls atop the main suspension cable lift deck sections into place, where workers attach them

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Dr. Babasaheb Ambedkar Technological University, Lonere

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. At specific points along the main cable (each
being the exact distance horizontally about the next) devices called "cable bands" are
installed to carry steel wire ropes called Suspender cables. Each suspender cable is
engineered and cut to precise lengths and is looped over the cable bands. In some bridges,
where the towers are close to or on the shore, the suspender cables may be applied only to the
central span.

Special lifting hoists attached to the suspenders or from the main cables are used to
lift prefabricated sections of the bridge deck to the proper level, provided that the local
conditions allow the sections to be carried below the bridge by barge or other means.
Otherwise, a traveling cantilever derrick may be used to extend the deck one section at a time
starting from the towers and working outward. If the addition of the deck structure extends
from the towers the finished portions of the deck will pitch upward rather sharply, as there is
no downward force in the centre of the span. Upon completion of the deck the added load
will pull the main cables into an arc mathematically described as a parabola, while the arc of
the deck will be as the designer intended usually a gentle upward arc for added clearance if
over a shipping channel, or flat in other cases such as a span over a canyon. Arched
suspension spans also give the structure more rigidity and strength.

The design of the deck also plays a major role in defining the stability of the
bridge. s If too low then it may hit the ships and if too high then it may not resist even low
turbulent air. If decks sway a bit then it may even result in large-scale destruction. Thus the
decks must be designed such that they are lightweight and can carry a heavy load. A
sufficient passage must be given for air to pass through them, such that it offers minimum
resistance. Also, space must be provided below the deck to carry out the maintenance work.
For this purpose box or triangular-based deck are most advisable. In the case of long-span
bridges, the deck must have a slight amount of swing to resist the turbulent air. One of the
major issues during design would be to see that the deck is given some limit to bulge so that
it won't collapse when the load imposed on it by the traffic above can be absorbed if it
exceeds the safe limit.

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Dr. Babasaheb Ambedkar Technological University, Lonere

Types of Suspension Bridge:

The followings are the types of suspension bridges:

1. Simple Suspension Bridge:

These bridges are ordinarily created with materials like a rope. These bridges do not
have towers or any huge foundations. Because of this, simple suspension bridges are not
effective. The materials used are simply swayed by wind most cannot last a natural
disaster of any kind. This can be why simple suspension bridges are accustomed to cowl
smaller distances and are not as well-liked as other suspension bridges.

Simple suspension bridge

2. Under-spanned Suspension Bridge:

The under-spanned span may be a rare variety of spans that was created within the
early 19th century. This bridge is comparable to the simple suspension bridge, but its
trusses are situated below the deck which will increase strength. This bridge also solely
spans smaller areas and is usually made up of iron.

Kellams Bridge

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Dr. Babasaheb Ambedkar Technological University, Lonere

3. Stressed Ribbon Bridge:

This Bridge contains cables embedded within the deck and contains a touch of an
arch. This span is stressed in traction which permits it to be a lot of durable than an easy
span. This bridge is created from steel rope, concrete, or treated wood.

Stressed Ribbon Bridge

4. Self-Anchored Suspension Bridge:

This span is extremely like common trendy suspension bridges. It is one major
distinction that is not anchored to concrete or rock-like different bridges, this bridge has
cables that are anchored to its deck. This puts a lot of tension on the bridge itself which
makes it less reliable within the finish.

Gushan Bridge

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5. Cable-stayed Suspension bridge:

The cable-stayed bridge, just like the span, supports the route with huge steel cables,
however in a very different manner. The cables run directly from the route up to a tower,
forming a singular "A" form.

Cable-stayed Suspension Bridge

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 Precautions while constructing suspension bridge:

1. The foundation type and depth must be the major attention that needs to be taken care of
since the entire load of the bridge are being transferred into the ground utilizing towers. The
towers must be designed such that it becomes easy for the maintainers to climb the tower for
purpose of painting etc. In the case of seismic-prone regions, steel towers are advisable since
they have an allowable swing on either side so the towers don't collapse during a sustainable
earthquake. Also, huge pendulums can be fixed within towers which help in maintaining the
tranquillity of the bridge, whenever there is an earthquake, by swaying in opposite direction
to that of the towers using hydraulics.

2. It must be accounted that the anchorages are strong enough to withstand the stress and
strain imposed on them by the master and suspended cables. Another major issue regarding
anchorages would be addressing the impact of weather conditions which often result in
rusting of the cables within anchorages. So it is evitable to make the entire anchorage airtight.

3. Once the master and suspended cables are in place the deck must be placed in position.
The hydraulics which moves the deck should be sufficiently lubricated so that they will not
fail in between, as it consumes a huge amount of time to repair them. As a precautionary
measure, the navigation of ships must be stopped when the decks are being placed. A major
issue that needs to be addressed during the placing of decks would be the safety of workers.

4. Three kinds of forces operate on any bridge: the dead load, the live load, and the dynamic
load. Dead load refers to the weight of the bridge itself. Like any other structure, a bridge
tends to collapse simply because of the gravitational forces acting on the materials from
which the bridge is made. Live load refers to traffic that moves across the bridge as well as
normal environmental factors such as changes in temperature, precipitation, and winds.
Dynamic load refers to environmental factors that go beyond normal weather conditions,
factors such as sudden gusts of wind and earthquakes. All three factors must be taken into
consideration when building a bridge.

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 Advantages of a suspension bridge:-

1. The center span may be made very long in proportion to the number of materials required,
allowing the bridge to economically span a very wide canyon or waterway.

2. It can be built high over water to allow the passage of very tall ships.

3. Neither temporary central supports nor access from beneath is required for construction,
allowing it to span a deep rift or busy or turbulent waterway.

4. being relatively flexible it can flex under severe wind and seismic conditions, whereas a
more rigid bridge would have to be made much stronger and so also heavier. Akashi kaikyo
bridge-Japan, the world's longest suspension bridge, its span extends about 1990mts.

 Limitations compared to other bridge types:-

1. Considerable stiffness or aerodynamic profiling may be required to prevent the bridge deck
from vibrating under high winds.

2. The relatively low deck stiffness compared to other types of bridges makes it more difficult
to carry heavy rail traffic where highly concentrated live loads occur.

3. Under severe wind loading, the towers exert a large torque force in the ground and thus
require very expensive foundation work when building on soft ground.

4. Considerable stiffness or aerodynamic profiling may be required to prevent the bridge

deck from vibrating under high winds.

5. The relatively low deck stiffness compared to other (non-suspension) types of bridges
makes it more difficult to carry heavy rail traffic where highly concentrated live loads occur.

6. 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

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Forces/ load acting on suspension bridge:

External Forces:
Two kinds of external forces operate on any bridge; the static (dead) load, and the dynamic
(live) load.

1. The static (dead) load refers to the weight of the bridge itself. Like any other
structure, a bridge tends to collapse simply because of the gravitational forces acting
on the materials from which the bridge is made.
2. The dynamic (live) load refers to traffic that moves across the bridge as well as
normal environmental factors such as changes in temperature, precipitation, winds,
and extreme environmental factors such as natural disasters.

Both factors must be taken into consideration when building a bridge. If not taken into
account, a suspension bridge will easily break down and can cause extreme danger to citizens
nearby.

Internal Forces:
There are four main types of internal forces acting upon suspension bridges; tension,
compression, torsion, and shear.

1. Tension: Tension is the pulling force that acts on the cables and suspenders of a
suspension bridge. Think about pulling an elastic band, you can see that a force is
acting on the band as you pull it. This is tension.
2. Compression: Compression is a pressing force that acts on the towers of a suspension
bridge as they are acted upon by gravity. Think about springs, as you compress a
spring it becomes smaller and the particles get closer together. This is similar to what
force is being exerted on the bridge.
3. Torsion: Torsion is a twisting force that can be seen when suspension bridges are put
under high wind speeds. The bridge will begin to move and twist if torsion is not
taken into consideration. An example of torsion could be wringing a cloth.
4. Shear: Shear forces occur when forces push or pull in opposite directions. This can
cause an object to bend or break. An example of shear force can be a tree that is
planted to the ground being hit by a strong gust of wind horizontally. This will cause
the tree to break. In a suspension bridge, the towers and cables can experience
opposite forces (cable pulling left/right while towers compress down) and if this is not
taken into account, the bridge will completely bend and break.

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CONCLUSION

These are the pinnacles of modern-day bridge technology. Longer spans of up to 2000
ft-7000 ft. are possible. They are ideal for covering busy waterways such as Gulf, Strait,
Lake, etc. These bridges are mainly meant for light & heavy roadways rather than railways.
The main forces in a suspension bridge are tension in the main cables and compression in the
pillars. In the future suspension bridges can be the tools that will test the engineer’s limits.

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REFERENCES:-

 www.google.com
 www.wikipedia.com
 www.studymafia.org
 www.pptplanet.com

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