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IJCSIET--International Journal of Computer Science inf ormation and Engg., Technologies ISSN 2277-4408 || 01102015-005
DESIGN OF CABLE – STAYED
RAILWAY BRIDGE
SARATH BABU J,ROLL NO:137K1D8716
M-TECH STRUCTURAL ENGINEERING DJR COLLEGE OF ENGINEERING &TECHNOLOGY
UNDER THE GUIDENCE OF
L.NAGARAJA (PH.D)
ABSTRACT
A bridge is a structure built to span physical obstacles
such as a body of water, valley, or road, for the purpose
of providing passage over the obstacle. Designs of
bridges vary depending on the function of the bridge, the
nature of the terrain where the bridge is constructed; the
material used to make it and the funds available to build
it. Bridge building is a complex art and science and
involves extensive knowledge, skill and expertise. It is
engineering in itself. Among the various kinds of
bridges existing cable –stayed bridges are found to be
highly economical and have great aesthetic appeal. These
are the preferred bridges for long spans in the recent
days. Cable – stayed bridges are easy to construct and
maintain. The history of bridges, bridge components and
bridge terminology has been clearly discussed. General
steps involved in any bridge construction are mentioned.
A brief literature report of cable – stayed bridges is also
presented.
In this report we designed a cable – stayed railway
bridge on River Gauthami between Yanam and
Yedurlanka. The various survey data has been collected
and thoroughly analyzed before proceeding to the design
of the bridge. A drawing of the alignment is also shown
in this report. The catchment area maps have been
studied before determining the hydrographical
particulars. The bridge is designed as per the IRS code
for MBG two tracks. Two plate girders are used and truss
shaped cross girders are placed at suitable spacing. Steel
pylons and cables are designed to support the deck and
transfer the forces effectively. The scour depth is
calculated in order to calculate the depth of the pile
foundation adopted. The bore – hole log and the soil
characteristics at different depths are studied before
determining the bearing capacity and type of foundation.
A cost estimate is made considering the material costs as
well as the construction costs. Finally the advantages of a
cable – stayed bridge for longer spans over other kinds of
bridges are discussed.
It is found that cable – stayed bridges have the following
advantages:
1) Greater stiffness than the suspension bridge,
so that deformations of the deck under live
loads are reduced.
2) Can be constructed by cantilevering out from
the tower - the cables act both as temporary
and permanent supports to the bridge deck.
3) It is very economical and has a huge aesthetic
appeal especially if very long spans are
involved.
1.1. INTRODUCTION
Bridges are defined as structures, which provide a
connection or passage over a gap without blocking the
opening or passageway beneath. They can be over
streams, canals and rivers, creeks and valleys or roads
and railways passing beneath. These days‟ bridges are
also being constructed over oceans to connect two or
more islands. The structure can be for passage/ carriage
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of persons, cattle, vehicles, water or other materials
carried across in pipes or conveyers. Bridges are a civil
engineering creation that holds enormous appeal and
fascination to the people. Bridge building is as old as
civil engineering. The design of bridges depends on
various factors like function of the bridge, the nature of
the terrain where the bridge is constructed, the material
used to make it and the funds available to build it etc…
The concept of bridge building came into existence
with the felling of wooden logs across small streams due
to natural forces. With the growth of civilizations, the
need for travel impelled mankind to find ways and means
of bridging gaps over deep gorges and perennial streams,
for walking across. Owing to the above fact timber can
be considered as the earliest material to be used for
bridging. This has been followed by bridges built with
stone and then of brick, used by themselves or in
combination with timber. Such bridges however have
been possible only for short spans. But with the
development of steel and iron as construction materials
bridge engineering has also expanded its horizons to span
longer distances.
Design of a bridge is an art involving immense
knowledge, skill and experience. It is a highly tedious job
which requires through expertise in all branches of civil
engineering like surveying, transportation, structural
engineering, geotechnical engineering material science
etc… There are three dimensions involved in the
planning of huge structures which are designed for a
period of 50 to 100 years. They are:
1) Scientific dimension
2) Social dimension
3) Technological dimension
Scientific dimension implies that every structure has
to perform in accordance with laws of nature. These laws
of nature are interpreted by scientists as formulas
containing relationships between various basic elements,
and engineers make use of such pre – existing formulas
to design the structures. Though the method of analysis
may differ depending on the structure and practice the
ultimate concept of design remains the same. The
scientific dimension helps the engineer in evolving
efficient structures.
Bridges are built for improving the mobility of people
and enhancing the quality of life of the society. Such man
– made structures may have some adverse effects on the
environment. Therefore bridges should satisfy both the
immediate and future demands of mobility and also be
acceptable to people in terms of visibility, noise and
pollution during and after construction. As construction
of bridges is a public welfare program, the society has to
pay for the cost of the structure in the form of taxes and
tolls. These aspects form the social dimension of any
project.
Technological dimension deals with the major
technological developments in evolution in different
forms of structures, materials of construction, design and
construction techniques and also machinery and plants
used for construction. Technology played a vital role in
finding and refining a number of alternative materials for
use in bridge building, like bricks, cast iron, wrought
iron, steel, cement etc… Because of such technological
research and advances bridges of longer spans at
challenging locations have become possible. A
technological development in the design and manufacture
of vehicles has also led to a need to increase the strength
and geometrical requirements of the bridges being built
as well as their standards of maintenance.
However for a structural engineer, the scientific
dimension is of primary importance, but it is also
necessary to balance the other two dimensions before,
during and after construction of the structure. It is the
responsibility of a structural engineer to evolve a form of
structure which is socially acceptable and at the same
time results in an economic, durable and efficient
product. For this he/she has to make use of the
technological developments in an optimal manner.
Some of the famous bridges in the world are listed
below:
1) Golden Gate Bridge, San Francisco (Fig 1)
2) Millau Bridge, France
3) Tower Bridge, London
4) Akashi-Kaikyo Bridge, Japan (Fig 3)
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5) Sydney Harbor Bridge, Australia
6) Brooklyn Bridge, New York
7) Howrah Bridge, India (Fig 2)
8) Chapel Bridge, Switzerland
9) Godavari Bridge, India
10) Antioch Bridge, USA
11) Royal Victoria Dock Bridge, London
Fig. 1 Golden Gate Bridge, San Francisco Fig. 2
Howrah Bridge, Calcutta
Fig. 3 Akashi-Kaikyo Bridge, Japan
1.2. BASIC BRIDGE FORMS
There are six basic forms of bridge structures:
1) Beam bridges
2) Truss bridges
3) Arch bridges
4) Cantilever bridges
5) Suspension bridges
6) Cable stayed bridges
Beam bridges are horizontal beams supported at each end
by abutments, hence their structural name of simply
supported. A beam bridge carries vertical loads by
flexure.
A truss bridge is a bridge composed of connected
elements (typically straight) which may be stressed
from tension, compression, or sometimes both in
response to dynamic loads. The truss bridge of simple
span behaves like a beam because it carries vertical loads
by bending. The top chords are in compression, and the
bottom chords are in tension, while the vertical and
diagonal members are either in tension or compression
depending on their orientation.
Loads are carried primarily in compression by the
arch bridge, with the reactions at the supports (springing)
being both vertical and horizontal forces.
A cantilever bridge generally consists of three spans,
of which the outer spans, known as anchor spans, are
anchored down to the shore, and these cantilever over the
channel. A suspended span is rested at the ends of the
two cantilevers, and acts as a simply supported beam or
truss. The cantilevers carry their loads by tension in the
upper chords and compression in the lower chords.
A suspension bridge carries vertical loads from the
deck through curved cables in tension. These loads are
transferred to the ground through towers and through
anchorages.
In cable stayed bridge, the vertical loads on the deck
are carried by the nearly straight inclined cables which
are in tension. The towers transfer the cable forces to the
foundation through vertical compression. The tensile
forces in the stay cables induce horizontal compression in
the deck.
1.3. BRIDGE COMPONENTS
The main components of a bridge structure are:
1) Decking, consisting of deck slab, girders,
trusses, etc.;
2) Bearings for the decking;
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3) Abutments and piers;
4) Foundations for the abutments and the piers:
5) River training works, like revetment for
slopes for embankments at abutments, and
aprons at river bed level;
6) Approaches to the bridge to connect the
bridge proper to the roads on either side; and
7) Handrails, parapets and guard stones.
Some of the components of a typical bridge are shown in
the figure below:
Fig. 4 Components of a typical bridge
The components above the level of bearings are
grouped as superstructure, while the parts below the
bearing level are classed as substructure. The portion
below the bed level of a river bridge is called the
foundation. The components below the bearing and
above the foundation are often referred as substructure.
1.4. BRIDGE TERMINOLOGY
An important first step in understanding the
principles and processes of bridge construction is
learning basic bridge terminology. Although bridges vary
widely in material and design, there are many
components that are common to all bridges. In general,
these components may be classified either as parts of a
bridge superstructure or as parts of a bridge substructure.
Fig. 5 Parts of Bridge
SUPERSTRUCTURE
The superstructure consists of the components that
actually span the obstacle the bridge is intended to cross
and includes the following:
1) Bridge deck
2) Structural members
3) Parapets (bridge railings), handrails,
sidewalk, lighting and some drainage features
The top surface of a bridge which carries the traffic is
called deck. The deck is the roadway portion of a bridge,
including shoulders. Most bridge decks are constructed
as reinforced concrete slabs, but timber decks are
occasionally used in rural areas and open-grid steel decks
are used in some movable bridge designs (Bascule
Bridge). As polymers and fibre technologies are
improving in the recent days, Fibre Reinforced Polymer
(FRP) decks are also being used these days. Bridge decks
are required to conform to the grade of the approach
roadway so that there is no bump or dip as a vehicle
crosses onto or off of the bridge.
The most common causes of premature deck failure are:
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1) Insufficient concrete strength from an
improper mix design, too much water,
improper amounts of air entraining
admixtures, segregation, or improper curing.
2) Improper concrete placement, such as failure
to consolidate the mix as the concrete is
placed, pouring the concrete so slowly that
the concrete begins the initial set, or not
maintaining a placement rate in accordance.
3) Insufficient concrete cover due to improper
screed settings or incorrect installation of the
deck forms and/or reinforcement
A bridge deck is usually supported by structural
members. The most common types are:
1) Steel I-beams and girders
2) Pre - cast, pre - stressed, reinforced concrete
bulb T beams
3) Pre - cast, pre - stressed, reinforced concrete I
beams
4) Pre - cast, pre - stressed, concrete box beams
5) Reinforced concrete slabs
Fig. 6 Bridge Deck
Secondary members called diaphragms are used as
cross-braces between the main structural members and
are also part of the superstructure. Bracing that spans
between the main beams or girders of a bridge or viaduct
and assists in the distribution of loads is called
diaphragm.
Parapets (bridge railings); handrails, sidewalks,
lighting, and drainage features have little to do with the
structural strength of a bridge, but are important aesthetic
and safety items. The materials and workmanship that go
into the construction of these features require the same
inspection effort as any other phase of the work.
SUBSTRUCTURE
The substructure consists of all of the parts that support
the superstructure. The main components are:
1) Abutments or end-bents,
2) Piers or interior bents,
3) Footings
4) Piling.
Abutments support the extreme ends of the bridge and
confine the approach embankment, allowing the
embankment to be built up to grade with the planned
bridge deck. When a bridge is too long to be supported
by abutments alone, piers or interior bents are built to
provide intermediate support. Although the terms may be
used interchangeably, a pier generally is built as a solid
wall, while bents are usually built with columns.
The top part of abutments, piers, and bents is called
the cap. The structural members rest on raised, pedestal-
like areas on top of the cap called the bridge seats. The
devices that are used to connect the structural members
to the bridge seats are called shoes or bearings.
Abutments, bents, and piers are typically built on spread
footings. Spread footings are large blocks of reinforced
concrete that provide a solid base for the substructure and
anchor the substructure against lateral movements.
Footings also serve to transmit loads borne by the
substructure to the underlying foundation material.
When the soils beneath a footing are not capable of
supporting the weight of the structure above the soil,
bearing failure occurs. The foundation shifts or sinks
under the load, causing structure movement and damage.
In areas where bearing failure is likely, footings are built
on foundation piling. These load bearing members are
driven deep into the ground at footing locations to
stabilize the footing foundation. Piling transmits loads
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from the substructure units down to underlying layers of
soil or rock.
Fig. 7 Abutment Types
SPANS AND SPAN LENGTH
The terms bridge and span are used interchangeable;
however, to avoid confusion and misunderstanding,
Technicians and construction personnel draw a
distinction between the two.
A bridge is made up of one or more spans. A span is a
segment of a bridge that crosses from one substructure
unit to the next, from abutment to abutment, from
abutment to pier, from pier to pier, or from pier to
abutment. Span length refers to either the length of any
individual span within the structure or to the total bridge
length. In most cases, span lengths are considered as the
distance between centrelines of bearing from one
substructure unit to the next.
The three basic types of spans are shown below. Any of
these spans may be constructed using beams, girders or
trusses.
Simple Span: A span in which the effective length is the
same as the length of the spanning structure. The
spanning superstructure extends from one vertical
support, abutment or pier, to another, without crossing
over an intermediate support or creating a cantilever.
Continuous Span: A superstructure which extends as
one piece over multiple supports is called a continuous
span.
Cantilever Span: A cantilever span is a span which
projects beyond a supporting column or wall and is
counterbalanced and/or supported at only one end.
Fig. 8 Types of Spans
2. THERITICAL ASPECTS
2.1. HISTORY OF BRIDGES
The history of development of bridge construction is
closely linked with the history of human civilization. The
efficiency and sophistication of design and the ingenious
construction procedures kept pace with the advances in
science, materials and technology. Since ancient times,
bridges have been the most visible testimony to the
contribution of engineers. Bridges have always figured
prominently in human history . Nature fashioned the first
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bridges. The tree fallen accidentally across a stream was
the earliest example of a beam type bridge. Similarly, the
natural rock arch formed by erosion of the loose soil
below and the creepers hanging from tree to tree
allowing monkeys to cross from one bank to the other
were the earliest forebears of the arch and the suspension
bridges, respectively. The primitive man imitated nature
and learnt to build beam and suspension bridges.
The earliest reference to a man made bridge goes as far
as 3306 BC, an 1100 – m – long wooden bridge built in
England. The oldest bridge still standing is a stone slab
pedestrian bridge across river Meles in Smyrna, Turkey
said to be 2500 years old. Swiss were the pioneers of
timber bridges, specially using trestle form. In known
history, the Chinese appear to be the earliest to build
stone bridges. Romans are believed to have built bridges
and aqueducts for carriage of water before even the start
of the first millennium. Romans are also credited to have
used timber pile bents for foundation and piers as early as
95 BC. Queen Nitocrin built a bridge in stone piers and
wooden deck in about 780 BC. Iron and lead were used
in this bridge to bind the stones together. Gordon River
Bridge built in 13 BC in France was a masonry aqueduct
49 m high, with three rows of superposed arches.
Etruscans are believed to have used vaults for bridge
construction as early as 600 BC. Europe is considered to
be one of the birthplaces of bridge design and
technology. Therefore it may also be said that they must
have been the earliest to develop bridge building as a
technique.
Gradually the Roman Bridge building art spread to
Middle East as far as India. Macro Polo is said to have
remarked „Indian cultures adopted their own tools under
this influence for bridge building and further developed
suspension bridges‟. Indians have built suspension
bridges with use of ropes for suspension and bamboo and
timber planks for decks in the hilly regions from early
days. They are also credited to have built cantilever type
of bridges laying stone slabs one over the other in a
progressive manner to bridge gaps, but have kept no
records. Russians used timber as main bridge building
material until the end of 15th century. China has built
some notable bridges using tied arch form and cable
stayed bridges. Two elegant examples are Dagu Bridge at
Tianjin and a railway bridge at high altitude on their
recently opened rail link to Tibet.
In the medieval times church has greatly influenced
bridge building. All the bridges in this age have been
built with stone, brick masonry and timber using
empirical methods for design. A typical example is the
first London Bridge built by Peter of Colechurch in 1176
– 1209 AD. This was masonry with 19 pointed masonry
arches on piers, none of them with same dimensions.
Wittengen Bridge built in 1758 in Germany was the
longest timber bridge in Europe with a span of 119 m in
those days.
It was during Renaissance period that the concept of
bridge building based on scientific basis came into
existence. The truss system based on the principle of
triangles, which cannot be deformed, was developed.
Andrea Pallaido (1508 – 1580 AD), evolved several truss
forms, including the king post type. Verrazino (1615)
had written about roads, machines, water wheels, bridges
including masonry arches with use of pre - stressing rods,
as well as suspension bridges and the use of iron bars for
suspension bridges. First metal bridge was
Coalbrookdale Bridge built in cast iron in the year 1776.
James was the person who patented suspension bridge
form and built some with steel chains. French Engineer
Vicat invented the aerial spun cables for suspension. This
type has become the major form for building longer and
longer spans today.
The industrial revolution ushered in the use of iron in
bridges in place of stone and timber. The first iron bridge
was built at Coalbrookdale in 1779 over the Severn in
England by Abraham Darby and John Wilkinson. It
consisted of five semicircular arch ribs in cast iron,
joined together side by side to form a single arch span of
30 m. The construction details of Iron Bridge followed
the spirit of timber and masonry construction practices.
Wrought iron replaced cast iron in bridge construction
during the period 1880 – 90. Wrought iron was ductile,
malleable and strong in tension. In 1808, James Finley in
Pennsylvania patented a design for suspension bridge
with wrought iron chain cables and level floor. Wrought
iron chains were used for a suspension bridge built by
Thomas Telford across the Menai Straits in Wales in
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1826 with a record – breaking span of 177 m. The Menai
Straits Bridge was the world‟s first iron suspension
bridge for vehicles and also the world‟s first iron
suspension bridge over sea water. Japanese also built iron
bridges in the same period. The longest cable suspension
bridge, Akashi Kaikyo Bridge, with a record span of
1991 m was built by them. Germany was the first to
introduce the concept of cantilever construction in the
modern days and incremental launching of concrete
decks, as well as the modern form of cable stayed
bridges.
Though steel is said to have been known in China by 200
BC and in India by 500 BC, its widespread use
materialized only in the latter half of nineteenth century
after the discovery of the Bessemer process in 1856. Eads
Bridge at St. Louis was the first bridge to be built with
extensive use of steel, as early as 1874. Firth of Forth
Railway Bridge in Scotland followed suit, with use of
tubular steel sections for main girders and columns. This
design had been appreciated for the bold attempt made to
span such lengths and shaping the structure so as to
follow clearly the force lines and giving an elegant look
for a viewer. Trend in 18 th and 19 th centuries for longer
span bridges especially in USA tended towards cable
stayed suspension bridges. The Golden Gate Bridge in
San Francisco, built in 1973 is the most famous of this
type. Use of wrought iron and steel as basic materials
instead of masonry and timber has revolutionized bridge
building for many centuries till the arrival of pre -
stressed concrete. The first Portland cement concrete
bridge to be built was the Grand Maitre Aqueduct across
River Vane in France built in 1867 – 1874. France is also
the birthplace of pre - stressed concrete, which is the
major form of bridge superstructures all over the world
today either by itself or in combination with steel.
The world‟s first modern cantilever bridge was built in
1867 by Heinrich Gerber across the river Main at
Hassfurt, Germany, with a main span of 129 m. The
world‟s most famous cantilever bridge is the Firth of
Forth Bridge in Scotland. The world‟s longest span
cantilever bridge was built in 1917 at Quebec, over St.
Lawrence River, with a main span of 549 m. The first
attempt to construct this bridge ended in failure due to
miscalculation of the dead load and buckling of the web
plates of the structure was rebuilt. The Howrah Bridge
over the Hooghly River at Kolkata, built in 1943 with a
main span of 457 m, has elegant aesthetics and possesses
pleasing proportions among the suspended span,
cantilever arms and anchor spans. It was a notable
achievement at the time of construction. Developments in
welding technology and precision gas cutting techniques
in the post Second World War period facilitated the
economical fabrication of monolithic structural steel box
girders characterized by the use of thin stiffened plates
and the closed form of cross section.
Franklin D. Roosevelt once said „there can be little
doubt that in many ways the story of bridge building is
the story of civilization. By it, we can readily measure a
progress I each particular country‟. Based on this saying,
the Indian civilization being one of the oldest, must have
built bridges well before Christian era. According to
records of Chinese travelers on Indian history, India
appears to have had a number of bridges. Firoze Shah
who ruled in Delhi is said to have built canals and
bridges. One can still see some old masonry arch bridges
built by the Portuguese in 16th or 17 th century in Goa.
One old bridge still in use is the stone slab bridge across
River Cauvery at Srirangapatnam built by Tipu Sultan.
India also has a number of old masonry and stone arch
bridges built in the middle of the 19th century on the
Railways, which bear testimony to the skill of the local
people in bridge construction. The British who built the
railways have brought the steel bridge girders and their
designs form UK, but they depended on the local skills
and expertise to build the others. Structural forms and
designs for longer spans also appear to have come from
the British. The technical knowledge within the country
has since kept pace with the developments abroad. The
use of reinforced concrete for road bridges has become
popular in India since the beginning of the twentieth
century. The bridge types adopted include simply
supported slabs, simply supported T – beam span,
balanced cantilever with suspended spans, arch and bow
string girder and continuous or framed structures. The
Third Godavari Railway Bridge built in 1996 with 28
spans of 97.5 m is a recent example of elegant concrete
bowstring girder bridges.
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A number of cable stayed bridges have been built in
India in the past two decades, the major one being the
VidhyasagarSethu across Hooghly at Kolkata and the
Naini Bridge on River Jamuna at Allahabad. The
railways are building a number of major bridges
including a large steel arch bridge in Jammu and
Kashmir. The Border Roads Organization has erected a
cable stayed bridge using Bailey bridge girders in early
part of this millennium, which bridge is claimed to be
only bridge of the type at highest altitude in the world at
the time of construction.
Fig. 9 Millennium Bridge, London (Steel Suspension
Bridge)
Fig. 10 Godavari Bridge, India (Steel Arch Bridge)
2.2. CLASSIFICATION OF BRIDGES
Bridge may be classified into different types depending
on various factors as listed below:
Function: Based on the purpose for which a
bridge is constructed it may be classified as
follows
Foot
Road
Railway
Road – cum – rail
Pipe line
Water conveying
(aqueduct)
Jetty
Material: Based on the material used for
construction bridges may be divided into the
following type
Stone
Brick
Timber
Steel
Concrete
Composite
Aluminium
Fibre
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Form: Based on the form of the superstructure
bridges may be classified as
Beam
Arch
Truss
Suspension
Cable stayed
Cantilever
Type of support: Bridges are also classified
based on the type of the structure it is
supported with
Simply supported
Continuous
Cantilever
Position of floor/ deck: The deck plays an
important role in classification of bridges.
Deck
Through
Semi – through
Usage: The time period for which a particular
bridge is used also aids in division of bridges
into different types.
Temporary
Permanent
Service (Army)
With respect to water level: They are
classified as follows
Causeway
Submersible
High level (normal case)
Grade separators: The purpose of separation
classifies bridges as
Road – over
Road under (subway)
Flyover (road over road)
With respect to connections: The joints used
in the bridge greatly affect the functioning and
analysis of bridge and based on this they may
be classified as
Pin jointed
Riveted/ bolted
Welded
Temporary Bridges: There are also many
types of temporary bridges.
Pontoon
Bailey
Callender – Hamilton
Fig. 11 Through Type Bridge
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Fig. 12 Simply Supported Bridge
Fig. 13 Temporary Bridge
2.3 CABLE STAYED BRIDGES
A cable stayed bridge is a bridge whose deck is
suspended by multiple cables that run down to the main
girder from one or more towers .The cable stayed bridge
is specially suited in the span range of 200 to 900 m and
thus provides a transition between the continuous box
girder bridge and the stiffened suspension bridge .It was
developed in Germany in the post war years in an effort
to save steel which was then in short supply. Since then
many cable stayed bridges have been built all over the
world, chiefly because they are economical over a wide
range of span lengths and they are aesthetically attractive
.The wide application of the cable stayed bridge has been
greatly facilitated in recent years by the availability of
high strength steels, the adoption of orthotropic decks
using advanced welding techniques and the use of
electronic computers in conjunction with rigorous
structural analysis of highly indeterminate structures. The
beauty and visibility of a cable stayed bridge at night can
be enhanced by innovative lighting schemes .The early
cable stayed bridges were mainly constructed using steel
for stay cables, Deck and towers. In some of the recent
constructions, the deck and towers have been constructed
in structural concrete or a combination of steel and
concrete.
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The Stromsund Bridge in Sweden, built in 1957 with
a main span of 183 m, and the Dusseldorf North Bridge
built in 1958 with a span of 260 m are early examples of
cable – stayed bridges. Another well known bridge in
this category is the Maracaibo Lake Bridge in Venezuela
designed by Ricardo Morandi of Italy and built in 1963.
The Sunshine Skyway Bridge (1987) designed by
Eugene Figg and Jean Muller over Tampa Bay in Florida,
has a main span of 360 m with pre – stressed concrete
deck and single – plane cables. The Dames Pont Bridge
at Jacksonville, Florida, built in 1987 with a span of 390
m is the longest cable stayed bridge in USA. Designed by
Howard Needles and Finsterwalder, the bridge features H
– shaped Reinforced Concrete towers and two – plane
cables supporting R.C deck girders. Currently, the Tatara
Bridge in Japan (1999) with a span of 890 m is the
longest cable – stayed bridge in the world. The Millau
Viaduct, completed in 2005, with six spans of 350 m and
two spans of 240 m, supported on towers up to 235 m
height is a unique cable – stayed bridge.
India‟s first cable – stayed vehicular bridge is the
Akkar Bridge in Sikkim completed in 1988 with two
spans of 76.2 m each. The Second Hooghly Bridge
(VidyasagarSetu), completed in 1992, with a central span
of 457.2 m and two side spans of 182.9 m each, is a
notable engineering achievement in India. The various
cable – stayed bridges are shown in the following table.
Basic concepts of the application and design of the cable
stayed bridges are presented here.
Table 1. List of Cable – Stayed Bridges
The main components of a cable stayed bridge are:
1) Inclined Cables
2) Towers (also referred as pylons)
3) Deck
In a simple form, the cables provided above the deck
and connected to the towers would permit elimination of
intermediate piers facilitating a larger width for purposes
of navigation. When the number of stay cables in the
main span is between 2 and 6 the spans between the stay
supports tend to be large (between 30 and 60 m)
Year Bridge Location Main
Span
(m)
Deck
Material
1999 Tatara Kamiura,
Japan
890 Steel
1994 Normandie Seine,
France
856 Steel
2001 Nanjing – 2 Nanjing,
China
628 Steel
1993 Yangpu Shanghai,
China
602 Composite
1997 Maiko Chuo Nagoya,
Japan
590 Steel
1999 Oresund Sweden 490 Steel
1992 VidyasagarSetu Kolkata 457 Composite
1996 Second Severn Bristol,
UK
456 Composite
1987 Rama IX Bangkok 450 Steel
1983 Luna Spain 440 Concrete
1975 St. Nazaire France 404 Steel
1978 Stretto di
Rande
Vigo,
Spain
400 Steel
1982 Luling Mississippi 372 Steel
1978 Dusseldorf
Flehe
Germany 367 Steel
1987 Sunshine
Skyway
Florida,
USA
366 Concrete
1970 Duisburg
Neuekam
Germany 350 Steel
1990 Tempozan Japan 350 Steel
1990 Glebe Island Australia 345 Concrete
2004 Millau Viaduct Millau,
France
342 Steel
1974 West Gate Australia 336 Steel
1978 Zarate - Brazo Argentina 330 Steel
1993 Karnali Nepal 325 Composite
1972 Kohlbrand Germany 325 Steel
1969 Kniebrucke Germany 320 Steel
1977 Brotonne France 320 Concrete
1971 Erskine Scotland 305 Steel
1959 Severins Cologne 302 Steel
1987 Dongying China 288 Steel
1976 WadiKuf Beida,
Libya
282 Concrete
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requiring large bending stiffness. The stay forces are
large and the anchorages of cables become complicated.
The erection of such bridges involves use of auxiliary
structures. On the other hand , the use of multiple stay
cables would facilitate smaller distances between points
of supports (between 6 and 10 m) for the deck girders ,
resulting in reduced structural depth and facilitating
erection by free cantilever method without auxiliary
supports .
The multiple stay cable system also permits easy
replacement of cables if needed and enhances
aerodynamics stability through increased damping
capacity. The deck can be supported by a number of
cables in a fan form (meeting in a bunch at the tower) or
in a harp form (joining at different levels on the tower) as
shown in the figure. The figure shows a typical fan-
shaped cable arrangement with the anchorages at the
tower distributed vertically down a certain length
(modified fan form). This arrangement facilitates easy
replacement of cables at a later date in case of accidents.
The fan type configuration results in minimum axial
force in deck girders. The harp form requires larger
quantity of steel for the cables. Includes the fan shape is
superior from a structural and economical view. The harp
shape possesses enhanced aesthetics. The harp
configuration cables also permits erection of the tower
and the deck to progress at the same time. Because of the
damping effect of inclined cables of varying lengths, the
cables stayed decks are less prone to wind –induced
oscillation than suspension bridges.
Fig. 14 Types of Cable Systems
Based on the span arrangement, the cable stayed bridge
can be one of four types:
1) Bridge with an eccentric tower, e.g. Hoescht
Bridge on main river
2) Symmetrical two – span bridge, e.g.
Ottmarshein Bridge in France
3) Three – span bridge, e.g. Brotonne Bridge,
France
4) Multi – span Bridge, e.g. Millau Viaduct,
France
TYPICAL CABLE STAYED BRIDGES
The first modern cable stayed bridge was the
Stromsund bridge in Sweden, built in 1956 with a main
span of 183m and two side span of 75m each .This
bridge consist of continuous plate girders supported by
two plane radial cables anchored to the tops of towers of
portal shape. The deck is of reinforced concrete slab
supported on strings and cross beams. The Dusseldorf
north bridge (1958) has harp type cables in two vertical
planes attached to single towers. The decking is of
orthotropic steel deck with box shaped main girders
stiffened by cross beams. The bridge has spans of 108-
260-108 m. The Severins Bridge (1959) in cologne has a
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single A-frame tower with fan type cables, converging at
the apex of the A- frame. The decking is of orthotropic
steel deck with two main girders of box section as in fig.
The Norderelbe Bridge (1962) in Hamburg was the first
bridge with cables arranged in star type in single plane.
The bridge has a box section at centre with one single
web girder on either side as in fig. The cable
configuration is justified more from aesthetic
considerations than on economic grounds. The Brotonne
Bridge built in 1977 with a main span of 320 m has a
single plane of cable stays and uses a precast pre -
stressed concrete box girder deck. The Yangpu Bridge in
Shanghai, china built in 1994 with a main span of 620 m
marked a significant development. This was surpassed in
the same year by the Normandie Bridge in France with a
main span of 856 m. The Sunniberg Bridge in
Switzerland built in 1999 with main spans of 140 m and
the Millau viaduct in France completed in 2005 with
main spans of 342 m are outstanding applications of
multi-span cable stayed bridges.
Akkar Bridge (2 spans of 79.0 m) and Hardwar bridge (2
spans of 65.0 m) are early examples of Indian cable
stayed bridges, essentially evolved as forerunners for
longer spans to follow .The second Hooghly bridge
(Vidyasagarsethu) completed in 1992 with a main span
of 457 m and side spans of 182.9 m each, using fan type
cable arrangement, is a land mark of bridge construction
in India. The Tatara Bridge on the Onomichi –Lmabari
highway route of the Honshu-shikoku bridge project in
Japan is the longest span cable stayed bridge in the world
with a main span of 890 m. The steel towers are 176 m
high above the bridge deck, corresponding to 0.2 of the
main span. The towers are shaped like an inverted Y after
examining the wind resistance, structural efficiency and
aesthetics. The stay cables have two-plane multi-fan
shape. The cables are anchored at spacing of 20 m at
deck level and at 3 m spacing at the tower. Based on
wind tunnel tests, the surface of the polyethylene cover
of the stay cables was provided with indentations, with a
view to prevent the turbulence that results from wind
blowing on rain water running on the surface of the long
stay cables. This innovation provides sufficient damping
and avoids the need for ties between the cables. The deck
is of streamlined steel box girder. The deck width is 28.1
m corresponding to width-to-span ratio of 1:31.7. The
center span was erected by the cantilever method. The
aesthetic appeal, the economic advantage and the ease of
construction make the cable stayed bridge the preferred
option in the span range of 200 to 900 m.
Fig. 15 Tatara Bridge, Japan
Fig. 16 Millau Viaduct Bridge, France
ARRANGEMENT OF CABLES
The cables may be arranged in one central plane (axial
suspension) as in Norderelbe bridge, in two vertical
planes with twin-leg tower as in Stromsund or
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Dusseldorf North bridges, or in two inclined planes as in
Severins bridge (lateral suspension) .The single-plane
system has the advantage that the anchorage at deck level
can be accommodated in the traffic median resulting in
the least value of required total width of deck. With the
two -planes system, additional widths are needed to
accommodate the towers and deck anchorages.
Aesthetically, the single-plane system is more attractive
as this affords an unobstructed view on one side for the
motorist. Other notable examples of single-plane system
are the Rama IX Bridge (1987) in Bangkok, Thailand,
the Sunshine Skyway Bridge (1987) in Florida, USA and
the Normandie Bridge (1994) in France. In the case of a
two-plane system of cables, a side view of the bridge
would give the impression of intersection of the cables.
The choice of the cable arrangement should be done with
care and diligence, so as to ensure an enhanced aesthetic
quality of the bridge through a system in harmony with
the environment.
The two inclined plane system of cables with the
cables radiating from the apex of an A-frame as in
Severins bridge facilitates the three-dimensional
structural performance of the superstructure and reduces
the torsional oscillations of the deck due to wind, thus
enhancing the aerodynamic stability of the bridge. The
torque due to eccentric concentrated loads would
necessitate the use of box section orthotropic deck for the
single-plane system. The decking is generally of
orthotropic plate system with box girders for the two-
plane system also, but can be of pre stressed concrete
girders as in Maracaibo bridge in Venezuela and Hoescht
bridge over main river in Germany. The Rama VIII
Bridge in Bangkok uses a combination of two-plane and
single plane systems. Using an inverted-Y pylon, the 300
m main span is supported with twin inclined stays while
the back span has a single plane system of stays.
DECK STRUCTURE
While the deck is merely supported by the cables in
suspension bridge, the deck of a cable stayed is an
integral part of the structure resisting the axial force and
bending induced by the stay cables. For bridge width
greater than 15 m and spans in excess of 500 m, the need
to reduce dead weight prompts the use of all-steel
orthotropic plate deck, as adopted for the Normandie
Bridge and the Tatara Bridge. Torsion box deck sections
in pre stressed concrete have been used with single-plane
system, as in Brotonne Bridge and the sunshine bridge.
Composite deck section have been employed in the
second Hooghly bridge at Kolkata, India and the Second
Severn crossing, U K. Special attention should be
devoted to the anchorage of cables to the deck. The
superstructure of the main span is normally constructed
using the segmental cantilever method.
The ratio of the side span (Ls) to the main span (Lm)
for the case of a bridge with towers on both sides of the
main span usually lies between 0.3 and 0.45.The ratio
Ls\Lm can be 0.42 for concrete highway bridge decks
and not more than 0.34 for Railway Bridge. This ratio
influences the changes in stress in the back stay cables
due to variation of live load. It further influences the
magnitude of vertical forces at the anchor pier, the
anchor force decreasing with increasing Ls\Lm. The
choice of Ls\Lm depends also on the local conditions of
water depth and foundation.
TOWERS
Towers carry the forces imposed on the bridges on the
bridge to the ground. They are not replaceable during the
life of the bridge. Hence they should be designed to be
structurally strong, constructible, durable and
economical.
The tower may take any one of the following forms:
1) Single free standing tower, as in Norderelbe
Bridge
2) Pair of free standing tower shafts, as in
Dusseldorf North Bridge
3) Portal frame, as in Severins Bridge and
Second Hooghly Bridge
4) A – frame, as in Severins Bridge or inverted
Y – shape as in Yangpu Bridge
5) Diamond configuration, as in Globe Island
Bridge, sydney
When the stay cables are in one plane, a single free
standing tower may be adopted. In this case, the pier
below the box girder should be sufficiently wide for
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bearings to resist the torsional moments of the
superstructure. For bridges with cables in two planes, the
towers can be a free standing pair, or a portal frame with
a slender bracing. An additional bracing may be
introduced below the deck. The A-shaped tower and the
inverted Y-shaped tower have been favoured for long
bridges having shallow box girder decks in regions of
strong wind forces. The land take at the base can be
reduced by adopting a diamond configuration, as used in
the Tatara Bridge. Typical arrangements of towers are
shown in figure.
Fig. 17 Types of Pylons
Since the tower is the most conspicuous component in
a cable stayed bridge, besides structural considerations,
aesthetics plays a prominent part in the selection of the
particular shape of the tower. For example, the proximity
of cologne cathedral influenced the adoption of the A-
frame for the Severins Bridge. Sometimes, an additional
height is provided for the tower above the point of
connection of the cable for architectural reasons, as in
Norderelbe Bridge. Anchorage of cables at the tower
should follow good order. Since the cables at the deck
level are anchored along a line along the edges or at the
middle of the deck, it is natural that these should end
along a vertical line at the tower head. In the case of A-
shaped tower, the anchorage line can be parallel to the
tower leg it is not desirable to spread the anchorages
transversely in one layer at the tower.
The single tower or towers consisting of a pair of
separate columns will be stable in the lateral direction
due to the restoring force provided by the cables in case
of lateral displacement due to wind forces, as long as the
cable anchorages are situated at a level above the base of
the tower. The towers may be designed to be hinged or
fixed at the base, depending on the magnitude of the
vertical loads and distribution of the cable forces. While
a tower with a fixed base induces a large moment, the
increased rigidity of the total structure resulting from a
fixed base at the towers and the relative ease in erection
as compared with a hinged base may be advantageous.
On the other hand, the hinged base results in reduced
bending moments in the towers and may be advantageous
with weak soil conditions. The towers should be slender
and should have a low bending stiffness in the
longitudinal direction so that back stay cables will be
functional in partially catering to live loads in the main
span. Towers should normally be vertical.
The height of the tower should be preferably being in
the range of 0.2 to 0.25 Lm. The higher the tower, the
smaller will be the quantity of steel required for the
cables and the compressive forces. But it is not
advantageous to increase the height beyond 0.25 Lm.
CABLES
The stay cables constitute critical components of a
cable stayed bridge, as they carry the load of the deck
and transfer it to the tower and the back stay cable
anchorage. The main requirements of stay cables are:
1) High load carrying capacity
2) High and stable Young‟s Modulus of
elasticity
3) Compact cross – section
4) High fatigue resistance
5) Ease in corrosion protection
6) Handling convenience
7) Low cost
The ultimate tensile strength of wire is of the order of
1600 MPa. A typical section of a stay cable is shown in
figure 18 (Fig.18). While locked coil strands have been
used in early bridges, the recent preference is towards the
use of cables with bundles of parallel wires or parallel
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long lay strands. The sizes of cables are selected to
facilitate a reasonable spacing at the deck anchorages.
Parallel wires cables using 7mm wires of high tensile
steel have been adopted in Second Hooghly Bridge.
Corrosion protection of the cables is of paramount
importance. For this purpose, the steel may be housed
inside a polyethylene (PE) tube which is tightly
connected to the anchorage. The cables are anchored at
the deck and at the tower. The anchorage at the deck is
fixed and has a provision for a neoprene pad damper to
damp oscillations. The length adjustment is done at the
tower end.
The cables are pre - stressed by introducing additional
tensile force is the cables in order to improve the stress in
the main girder and tower at the completion stage, to
prevent the lowering of rigidity due to sagging of cable,
and to optimize the cable condition for the erection. The
magnitude of the pre - stress is determined by taking into
consideration the following factors: I) the horizontal
component of each cable tension in balanced such that
there is no in-plane bending of the tower due to balanced
horizontal force due to dead load at the completion stage:
and ii) the net force on the main girder member at the
connection of the cable at the completion stage be zero.
Currently the steel used for cables have ultimate
tensile of the order of 1600Mpa. Carbon fibre cables
having UTS of about 3300Mpa are under development.
The latter cables are claimed to have negligible corrosion
and to possess high fatigue resistance. However, carbon
fibber cables are presently very expensive.
Fig. 18 Typical cross section of Stay Cable
ANALYSIS
The cable stayed bridge with multi – stay
configuration is a statically indeterminate structure with a
high order of indeterminacy. The deck acts as a
continuous beam on elastic supports of varying stiffness.
Bending moments in the deck and pylons increase due to
second order effects due to deflection of the structure.
The effects of creep and shrinkage during construction
and service life should be considered for concrete and
composite decks. The internal force distribution in the
deck and tower can be managed to be compression with
minimum bending, by adjustment of the forces in the
stay cables. A rigorous analysis considering three –
dimensional space action is quiet complex. Approximate
designs can be made using a two – dimensional
approach. Though the cable stays show a non – linear
behaviour due to large displacements, sag in cables and
moment – axial force interactions in stays, girders and
towers, an approximate analysis assuming linear
behaviour leads to satisfactory results in most cases.
However, a non – linear analysis is essential for very
long span bridges.
CONSTRUCTION BY CANTILEVER METHOD
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The cantilever method is normally adopted for the
construction of long span cable stayed bridges. Here the
towers are built first. Each new segment is built at site or
installed with precast segment, and then supported by
one new cable or a pair of new cables which balance its
weight. The stresses in the girder and the towers are
related to the cable tensions. Since the geometric profile
of the girder or elevation of the bridge segments is
mainly controlled by the cable lengths, the cable length
should be set appropriately at the erection of each
segment. During construction, monitoring and
adjustment of the cable tension and geometric profile
require special attention.
A notable example of construction of a major cable
stayed bridge by cantilever method is the Yangpu Bridge
in Shanghai, China, built in 1994 with a main span of
602 m. The composite girders of this bridge consisted of
prefabricated, wholly welded steel girders and precast
reinforced conceret deck slab.
Depending on the bridge site, cable stayed bridges can
have any one of four general layouts of spans:
1) Cable stayed bridges with one eccentric
tower, eccentric with respect to the gap to be
bridged, e.g. Severins Bridge
2) Symmetrical two – span cable stayed bridges
e.g. Akkar Bridge
3) Three – span cable stayed bridges, e.g.
Second Hooghly Bridge
4) Multi – span cable stayed bridge e.g. Millau
Viaduct.
Of these the most common type is the three – span
cable stayed bridge, consisting of the central main span
and the two side spans. Temporary stability during
construction is a major problem, particularly just prior to
closure at mid span. The structure must be able to
withstand the effects due to wind and accidental loads
due to mishaps during erection. When intermediate piers
are provided in the side spans, the stability is very much
enhanced. In this case, the side spans are built first on the
intermediate supports, and later the long cantilevers in
the main span.
2.4 GENERAL STEPS IN BRIDGE DESIGN
The sequence of planning for bridges forming part of
a new highway or railway project will form part of that
particular project planning. But, in case of a major
crossing across a large or important river or a major road
intersection, a more detailed planning for the particular
bridge itself will be required.
Different steps involved in planning for such a bridge
and for major links are:
1) Study the need for a bridge
2) Assess the traffic requirement
3) Location study
4) Study of alternatives
5) Short listing feasible alternatives
6) Developing concept plans for alternatives
including choice of form, materials, span
arrangements etc...
7) Preliminary design and costing
8) Evaluation of alternatives, risk analysis and
final choice
9) Finding resources, detailed survey and design
10) Implementation including final preparation of
bid documents, fixing agency, construction
and commissioning
A new highway or railway line may need to be
provided as part of development of an area; linking two
or more places of commercial or tourist interest and
strategic importance; as a link to a port, mines, industrial
area and/or a large thermal power plant. Their need is
usually established by evaluating their socio – economic
and/or financial viability.
Once the need for the project is established the
various details have to be worked out. Any highway or
rail line will be crossing a number of small and large
streams, canals, rivers and lakes, over which culverts or
bridges will have to be provided. Need for a culvert or
bridges become automatically established in such cases
during preparation of project sheets. When a bridge
forming part of such new road or rail projects has to be
provided over a large river, a more detailed initial
planning is required involving various steps listed above.
Apart from this, need may arise to provide additional
bridges across major rivers, for linking two major
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highways or rail lines or a network of roads in an urban
area, including grade separators at busy road
intersections in urban areas. Similar planning approach is
required in such projects.
LOCATION OF BRIDGE
Cross drainage works on alternative alignments of a
road or rail alignment can differ considerably and since
they tend to form 15 to 20 % of the cost of the total
project, it is essential to analyze and consider the effect
of all the CD works on the alignment, before choosing
the alignment. While fixing the horizontal alignment of
the line/road, it is desirable to select a bridge site such
that the bridge/culvert is:
On a straight reach of the stream avoiding any
bends or meanders
Clear of the confluence of any tributaries or
branches
Confined within well defined banks
With the road approach on either side straight
to maximum extent and
With the crossing normal to the road alignment
and if skew is unavoidable, limit the skew
angle
In addition, major river crossings should satisfy
following conditions to the maximum extent.
River Regime
The upstream reach of the river, should be
straight, and any sharp bend downstream
should be avoided.
The river in the reach should have a regime
flow free of whirls, eddies and excess current.
It should preferably not have any confluence of
streams immediately upstream.
The channel in the reach should be well
defined and as narrow as possible.
It should have firm high banks which are fairly
inerodable (the ideal site is at a gorge).
In a meandering river, it should be at a nodal
point
Where artificial gorging is necessary due to absence of
firm inerodable high banks, it should be possible to build
protection works like guide bunds on a dry location or in
shallow water if unavoidable.
Approaches
The approach bank should be secure, and not
be liable to flash flood attacks or major spills
during floods.
It should not be too high or too expensive to
build; it should not pass through high hills or
major drainage basins or built up areas or
religious structures.
It should have reasonable proximity to the
main road or railway to be served without need
for long or costly connecting links.
It should be such as to avoid excessive
construction works under water or over marshy
lands.
Approaches and protection works should be
such as to involve minimum recurring
maintenance expenditure and be reasonably
safe from flood damages which would
otherwise put the bridge out of use for long
periods.
INVESTIGATIONS FOR MAJOR BRIDGES
Planning and design of major bridges call for more
detailed survey and collection of data. Such requirements
are detailed in ensuing paragraphs:
TOPOGRAPHIC DETAILS `
The survey of the river course should extend up to the
firm banks or up to the HEL line, if it over-tops the banks
and water spreads out. It should cover a distance of about
2 km upstream and 2 km downstream of the alignment in
the case of smaller streams and 5 km upstream and 2 km
downstream for larger rivers. This is necessary for
locating a straight reach of river where a good normal
crossing can be provided. The extent of water course at
flood time (excluding, of course, spill) will give an idea
of` the minimum width of waterway that will be required
for providing a bridge with minimum obstruction to the
natural flow.
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The plan of the water course should be drawn to a
scale of l : 2000 for smaller rivers and l : 5000 for larger
ones. Cross sections should be taken at the proposed
crossing, one upstream and one downstream, each about
2 km apart. They should be drawn to the same horizontal
scale as that of the plan. In case it is fairly flat, the cross
section can be plotted to different vertical and horizontal
scales (the latter being the same as that of the plan), with
the proportion between the two not less than 1:10 this
will give a better idea of the area of section of flow for
working out the observed discharge and also correctly,
determining the position of the bridge so as to cover the
deeper and perennially flowing channels.
The plan should cover the details of all streams
joining the main stream river within the reach surveyed,
the location and value of any benchmark, and the closest
inhabited locality; it should also provide sufficient spot-
levels for drawing contours, and indicate clearly the
alignment and position of the proposed bridge with its
change marked. The low water level, highest flood level
(HFL) and ordinary flood level (OFL) should be marked
on the cross section. The position of any borings and trial
pits should be indicated on the plan while the details of ̀
bore data should be indicated on the section. The position
of GTS benchmarks with their values and also any
survey reference pillars left by the survey party should be
marked, indicating, by the side, benchmark values. Such
detailing will give an idea of all related data governing
the siting of the bridge at a glance. If any checking
becomes necessary later, referencing will be easy and
quick, without any need to refer to a number of detailed
drawings.
The survey can be conducted by triangulation in the
case of a small stream or by a closed traverse in the case
of larger streams. (For detailed procedure for
triangulation and other detailed surveys, in order to
achieve a good accuracy of the plan, suitable cross
checks in the case of running open traverses along the
bank should be established. Some spot-levels should be
taken so that the contours can be plotted on the plan also.
This plotting of contours will help in proper location of
the axis of bridge with respect to the stream and
determining the sew angle, if any. Any marginal bunds,
other flood protection works, and any tanks, lakes and
irrigation works in the vicinity should be clearly
indicated. The direction of flow of the stream and the
north line are the most important markings that are
sometimes omitted by oversight.
CATCHMENTS AREA MAP
The catchments area map for major bridges can be
prepared from the available topographic sheets of the
most recent survey made in the area. The Survey of India
has prepared maps for the entire country to scale 1” to a
mile or 1: 50,000. They are now preparing maps to scale
1: 25,000 and such maps for some areas are available.
The map available to the largest scale should be obtained
from them. All these maps contain contours for areas
covered. Hence, the ridge line bounding the watershed
contributing to the flow to a particular stream/river can
be easily traced on such maps. A tracing showing the
river, tributaries and the ridge or catchments boundary
line prepared will form the catchments area map. The
area bounded can be worked out either by a planimeter or
using squared paper to proper scale. Where the
catchments is small, close enough contours may not be
available on these topographical sheets and a tracing of
the ridge line may be difficult.
Additional details to be marked on the catchments area
map are:
1) All irrigation tanks and reservoirs in the
catchment area intercepting the contributing
streams and which are likely to affect the
bridge if any of them is damaged
2) Rain gauge stations
3) Discharge observations
4) River bed levels along the river up to the
source, as may be available
5) Levels of peaks on ridges and peaks of
isolated hillocks falling within
If possible, the heaviest intensity of rainfall recorded
at the rain gauge stations can be indicated on the
catchments plan. The work of preparation of this map can
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be done in the office itself and supplementary
information obtained from enquiries from other
departmental officers.
HYDROLOGIC PARTICULARS
Some particulars which come under this heading have
been covered under `topographic details‟ and catchments
area‟. The hydrographical, i.e., gauging and discharge
details available for the bridge site or the nearest
available site should be collected for the longest periods
available, either from irrigation or flood control
engineers. If not available, some short – term
observations for velocity and discharge can be made by
the survey team. Enquiries should be made regarding the
data, formulae and coefficients adopted for working out
the design discharge for the same or similar
streams/rivers in the same area by other engineers. The
size of the openings which have been provided for
existing bridges on the same river upstream and
downstream should be ascertained along with
information on past experience regarding their adequacy
or otherwise. Hydrographical details available for such
bridges can be of much help.
GEO-TECHNICAL DETAILS
The scope of geo-technical investigations should be
such as to enable the designer determine or comprehend
the following:
1) Location and extent of soft layers and gas
pockets, if any, especially in apparent hard
founding strata
2) The type of rock, dips, faults and fissures
3) Possibility of subsidence due to mining in the
neighbourhood
4) Sub – soil water level and artesian conditions
5) Quality of ground water
6) Particle size and classification of the soils at
various levels
7) Physical properties of the soil to determine
the bearing capacity
8) Settlement characteristics of the soil for
determining the settlement and differential
settlement
9) Frictional and porosity properties for
determining sinking or driving effort
10) Any possible constructional difficulties
SEISMOLOGY OF THE AREA
India is divided into five seismic zones based on the
likely intensity and frequency of earthquakes. The
coefficients to be used for arriving at horizontal and
vertical seismic forces induced on the structure in
different zones are covered in IS: 1893. It may be
necessary to modify the coefficients in specific cases or
carry out model studies to determine these coefficients in
the case of very long spans in highly earthquake-prone
areas, particularly for the sub – Himalayan zone, the
entire north eastern India and in some areas like Koyna
where there is a past history of occurrence of disastrous
earthquakes. Detailed information can be obtained from
the Geological Survey and Meteorological Departments
regarding the seismic history and intensity as well as of ̀
damages caused by past earthquakes in the area. This
information can be used for modifying the coefficient of
design of structures or Carry out model studies. In case
there is any geological fault along the river course, the
Meteorological Department should be consulted
regarding the likelihood of its buffing action.
NAVIGATIONAL REQUIREMENTS
There may be some plans by the Inland Navigation
Department of the State or Union Government for
introducing navigation in the water course to be bridged
in the foreseeable fixture. Provision should be made for
adequate headroom above the OFL (or normal HFL) in
such cases. The general standards suggested for different
types of craft (boats and barges) used in inland
navigation are indicated in table 2.
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Table 2 Navigational Clearance for Bridges
Tonnag
e of
Vessel
Lengt
h (m)
Bea
m
(m)
Draf
t
(m)
Minimu
m clear
span (m)
Minimu
m
headroo
m over
mean
HFL (m)
50 18 5.0 1.5 15 2.0
100 24 5.0 1.5 17 3.0
300 35 6.5 2.0 25 3.9
600 60 7.0 2.0 30 6.0
900 and
more
75 10.0 2.0 90 to
110
10 to 12
CONSTRUCTION RESOURCES
During the field survey, sufficient information should
be collected to have an idea of the type of labour that will
be available locally and if they will have to be
supplemented by bringing people, particularly skilled,
from outside. If all the required labour is locally
available, they should be able to come to the site, from
their homes; they will not need much of site
accommodation and may only need some transport
arrangement. If any type of skilled and other labour has
to be brought from outside, residential and! Or camp
accommodation will have to be provided for them as
close to the site as possible. Such people will have to be
paid higher rates of wages and given additional leave and
also travel expenses from and to i - their homes.
Information collected will give an idea of the extent
which temporary accommodation for such imported
labour will have to be provided and also to arrive at
correct unit rates for the labour component of the
various items of work. The availability of construction
materials, particularly bricks, quarry materials like stone
and aggregate, and good quality sand and timber in the
vicinity will have to be found out to determine the extent
to which transportation will be involved in carrying these
to the site. While doing so, the existing roads, pathways,
the availability of the various types of transport, their
cost, etc. should be considered. This will give a better
idea to work out to the unit cost to be adopted for
material components of various items of work and also
the infra-structure requirement for transporting materials
and equipment.
PARTICULARS OF NEAREST BRIDGES
During the investigations, particulars with regard to
foundation details, clearances and other physical
features of the bridges that might have been constructed
on the same water course on the nearest railway line or
road should be obtained. Enquiries should also be made
of the bridges that have been overtopped or breached
since their construction or any other type of failure to the
structure. In the case of bridges of smaller magnitude, it
should be sufficient if particulars of such bridges within
about l0 km radius are obtained. In the case of larger
bridges, particulars should be gathered for those situated
even 50 to 60 km away.
TRAFFIC FORECAST
If the bridge forms part of an overall project like the
construction of a new railway line or construction of a
new road, the traffic forecast would have been already
made earlier. If not made already, this forecast will have
to be done for purposes of:
1) Determining the size of the bridge, i.e., the
number of lanes or tracks to be provided and
whether a footpath has to be provided.
2) Working out the benefits that will accrue by
providing such a bridge (if it is a project by
itself).
There may be some traffic across the stream already at
the location, but that may be using some other mode like
a ferry, or a crossing may be made only when the water
level is low. It may also be taking the route over a bridge
already existing over the stream by a detour Hence, an
assessment has to be made first of the diversion of the
existing traffic which will use the bridge after it is
provided. Second, the provision of the bridge itself can
create development opportunities on either side and also
increase the inter- flow, and this will have to be forecast
taking into consideration the economic and social
conditions of the area. The structure to be provided
should be for a volume of traffic that will develop over a
foreseeable future so that no additional work or
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reconstruction will be called for in that period. A time
space of 40 to 50 years is generally advisable for this.
REPORT AND DRAWING
The documents to be prepared for the bridge project
will comprise a brief report giving the salient features for
aiding in detailed design and an estimate along with the
under mentioned drawings:
a) An index map to scale, 1; 50,000 in the case of
small rivers and 1: 2, 50,000 in the case of
larger ones. It should show the road/rail
alignment, the position of the proposed bridge
with the chainage, general topography of the
area, existing communication lines. Important
towns and villages, rivers, canals and other
irrigation works.
b) A survey plan showing all topographical
features in the immediate vicinity for sufficient
distance on either side of the proposed bridge
showing the contours at 1 to 2m intervals
should be prepared; All alternative sites should
be marked on this plan. All features that can
influence the design of the bridge should also
be marked on this plan. A longitudinal section
along the proposed alignment to the same
horizontal scale as that of the plan and one-
tenth of the same as the vertical scale should be
drawn. The line showing the top of the
proposed formation should be marked in red on
the same sheet. These distances can be reduced
for artificial (like men-made irrigation and
navigation canals) and in difficult countries by
the engineer to suit site conditions. The Indian
Roads Congress requires this sheet to cover
details for distances on either side and to scales
as indicated below.
Catchment areas less than 3 sq. km:
100 metres and to scale
1:1000
Catchment areas of 3 to 15 sq. km:
300 metres and to scale
1:1000
Catchment areas over 15 sq. km:
1.5 km and to scale
1:5000
c) A site plan to a suitable scale should show the
selected site and ground details for a distance
of 100 m upstream and 100 m downstream for
small bridges and 500 m on either side for
larger bridges. It shall contain the following
details:
1) Name of the channel and road, chainage
and identification mark (number etc...)
allotted to the crossing
2) Direction of flow, maximum and
minimum discharges
3) Existing and proposed alignments, if it
is in replacement
4) Angle and direction of skew
5) Name of the nearest identifiable town at
either end of the road
6) Position of any bench marks and their
values
7) Reference to the value of the bench
marks, the mean sea level taken as
datum
8) Location of cross – section lines
9) Longitudinal section (i.e., alignment
centre) line with reduced levels marked
10) Location of trial pits and bore holes with
identification numbers/ marks
11) Location of any obstruction for road
alignment, such as nullahs, buildings,
wells, outcrops of rocks etc... (the scale
should preferably be to 1:1000) the
alignment line shall be shown in form of
a thick green line.
d) Cross Sections, one along the proposed
alignment and two others, one upstream and
one down- stream at a suitable distance to scale
1 : 1000 horizontal and 1 : 100 vertical. It
should contain the following:
1) Bed levels up to tops of banks/ bunds
and ground levels for sufficient distance
beyond, the intervals being such as to
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give a clear idea of the uneven features
both in the bed and ground
2) Location and depth of trial pits or
borings and nature of soil in bed, banks
and approaches (in cases of smaller
bridges and shallow bores, the section of
soil profile at each bore/ pit can be given
on this itself).
3) HFL, OFL, LWL on the entire three
cross – sections.
e) In addition, a few more cross sections to the
same horizontal and vertical scales as that of
the site plan-
f) A longitudinal section along the channel,
showing the site of the bridge, to the same
horizontal scale as the survey plan (in the case
of small bridges this can be plotted on the
survey plan itself), the vertical scale not less
than 1: 1000.
g) Typical cross sections along alternative sites
considered, if any, with a brief note giving
reasons for selection of the proposed site to the
same scale as in (d) above.
The purposes of these drawings are given below briefly:
Index map: The index map will indicate the
geographical location of the bridge and the
nature of the area served by the bridge
Survey plan: It will give an idea of the nature
and direction of flow of the river and will help
in choosing a location that will ensure a
straight flow through the bridge. The
catchment area map is required to assess the
catchment areas and the type of terrain to work
out the design discharge using a flood formula
Plan: The plan will show the exact location
and lay out of the bridge for the purpose of
future setting out
Cross – sections: These are required firstly for
working out the area of flow as existing and the
slope of the river with which the volume of
discharge that has passed over the site can be
worked out. It is also used for centring the
bridge opening, and locating abutments and
piers so that the bridge covers the deepest and
perennially flowing channel. The flood levels
are required for the purpose of determining the
deck level of the bridge after allowing for the
necessary clearance. The low water level helps
in fixing the top of the well or pile foundations
taking into consideration the working
conditions.
Collection of data: The collection of data with
reference to construction resources and the
details of the nearest bridge across the same
river are required for obtaining an idea of the
construction problems that are likely to be met
with and working out the unit cost for
preparation of the estimate for the cost of the
bridge and appurtenant works. Forecast of the
traffic is likely to use the bridge is made for
determining the width of the bridge and in the
case of a costly alternative, for working out the
relative cost – benefit ration also.
Report: After the collection of these data and
working out the details, a report has to be made
out bringing the salient features of the bridge,
its estimated cost, cost – benefit ration, etc...
for helping to obtain the sanction. This report
should, as far as possible, be so detailed that
when work is sanctioned, the site work can be
commenced immediately.
3.DESIGN PROCEDURE
3.1 EXISTING BRIDGE DESCRIPTION
A road bridge presently exists across Gauthami River,
a tributary of Godavari between Yanam and Yedurlanka.
It is named as “BalayogiVaradhi”. It is a simply
supported Pre – stressed Concrete (PSC) box girder
bridge with 43 spans @ 40 m and 2 end spans @39.4 m
between centre to centre (c/c) of pier/ face of dirt wall.
The total length of the bridge is 1798.8 m between the
river faces of dirt walls. The bridge was constructed by
M/s. Navayuga Engineering Company Limited,
Visakhapatnam under Build Operate and Transfer (BOT)
vide Government Order (G.O) as a part of construction
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works of National Highway (NH) 214. The work was
supervised by Consulting Engineering Services (India)
Private Limited (CES) and PWD, Andhra Pradesh.
Now using the topographical survey data,
hydrographical survey data, sub – soil investigation data
conducted by PWD, Andhra Pradesh for the construction
of this bridge we are designing a Cable – Stayed Railway
Bridge in the same location is proposed in this report in
order to compare the advantages of each bridge.
3.2 AREA DESCRIPTION
The bridge is located between yanam (one of the four
districts of the Union Territory of Pondicherry in India)
and Yedurlanka (East Godavari District of Andhra
Pradesh in India) on Gauthami River, a tributary of the
perennial river Godavari. An extensive survey of the
area was done before deciding on the site for the bridge.
Yanam (Latitude16°42' N – 16°46' N; Longitude:
82°11' E – 82°19' E)is a town in the Indian union
territory of Pondicherry; it is located in Yanam district.
Yanam has about 300 years of history and it was
transferred to India in 1954. It forms a 30 km² enclave in
the district of East Godavari in Andhra Pradesh. It
occupies the delta of Godavari River, the town is situated
where the River Coringa(Atreya) branches off from
Gauthami into two parts, 9 km from the Bay of Bengal in
the Coromandel Coast. It has a population of about
32,000. Due to some relaxation in Tax and other
exemptions, lots of Business activities go on in Yanam.
Many people are getting employment in these
industries/firms. The major business areas in this region
are Coconut Dwelling, Rice Mills, Fishing and Other
Traditional Occupations. According to the 1995–2005
Development Records it was the first best constituency
in Pondicherry, which is moving forward in the
development sector, and also one of the best
constituencies in India.
Yedurlanka (Latitude 160 42‟ N; Longitude 820 12‟ E) a
village in East Godavari district of Andhra Pradesh is
situated on the banks of Gauthami River. It is located
28.7 km from the district main City Kakinada in
I.Polavarammandal. The District is known as rice bowl
of Andhra Pradesh with lush paddy fields and coconut
groves. The village is located in the Godavari Delta
region. The main soils in the area are alluvial red soil,
sandy loam and sandy clay.
3.3 NEED FOR BRIDGE
During the time of bridge construction there was no
bridge existing across the river Gauthami, branch of
Godavari River, connecting Yanam to Yedurlanka in
East Godavari District in Andhra Pradesh. Ferry service
was being operated for crossing the river. However, since
the width of the river is very large between the banks, it
was a huge inconvenience to the large crowds who had to
cross the river daily. It was therefore felt necessary to
construct a high level bridge at this location for smooth
flow of daily traffic. Before finalizing the project bridge
expert from CES along with representatives of PWD,
visited the site to have firsthand information about the
work and local conditions.
This stretch from Yanam – Yedurlanka is now a part of
NH 214. At the time of survey the people in Konaseema
area covering Yedurlanka, Muramalla, Polavaram,
Pallamkurru and other villages up to Amalapuram of East
Godavari District had to cross the Gauthami, Branch of
Godavari River by ferry service from Yedurlanka in
Konaseema area to Yanam in Pondicherry Union
Territory and then proceed by road to Kakinada, the
District Head Quarters of East Godavari District. When
the river is in floods, there is much inconvenience in
transportation of their agricultural produce to the markets
of Yanam and Kakinada and vice – versa. Yanam is fast
growing Industrial center and several Gas Based
Industries are now coming up around Yanam and
Kakinada. Kakinada port is fast developing and two
Major fertilizer Companies i.e., Godavari and Nagarjuna
fertilizers are located at Kakinada.
If a high level bridge is constructed across Gauthami
branch of river Godavari between Yanam and
Yedurlanka, it will help in development of 2 regions i.e.,
Yanam of Pondicherry State and Konaseema area
(Central Delta) of Andhra Pradesh. As the O.N.G.C
activities in Godavari basin are in full swing it will
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facilitate government and transportation of their vehicles
and also supply of Natural Gas to Yanam and other
industrially developing centers in East Godavari District.
Also several raw materials particularly, that required for
coir industries and edible oil units can be easily
transported from Konaseema area to Yanam. Further the
construction of the bridge reduces the distance between
Amalapuram and Kakinada by 40 km, by road. Apart
from the above mentioned facts, the fact that both Yanam
and East Godavari Districts are centers of tourism is well
known. Therefore providing a bridge in this region will
enable the development of tourism in this area.
Railway track always leads to improvement in
the economy of any area. The provision of this railway
link between Yanam and Yedurlanka will prove
advantageous both to the industries in the area as well as
tourism.
3.4 TOPOGRAPHICAL SURVEY
A detailed survey has been carried out in the area in
order to decide the alignment of the bridge. Due to lack
of resources and permissions we could not conduct the
survey, but as a part of the project we have collected the
data from R&B, Andhra Pradesh and made a detailed
study of the same. The method used in surveying and the
various facts observed thereby in the finalization of the
alignment are briefly described below.
The survey of the river course was done up to the
High Flood Level (HFL) line covering a distance of
about 5 km upstream and 2 km downstream for locating a
straight reach of river where a normal crossing can be
provided. Hydrographic surveying was used for
surveying over the body of water and determining the
levels at various points across the stream. A contour
survey i.e., triangulation by closed traverse was also
carried out to determine the levels at various points near
the river. Before deciding the alignment the soil profile at
all the suitable sites was taken into account. A
reconnaissance survey team has carried out the on field
survey to assess the site and determine the final
alignment. The alignment of the bridge was proposed
along the ridge line in order to reduce the Cross Drainage
(CD) works. The skew angle is also taken into
consideration. Skew angle is the angle between the major
axis of the substructure and a perpendicular to the
longitudinal axis of the superstructure. The
representatives of CES, with assistance from PWD,
examined the site. The site was inspected from both
Yedurlanka and Kakinada sides.
The alignment proposed after analyzing all the
contributing factors is shown in “Drawings”. The
proposed alignment is shown in green. The reasons for
selecting this particular alignment are as listed below:
1) This gives shorter bridge length
2) This is at a smaller skew angle
3) This lies beyond the territory of pondicherry
The alignment starts at km. 22/2 +175 of Kakinada –
Yanam road (now part of NH214) goes round of Yanam
town and crosses the river Gauthami in a normal
direction (1 km upstream side of the existing ferry point)
and then turns right and runs parallel to flood bank before
joining the Amalapuram – Yedurlanka road at km 24/6.
The total width of the river at site of crossing is 2.331 km
form flood bank to flood bank. The approach length of
Yanam side is 5.285 km and on Yedurlanka side is 4.115
km.
3.5 HYDROGRAPHIC PARTICULARS
The terms necessary for the hydrologic design of a
structure are as follows:
AFFLUX (h) is the rise in water level upstream
of a bridge as a result of obstruction to natural
flow caused by the construction of the bridge
and its approaches.
CAUSEWAY or Irish bridge in a dip in the
railway track which allows floods to pass over
it.
CLEARANCE(C) is the vertical distance
between the water level of the design discharge
(Q) including afflux and the point on the bridge
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super-structure where the clearance is required
to be measured.
DEPTH OF SCOUR (D) is the depth of the
eroded bed of the river, measured from the
water level for the discharge considered.
DESIGN DISCHARGE (Q) is the estimated
discharge for the design of the bridge and its
appurtenances.
DESIGN DISCHARGE FOR FOUNDATIONS
(Qf) is the estimated discharge for design of
foundations and training/protection work.
FREE BOARD (F) is the vertical distance
between the water level corresponding to the
Design Discharge (Q) including afflux and the
formation level of the approach banks or the
top level of guide banks.
FULL SUPPLY LEVEL (FSL) in the case of
canals is the water level corresponding to the
full supply as designed by canal authorities.
HIGHEST FLOOD LEVEL (HFL) is the
highest water level known to have occurred.
LOW WATER LEVEL (LWL) is the water level
generally obtained during dry weather.
The department has also carried out the
hydrographical survey and has provided the hydraulic
details such as HFL, Design Discharge, HTL, LTL, and
Water Current Velocity etc… The catchment area maps
of the region have been collected and studied carefully
before determining the hydrographical details. The
discharge data has been collected as per the specification
in Indian Railway Standard Code of Practice for Design
of the Substructures and Foundations of Bridges (Bridge
Sub-structure and Foundation Code). The particulars are
as shown below:
Deepest Bed Level =
14.860 m
Maximum flood level =
+4.58 m
T.B.L =
+4.94 m (on Yedurlanka side)
+5.060 m (on Yanam side)
Free board
= 3.66 m for central spans for
navigational purpose and
varying
from 3.66 m to 1.5 m
LTL
= -0.60 m
HTL
= +1.20 m
Width of river at gorge portion
= 520 m
Scour level : Normal
condition = -29.00
Seismic condition
= -25.62
Width of river in at M.F.L
= 2331 m
Flood discharge
= 56,700
cumecs
Water current velocity
= 3.5 m/s
On Yanam side, there is a flood bank approximately
600 m form the water line. From this bank about 600 m
length of the river bed is generally dry except for the
HFL condition which occurs once in 10 years or so. This
600 m stretch of bank is cultivated and there are a large
number of coconut and other trees. It shows scouring
does not take place in this portion. The average bed level
in this stretch is little above Reduced Level (RL) 3.00 m.
There after the waterline starts and well demarcated bank
line have been noticed, where ferry has been operated.
This bank of the river has been protected by stone
pitching at number of stretches and to prevent further
erosion, spurs also exist.
In the middle portion of the river there is an island
which separates the shallow channel from the deep
channel. The river on Yedurlanka side channel is much
deeper (about 18 to 19 m in depth), whereas on
Yanamside channel is shallower (about 4 to 6 m in
depth). The total waterway width under HTL condition
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will be approximately 1700 m whereas only during HFL
condition the waterway spreads further and waterway
width becomes about 2300 m.
The proposed bridge site is located near the sea and is
subject to tidal variation. Under HTL condition, the
waterway is restricted between the banks which are about
1700 m apart.
Because of close proximity to the sea, during the rainy
season when the high flood occurs, the river cannot
generate a very high velocity with the result the water
spreads on the banks and the highest flood known is only
1.5 m above the ground. There are lots of trees and
cultivation is going on in all the seasons and there is no
visible sign of any scour that has taken place in this
stretch.
Under HFL condition Lacey‟s waterway comes out to
1370 m which is less than 1700 m. Therefore, the bridge
length of 2150 m as suggested by the survey team is not
necessary. It is proposed to have 1798.8 m length of
bridge between river faces of dirtwalls, which is more
than Lacey‟s waterway, thus excluding the 600 m
shallow depth stretch towards Yanam side from the
existing bund up to the Ferryghat, over which approach
embankment of suitable height may be provided, to
minimize the cost.
On Yanam side well demarcated bank line starts at
chainage 580. This bank is being pitched by the Irrigation
Department to protect it. Hence, we propose the
abutment A1 at chainage 560.3 (19.7 m beyond the bank
line). Pitching around the abutment is proposed to be
done after construction.
On Yedurlanka side flood bank (bund) is at chainage
2320 with top of bund at RL + 4.94. We propose the
abutment A2 at chainage 2359.7 i.e., 40 m away from the
bund.
It is suggested that vertical clearance for navigational
purposes i.e., 3.66 m above the exceptional HFL of +4.58
may be provided at the center of the bridge and towards
the ends the bridge may be sloped on either side with a
gradient of 1 in 50 with introduction of vertical curve of
50 m. Minimum abutment clearance of 1.5 m is provided
at the abutment ends thus providing formation level at
abutments as +8.73
3.6 SCOUR DEPTH AND LINEAR WATERWAY
CALCULATION OF SCOUR DEPTH
The scour depth is calculated in accordance with IRS
substructure code.
As per Lacey‟s formula
Where Q = Discharge = 56700 cumecs
f = Lacey‟s silt factor for bed material
D = mean scour depth
“f” shall be determined for representative samples of bed
material collected from scour zone.
Where m = weighted mean diameter of the bed material
in mm.
For standard silt
Particle size = 0.32mm
Therefore silt factor (f) =1
The depth calculated has to be increased to obtain
maximum as per clause 4.6.6 of IRS sub-structure.
Hence depth is increased by 1.25D
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Therefore
= 18.17 say
18.2 m
Afflux for non – erodiable beds
Where H = afflux
v = velocity in m/s
A = un-obstructed sectional area in sq.m
a = sectional area of river at obstruction
in sq.m
For piers scour level is given by
= H.F.L-2(mean scour depth)
= 4.58-2D
= 4.58-2(18.17)
= -31.8 (RL)
For seismic scour level, it has to be multiplied by a factor
of 0.9.
Seismic scour level = HFL-(2(D) 0.9)
= 4.58-2(18.2) (0.9)
= -2.82 (RL)
Minimum grip length =
=
= 12.13 say 12.2m
Corresponding
founding level =
= - 44.0
(RL)
However in design, considering factor of safety founding
level is kept at
RL - 45.8 m
For abutment, scour level =
= 4.58-1.27(18.17)
= -18.50
However in design of abutment wall, we consider 2
cases.
1) Earth protected by pitching
2) Scour all around
CALCULATION OF EFFECTIVE WATERWAY
As per IRS substructure code the linear waterway is
calculated as below.
Linear waterway,
Where Q = 56700 cumecs.
C = 4.8
=1143 m
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Effective width of pier foundation =
= 4.91 m
Effective width of abutment foundation =
= 2.92 m
Total obstruction =
= 222.9 say 222m
Total waterway = 1143 + 222
=1365m say 1370m
The distance between flood banks at site of crossing is
2331 m but water spread under HTL condition is about
1700 m only. On Yanam side there is a flood bank
approximately 600 m form the water line. From this bank
about 600 m length of the riverbed is generally dry
except for the H.F.L. condition, which of course occurs
once in 10 years or so. This 600 m stretch of bank is
being cultivated. Thus it shows clearly that scouring does
not take place in this portion. The total length of the
bridge proposed by contractors is 1800 m (leaving 530 m
for approaches) which is more than the water spread
under HTL condition. The lacey‟s regime width for the
discharge was worked out to be 1143 m. The clear water
way proposed was (1800 – 224) m = 1576 m is therefore
more than the lacey‟s regime width. The additional water
will cater for the flow in the portion of the river, which is
proposed to be blocked by the approach embankment
under HTL condition.
The proposed bridge site is located near the sea and
subject to tidal variation. Under HTL condition, the
waterway is restricted between the banks which are about
1700 m apart. Because of close proximity to the sea,
during the rainy season when the high flood occurs, the
river cannot generate very high velocity with the result
the water spreads on the banks and the highest flood is
only 1.5 m above the ground. There is cultivation is
going on in all the seasons and there is no visible sign of
any scour that has taken place in this stretch. Under HFL,
condition Lacey‟s waterway comes out to 1370 m which
is less than 1700 m.
Therefore it is felt that as proposed earlier a bridge
span of 2150 m is not really required. It is proposed to
have 1798.8 m length of bridge between river faces of
dirtwalls, which is more than Lacey‟s waterway, thus
including 600 m shallow depth stretch towards Yanam
side from the existing bund up to the Ferryghat, over
which approach embankment of suitable height may be
provided, to minimize the cost.
3.7 SOIL PARTICULARS
The scope of this soil investigation includes
exploration of subsoil using 150 mm diameter bore holes
form ground surface to hard rock or 1m below refusal
resistance. It includes conducting various field tests,
collection of samples from the field and conducting
various laboratory tests analyzing the results and
preparation of soil investigation report and
recommendations. The field tests conducted include SPT
at all depths where change of strata occurs and VST for
soft marine clay in bore holes at specified depths in
enclosed bore logs.
The field tests also include collection of US form bore
holes, and DS were collected at every meter depth
intervals or where the strata changes. The samples
collected from the field are subjected to various
laboratory tests including atterberg limits, NMC, dry
density, bulk density, void ratio and specific gravity tests
at each bore. The laboratory test program also includes
grain size analysis tests, undrainedtriaxial tests and
consolidation tests in each bore. The results of laboratory
and field investigations are used for classification of soil,
determination of shear parameters and appropriate
recommendations for foundation.
FIELD INVESTIGATIONS
Actual field investigations were carried out with 14
bore holes using power driven mechanical auger and
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wash water. In this method, water was forced under
pressure through an inner tube which is rotated inside a
casing pipe. The slurry flowing out gives an indication of
the soil type. Whenever a change in strata is indicated by
the slurry flowing out, washing was stopped and a tube
sampler was attached to the end of the drill rod. Soil
samples were obtained by driving the sampler into the
soil. The entire boring operation was conducted in
accordance with the provisions laid in IS: 1892 – 1962
The diameter of the bore hole was 150 mm and casing
was used to support the walls of the bore hole. US
samples were collected using seamless thin walled
sampling tubes at all bore holes. The thin walled sampler
is 100 mm in diameter and 1.7 mm in wall thickness. The
inside and outside clearances as well as the area ratio of
the sampling tube are within the permissible limit. The
collection of UD samples was done as per provisions laid
in IS: 2132 – 1963.
SPT were conducted at change of strata in each bore
hole, depending on the soil strata, throughout the depth
of exploration, the SPT was conducted by driving a split
spoon sampler under the blows of 65 kg weight with a 75
cm free fall. The initial 15 cm penetration was taken as
the seating drive. The number of blows required to drive
the sampler 30 cm beyond the seating drive is taken as
the SPT „N‟ value. Refusal is considered to have been
reached when the rate of advance is less than 2.5 cm for
50 blows. All standard penetration tests are carried out as
per provisions in IS: 2131 – 1963. DS samples were
collected at required intervals to assess the nature of soil
and to evaluate geo – technical properties in the
laboratory.
Field VST is conducted as per IS: 4434 – 1978 code
to determine inplace shearing resistance in saturated soft
marine clay. The test was conducted at various depths.
For conducting tests, the shear – vane is pushed into the
ground up to a depth of 4 times the diameter of the bore
hole or 50 mm whichever is more below the bottom of
the bore hole. It was ensured that no torque is applied to
the torque rods during the thrust. No hammering was
permitted. A minimum period of 5 minutes was allowed
after insertion of the vane. The gear handle is turned so
that the vane is rotated at the rate of 0.1 /sec. the
maximum dial reading attained is noted. From initial and
final dial gauge readings the deflection may be found out.
Torque may be obtained from the calibration chart. By
the torque value shear strength of the soil may be
computed from the height diameter ratio is 2 for the
apparatus using in the field.
SOIL PROFILE
At bore hole number LB – 3, the soil profile consists
of yellowish clay with sand from + 2.632 m to + 2.032 m
followed by soft clay from + 2.032 to – 1.268. Medium
sand from -1.268 to – 3.618, soft clay from – 3.618 to –
6.868, medium sand from – 6.868 to – 12.268, soft clay
from – 12.268 to – 17.268, stiff clay from – 17.268 to –
26.468, coarse sand from – 26.468 to – 30.368, stiff clay
from – 30.368 to – 36.868, yellowish very stiff clay from
– 36.868 to – 47.088, yellowish very stiff clay with
pebbles from – 47.088 to – 55.368, very stiff clay from –
55.368 to – 57.768. Where the bore was terminated the
water table was met with at a depth of 2.5 m below the
EGL.
At bore hole number LB – 4, the soil profile consists
of yellowish silty sand from + 3.076 to + 0.576 followed
by soft clay from + 0.576 to – 6.424, sandy clay from –
6.424 to – 9.674, stiff clay from – 9.674 to – 16.424, stiff
clay with pebbles from – 16.424 to – 23.424, coarse sand
from – 23.424 to – 26.674, yellowish stiff clay from –
26.674 to – 39.174, yellowish very stiff clay with pebbles
from – 39.174 to – 46.124, yellowish very stiff clay with
pebbles and traces of mica from – 46.124 to – 50.924,
very stiff clay with pebbles from – 50.924 to – 57.374.
At bore hole number MB – 1, the soil profile consists
of sandy clay from – 2.268 to – 3.268, medium sand –
3.268 to – 6.768, coarse sand with pebbles sand shells
from – 6.768 to – 17.268, soft clay from – 17.268 to –
27.268, sandy clay from – 27.268 to -29.268, yellowish
very stiff clay from – 29.268 to – 51.968, yellowish very
stiff clay with pebbles from – 51.968 to – 63.018 where
the bore was terminated.
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At bore hole number MB – 2, the soil profile consists
of sandy clay from – 4.297 to – 4.797, medium sand with
traces of silt from – 4.797 to – 10.297, medium sand with
traces of silt and clay from – 10.297 to – 15.047, medium
sand from – 15.047 to – 21.797, soft clay – 21.797 to -
32.297, yellowish stiff clay from – 32.297 to -65.247,
where the bore was terminated.
At bore hole MB – 3, the soil profile consists of sandy
clay from – 1.496 to – 2.246 followed by medium sand
with traces of silt from – 2.246 to – 7.746, medium sand
with traces of silt and clay from – 7.746 to – 13.246,
medium sand – 13.246 to – 17.596, soft clay from –
17.596 to -17.796, medium sand from – 17.796 to –
20.996, soft clay from – 20.996 to – 26.996, yellowish
very stiff clay from – 26.996 to -62.996, where the bore
was terminated.
At bore hole MB – 4, the soil profile consists of sandy
clay from – 1.413 to – 2.163 followed by medium sand –
2.163 to -20.913, coarse sand with shells from – 20.913
to – 23.913, soft clay – 23.913 to – 30.413, yellowish
very stiff clay with pebbles from – 30.413 to – 62.563
where the bore was terminated.
At bore hole MB – 5, the soil profile consists of sandy
clay from – 1.768 to – 3.768 followed by medium sand -
3.768 to – 20.768, soft clay from – 20.768 to – 25.768,
soft clay with pebbles from – 25.768 to – 28.768,
yellowish stiff clay with pebbles from – 28.768 to –
62.718 where the bore was terminated.
At bore hole MB – 6, the soil profile consists of sandy
clay from – 3.925 to – 4.825 followed by medium sand
from – 4.825 to – 24.925, coarse sand with shells –
24.925 to -26.925, soft clay from – 26.925 to – 30.925,
yellowish very stiff clay with pebbles from – 30.925 to -
64.875 where the bore was terminated.
At bore hole MB – 7, the soil profile consists of sand
clay with silt from – 5.168 to – 6.918 followed by
medium sand – 6.918 to – 9.268, sandy clay – 9.628 to –
9.668, medium sand with traces of silt from – 9.668 to -
24.168, soft clay with pebbles from – 24.168 to – 27.168,
yellowish clay with pebbles from – 27.168 to -66.318
where the bore was terminated.
At bore hole MB – 8, the soil profile consists of sandy
clay from – 4.113 to – 5.613, medium sand with traces of
silt from – 5.613 to – 22.613, soft clay with pebbles from
– 22.613 to – 28.113, yellowish very stiff clay with
pebbles from – 28.113 to – 64. 963, where the bore was
terminated.
At bore hole MB – 9, the soil profile consists of
medium sand from – 10.250 to – 17.45 followed by soft
clay with pebbles from – 17.45 to – 31.05, yellowish
very stiff clay with pebbles from – 31.05 to – 72.20
where the bore was terminated.
At bore hole LB – 5, the soil profile consists of
brownish stiff clay from + 2.892 to – 1.908 followed by
medium sand with traces of silt from – 1.908 to -13.108,
soft clay from – 13.108 to – 25.608, yellowish very stiff
clay with pebbles from – 25.608 to – 57.858 where the
bore was terminated.
LABORATORY TEST RESULTS
The various soil samples namely the US samples, the
DS samples and SPT samples were used to determine the
following soil properties.
1) Grain size distribution by wet mechanical
analysis and hydrometer
2) The Atterberg limits i.e., liquid limit, plastic
limit and plasticity index
3) The specific gravity of soil solids
The US samples collected from the field are testes for the
following soil properties
1) Natural moisture content, bulk density and
dry density hence void ratio
2) Shear parameters using triaxial tests in
undrained conditions
3) Consolidation characteristics using
consolidation tests.
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All the soil properties in laboratory are determined as
per the provision in relevant Indian Standards IS: 2720 –
part I through VIII. Using the test results of the soil
sample, the soil at the site at different bores and at
different depths has been classified as per IS: 1498 –
1970.
SILT FACTOR:
Most of the bore holes have medium sand / sandy clay in
the upper region. The mean particle diameter is obtained
from the average grain size distribution curve as shown
in the figure.
Mean particle size
Silt factor
=
1.27
SCOUR DEPTH:
As per Lacey‟s formula
Where Q = Discharge = 56700 cumecs
f = Lacey‟s silt factor for bed material = 1.27
D = mean scour depth
= 16.78 m
Pier scour level
= 4.58 – 2
Dsm
= 4.58 – (2)
16.78
= - 28.98 m
(RL) say – 29 m
Seismic scour level
= 4.58 – (2
* 16.78 * 0.9)
= - 25.62 m
Abutment scour level
= 4.58 –
(1.27 * 16.78)
= - 16. 73
say – 16.75 m
After critical review of field conditions, laboratory
test results and probable foundation systems for bridge
sub – structures, Bored cast – in – situ piles are selected
for bridge abutments and pylons. All piles should be
designed as friction cum end bearing piles
The depth of pile foundation is taken from
consideration of scour, settlement and overall stability.
The depth of foundation is generally governed by scour
depth. All the piles shall be taken up to at least – 29 m
(RL) at all the bore holes investigated for major bridges.
As soft clay layers of large thickness are encountered
in most of the bore holes, considerable amount of
negative drag is expected on piles. However the negative
drag need not be considered under scour conditions. It is
advised to use larger diameter piles which would increase
the negative friction linearly but would also improve the
vertical load carrying capacity at a higher rate that the
negative drag. A factor of safety of 2.5 shall be used on
the ultimate capacity estimated by static formula. In stiff
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clay layer the „C‟ values to be adopted for the pile
capacity are shown in the table 3.
Table 3 „C‟ values for stiff clay layer
Depth (RL) „C‟ value (t/m2)
-30 to -35 m 63.0
-35 to -40 m 54.0
-40 to -45 m 43.5
-45 to -50 m 48.0
-50 to -55 m 48.0
-55 to -60 m 75.0
-60 to -65 m 95.0
3.8 LOAD COMPUTATIONS
The loads to be considered on the bridge are taken as
specified in IRS Bridge rules. The track is a BG-main
line. The width of the track is taken as 1676 mm. The
various load considered to compute stress in the bridge
members as follows:-
1) Dead load
2) Live load
3) Dynamic effects
4) Temperature effect
5) Frictional resistance of expansion bearings.
6) Longitudinal forces
7) Racking force
8) Wind pressure effect
9) Forces on parapets
10) Erection forces and effects.
DEAD LOAD: The dead load is the weight of the
structure and any permanent load fixed thereon. The dead
load is initially assumed and checked after design is
completed.
In this case
=47539.2*1000/400
=118848 kN/m
LIVE LOAD: The actual loads i.e., live loads consist of
axle loads from engine and (i.e., cable stayed bridge of
span 1000m) bogies. In this case it is necessary to
proceed from the basic wheel loads. The EUDLS for
bending moment and shear force for BG main line
loading are obtained by regression analysis
For bending moment
For shear force
Where L is the effective span for bending moment and
loaded length for the maximum effect in the member
under consideration of shear.”L” is expressed in „m‟
In this case the wheel loads are considered as shown in
fig.20.
The train length is taken as 910metres in level grade.
From the entire length of the train the segment of loading
producing worst effect in the constituent members of the
bridge is taken as shown in the fig 20 by trial and error.
It is clearly evident that the load is moving load and the
shear force and bending moments at various sections are
computed by ILDs.
SHEAR FORCE AND BENDING MOMENT
It is evident that the maximum shear force will occur
at the centre of the span. Maximum shear force will
occur when the entire moving load of span 910m is on
the deck.
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In order that maximum positive shear force is
produced the leading load should be at section X which is
at a distance of „x‟ as shown in the figure
Positive shear = 33701.42 kN
In order that maximum negative shear force is
produced the trailing load should be at section „X‟
Negative shear = - 3095.42 kN
As the load crosses the centre of the span „c‟ the negative
shear produced at „c‟ is
F = - 931.814 kN
Now in order to find the absolute bending moment we
should find the centre of gravity of load system.
C.G of load system = 364.53m from first train load as
shown in the figure
It is also known that the absolute maximum bending
moment will occur when the heaviest load is close to the
centre of span.
Assume maximum bending moment will occur under
245.2kN load.
For maximum bending moment to occur the load
should occupy such a position on the beam, that the
centre of the span is midway between the centre of
gravity of the load and 245.2kN load.
Therefore 245.2kN load should be at a distance
= 461m from A as shown in the
figure
The ordinate at the centre is
The absolute maximum bending moment
M = 4536767.218kN-m
The distribution of wheel loads on steel troughing or
steel or wooden beams spanning transversely to the track
and supporting the rails directly shall be designed in
accordance with the constant elastic support theory.
DYNAMIC EFFECTS
When a train moves over a bridge an additional impact
load is caused due to factors such as fast travel of load,
uneven track, rough joints, imperfectly balanced driving
wheels and lateral sway. The increase in load due to
dynamic effects should be considered by adding a load
equivalent to a CDA multiplied by the LL giving the
maximum stress in the member under consideration. The
speeds up to 160 km/h are taken for BG.
For main girders of double track spans with 2 girders
Where „L‟ is loaded length of span in meters for the
position of the train giving the maximum stress in the
member under consideration
L = 910 m
= 0.114
Increase in load due to dynamic effect = 0.114 (47539.2)
= 4519.468 kN/m
TEMPERATURE EFFECT
Where any portion of the structure is not free to
expand (or) contract under variation of temp., allowance
should be made for stresses arising from this condition.
In computing these stresses, the co-efficient of expansion
is assumed as 11.7*10-6 per degree centigrade for steel.
Increase in load due to temperature effects =
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= 0.556 kN/m
FRICTIONAL RESISTANCE OF EXPANSION
BEARINGS
Frictional resistance of expansion bearings has to be
taken into account; the co-efficient of friction for steel
bearings of steel on steel (or) cast iron is 0.25.
For expansion (or) contraction of the structure, due to
variation of temperature under dead load, the friction on
the expansion bearing shall be considered as an
additional load throughout the chord to which the bearing
plates are attached.
Load increase due to frictional resistance of expansion
bearings
=
11884.8 kN/m
LONGITUDINAL FORCES
Longitudinal loads are caused due to one (or) more of the
following Causes:
1) The tractive effort of the driving wheels of
locomotives = 490.3kN/m
2) The braking force due to application of the
brakes to all braked vehicles.
Braking force per locomotive =
0.25*axle load
= 0.25*245.2
= 61.3kN/m
Braking force per train load =
0.25*train load
= 0.25*80*150
= 3000kN/m
3) Resistance due to the movement of bearings
due to change in temperature. These forces are
considered as acting horizontally through the
grider seat where the girders have sliding
bearings. For spans supported on sliding
bearings, the horizontal loads are d ivided
equally between the two ends.
The loaded length „L‟ is taken as follows:
1) The length of one span, when
considering the effect of the
longitudinal loads on
The girders
The stability of abutments
The stability of piers under
the condition of span
loaded, or when piers carry
one fixed and one roller
bearing.
2) The length of two spans, when
considering the stability of piers
carrying fixed or sliding bearing for
the condition of both spans loaded. In
this latter case, the total longitudinal
force is to be divided between the 2
spans in proportion to their lengths.
3) For determining the value of tractive
effort, L should not be taken to
exceed 29m for BG. Where the
structure carries more than one track,
the longitudinal loads shall be
considered to act simultaneously on
all tracks. The maximum effect on
any girder with 2 tracks so occupied
should be allowed for, but with more
than 2 tracks a suitable reduction
may be made on the loads for the
additional tracks.
Total longitudinal loads =
3551.6 + 887.9
=
4439.5kN/m
RACKING FORCES
Lateral bracings of the loaded deck of railway spans
should be designed to resist, in addition to the wind and
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centrifugal forces, a lateral load due to racking forces of
5.9KN/m is treated as a moving load. This lateral load
need not be considered for computing the stresses in
chords or flanges of main members.
FORCES ON PARAPETS
Railings or parapets should have a minimum height
above the adjacent roadway or footway surface of 1m
less one half of the horizontal width of the top rail (or)
top of the parapet. They are to be designed to resist a
horizontal force and the vertical force each of 1.5 kN/m
applied simultaneously at the top of the railing (or)
parapet.
WIND PREESURE EFFECT
The basic wind pressure is obtained from IS: 875. No
live load on the bridge need to be considered when the
basic wind pressure at deck level exceeds for BG bridges
1.5kN/m2.
a) For unloaded spans:
One and half times the horizontal
projected area of the spans for decks
other than plate girders.
For plate girders, the area of the
windward girder plus a fraction as
below of the area of the leeward
girder.
For spacing of leeward girder.
Less than half its depth 0
Half depth to full depth 0.25
Full depth to 1.5 depth 0.50
1.5 depth to 2.0 depth 1.00
b) For loaded spans:
The area as above for the unloaded portion, plus the area
of the windward girder above and below the moving load
plus the horizontal projected area of the moving load. For
railway bridges, the height of the moving load is the
distance between the top of the highest stack for which
the bridge is designed and the rail level, less than
600mm. In case of foot bridges, the height of moving
load is to be taken as 2m throughout the span.
3.9 DECK DESIGN
Generally, the main girders require web stiffening
(either transverse or both transverse and longitudinal) to
increase efficiency. Sometimes variations of bending
moments in main girders may require variations in flange
thickness to obtain economical design. This may be
accomplished either by welding additional cover plates
or by using thicker flange plate in the region of larger
moment. In very long continuous spans (span > 50 m)
variable depth plate girders may be more economical.
MAIN GIRDER DESIGN
The main span is taken as 1000 m and the side spans are
399.4 m each. The plate girders for the main span are
designed here and a similar consideration is considered
for the side spans also. The loads are moving loads and
the deck is designed to withstand the worst position of
loading i.e., the maximum bending moment is considered
for the loading which causes maximum stress in the
members.
Total Load = 188731.52 kN/m for 1000 m
As it is not feasible to design a plate girder for the entire
span the span of each plate girder is taken as 200 m.
Effective span (L) = 200m
Maximum Bending Moment is calculated as follow
As the effective span is taken as 200 m the load should
also be considered proportionally in order that the design
is economical
Therefore, for 200 m span length
Total Load = 37746.304 kN/m
L = 200m
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=
188731520 kN –
m
=
188731520*106 N – mm
Maximum shear force is calculated as follows
= 3774630.4 kN
Design of Web
Assume
= 372025.690 mm
= 372.025 m
= 101.46 say 110
mm
Provide a web of =
(372025.690*110) mm
=
40922825.9 mm2
=
40.92 m2
Design of Flange Plate
=
307459
2.494
mm2
Assume b=1500 mm
= 2049.7
mm say 2100 mm
Therefore provide 4 plates
Thickness of plates = 525mm
Size of the flange plate = (1500*525) mm2
= 376225.68 mm
= 376.225 m
=188112.84 mm
=
188112.84 +
1050
=189162.845
mm
= 5.283447875*1017 mm4
= 67.19 N/mm2
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67.19 < 165 N/mm Hence
it is safe
Curtailment of Flange Plates
X1 = 308.67 mm say 310 mm
X2= 436.525 mm say 440 mm
X3 = 534.632 mm say 535 mm
Design of Stiffeners
It is essential to provide stiffeners at appropriate
distance in order that the plate girder is safe.
Bearing Stiffeners:
The permissible bearing stress is given by
= 187.5 N/mm2
= 20131362.13
mm2
= 20.13 m2
Therefore t = 0.91 m
b = 10.9 m
Hence provide a plate (10.9 * 0.91) m as bearing stiffener
Intermediate Stiffener:
= 92.23 N/mm2
1) = 2107.301 mm
2) = 2225.392 mm
3) = ( ) / 85 = 2225.392 mm
The actual thickness of web 110 mm is less than the
above values hence vertical stiffeners should be
provided.
In case only vertical stiffeners are provided then the
thickness of web required is as follows:
1) = 934.664 mm
2) = 945.814 mm
But the actual web thickness of web is 110 mm,
therefore both vertical and horizontal stiffeners are
necessary. After providing the stiffeners the
values should be as follows:
1) = 747.73 mm
2) = 756.651 mm
Since even now the actual thickness of web is less the
required thickness a horizontal stiffener should be
provided at the Neutral Axis. The stiffeners are steel
plates of (10.9 * 0.91) m size with a spacing of 5 m
between each other. All the connections are butt welds
and the design procedure is done in accordance with IS:
800.
CROSS GIRDER DESIGN
A cross girder is very important as it connects the two
main girders and is also responsible for supporting the
bridge against all the lateral forces. A cross girder is a
beam column acted upon by the lateral forces i.e., the
wind pressure etc... and also a concentrated transverse
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load at the centre of the span of the girder. As the bridge
is a double track BG bridge the width of the bridge is 4.5
m.
For a span of 1000 m 100 cross girders placed at a
spacing of 10 m are found necessary to balance the
lateral force and provide bracing of the main girders.
Total Lateral Force is found out to be P = 8900 kN
The concentrated load acting on each cross girder is W =
475.392 kN
The maximum bending moment is found using the
following formula
= 9334869.523
N – mm
= 9.33 kN –m
Steel ISHB sections are provided as cross girders.
3.10 PYLON DESIGN
The towers are the most visible elements of a cable-
stayed bridge. Therefore, aesthetic considerations in
tower design are very important. Generally speaking,
because of the enormous size of the structure, a clean and
simple configuration is preferable. Though concrete is
the best choice for the pylon, in this case a steel tower is
considered as the loads are large and concrete is not
economical. Steel tubular members are therefore
considered in the pylon design. The pylon is subjected
primarily to compressive loads. The forces acting on the
deck are transferred to the cable in the form of tension;
these tensile forces in turn are transferred to the pylon.
Therefore the pylon is designed to withstand the axial
forces to which it is subjected. Tubular sections are an
economical choice especially if the member is designed
to resist axial forces. The round tubular sections have 30
to 40 percent less surface area than that of an equivalent
rolled steel shape. Therefore, the cost of maintenance,
cost of painting reduces considerably. The tubular
sections are used to advantages in structural designed for
material handling equipment like a bridge where weight
savings are a substantial economic consideration. Apart
from these moisture and dirt do not collect on the smooth
external surface of the tubes thereby reducing the
possibility of corrosion. The steel tubes here are taken as
per the specifications in IS: 228 and steel tables.
The direct stress in compression on the cross sectional
area of axially loaded steel tubes should not exceed the
values of 𝞼c as per IS: 806. The maximum shear stress in
a tube is calculated by dividing the total shear by an area
equal to half the net cross – sectional area of tube, and
this should not exceed the 𝞼b values mentioned in IS:
806.
The round tubular sections provide the most efficient
cross – sectional shape for compression members having
lateral restraint in all directions normal to the axis of the
member. The diameter of the member should be as large
as possible with the additional requirement of d/t should
be small enough to assure that pressure failure by local
buckling will not occur.
The local buckling strength of very short perfect tubes
depends primarily on L/d ratio. The local buckling is
obtained using the following equation.
Where C is approximately equal to 0.6
In the design of steel tubular compression members
attention is to be paid against crinkling and heat
treatment. The yield strength of mild steel considerably
reduces by any heat treatment which it receives such as
welding. On this account, the precautions should be taken
to prevent heat treatment or the strength must be taken as
that of the annealed material. The former is the better
option as it is both safe and economical. The effective
length is taken as per the specifications in IS: 800. In
addition to this the member should also satisfy minimum
thickness requirement.
The steel tubes used for construction exposed to the
weather should not be less than 4 mm thick and for
construction not exposed should not be less than 3.2 mm
thick when the structures are not readily accessible for
maintenance, the minimum thickness should be 5 mm.
The thickness can be found out using the following
formula.
= 1.47 m
The crinkling is calculated as follows
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Where p is stress causing the collapse
t is thickness of the tube
R is Mean radius of tube
3.11 CABLES DESIGN
The basic element for all cables to be found in modern
cable supported bridges is the steel wire characterized by
a considerably larger tensile strength than that of
ordinary structural steel. In most cases, the steel wire is
of cylindrical shape with a diameter between 3 and 7
mm. typically, a wire with a diameter of 5–5.5mm is
used in the main cables of suspension bridges whereas
wires with diameters up to 7mm are used for parallel
wire strands in cable stayed bridges. In the present cable
– stayed bridge a harp type cable system is adopted.
Cables are the most important elements of a cable-stayed
bridge. They carry the load of the girder and transfer it to
the tower and the back-stay cable anchorage. The actual
stiffness of an inclined cable varies with the inclination
angle, a, the total cable weight, G, and the cable tension
force, T.
EA(eff) = EA {1+ G2 EA cos2 a(12 T3)}
Where E and A are Young‟s modulus and the cross-
sectional area of the cable. And if the cable tension T
changes from T 1 to T 2, the equivalent cable stiffness
will be
EA(eff) = EA {1+ G2 EA cos2 a(T1+ T2) (24 T12 T22 )}
In most cases, the cables are tensioned to about 40%
of their ultimate strength under permanent load
condition. Under this kind of tension, the effective cable
stiffness approaches the actual values, except for very
long cables. However, the tension in the cables may be
quite low during some construction stages so that their
effectiveness must be properly considered.
A safety factor of 2.2 is usually recommended for cables.
This results in an allowable stress of 45% of the
guaranteed ultimate tensile strength (GUTS) under dead
and live loads. It is prudent to note that the allowable
stress of a cable must consider many factors, the most
important being the strength of the anchorage assemblage
that is the weakest point in a cable with respect to
capacity and fatigue behaviour. Therefore consider steel
tables of 7 mm diameter spanning 30 m on the deck.
Hence the number of cables is 17.
3.12 FOUNDATION DESIGN
Taking into consideration the soil characteristics it is
found that pile foundations are the most suitable type of
foundation in this case. The bearing capacity for pylons
and abutments is as under.
For pylon locations, bearing capacity is calculated with c
= 0.8 kg/cm2 and ѳ = 80
Calculation of Bearing Capacity for Pylon Foundation
For C = 0.8 kg/cm2
ѳ = 80
Nc = 7.606
Nq = 2.110
Nr = 0.912
Therefore
Sc = 1.3
Sq = 1.2
Sr = 0.6
Hence
dq = dr = 1 for ѳ = 80
For α = 0
ic = iq = ir = 1
w‟ = 1
= 141.79 t/m2
𝞼all = 141.79 /2.5 = 56.7 t/m2
Addition should be made to the bearing
capacity for skin friction
As per IS: 2911 for cohesive soil,
Skin friction (Qs) = α C
As
Here α = 0.3
C = 8
t/m2
Hence Qs = 0.3 * 8 *λ DH = 636.4
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D = 6 m and H =
14 m
𝞼all = 7.1 t/m2
Therefore total all = 7.1 + 56.7 = 63.8 t/m2
Surcharge = 14 * 1 = 14 t/m2
Total gross bearing capacity = 63.8 + 14 = 77.8 t/m2
Under seismic / wind case, bearing capacity = 77.8 *
1.25 = 97.25 t/m2
Calculation for Bearing Capacity for Abutment A1
For C = 0.1 kg/cm2
ѳ = 220
Nc = 17.19
Nq = 8.1
Nr = 7.59
Therefore
Sc = 1.3
Sq = 1.2
Sr = 0.6
Hence
For α = 0
ic = iq = ir = 1
w‟ = 1
= 210.83 t/m2
𝞼all = 210.83 /2.5 = 84.3 t/m2
Surcharge = 14 * 1 = 14 t/m2
Total gross bearing capacity = 84.3 + 14 = 98.3 t/m2
Calculation for Bearing Capacity for Abutment A2
For C = 0.45 kg/cm2
ѳ = 190
Nc = 14.060
Nq = 5.908
Nr = 4.842
Therefore
Sc = 1.3
Sq = 1.2
Sr = 0.6
Hence
For α = 0
ic = iq = ir = 1
w‟ = 1
= 246.95 t/m2
𝞼all = 246.95 /2.5 = 98.8 t/m2
Surcharge = 14 * 1 = 14 t/m2
Total gross bearing capacity = 98.8 + 14 = 112.8 t/m2
Founding level is at 14 m below scour level and due
to this large embedment, deep seated failure theory
which is nothing but “General shear failure theory” will
be applicable as there is no chance of punching shear
failure. On this basis bearing capacity has been
calculated by “General shear failure theory” as per IS:
6403. The foundation system adopted is group piles at –
45 m reduced level.
CONCLUSION
The main aim of this report is to highlight the advantages
of cable – stayed bridges. The design of a cable – stayed
bridge is presented taking into consideration all the active
forces of nature. The designs have been computed as per
the specifications in the various Indian Standard Codes
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and Indian Railway Standards. The cable-stayed bridge is
related to the cantilever bridge. The cables are in tension,
and the deck is in compression. The spans can be
constructed as cantilevers until they are joined at the
centre. A cable stayed-bridge lacks the great rigidity of a
trussed cantilever, and the continuous beam compensates
for this. Indeed, while a long cable-stayed span is under
construction, there can be great concern about possible
oscillations, until the cantilevers are joined.
Advantages of cable-stayed bridges are that the two
halves may be cantilevered out from each side. There is
no need for anchorage's to sustain strong horizontal
forces, because the spans are self-anchoring. They can be
cheaper than suspension bridges for a given span. Many
asymmetrical designs are possible. They can be built
with any number of towers. The number of cables
required is also less and the time taken for construction is
also very less. The aerodynamic design of a cable stayed
bridge is very effective and in spite of very long spans
the lateral sway due to wind pressure can be easily
countered by a cable – stayed bridge. These bridges do
not block the waterway and thereby provide greater
width and height for navigation.
Disadvantages of cable-stayed bridges are that in the
longer sizes, the cantilevered halves are very susceptible
to wind induced oscillation during construction. The
cables require careful treatment to protect them from
corrosion.
REFERENCES
1) Victor Johnson. D., „Essentials of Bridge
Engineering‟, Sixth Edition.
2) Ponnuswamy. S., „Bridge Engineering‟,
Second Edition.
3) Niels J. Gimsing., „Cable Supported Bridges
Concept and Design‟, Third Edition
4) Bridge Engineering Handbook
5) Indian Railway Standard Bridge Rules
6) Indian Railway Standard Bridge Sub –
structure Code
7) Indian Railway Standard Steel Bridge Code
8) Khurmi .R.S, Theory of Sturctures, Revised
Edition
9) Ram Chandra, Design of Steel Structures,
Volume – 1
10) Troitsky, M.S., „Cable stayed bridges –
Theory and Design‟
11) Department of Roads and Building,
Government of Andhra Pradesh