project report on truss bridge

Department of Civil Engineering Page 1

1. SYNOPSIS

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.


2. ABSTRACT

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

bridge.


3. INTRODUCTION

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

connections.


4. HISTORY 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


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

K-truss bridges

Continuous truss bridges


6.1 Kingpost and Queenpost Kingpost: • 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

beam below.

• Kingpost braces are in compression, and the Kingpost, in

compression. CHECAR

Figure1 KINGPOST

Queenpost:

• It has two vertical post.

• Very strong and stable.

• It´s more stable and can support a wider span than a kingpost

Figure2 QUEENPOST

6.2Howe Truss Bridge • William Howe, 1840.


• 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


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

compressional force.


• The design is complicated and it is considered to be one of the

hardest bridges to build.

Howrah Bridge, Kolkata, India.

Figure5 K-TRUSS 6.6 Continuous Truss Bridges.

• Comparatively, it is more rigid and statically indeterminate

structure.

• 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.


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

trusses.


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.


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

required.

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


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

grades.

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.


Figure 8 Typical cross-section for truss members

9. DESIGN OF VERTICAL AND DIAGONAL

MEMBERS

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.


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.


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.

11. CONNECTIONS

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


of gussets are lesser (6 or 8 mm) and for longer span lengths (> 30 m)

the thickness of gussets are larger (12 mm).

Figure 9

12. LOADS ON BRIDGES

The following are the various loads to be considered for the purpose

of computing stresses, wherever they are applicable.

Dead load

Live load

Impact load

Longitudinal force

Thermal force

Wind load

Seismic load

Racking force

Forces due to curvature

Forces on parapets

Frictional resistance of expansion bearings

Erection forces


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


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

given by

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

accelerating.

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,


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

loadings

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


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.

TABLE 1

BRIDGE

LOADING

LOADING

IMPACT

FACTOR (I)

Railway

bridges

according to

bridge rules

Broad

gauge and

Meter

gauge

(a) Single track

(b) Main girder

of double track

with two girders

.72

(c) Intermediate

main girder of

multiple track

spans

.60

(d) Outside

main girders of

multiple track

spans

Specified in

(a) or (b)

whichever

applies

(e) Cross girders

carrying two

or more tracks

72

Broad

gauge

Rails with ordinary

fish plate joints and

supported directly on

sleepers or transverse

steel troughing

Meter

gauge

Narrow

gauge

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


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


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

design loads.

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

bridges.

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:


Method of fabrication and erection to be followed, facility for shop

fabrication available, transportation restrictions, field assembly

facilities.

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

of material.

14. ADVANTAGE OF STEEL IN BRIDGE

CONSTRUCTRION

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


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

earthquake-prone areas.

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,


such as water, aggregates etc may sometimes not be easily available

at sites on this project, for the purpose of production of concrete.

14.4 VERSATILITY 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.

14.5 RECYCLING

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

operations.

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.

14.7 AESTHETICS

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.


14.8 DURABILITY

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

major maintenance.

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

Figure 11

15.1 Top chords:

• The most highly stressed compression members.

• Need special attention while proportioning and detailing.


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”


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

level.

Figure15

15.5 Bracing system: • Considered as secondary members, but in fact, vital for the

successful performance of the primary members.


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.

Lateral bracing:

• Placed between the top chords and bottom chords of a pair of

trusses.

• 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

system.

• 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.


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

structure.

• Method of Joints.

• Method of Sections.

For indeterminate truss:

• Virtual work.

Another methods:

• Finite Element Method

• Computer Analysis.


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.


Figure 20

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.

L= length.

A= area.

E= Young´s Modulus of Elasticity.


• 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

the joints.

A truss may also be unstable if b + r ≥2j. In this case, stability will

be determined by inspection


b + r < 2j Unstable

b + r 2j Unstable if reactions are concurrent, parallel, or collapsible

mechanics

21.1 External stability - a structure (truss) is externally unstable if its

reactions are concurrent or parallel

Figure21

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

by inspection.

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.

Internal stability

Externally stable Internally stable

Figure22


Collapsible mechanism

Externally stable Internally unstable

Figure23 Collapsible mechanism

Figure24 Externally stable Internally unstable

22. TRUSS MEMBERS ARE CONNECTED BY

SMOOTH PINS

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.

Figure25


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

force.

In general, compression members are bigger to help with instability

due to buckling.

24. CLASSIFICATION OF COPLANAR

TRUSSES

24.1Simple Truss

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

elements.

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

together.


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

trusses

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.


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

span.

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

through bridge

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.


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

length

Figure28


30. CONCLUSIONS 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

40%(max).

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

and Verticals.

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

Bridge.


31. SUGGESTION 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.


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!

http://www.holidayiq.com/destinations/Howrah-Overview.html

http://www.holidayiq.com/states/West-Bengal-Overview.html

http://www.holidayiq.com/destinations/Kolkata-Overview.html

http://www.holidayiq.com/destinations/Kolkata-Know.html

http://www.holidayiq.com/Kolkata-Sightseeing.html


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

was used

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

side

No nuts & bolts

Total 8 articulation joints, 3 at each of the

cantilever arms, and 2 in the suspended

portions

Carriageway Minimum headroom is 5.8 m

River traffic freeboard is 8.8 m

Ranks sixth in World’s top 10 longest

Cantilever bridges

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

route Kolkata.

http://en.wikipedia.org/wiki/Cantilever_bridge

http://www.holidayiq.com/Kolkata-Route-Map-Road-Driving-Directions.html


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 service.

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.

http://en.wikipedia.org/wiki/Truss_bridge

http://en.wikipedia.org/wiki/Godavari_river

http://en.wikipedia.org/wiki/Godavari_river

http://en.wikipedia.org/wiki/Rajahmundry

http://en.wikipedia.org/wiki/India

http://en.wikipedia.org/wiki/Kansai_International_Airport

http://en.wikipedia.org/wiki/Kansai_International_Airport

http://en.wikipedia.org/wiki/Osaka

http://en.wikipedia.org/wiki/Godavari_River


http://en.wikipedia.org/wiki/Godavari_Arch_Bridge


http://en.wikipedia.org/wiki/Tied-arch_bridge


http://en.wikipedia.org/wiki/Single_track_(rail)

http://en.wikipedia.org/wiki/Grafton_Bridge,_New_South_Wales

http://en.wikipedia.org/wiki/Grafton_Bridge,_New_South_Wales

http://en.wikipedia.org/wiki/New_South_Wales

http://en.wikipedia.org/wiki/Australia



http://en.wikipedia.org/wiki/Arts

http://en.wikipedia.org/wiki/Mass_media

http://en.wikipedia.org/wiki/Culture


http://en.wikipedia.org/wiki/File:Godavari_bridge_panorama.jpg


33. IMAGES





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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,

BIS ,N.Delhi

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

8. www.mittalsteel.com

9. www.nsba.com

10. www.iitr.ac.in/departments/CE/abe/413-419.pdf

11. www.fhwa.dot.gov/bridge

12. www.corusgroup.com

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.