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Since inception of Indian Railways in 1853 the Railway Engineers
has history of more than 250 years of construction and maintenance of
railway bridges. During the long journey they had achieved several
heights and continuing to excellence. Recently Indian Railways has
either constructed several worldclass bridges or they are in the
process of construction. A technical review of design and construction
of recent bridges (viz. Bogibeel, Chenab and New Jubilee Bridges)
through light on the recent technological advancements attained by
the Indian Railways in the field of bridge engineering.
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The joints of Riveted Steel Truss Railway Bridge consist of gusset
plates which lose their rigidity due to repeated passages of train loads;
therefore the loss of rotational rigidity is to be taken into account in
analysis of Bridge. This joint flexibility tends to alter the vibration
characteristics of the Bridge system and each component of the bridge
responds dynamically to the rapidly varying loads and thus the time
history obtained is a function of load variation and dynamics of the
structure, which consequently affects fatigue life of the bridge
components. In past, effect of semirigid joints has been studied in
case of building frames.
So here the knowledge of semirigid joints on building frame
has been extended to Steel Truss Railway Bridge. This present
article tries to study the influence of joint flexibility on the fatigue
life of 76.2 m Truss bridge due to moving load at different
speeds. The joint rotational stiffness is reduced by 5%, 10%, 25% and
50%. The result of preliminary studies conducted on Steel Truss
Bridge is presented. It is prime facia that upto 50% reduction in
rotational Stiffness of the joints does not affect the stability of the
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In India, Economic progress mainly depends on the railway and is
considered as the Life line of the Nation. India has the second largest
rail network in the world, transporting over four billion people
annually and the total figure of existing railway bridges are approx.
1,20,000. Out of these, 731 are long span open girders, 19014 are
rolled steel joist or plate girders. So it can be seen that more than
20% are Steel girder bridges. Due to continuous movement trains, the
members and their connections are subjected to repeated loadings due
to which the stiffness of the joint gets reduced, which are more prone
to fatigue damage. The conventional static, dynamic or stability
analysis of Steel Trusses bridges assumes that their members are
connected at rigid or hinged joints. However in reality Steel Trusses
are reinforced at their joints by Gusset plates, which possess rotational
flexibility. The presence of this gusset plates has an appreciable effect
on the stiffness of the members of the Bridge and consequently on its
behavior to Static and Dynamic loading. However, the behavior of
connections is neither rigid nor pinned. Structures having such
flexible Joints in which Joint flexibility becomes important are called
as semirigid frame members. In fatigue assessment of the bridge
components the joints are assumed to be rigid as per RDSO, where
joint flexibility is neglected which may affect the dynamic behavior
of the bridge component, consequently its fatigue life. Therefore it
is necessary to evaluate the bridge components for semirigid
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Basic bridge designs are developed from natural bridges- a tree trunk
has fallen across a stream, vines hanging over a river, or stones that
make a stepping-stone path across a shallow stream. These natural
bridges were probably built upon by ancient bridge builders. For
example, someone may have built up the steeping stones, placed flat
stone slabs or logs on top of them, and connected the stones to make a
low bridge. This type of bridge was called a “clapper bridge.” It is one
of the earliest bridge constructions. Such simple bridges are probably
still built today in many places. In general, though, bridge
construction has changed greatly.
The ancient Romans refined bridge building with two important
contributions. Nearly all of their bridges used the arch design- a
structure that can support more weight than a flat surface can. Also,
the Roman’s discovery of natural cement allowed them to build
strong, long-standing bridges. Many of these ancient Roman bridges
are still standing today.
There were excellent bridge builders in Asia, too. Some early
bridges in Asia used a cantilever design. This design enabled the
builder to make simple, long-span bridges across fairly wide rivers.
One famous bridge in China, built about 1300 years ago, is the Great
Stone Bridge. Its graceful arch shape is not the same type of arch
shape used by the Romans. Instead, this bridge is quite low, and the
arch is very shallow.
The Renaissance brought new scientific ideas to bridge building.
Leonardo da Vinci and Galileo developed theories about the strength
of building materials. Their theories have helped architects understand
how to make strong structures from lightweight materials. Bridge
building became more exact as people began to use more
mathematical theories about it. Another new development that
changed bridge building was the development of metal.
About 200 years ago, the first cast-iron bridge was built. This
was the Iron Bridge at Coal brookdale in England. Before that time,
bridges were made of stone, brick, clay, or timber. Eventually,
wrought iron was used instead of cast iron. Much later, steel was
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used. Many new bridges were created and tested during this time. The
Britannia Tubular Bridge, completed in 1850, showed one such new
development. It was built from rectangular tubes of wrought iron.
Similar bridges are often used today.
Other important developments came with the truss bridge and
the suspension bridge designs. The truss is an old design, but it was
improved when scientists and engineers knew enough about science
and mathematics to work out the mechanics of the design. Covered
bridges were usually built on the truss design. Truss bridges were
improved even more when metal was used. The suspension bridge
was another basic design that was changed by the use of metal. The
Brooklyn Bridge is one famous suspension bridge built during that
time. It uses steel wires for the suspension cables.
About a hundred years ago, engineers began using concrete for
bridges. A new method called “prestressing” helps prevent concrete
from cracking after a structure is built. Today, most new bridges are
made of prestressed concrete and steel.
5. GENERAL INFORMATION ABOUT TRUSSES.
A truss is a structure comprising one or more triangular units
constructed with straight members whose ends are connected at
joints or nodes.
If all the bars lie in a plane, the structure is a planar truss
The main parts of a planar truss.
In other words, Trusses are designed to form a stable structure.
6. TYPES OF TRUSSES.
Kingpost and Queenpost
Howe truss bridge
Pratt truss bridge
Warren Truss Bridges
Continuous truss bridges
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6.1 Kingpost and Queenpost
• It is used for simple short-span bridges: 40 feet. (probably, it was
the first used with small open-work bridges).
• Fewest number off truss members.- two diagonal members,
kingpost braces, that meet at the apex of the truss, one horizontal
beam and the king post which connect the apex to the horizontal
• Kingpost braces are in compression, and the Kingpost, in
• It has two vertical post.
• Very strong and stable.
• It´s more stable and can support a wider span than a kingpost
6.2Howe Truss Bridge
• William Howe, 1840.
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• It became very popular and was considered one of the best designs
for railroad bridges back in the day.
•Wooden beams for the diagonal members, which were in
compression. It used iron (and later steel) for the vertical members,
which were in tension.
Figure3 HOWE TRUSS
6.3 Pratt Truss Bridge (1844)
• Very common type but has many variations (Baltimore,
Pennsylvania, and the Parker)
• The basic identifying features are the diagonal web members which
form a V-shape. (Howe truss bridge has a A-shape).
• Maximum length of the this bridge can be 250 feet.
• Commonly used for supporting railways.
•The Pratt truss’s verticals functioned as compression members and
diagonals functioned as tension members.
• The Pratt truss required more iron than a Howe truss, and due to the
increased cost and less rigid construction, builders did not extensively
use it for wooden trusses
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Figure 4 PRATT TRUSS
6.4 Warren Truss Bridge (1848)
• It uses equilateral triangles to spread out the loads on the bridges.
The equilateral triangles minimize the forces to only compression and
tension. The forces for a member switch form compression to tension,
especially to the members near the center of the bridge.
• This bridges are often used with verticals to reduce the panel
size.Without vertical present aesthetically pleasing appearance.
6.5 K-truss Bridges
• The length of members undergoing compression is reduced.
•This reduction in length enables components of bridges to endure the
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• The design is complicated and it is considered to be one of the
hardest bridges to build.
Howrah Bridge, Kolkata, India.
6.6 Continuous Truss Bridges.
• Comparatively, it is more rigid and statically indeterminate
• Only suitable in case where the differential settlements of abutments
and piers are not significant.
• A continuous truss bridge may use less material than a series of
simple trusses, because a continuous truss distributes live loads across
all the spans; in series of simple trusses
• This have been used in span ranges of 150m to 400m.
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Figure6 CONTINUOUS TRUSS
7. TRUSS BRIDGES
Trusses are used in bridges to transfer the gravity load of moving
vehicles to supporting piers. Depending upon the site conditions and
the span length of the bridge, the truss may be either through type or
deck type. In the through type, the carriage way is supported at the
bottom chord of trusses. In the deck type bridge, the carriage way is
supported at the top chord of trusses. Usually, the structural framing
supporting the carriage way is designed such that the loads from the
carriage way are transferred to the nodal points of the vertical bridge
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Figure 7 Some of the trusses that are used in steel bridges
Truss Girders, lattice girders or open web girders are efficient and
economical structural systems, since the members experience
essentially axial forces and hence the material is fully utilised.
Members of the truss girder bridges can be classified as chord
members and web members. Generally, the chord members resist
overall bending moment in the form of direct tension and
compression and web members carry the shear force in the form of
direct tension or compression. Due to their efficiency, truss bridges
are built over wide range of spans. Truss bridges compete against
plate girders for shorter spans, against box girders for medium spans
and cable-stayed bridges for long spans. Some of the most commonly
used trusses suitable for both road and rail bridges.
For short and medium spans it is economical to use parallel chord
trusses such as Warren truss, Pratt truss, Howe truss, etc. to minimize
fabrication and erection costs. Especially for shorter spans the warren
truss is more economical as it requires less material than either the
Pratt or Howe trusses. However, for longer spans, a greater depth is
required at the centre and variable depth trusses are adopted for
economy. In case of truss bridges that are continuous over many
supports, the depth of the truss is usually larger at the supports and
smaller at midspan.
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As far as configuration of trusses is concerned, an even number of
bays should be chosen in Pratt and modified Warren trusses to avoid a
central bay with crossed diagonals. The diagonals should be at an
angle between 50° and 60° to the horizontal. Secondary stresses can
be avoided by ensuring that the centroidal axes of all intersecting
members meet at a single point, in both vertical and horizontal planes.
However, this is not always possible, for example when cross girders
are deeper than the bottom chord then bracing members can be
attached to only one flange of the chords.
8 GENERAL DESIGN PRINCIPLES
8.1 Optimum depth of truss girder
The optimum value for span to depth ratio depends on the magnitude
of the live load that has to be carried. The span to depth ratio of a
truss girder bridge producing the greatest economy of material is that
which makes the weight of chord members nearly equal to the weight
of web members of truss. It will be in the region of 10, being greater
for road traffic than for rail traffic. IS: 1915-1961, also prescribes
same value for highway and railway bridges. As per bridge rules
published by Railway board, the depth should not be greater than
three times width between centres of main girders. The spacing
between main truss depends upon the railway or road way clearances
8.2 Design of compression chord members
Generally, the effective length for the buckling of compression chord
member in the plane of truss is not same as that for buckling out-of-
plane of the truss i.e. the member is weak in one plane compared to
the other. The ideal compression chord will be one that has a section
with radii of gyration such that the slenderness value is same in both
planes. In other words, the member is just likely to buckle in plane or
out of plane. These members should be kept as short as possible and
consideration is given to additional bracing, if economical. The
effective length factors for truss members in compression may be
determined by stability analysis. In the absence of detailed analysis
one can follow the recommendations given in respective codes. The
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depth of the member needs to be chosen so that the plate dimensions
are reasonable. If they are too thick, the radius of gyration will be
smaller than it would be if the same area of steel is used to form a
larger member using thinner plates. The plates should be as thin as
possible without losing too much area when the effective section is
derived and without becoming vulnerable to local buckling. Common
cross sections used for chord members are shown in Fig. 7.
Trusses with spans up to 100 m often have open section compression
chords. In such cases it is desirable to arrange for the vertical posts
and struts to enter inside the top chord member, thereby providing a
natural diaphragm and also achieving direct connection between
member thus minimizing or avoiding the need for gussets. However,
packing may be needed in this case. For trusses with spans greater
than about 100 m, the chords will be usually the box shaped such that
the ideal disposition of material to be made from both economic and
maintenance view points. For shorter spans, rolled sections or rolled
hollow sections may be used. For detailed design of compression
chord members the reader is referred to the chapter on Design of
axially compressed columns.
8.3 Design of tension chord members
Tension members should be as compact as possible, but depths have
to be large enough to provide adequate space for bolts at the gusset
positions and easily attach cross beam. The width out-of-plane of the
truss should be the same as that of the verticals and diagonals so that
simple lapping gussets can be provided without the need for packing.
It should be possible to achieve a net section about 85% of the gross
section by careful arrangement of the bolts in the splices. This means
that fracture at the net section will not govern for common steel
In this case also, box sections are preferable for ease of maintenance
but open sections may well prove cheaper. For detailed design reader
is referred to the chapter on Design of Tension members.
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Figure 8 Typical cross-section for truss members
9. DESIGN OF VERTICAL AND DIAGONAL
Diagonal and vertical members are often rolled sections, particularly
for the lightly loaded members, but packing may be required for
making up the rolling margins. This fact can make welded members
more economical, particularly on the longer trusses where the packing
operation might add significantly to the erection cost. Aesthetically, it
is desirable to keep all diagonals at the same angle, even if the chords
are not parallel. This arrangement prevents the truss looking over
complex when viewed from an angle. In practice, however, this is
usually overruled by the economies of the deck structure where a
constant panel length is to be preferred. Typical cross sections used
for members of the truss bridges
are shown in Fig. 8
9.1 Lateral bracing for truss bridges
Lateral bracing in truss bridges is provided for transmitting the
longitudinal live loads and lateral loads to the bearings and also to
prevent the compression chords from buckling. This is done by
providing stringer bracing, braking girders and chord lateral bracing.
In case of highway truss bridges, concrete deck, if provided, also acts
as lateral bracing support system.
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Figure 9 Lateral bracing systems
The nodes of the lateral system coincide with the nodes of the main
trusses. Due to interaction between them the lateral system may cause
as much as 6% of the total axial load in the chords. This should be
taken into account. Fig.8 shows the two lateral systems in its original
form and its distorted form after axial compressive loads are applied
in the chords due to gravity loads. The rectangular panels deform as
indicated by the dotted lines, causing compressive stresses in the
diagonals and tensile stresses in the transverse members. The
transverse bracing members are indispensable for the good
performance of St. Andrew’s cross bracing system.
In diamond type of lateral bracing system the nodes of the lateral
system occur midway between the nodes of the main trusses [Fig.8].
They also significantly reduce the interaction with main trusses. With
this arrangement, “scissors-action” occurs when the chords are
stressed, and the chords deflect slightly laterally at the nodes of the
lateral system. Hence, diamond system is more efficient than the St.
Andrew’s cross bracing system. It is assumed that wind loading on
diagonals and verticals of the trusses is equally shared between top
and bottom lateral bracing systems. The end portals (either diagonals
or verticals) will carry the load applied to the top chord down to the
bottom chord. In cases, where only one lateral system exists (as in
Semi through trusses), then the single bracing system must carry the
entire wind load.
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10. HOW BRIDGES WORK?
Every passing vehicle shakes the bridge up and down, making waves
that can travel at hundreds of kilometers per hour. Luckily the bridge
is designed to damp them out, just as it is designed to ignore the
efforts of the wind to turn it into a giant harp. A bridge is not a dead
mass of metal and concrete: it has a life of its own, and understanding
its movements is as important as understanding the static forces.
Members of trusses can be joined by riveting, bolting or welding. Due
to involved procedure and highly skilled labour requirement, riveting
is not common these days, except in some railway bridges in India. In
railway bridges riveting may be used due to fatigue considerations.
Even in such bridges, due to recent developments, high strength
friction grip (HSFG) bolting and welding have become more
common. Shorter span trusses are usually fabricated in shops and can
be completely welded and transported to site as one unit. Longer span
trusses can be prefabricated in segments by welding in shop. These
segments can be assembled by bolting or welding at site. This results
in a much better quality of the fabricated structure. However, the
higher cost of shop fabrication due to excise duty in contrast to lower
field labour cost frequently favour field fabrication in India.
If the rafter and tie members are T sections, angle diagonals can be
directly connected to the web of T by welding or bolting. Frequently,
the connections between the members of the truss cannot be made
directly, due to inadequate space to accommodate the joint length. In
such cases, gusset plates are used to accomplish such connections
(Fig. 9). The size, shape and the thickness of the gusset plate depend
upon the size of the member being joined, number and size of bolt or
length of weld required, and the force to be transmitted. The thickness
of the gusset is in the range of 8 mm to 12 mm in the case of roof
trusses and it can be as high as 22 mm in the case of bridge trusses.
The design of gussets is usually by rule of thumb. In short span (8 –
12 m) roof trusses, the member forces are smaller, hence the thickness
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of gussets are lesser (6 or 8 mm) and for longer span lengths (> 30 m)
the thickness of gussets are larger (12 mm).
12. LOADS ON BRIDGES
The following are the various loads to be considered for the purpose
of computing stresses, wherever they are applicable.
Forces due to curvature
Forces on parapets
Frictional resistance of expansion bearings
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12.1 DEAD LOAD
The dead load on a bridge consists of the weight of all its structural
parts and all the fixtures and services like deck surfacing, kerbs,
parapets, lighting and signing devices, gas and water mains,
electricity and telephone cables. The weight of the structural parts has
to be guessed at the first instance and subsequently confirmed after
the structural design is complete.
12.2 Live loads
Bridge design standards of different countries specify the design loads
which are meant to reflect the worst loading that can be caused on the
bridge by traffic permitted and expected to pass over it. The
relationship between bridge design loads and the regulations
governing the weights and sizes of vehicles is thus obvious, but other
factors like traffic volume and mixture of heavy and light vehicles are
also relevant. Short spans, say up to 10m for bending moment and 6m
for shear force, are governed by single axles or bogies with closely
spaced multiple axles. The worst loading for spans over about 20m is
often caused by more than three vehicles.
The worst vehicles are often the medium-weight compact vehicles
with two axles, and not the heaviest vehicles with four, five or six
axles. The criteria thus change from axle loads to worst vehicles as
the span increases, with the mixture of vehicles in the traffic being an
important factor for the longer spans. When axles or single vehicles
are the worst case, the effect of impact has to be allowed for, but
several closely spaced vehicles represent a jam situation without
significant impact. The adjacent lanes of short span bridges may all be
loaded simultaneously with the worst axles or vehicles, but this is less
likely for long spans. Apart from the design loading for normal traffic,
many countries specify special bridge design loading for the passage
of abnormal vehicles of the military type or carrying heavy indivisible
industrial equipment like generators. The passage of such heavy
vehicles on public roads usually involves special permits from the
highway authorities and often supervision by the police. In addition to
these legal heavy loads, there is the growing problem of illegal
overweight vehicles weighing as much as 40% over their legal limits.
In each country traffic regulations limit the wheel and axle loads and
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gross vehicle weights, and impose dimensional limits on axle spacing
and size of vehicles. Goods vehicles may be of the following types:
vehicles with two axles
rigid vehicles with three or more axles
articulated vehicles with two or three axles under the tractor and
one or more axles under the trailer
road trains comprising a vehicle and trailer.
12.3 Longitudinal forces
Longitudinal forces are set up between vehicles and the bridge deck
when the former accelerate or brake. The magnitude of the force is
where W is the weight of the vehicle, g is the acceleration due to
gravity (¼9.81 m/s2) and V is the change in speed in time t.
Usually the change in speed is faster during braking than while
Railway bridges: Railway bridges including combined rail and road
bridges are to be designed for railway standard loading given in
bridge rules. The standards of loading are given for:
· Broad gauge - Main line and branch line
· Metre gauge - Main line, branch line and Standard C
· Narrow gauge - H class, A class main line and B class branch line
The actual loads consist of axle load from engine and bogies. The
actual standard loads have been expressed in bridge rules as
equivalent uniformly distributed loads (EUDL) in tables to simplify
the analysis. These equivalent UDL values depend upon the span
length. However, in case of rigid frame, cantilever and suspension
bridges, it is necessary for the designer to proceed from the basic
wheel loads. In order to have a uniform gauge throughout the country,
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it is advantageous to design railway bridges to Broad gauge main line
standard loading. The EUDLs for bending moment and shear force for
broad gauge main line loading can be obtained by the following
formulae, which have been obtained
from regression analysis:
For bending moment:
EUDL in kN = 317.97 + 70.83l + 0.0188l2 ≥ 449.2 kN (7.1)
For shear force:
EUDL in kN = 435.58 + 75.15l + 0.0002l2 ≥ 449.2 kN (7.2)
Note that, l is the effective span for bending moment and the loaded
length for the maximum effect in the member under consideration for
shear. 'l ' should be in metres. The formulae given here are not
applicable for spans less than or equal to 8 m with ballast cushion. For
the other standard design loading the reader can refer to Bridge rules.
12.4 Impact load
Figure 10 Impact percentage curve for highway bridges for IRC class A and IRC class B
The dynamic effect caused due to vertical oscillation and periodical
shifting of the live load from one wheel to another when the
locomotive is moving is known as impact load. The impact load is
determined as a product of impact factor, I, and the live load. The
impact factors are specified by different authorities for different types
of bridges. The impact factors for different bridge for different types
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of moving loads are given in the table 1. Fig.10 shows impact
percentage curve for highway bridges for class AA loading. Note that,
in the above table l is loaded length in m and B is spacing of main
girders in m.
(a) Single track
(b) Main girder
of double track
with two girders
main girder of
main girders of
(a) or (b)
(e) Cross girders
or more tracks
Rails with ordinary
fish plate joints and
supported directly on
sleepers or transverse
12.5 Thermal forces – The free expansion or contraction of a
structure due to changes in temperature may be restrained by its form
of construction. Where any portion of the structure is not free to
expand or contract under the variation of temperature, allowance
should be made for the stresses resulting from this condition. The
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coefficient of thermal expansion or contraction for steel is 11.7 x 10-6
12.6 Wind load – Wind load on a bridge may act
· Horizontally, transverse to the direction of span
· Horizontally, along the direction of span
· Vertically upwards, causing uplift
· Wind load on vehicles
Wind load effect is not generally significant in short-span bridges; for
medium spans, the design of sub-structure is affected by wind
loading; the super structure design is affected by wind only in long
spans. For the purpose of the design, wind loadings are adopted from
the maps and tables given in IS: 875 (Part III). A wind load of 2.40
kN/m2 is adopted for the unloaded span of the railway, highway and
footbridges. In case of structures with opening the effect of drag
around edges of members has to be considered.
12.7 Racking force – This is a lateral force produced due to the
lateral movement of rolling stocks in railway bridges. Lateral
bracing of the loaded deck of railway spans shall be designed to
resist, in addition to the wind and centrifugal loads, a lateral
load due to racking force of 6.0 kN/m treated as moving load.
This lateral load need not be taken into account when
calculating stresses in chords or flanges of main girders.
12.8 Forces on parapets - Railings or parapets shall have a
minimum height above the adjacent roadway or footway surface
of 1.0 m less one half the horizontal width of the top rail or top
of the parapet. They shall be designed to resist a lateral
horizontal force and a vertical force each of 1.50 kN/m applied
simultaneously at the top of the railing or parapet.
12.9 Seismic load – If a bridge is situated in an earthquake prone
region, the earthquake or seismic forces are given due
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consideration in structural design. Earthquakes cause vertical
and horizontal forces in the structure that will be proportional to
the weight of the structure. Both horizontal and vertical
components have to be taken into account for design of bridge
structures. IS:1893 – 1984 may be referred to for the actual
12.10 Forces due to curvature - When a track or traffic lane on a
bridge is curved allowance for centrifugal action of the moving
load should be made in designing the members of the bridge. All
the tracks and lanes on the structure being considered are
assumed as occupied by the moving load.
12.11 Erection forces – There are different techniques that are used
for construction of railway bridges, such as launching, pushing,
cantilever method, lift and place. In composite construction the
composite action is mobilised only after concrete hardens and
prior to that steel section has to carry dead and construction live
loads. Depending upon the technique adopted the stresses in the
members of the bridge structure would vary. Such erection
stresses should be accounted for in design. This may be critical,
especially in the case of erection technologies used in large span
13. ECONOMY OF TRUSSES
Trusses consume a lot less material compared to beams to span the
same length and transfer moderate to heavy loads. However, the
labour requirement for fabrication and erection of trusses is higher
and hence the relative economy is dictated by different factors. In
India these considerations are likely to favour the trusses even more
because of the lower labour cost. In order to fully utilize the economy
of the trusses the designers should ascertain the following:
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Method of fabrication and erection to be followed, facility for shop
fabrication available, transportation restrictions, field assembly
Preferred practices and past experience.
Availability of materials and sections to be used in fabrication.
Erection technique to be followed and erection stresses.
Method of connection preferred by the contractor and client
(bolting, welding or riveting).
Choice of as rolled or fabricated sections.
Simple design with maximum repetition and minimum inventory
14. ADVANTAGE OF STEEL IN BRIDGE
The steel is a very versatile material having many advantages over the
other material. Presently, the mega bridge projects being undertaken
by the Railways involves steel super structures. For longer spans, the
railway has shown more confidence in steel compared with PSC. This
is basically due to the fact that in case of steel bridges, rehabilitation
procedures are easier and involve lesser delays, inspections are easier
as it allows the deformations to be seen and easily
evaluated/measured besides the basic fact that Railway engineers feel
comfortable in constructing and maintaining steel bridges. Generally
speaking, steel bridges may have the following advantages when
compared to concrete/PSC bridges:
-_Reduced dead loads.
-_More economic foundations.
-_Simpler erection procedures.
-_Shorter execution time.
-_Faster and easier rehabilitation.
When constructed in insurgency affected areas like North-East and
J&K and in high seismicity areas where damage to the bridges is
more likely, steel bridges provides easier and faster options for
rehabilitation. More over, structural redundancies can be easily inbuilt
in steel bridges. A disadvantage of steel when compared to concrete is
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the maintenance cost for the prevention of corrosion. However, it is
now recognized that concrete bridges also have problems relating to
maintenance i.e. relating to the effects of corrosion of steel
reinforcement on the durability of the structure. In addition to the
various points cited above, structural steel as the basic bridge
construction material involves following advantages which have also
played an important part in this shift of Railway engineer’s ideology
from concrete/PSC bridge construction to steel bridge construction: -
14.1 HIGH STRENGTH TO WEIGHT RATIO
High strength to weight ratio of steel minimizes substructure costs,
which is particularly beneficial in poor ground conditions. Minimum
self weight is also an important factor in transporting and handling of
bridge components specially in hilly areas like North-East and Jammu
& Kashmir. In addition, it facilitates very shallow construction depth
for girders, which over come problems with headroom and flood
clearances and minimizing the length of approach ramps. The low self
weight also minimizes foundation work in case of bridges being
constructed near existing rail lines.
14.2 HIGH QUALITY MATERIAL
Steel is a high quality material, which is readily available world wide
in various certified grades, shapes and sizes. The testing regime
carried out at the steel mills imparts confidence to the engineers for
the bridge projects. Prefabrication in controlled shop condition leads
to high quality work at minimum cost. The quality control extends
from the material itself and follows on through the processes of
cutting, drilling, welding and fit-up. The total weight of steel
constructions is a fraction of the total weight of concrete bridges.
Therefore steel bridges can be used with long spans, even in
14.3 SPEED OF CONSTRUCTION
The prefabrication of the components means that construction time on
site in hostile environment is minimized. The light weight nature of
steel permits the erection of large components. Besides this, resource,
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such as water, aggregates etc may sometimes not be easily available
at sites on this project, for the purpose of production of concrete.
The prefabrication of the components means that construction time on
site in hostile environment is minimized. The light weight nature of
steel permits the erection of large components. Besides this, resource,
such as water, aggregates etc may sometimes not be easily available
at sites on this project, for the purpose of production of concrete.
Steel suits a wide range of construction methods and sequences.
Installation may be by cranes, launching, slide-in-techniques or
transporters. For example, in Jammu & Kashmir area on Katra-
Quazigund section, the erection of the main steel arch of Chenab and
Anjikhad bridge is being planned by mechanized rope way. Steel
gives the engineer flexibility in terms of erection sequence and
programme. Components can be sized to suit access restriction at site,
and once erected the steel girders provide a platform for subsequent
14.6 REPAIR & REHABILITATION
Steel is a ‘sustainable’ material. When a steel bridge reaches the end
of its useful life, the girders can be cut into manageable sizes to
facilitate demolition, and returned to steelworks for recycling. The
increased emphasis of the green techniques for construction, steel is
lot ‘Greener’ than concrete for bridges.
Steel bridges can readily be repaired after accidental damages. In case
of damage to the bridge due to derailment/accident, damage due to a
terrorist activity or damage due to natural causes such as earthquakes,
floods etc. complete steel spans can be replaced without much delay
which is not the case with PSC super structures. This aspect is very
important in the case of Railways where longer disruption to rail
traffic can not be afforded.
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Steel bridges now have a proven life span extending to well over 100
years. In fact, old steel girders of vintage 1854 etc are also in use on
branch lines. Steel has a predictable life, as the structural elements are
visible and accessible. Any signs of deterioration are readily apparent,
without the need for extensive investigations. Direct measurements of
stresses are possible as all the parts / members are accessible and
thickness of members is less. Corrosion is a problem, which can,
however, be addressed by timely painting. In addition, the latest
coatings are anticipated to last well beyond 30 years before requiring
The potential durability of steel may be summarized in the following
quote by a Mr. J.A.Waddell in 1921:
“The life of a steel bridge that is scientifically designed, honestly and
carefully built, and not seriously overloaded, if properly maintained,
is indefinitely long.”
15. COMPONENTS OF SUPERSTRUCTURE OF
A STEEL TRUSS BRIDGE
15.1 Top chords:
• The most highly stressed compression members.
• Need special attention while proportioning and detailing.
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Figure 12 Common cross-sections of top chords
15.2 Bottom chords:
• The most highly stressed tension members.
• Connections may be welded, bolted or riveted.
Figure 13 Common cross-sections of bottom chords
15.3 Web members.
• These members could be diagonals and verticals and may be
subjected to tension and some to compression. ( it depends on the type
of the truss)
• Vertical members working at compression are termed “post” and
those in tension are called “hangers”
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Figure 14 Typical crosssections of web members .
15.4 End posts or rakers:
• These are located at the ends of a truss to carry the lateral forces
from the top chord level to the bridge bearings.
• For this purpose portal bracings are fixed onto them at the upper
15.5 Bracing system:
• Considered as secondary members, but in fact, vital for the
successful performance of the primary members.
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Bracings are designed to resist two types of forces:
• Lateral forces: those acting transverse to the axis of the bridge.
• Longitudinal forces: Those acting along the axis of the bridge.
• Placed between the top chords and bottom chords of a pair of
• In a road bridge, the deck slab can act as a stiffening member
between the trusses.
Figure 16 Lateral bracing systems
15.6 Sway bracing or cross bracing:
• Placed between trusses.
• Are provided for distributing the transverse loads to the lateral
• Also for providing torsional rigidity to the truss frame
Figure 17 Sway bracing
15.7 Portal bracings :
• Located at the end posts or rakers.
• Provide end supports to the top lateral bracing system.
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Figure 18 Portal bracing
16. ANALYSIS AND DESIGN OF TRUSSES
Stability and Determinacy
• A stable and statically determinate plane truss should have at
least three members, three joints and three reaction components.
• To form a stable and determinate plane truss of “n” joints:
• The number of members (m) = 2n-3
• If the stable, simple, plane truss has more than three reactions
components, the structure is externally indeterminate.
• If it has more than (m>2n-3) members, the structure is internally
indeterminate and hence all of the member forces cannot be
determined form the 2n available equations of static method of joints.
• Truss analysis gives the bar forces in a truss, for a statically
determined truss, these bar forces can be found by employing
the laws of statics to assure internal equilibrium of the
• Method of Joints.
• Method of Sections.
For indeterminate truss:
• Virtual work.
• Finite Element Method
• Computer Analysis.
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17. METHOD OF JOINTS.
• Chose a node whose number of unknown forces doesn’t exceed two,
then study its equilibrium using static equilibrium equations to
determine these forces:
• ΣFx = 0, ΣFy = 0 and ΣM = 0
• Go to next node and study its equilibrium using the evaluated forces
from the previous node then go to the next node and so one.
Figure 19 Method of Joints.
18 METHOD OF SECTIONS
• If only a few member forces of a truss are needed, the quickest way
to find these forces is by this method.
• In this method, an imaginary cut (section) is drawn through a stable
and determinate truss. Thus, a section subdivides the truss into two
separates parts. Since the entire truss is in equilibrium, any part of it
must also be in equilibrium.
•ΣFx = 0, ΣFy = 0 and ΣM = 0.
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19. DEFLECTION OF A TRUSS
• The virtual work method can be used to determine the
deflection of trusses.
• with n equal to the virtual force in the member and
equal to the change in length of the member.
• A deflection is caused by three ways:
• Applied loads acting on each member.
• Temperature changes
• Fabrication errors.
• Axial deformation:
• When the force of all the members are known we can determine the
axial deformation of each member by using the equation:
• If we modified the value of into the equation for the deflection.
m= number of members.
n= the force in the member due to the virtual load.
N= the force in the member due to applied load.
E= Young´s Modulus of Elasticity.
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• Temperatures Changes= is the coefficient of thermal expansion
L = length of the member
AT= Change in temperature.
• Fabrication Errors:
• The original equation for deflection of a truss can be modified
K= number of members undergoing fabrication errors.
n= force in the member due to the virtual load
= the change in length of the member due to fabrication errors.
Total deflection of a Truss (Hibbeler, Structural Analysis)
20. DETERMINACY OF COPLANAR TRUSSES
Since all the elements of a truss are two-force members, the
moment equilibrium is automatically satisfied.
Therefore there are two equations of equilibrium for each joint, j,
in a truss. If r is the number of reactions and b is the number of bar
members in the truss, determinacy is obtained by
b + r = 2j Determinate
b + r > 2j Indeterminate
21. STABILITY OF COPLANAR TRUSSES
If b + r < 2j, a truss will be unstable, which means the structure
will collapse since there are not enough reactions to constrain all
A truss may also be unstable if b + r ≥2j. In this case, stability will
be determined by inspection
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b + r < 2j Unstable
b + r 2j Unstable if reactions are concurrent, parallel, or collapsible
21.1 External stability - a structure (truss) is externally unstable if its
reactions are concurrent or parallel
21.2 Internal stability - may be determined by inspection of the
arrangement of the truss members.
A simple truss will always be internally stable
The stability of a compound truss is determined by examining how
the simple trusses are connected
The stability of a complex truss can often be difficult to determine
In general, the stability of any truss may be checked by performing
a complete analysis of the structure. If a unique solution can be
found for the set of equilibrium equations, then the truss is stable.
Externally stable Internally stable
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Externally stable Internally unstable
Externally stable Internally unstable
22. TRUSS MEMBERS ARE CONNECTED BY
The stress produced in these elements is called the primary stress.
The pin assumption is valid for bolted or welded connections if the
members are concurrent.
However, since the connection does provide some rigidity, the
bending introduced in the members is called secondary stress.
Secondary stress analysis is not commonly performed.
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23. ALL LOADING IS APPLIED AT THE JOINTS
OF THE TRUSS
Since the weight of each members is small compared to the
member force, the member weight is often neglected.
However, when the member weight is considered, it is applied at
the end of each member.
Because of these two assumptions, each truss member is a two-
force member with either a compressive (C) or a tensile (T) axial
In general, compression members are bigger to help with instability
due to buckling.
24. CLASSIFICATION OF COPLANAR
The simplest framework that is rigid or stable is a triangle.
Therefore, a simple truss is constructed starting with a basic
triangular element and connecting two members to form additional
As each additional element of two members is placed on a truss,
the number of joints is increased by one.
Figure26 SIMPLE TRUSS
24.2 Compound Truss
This truss is formed by connecting two or more simple trusses
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This type of truss is often used for large spans.
There are three ways in which simple trusses may be connected to
form a compound truss:
1. Trusses may be connected by a common joint and bar.
2. Trusses may be joined by three bars.
3. Trusses may be joined where bars of a large simple truss, called the
main truss, have been substituted by simple trusses, called secondary
Figure27 COMPOUND TRUSS
25. FORCE ANALYSIS (TRUSS)
• Loads members in tension and compression.
• Members are pinned at joints (Moment = 0).
• Triangles provide stability and strength.
• Top members in Compression.
• Bottom members in Tension.
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26. APPLICABLE SPAN OF TRUSS BRIDGE
Truss brides are generally applied within the following range.
1. Simple truss bridge is in the range of 55 meter to 85 meter span.
2. Continuous truss bridge is in the range of 60 meter to 300 meter
3. Cantilever truss bridge is in the range of 300 meter to 510 meter
span (in Japan, only one bridge has longer span than 200 meter,
and that is Minato Ohashi, with 510 meter span.)
Among the 3 types of bridges, the simple truss bridge or the
continuous truss bridge, either with approximate 60 meter to 100
meter span is usually applied.
27. ADVANTAGES OF TRUSS BRIDGE
27.1 Mountain Region
1. When the members are difficult to be transported to the site and
when the conditions of construction is restricted.
2. When a bridge in a curve alignment is required, a horizontal bent
continuous bridge or a deck truss bridge with brackets can selected.
27.2 Open Field Region
1. Assurance of space below the bridge soffit due to adoption of
2. Long span bridge over the mouth of a river or the coast.
3. Double-deck bridge having upper and lower 2-layer road surface.
28. DISADVANTAGES OF TRUSS BRIDGE
1. As it is composed by many members, maintenance requirements
such as repainting is needed.
2. The sight view from the driver in case of through bridges.
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29. OVER ALL HEIGHT OF TRUSS
AND LENGTH BETWEEN PANELS
1. For simple truss, the truss height is 1/7-1/8 time the span length
and the length between panels is 1/6-1/8 time the span length.
2. For continuous truss, the truss height is 1/9- 1/10 time the span
length and the length between panels is 1/8-1/10 time the span
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1. Bridge components are having substantial fatigue life even after
considering the Joint Flexibility.
2. Joint flexibility tends to alter the vibration characteristics of
each component to loading environment in presence of realistic
damping of 2%, thus the damage
potential of each member which depends upon the stress range and
cycle counts is also got affected ,however the change was only
3. In most of members fatigue life got increased, however life of
some component got decreased, the maximum decrease observed is
about 40% in one of Bottom Chords
4. The variation in passage to failure exhibited by each component
with the different speeds makes it difficult to find a particular
trend, however the trend is similar at
different flexibilities with change only in magnitudes.
5. It can be concluded that the reduction of joint rotational stiffness
up to 50% has less effect on structural stability of Steel Truss Railway
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In comparison to the developed countries, the steel being used in
Indian railways is of inferior quality. Following suggestions
recommendations are given for early adoption of High Performance
steel over Indian Railways:-
1. Furthermore studies should be conducted for the adoption of HPS
or any other type of steel which suits Indian conditions and economy.
2. The Railway Board in consultation with RDSO may jointly discuss
the issue for convincing the steel industry including SAIL for
producing the special type of steel for Indian bridges.
3. A pilot project should be given to each railways for applying the
High Performance steel in at least one bridge so that experience in the
same can be gained.
4. The major supplier of HPS in US and Europe is Arcelor-Mittal
steel and Corus Group (recently tied up with Tata steel) and they have
a very good Indian connection. These groups may be approached for
their help and guidance.
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32.Marvels of truss bridge in India:
Howrah Bridge – The Bridge without Nuts & Bolts!:
How about visiting a vintage bridge which has no nuts & bolts
in its construction but still standing tall for the last 66 years?
Hard to believe? The Bridge in concern - one of the busiest in
the world - is located at Howrah in West Bengal. The Howrah
bridge, the sixth longest of its type, has been an emblem of the
city of Kolkata from its inception. So much so that the world
knows Kolkata by its trams, the Victoria Memorial, and of
course the Howrah Bridge. Opened to traffic in 1943, the
construction of the bridge was started in 1937. The bridge has
remained one of the most renowned landmarks of Kolkata.
More than 150,000 vehicles and 4,000,000 pedestrians cross
over the bridge every day. Technically speaking, Howrah
Bridge is a "Cantilever Truss" bridge, constructed entirely by
riveting, without nuts or bolts!
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Notable features of howrah bridge:-
705 meters in length, 97 feet in width, 82
meters in height
26,500 plus mega tonne of high-tensile steel
Suspension type Balanced Cantilever
325 ft, length of each anchor arm
468 ft, length of each Cantilever arm
564 ft, suspended span
Deck width 71 ft, footpath 15 feet on either
No nuts & bolts
Total 8 articulation joints, 3 at each of the
cantilever arms, and 2 in the suspended
Carriageway Minimum headroom is 5.8 m
River traffic freeboard is 8.8 m
Ranks sixth in World’s top 10 longest
What is a “Cantilever Truss” bridge?
A Cantilever bridge is a type of bridge constructed using cantilevers
only. Cantilevers are constructions that protrude horizontally into
space, secured on one end of the structure! In case of small
footbridges the technique is quite simple however, for huge bridges
the volume of work is enormous. Steel truss cantilever (STC) was one
of the newer technologies in the 1930s which was used in building
Howrah Bridge. The advantage of STC was the lack of complexity in
designing and implementation that included little or no falsework.
Falsework, in layman’s term, is the temporary construction, provided
externally to a structure, till the time it needs no extraneous support to
stand on its own architecture. For lovers of vintage architecture, the
finest example of the above mentioned architecture lies en
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The Godavari Bridge or Kovvur-Rajahmundry Bridge
The godavri bridge is a truss bridge spanning Godavari
river in Rajahmundry, India. It is Asia's second longest road-cum-rail
bridge crossing a water body, after the Sky Gate Bridge in Kansai
International Airport, Osaka. It is second of the three bridges that
span the Godavari River at Rajahmundry. The Havelock Bridge being
the earliest, was built in 1897, and having served its full utility, was
decommissioned in 1997. The latest bridge is the Godavari Arch
Bridge, a bowstring-girder bridge, built in 1997 and presently in
The bridge is 2.7 kilometers long consisting of 27 spans of 91.4 m and
7 spans of 45.72 m of which 6 spans of 45.72m are in 6 deg. curve at
long Rajahmundry end to make up for the built up area. The bridge
has a road deck over the single track rail deck, similar to the Grafton
Bridge in New South Wales, Australia. This bridge, in addition
to Godavari Arch Bridge, has been widely used to
represent Rajahmundry in arts, media, andculture. It is one of the
recognised symbols of Rajahmundry.
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1. “Use and Application of High-Performance steels for steel
structures” Structural Engineering documents No 8 Published by
IABSE Oct 2005.
2. NBSA White paper, “Advances in High performance steels for
Highway bridges” by Alexander D Wilson, manager customer
technical service, Mittal Steel USA
3. “Prospects of High-Performance welded steel bridge” , Advances
in bridge Engineering, Mar 24-25, 2006 by
P.K.Ghosh,Professor,Department of Metallurgical and Material
Engineering, IIT , Roorkee.
4. “Improvements to High Performance steel “ by A. D. Wilson ,
Mittal Steel USA
5. IS 2062:1999 “Steel for general structural purposes”- specification,
6. IS 8500 1977 “Specification for Weld able structural steel”
( Medium and High strength qualities ) , BIS, N. Delhi
7. “Steel Structures, Design and Behaviour” by Charles G. Salmon &
John E.Johnson , Harper & Row
13. Anon, “Design of Composite Trusses”, Steel Construction
Institute”, Ascot, 1992.
14. Anon “Constructional Steel Design: An International Guide”,
Elsevier, London, 1993.
15. S.Venkateswara Rao, and M.Prabhakar, Prestressingfor
economical Bridge construction”, Indian Highways (IRC), 1990.
16. Vallenilla, C.R., and Bjorhovde, R., “Effective Width Criteria for
Composite Beams”, Engineering Journal, AISC, Fourth Quarter,
1985, pp. 169-175.