Sr.
No.
Topic Name
Page No.
1
INTRODUCTION
3-8
2
CLASSIFICATION OF BRIDGE
9-24
3
SITE SELECTION, DISCHARGE, AFFLUX
25-37
SCOURING
4
5
6
38-40
PIERS, ABUTMENT, APPROACHES
FOUNDATION AND LOADING
7
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BRIDGE, BEARING
41-44
45-53
54-57
Page 2
Chapter - 1
INTRODUCTION
Bridge: It is an arrangement made to cross a obstacle in the form of a river, system valley, traffic etc.
Development:

Most earliest bridge was build by king of Egypt about 2550 BC

Lake dwellers of Switzerland are said to the pioneers of timber trestle constructions

In the eighteen country, France was most powerful country of conational Europe which construct finest
bridge

1st from bridge was developed in England in1979 of spam 30.sm

1st vehicular bridge was build in china around 600 Ad near bailing of single spam of 37.4m
Development in India:

Still cantilever bridge ever river hoogly at Calcutta constructed in1943, with a main spam 457m is
considered to be the one of the longest cantilever bridge in the world.

In British India mainly bridge over constructed for railway are arch type which are still in good
conditions. Later in twentieth century, Road Bridge comes in focus.

Construction of third ‘Godavari railway bridge’ is perhaps the only bridge of its kind the world were
bow string arch girder using concrete has been constructed of spam of 97.55m
Component of a bridge :
Components are of 3 types :
1) Sub-structure
2) Superstructure
3) Adjoining structure
1. Sub structure : The component of bridge up to the level of bearings are called “sub-structure; of
bridge E.x. piers, abutments, Wings walls and foundations.
2. Super structure: Component of bridge above the level of bearings are called “super structure’.
Ex. Beams Girders, Arches and cables, flooring, parapet wall, railing, handrails, etc.
3.
Adjoining structure: The components of Bridge like approaches, bearings ,river training works,
guard stone, revetment for sloops at abutments etc.
Definitions:
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1.
Approaches:

The portion of roadway or railway on both the end of bridge effected by the design, layout of a bridge
are known as ‘approaches’.

Its function is to enable vehicles running on a road or railway track at normal level to approach the
level of bridge floor.
Note: length of approach on either side shall be Min = 15m
2.
Abutments : Ends support of a superstructures of a bridge.
Functions : i) To support superstructure.
ii) To retain the Earth pressure on their back.
3) Piers: Intermediate support of a super-structure of a bridge
Functions:
i) To support super structure
ii) Transfer load from super structure to foundation below
iii) Divide the length of bridge into suitable spam
Note: Length of a bridge bet” its abutment should be >6m.
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4) Wing wall: The wall constructed on both sides of butments to retain the earth banks of the river or the
bridge approaches.
Functions: i) To protect earth bank from action of water
ii) To retain earth bank of approaches in this case they are called ‘returns wings wall’
5) Length of bridge: Overall length measured along center line from end of the bridge deck
6) Span: Centre to centre distance between two adjacent supports is called Effective spam, Its may be c/c
Distance between abutments and pier or pier to pier.
7) Clear span: Clear Distance between adjacent supports of bridge.
8) Total Span: c/c distance between end adjustments of bridges.
9) Economic span: Span for which total cost of structure is minimum.
10) Waterways: Sectional area at bridge sides through which water flows.
11) Natural waterways: Unobstructed natural flows at bridge sides.
12) Linear waterways: The Length Unavailable Between Extreme edges of water at highest flood level
measured at right angel to the abutments.
∴ linear waterways 3𝑙 + 2𝑏
b = width of pier
𝑙 = clear spam
13) Artificial waterway : Sectional area provided under bridge superstructure to flow of water is called.
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14) Effective liner waterways: 4l(linear waterways –width of piers)
15) Apron: It is layer of concrete masonry stone etc. placed like flooring at entrance or at outlet of a culvert
to protect scouring.
16) Curtain wall: It is the thin wall use as a protection against scouring action of a steam.
1) Afflux: Heading up of the water above its normal level while passing under the bridge. This caused
caused due to the construction of pier, abutments etc.
Need of determination of afflux:
i) To provide sufficient free board and clearance under the bridge site.
ii) To assertion increased velocity of flow due to afflux which should be always less than permissible
velocity under bridge.
iii) To assign river training works.
17) Free boards : The difference between the highest hood level after allowing for afflux is any and lowest
point on the underside of the bridge super structure or its approaches is called free board’
Note: Free board shall not be less than 600mm for high flood level bridge and if used for navigator then
>3m.
18) Clearance: Minimum distance between boundaries at specified position of bridge structure , it is nothing
but spacing between lowest point of super structure of HFL or if any navigable vehicle is travelling.
Note:
Clearance is associated with the lowest point on super structure and the high flood level. Were as
free board is clear vertical distance between the formation level of road and HFL.
Necessity of providing clearance:
i)
For afflux: Due t o afflux and high flood at bridge site there may be chances of submergence of the
bridge floor under water : need to provide sufficient clearance
ii)
Floating Debris: Due to flood there is debris with flood water with high level flood wave’s .If
clearance in not provided then these debris may block flow of flood.
iii)
Navigation: For clear navigation of vehicle or boats there should be sufficient clearance.
Provisions : Clearance or free board should be as given.
Note:
Arch bridge
=
300mm
Girder bridge
=
600mm or 900mm
High level bridge
=
600mm
Navigable bridge
=
2400mm to 3000mm or >3m.
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With respect to discharge OF FLOW
No.
Discharge in M3. Per sec
Minimum vertical clearance mm
Road bridge
Railway bridge
1
Below 0.3
150
600
2
0.3 to 3
450
600
3
3 to 30
600
600
4
30 to 300
900
600
5
300 to 3000
1200
1200
6
> 3000 M3/sec
1500
1800
Note: 1) Structure provide with metallic bearing, the clearance between the highest flood level including
afflux and the base or haring should not be less than 500 mm
2) For arch bridge the clearance below the crown of the intradoses of the arch should not be less
than result of equations
Clearance =
× maximum depth of water + × rise of the arch.
19) Headroom: Is the vertical distance between the highest point of vehicle or vessel and the lowest point of
any projecting structure of bridge.
20) Low water level: The low water level is the level of water obtained in dry season(LWL)
21) Ordinary flood level: (OFL) It is the ova level of flood which s expected to occurred every years.
22) Highest flood level: (HFL): It is the level of highest flood every recorded or the calculated level for the
highest possible flood.
23) Scour: Vertical cutting of river bed due to action of flowing water.
Probable max depth of sources should be use for designing foundation of piers, abutments etc.
Identification of Bridge :
IRC given a specific number system for bridges and cross drainage works.
Method of numbering:
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i) Numerator should donate the no of kilometer in which the structure is situated
ii) Denominator donates the kilometer wise serial number of the structure.
Ex.: Third bridge in the fifth kilometer (i.e. between kilometer stone 4 and 5) should be desigh as 5/3
and fifth structure is 12th kilometer as 12/6

In case of any new bridge is constructed between these existed bridges then that can be given no. aS
suppose new bridge is constructed between 3rd and fourth bridges Of 12th kilometer road, then it is
numbered as

/
, / like this.
This number of the structure should be inscribed near te top of left hand side parapet wall
Note: This numbering practice is given in IRC: 7-1971
IRC Guideline code for standers and uniform bridge design are:
1.
IRC :5-1998 → Standers specification and code of practice for road bridge, selection I general feathers of
design.
2.
IRC: 6-2000 →
Section II → load and stresses.
3.
IRC : 18-2000 →
Design criteria for presented concrete road bridge ( Post Tensioned Concrete.)
4.
IRC : 21-2000 →
Section III → cemetal concrete (plain And reinforced).
5.
IRC : 22-1986 →
Section VI → composite construction.
6.
IRC : 24-2001 →
Section V → still road bridges.
7.
IRC : 40-2002 →
Section IV → brick stone and block emissary.
8.
IRC : 78-2000 →
Section VII → foundation and substructure.
9.
IRC : SP: 40 →
Gridline on technique for straightening and rehabilitation of bridge .
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Chapter : 2
CLASSIFICATION OF BRIDGE
1) According to life :i) Temporary Bridge:
Which has short life

Constructed and maintain at low cost

Generally life is about 10 yeas

Constructed when budget is low
Ex : Timber Bridge, Still Bridge
ii) Permanent Bridge :
Which has long life generally 15 years

Constructed and maintain at high cost

Constructed when found is more
Ex. RCC bridge, suspension bridge etc.
2) According to purpose:a) Aqueduct:- A bridge for caring water across a valley of low ground
b) Viaduct:- Bridge constructed across a deep valley without perennial river is called ‘viaduct’. Use for
crying traffic.
Note: Such a bridge constructed of single span without any intermediate supports.
c) Grade seperations : The bridge constructed when a road across another road at different level.
d) Foot bridge :- Vridge exclusively use for caring pick strains, cycle across animal any abstractions
e) Highway bridge :- Use for vehicular traffic along a road
f) Railway bridge :- Constructed for a highway
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3) According to material :i)
Timber bridge
ii)
Masonry bridge
iii)
Iron bridge or still bridge
iv)
RRC bridge
V)
Prestressed bridges.
4) According to spam length:This are classified by two ways
A. By road engineers
i) Callyents:- having spam length less than 6m
ii) Minor bridge:- span 6miter to30m.
iii) Major bridge :-30m to120m
iv) Long Spam Bridge:-Span length 120m.
B. By railway engineers:This are classified based on liner waterways
i) Major bridge : Total water ways more than 18 meter or having clear waterway of12 m or more
ii) Minor or bridge : Total waterways less than 18 m or having clear waterways less than 12m
iii) Important bridge :Total waterways of 18 meter or more and bridge area more and having area
more than 118m2
iv) Culvert : Having maximum 12m linear waterway.
5) According to alignments:
i)
Straight bridge or square bridge:Bridge having centre line right angel to the axis of the river is known as straight or square bridge.
ii)
Skew bridge: Center line bridge or axis of bridge is not right angel to the river axis.
Note: Screw restricted to 30°
6) According to level of bridge Floor :Classification into 3 type :
i) Deck bridge :

It is the bridge in which bridge floor is supported at the top of superstructure

Provided were a[approaches provides in cutting .
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
There should be sufficient distance between HFL and bottom of floor to accumudate the superstructure with a
suitable freeboard.
Note: Deck bridge os more economical due to effects of various like wind tractive is min on super
structure.
ii) Through bridge :

Bridge having its floor at bottom of superstructure

Provided were floor is to be provided in filling
iii) Semi-through bridge :- Bridge in which it’s a supported at the same intermediate level.
7) According to position of high flood level:i) Submersible Bridge :

In this bridge high floor overtops the bridge floor

Constructed on less important road were fund are not available

Level of bridge flooring is positioned that does not interrupt the traffic for not more 3 days during
flood and not more than in a year.
ii) Non submersible bridge:

This Bridge does not allow high flood water to overtops the bridge floor.

Also called as high level bridge’

Construced on important road were fundas are available
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Ex: Deck bridge
8) According to IRC loading class:
i) Class ‘A’ bridge : Bridge designed and constructed for IRC class A loading permanent bridge.
ii) Class ‘AA’ Temperory bridges
iii) Class ‘B’
iv) class 70 R → permanent bridges
9) Nature of superstructure action :
i) Beam type bridge : such as RRC The beam balanced contilever, stio girder, plate girder box girder
truss and portal frame bridge.
ii) Arch type: Spandrel, filled, spandrel, barrel, and rib type bridge.
iii) Suspension type bridge: Ramp Bridge, trestle bridge, sling bridge.
10) According to method of clearance for navigation :
For navigable channel were permanent and suffix clear water ways can not be provided ,the following
movable bridge are used
i) Swimming bridge
ii) Bascule bridge
v) Transporter bridge
vi) Traversing bridge
iii) Lift bridge
iv) Cut boat
11) Degree of Redundancy:
i) Determinate bridges ii) Inderminate bridges
12) Type of connection : Under this category still bridge are classified as :
i) Pinned connection
ii) Riveted or bolted or welded connection bridges.
Classification based on interspan relations :
i)
Simple bridge or beam bridge : Bridge in which span is simply supported over supports, no
continuous joint between two spans.
Note: It is suitable for span upto 8m
Ex. Beam, Girder or Truss bridges.
ii)
Continuous bridge : Bridges which continuous over two or more spans. They are used for large
span and were unyielding foundation are available.
iii)
Simple cantilever bridges: Bridge which are more or less fixed at one end and free at the other
end I.
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
Can be used for span varying from 8mto 20

It has advantages of both simply supported span and a continuous span
Note : Long span and deep valley and at plasess were it will not possible to use converting at that place
cantilever bridge are more suitable.
iv) Balanced Type Cantilever: In this bridge hinges are provided at the point of contra flexure of
a continuous span and an intermediate simply supported span is suspended between hangs.
Note : It is suitable for span up to 60m.
According to the superstructure:
1) Arch bridge :
In arch bridge rise of arch is kept high as much as possible. So that horizontal thrust produced as
support should be min.
Note: Rise of arch generally kept greater than of span. But in no case should be less than 𝑡ℎ of span.
More rise, less – thrust, it reduces cost of construction.
Note : Arch bridges are used to span of 20min.
Arch bridge are classified as :
a) According to condition of spandrel :
i.e. Depending on the space between arch above and the formation level classified as:
i) Filled spandrel arch: Spandrel is filled by earth or masonry fully.
Suitable when rise to span is small if ratio increases up to certain limit then we needs heavy sides wall
and also a heavy fillings material.
ii) Open spandrel arch: Spandrel is not filled.
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
Suitable when to span is high and it is economical than filled spandrel for high ratio.

When ratio is high then if we filled the spandrel then dead weight of arch increases for high ratio use
open spandrel.
b) According to number of hings:
i)
Three –hinged arch
ii)
Two-hinged arch
iii)
One hinged arch- contain the hinge at crown
iv)
Hingless or fixed arch : Used were hard unyielding soil is available at support .it mst
commonly used arch and most economical in all above .only difficulty in anylisis of arch.
c) According to shape:
i) Semi-circular shape arch bridge : Where rise is more
ii) Segmental or parabolic : For medium span length
iii) Multi-centered : Use for long spans.
d) According to width :
i)
Barrel type arch: In this bridge width of arch and bridge are same .
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ii)
Rib type arch : Here width of bridge and arch are different .arch is uses to support the formation
level
Note : Arch bridges are used upto span of 20m.
Advantages of arch bridge :
1) We can give good aesthetic effect by the desired shape .
2) Arch section need to design only for normal trust and radial shear. No need to design for banding
moment.
3) Suitable for deep gorges with hard rock at support
4) Maintenance of arch bridges is easy.
5) Due to heavy mass of structure, impact due to vibrations is completely eliminated. Also noise like in
steel bridges is also minimised.
2) Bow-string Girder Bridges :

In this bridge arch and tie respectively resemble bow and string.

Flooring rest on ties and the load is transmitted to the arch rib through suspenders.
Note :
1) In this bridge horizontal thrust is resisted by ties.
2) Very suitable for multiple span bridges and at places where the available clearance is restricted as
bow- string girders are projected above the formation level of road.
 It may be R.C.C. bow-string girder bridge : can be adopted for spans of 30m to 45m.
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 or steel bow-string girder bridge : used for span upto 120m to 240m.
3) Rigid Frame Bridges :
 In these bridges, there is monolithic from for super-structure + sub-structure.
 In this floor slab is cast monolithically with abutment wall.
 Used when span is small, roadway is wide, bearing capacity of soil is low.
4) Suspension Bridges :

The suspension bridge consists of a hanging cable which is anchord at the two ends.

Cable take catenary shape between two supports.

The vertical members, known as ‘suspenders’ are provided to transfer the load from bridge floor to the
suspension cable.

Cable are made from chains or steel wire ropes and suspenders are made of twisted wire ropes and
connected to the cable by loops.

Depending on site condition, ratio of side span to the main span varies from 0.17 to 0.50.
Can be classified as :
i) Unstiffened suspension bridge : Used for right verticular traffic and for foot bridges.
ii) Stiffned suspension bridges : Used for heavy traffic.

Here load of vehicle from floor is transferes to the stiffening girder and by suspenders it is transferred
to the cable.
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
Ideal for hilly areas

Compitable for large spans.
Advantages :
1) No pier
2) Simple construction
3) Economical compare to other
4) Light weight
5) Can be used for span > 600m
6) Better architectural effect
7) Fast erection
Disadvantages :
1) Parts are rare ∴ repair is not easy.
2) Painting required frequently
5) Cable stayed bridges :
Similar to the suspension bridges except in suspension bridges we used suspenders but not in cable
stayed cables are directly stretched from towers to connect with the deck. ∴ no need of anchorage cable
as that of suspension bridge.
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Principal elements of cable stayed bridges are :
1) Bridge deck
2) Pylons or towers
3) Stay-cables
Low cost bridges :
1) Causeways
2) Culverts
3) Timber bridges
1) Causeways : In this type of bridge, bridge floor is little above the bed of stream which allows flood
water to pass always over it.
It has two types :
a) Low level causeway : It is a causeway in which bridge floor is directly rest on bed of stream.
 No vent way for drainage
 It is also called as ‘metal dip’ or ‘flush causeway or ‘Irishbridge’
b) High level causeway :
In this causeway, vent pipes are provided for drainage.
 In both causeways approaches are provided in cutting
 These are sub mersible bridge, constructed to reduce the cross drainage work.
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Low cost bridges are such bridges which are constructed at low cost and are capable of being maintained at
low cost. Low cost bridges are constructed by using local available material near bridges site.
Movable span bridges :
These are generally used for navigation purpose.
i) Bascule Bridges :
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
These are rotated about horizontal axis in vertical plane upto an angle of 70° to 80° with horizontal.
(whole bridge is rotated)

Only disadvantage is that it get disturbed due to fast moving winds.
Advantages over swing bridge :
1) Opened easily and rapidly
2) If needed another bascule can be constructed near to previous.
3) Opening can be adjusted for any vessels i.e. large or small.
4) When in fifted position, entire span of bridge available for navigation.
Note : Double bascule bridge
ii) Cut-boat bridges :
 In this bridge super structure is resting on boats.
 Cut-boat bridge is provided when it is necessary to provide some passage for navigation traffic. When
ships pass out, the movable span is pulled back and it is placed in it’s normal position.
iii) Flying bridges :

Used for transportation of goods and material from 1 end to other end and used where funds are not
available.

Boat is attached to the suspended cable by swinging cable at 55° to the direction of flow.
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iv) Lift bridges :

Whole bridge is lifted with help of pulleys and suitable arrangement.

Wind velocity does not disturb the working of this bridge.
v) Swing bridges :

Central pier is provided with suitable bearing or rollers.

Superstructure can be rotated in horizontal plane with help of central steel pier.
Disadvantages :
1) Bridge divides the wider waterway into two narrower waterway which reduces navigation capacity.
2) Obstruction to the flow of water due to central big pier.
3) Time for rotation is large.
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vi) Transporter Bridge :

In this bridge cage is used for transportation of persons and goods 1 end to the other end of a harbor.
This cage is suspended by use of cable to a overhead truss with help of bearings.

These bridges are used in harbour area for transportation of persons and goods.
vii) Traversing bridge :
 Bridge deck provided with rollers so that it can traverse forward or back word fully or partly.
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 These bridges now a days not used.
CULVERTS AND CAUSEWAYS
1) Culverts : span < 6m
It also called as cross drainage work for flow of water from 1 to other end.
Types :
1) Arch culvert
2) Box culvert
3) Pipe culvert
 Dia of pipe used > 300mm
 Used for discharge upto ‘70 m3/sec’
 Earth cushion of min depth of 450m should be provided at the top of pip
 Concrete bedding also required.
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4) Slab culvent
5) Scuppers : - It is chepest type of culvert
- Provided when width of river to be crossed is about 900mm to 1000mm
- It is constructed by using locally available material.
Floating bridges:
Most useful during wars such that it can be constructed and dismental easily within short time without
knowing to the enemy.
Types :
1) Boat bridges
2) Pontoon bridges
3) Raft bridges
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CHAPTER – 3
SITE SELECTION, DISCHARGE, AFFLUX
Economic span
Selection of Bridge Site :
Following points should be considered while selecting best site for bridge :
1) A well defined and narrow channel :
This leads to help in providing least possible length of bridge, thus resulting in economy in initial cost of
construction as well as maintenance cost.
2) A straight reach :
Stream reach should be straight at site. Due to straight reach it does not change flow path which will be
beneficial to utilize the bridge site for it’s design life.
3) Good foundation bed at a short depth :

A good unyielding and non-crodable bed should be available at a short depth for providing foundations
for piers and abutment.

Due to shallow depth it reduces cost of foundation, material, labour and time.
4) Suitable high banks :
Which reduces changes of overflow of water. It should be soild, sound and straight.
5) Minimum angle of crossing or minimum skew angle : 𝜃 ≯ 30°
min. skew-angle reduce cost of construction.
Also it provides shortest length of bridge span as well as length of bridge pier and abutment.
6) Absence of scouring and silting :
 It reduces cost of maintenance
 Site should be in steady regime condition
7) Location of river tributaries :
Bridge site should be away from the large no. of tributaries at site. It reduces harful effect of tributaries on
bridge site.
8) Minimum obstruction to waterway :
It will reduces afflux and scouring at site.
9) Sound economical and straight approaches :
Curved approaches difficult to constructs
10) Absence of costly river training work :
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This will economical in initial cost of bridge and also reduces cost of maintenance.
11) Minimum obstruction work inside water :
 Stream at bridge site should be such that no excessive work need to be carried out inside water.
 It yield economy.
12) Proximity to the alignment of communication route :
This will reduces long approaches and will be economical.
13) Availability of sufficient free board :
For better navigation of boats, ships there should be free board available.
14) Availability of labour and construction material : Easy availability at or near
Additional points :

Approaches at bridge site should be not be in cutting, which create problem of acquisition and which
increases the expenditure. Approaches should be try and hard.

Skew bridges are avoided because : Difficult to construct, depth of foundation will be more as it is
subjected to scour, maintenance is difficult, passage of water is not smooth, formation of whirls or
currents, piers of skew bridge subjected to excessive pressure.

Velocity of flow : Velocity should be such that there should not be scouring or silting at bridge site.
Therefore velocity i.e. permissible velocity depends on the nature of bed. Permissible velocity is more
for rock and boulder than the fine sands.
∴ 𝑉
for i) rock = 4.2 to 6 m/sec
ii) rocky soil = 3 m/sec
iii) fine gravel = 1.5 to 1.8 m/sec
iv) clay = 2.1 m/sec
v) sandy clay = 1.5 m/sec
Collection of data :
Following data should be necessary for safe and economical design of a bridge :
i) General Data : It includes maps, plans, topographical features etc
a) Index map : It shows location of proposed bridge site, town near bridge site, alternate. Bridge sites
and rejected alignments, general topographical.
It’s scale of drawing should be 1cm : 500m
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b) Contour plan : It shows topographical features of stream which affects the design data at U/s and
d/s upto certain distance.
∴ By IRC, the distance should be covered by the plan on either side of bridge site for catchment area of
3m2, 15m2 and over 15km2 should be respectively 100m, 300m and 1500m.
c) Catchment area plan :

It indicates the proposed catchment area which contributes it’s flow at bridge site.

It is taken from survey of India topographical plan.

It is drawn to a scale of 1/50,000

Used to determine flood discharge at bridge site.
d) Cross-section :
It gives info about low water level, high flood level, nature of sub soil by taking suitable cross section
at U/s and d/s. Also gives info max discharge, avg velocity of flow, scour depth.
e) Longitudinal section :
Shows high flood level, low water level, bed level at different locations etc.
Cross section should give info of :
a) Depth of scour below highest flood level.
b) Maximum discharge and velocity of flow at the bridge site.
c) Name of road with discharge
d) Name of the stream or river
e) Position of low water level, ordinary flood levels highest flood level.
f) Soil profile : The borings should be taken at the approximate location of abutment and piers along the
bridge so as to get idea of sub-soil conditions indicating water level, it’s depth below ground, thickness
and composition of different strata below ground.
For major bridges soil investigation is done upto a depth of 1.5 times the proposed width of foundation
below the proposed bottom of foundation.
g) Site plan : Showing details upto 100m on both U/S and d/s of site. Prepared by using scale of 10cm =
100 and for long span bridges information of about 500 site on both side should be given in site plan.
It includes following information :
i) Catchment area
ii) Direction of flow of water
iii) Formation level of road
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iv) HFL, OFL, and low water level
v) Maximum clear linear waterway required
vi) Maximum discharge in the river
vii) Maximum expected or observed score depth
viii) Maximum velocity of flow of water
ix) Name of river
x) Name of towns on either side of bridge
xi) North line
xii) Width of approaches, bridge
2) Hydraulic data for particular bridge site
3) Geological data
4) Climatic data
5) Loading and other data
Bridge Alignment :

After selecting bridge site next is to check for bridge alignment.

Following points should be considered :
i) Alignment should be perpendicular to the bridge axis. i.e. square alignment should be prefered.
ii) Alignment should not be curved
iii) Despite of disadvantages of skew-alignment is to be provided sometimes to avoid costly and
unsafe approaches.
iv) Skew angle for skew alignment restricted to 30°disadvantages of skew-alignment is that :
a) Difficulty in construction
b) Due to more scour, more depth of foundation
c) Maintenance is difficult
d) Passage of water is not smooth which creates extra currents and turbulence.
e) Piers need to resist excessive water pressure.
Traffic Requirement of Highway Bridges :
1) Central verge of median :

To divide traffic flow and also for safety we use central verge or median not less than 1.2m of
width.
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
Generally it’s width is kept minimum from economical point.
2) Footpath :

It is provided on both side of a carriageway.

It reduces merging of pedestrian on main road and reduces traffic accidents.

The width of footpath will be decided by the volume of pedestrian traffic and importance of the
bridge.
Note :
a) For rural areas, the minimum width of footpath should be as 1500 mm and it increases in urban area
with volume.
b) Capacity of 150mm footpath is 108 person per minute and it should be increased at rate of 600mm
for every addition capacity of 54 persons per minute.
3) Parapets and Handrails :
Min. height should be 600mm and not more than 1m.
4) Roadway width :
Min width for traffic as well as cycle is as given :
No.
Type of traffic
Minimum width
in cm
1.
Vehicular traffic
i) Single lane bridge
425
ii) Two-lane bridge
750
iii) Multi-lane bridge
350 for every
additional lane
over two lanes
2.
Cycles
i) Without overtaking
200
ii) With overtaking
300
Note :
1) For 100 vehicles per hour per lane required width = 3750mm
2) 3600 cycles per day for two lane, width = 2000m
5) Safety kerb : Use kerb of size – 600mm × 225mm on either side of road.
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6) Sight distance :

For N.H/SH with design speed of 100 kmph. Use 150 m as a sight distance.

For MDR of design speed of 180 kmph use 110m.
o For village road of design speed of 150 kmph use 160m.
Determination of Design Discharge :
IRC recommends to determine design discharge by at least two methods given below and adopt max
value as design discharge.
1) From records available at site or near by site
2) From rainfall and characteristic of catchment area :
i) By empirical methods
ii) By rational methods
iii) By area velocity method
iv) By unit hydrograph method
i) Emperical methods : gives fairly accurate result but not used universally.
a) Dicken’s formula : For central India and North India.
Q = 𝑐𝑚 /
b) Ryve’s formula : used for southern India.
Q in m3/sec
Q = 𝑐𝑚 /
M in km2
c) Inglish formula : for Maharashtra,
𝑄=
√
.
ii) By rational method : used for area <50km2
𝑄=
×𝑘×𝐴×𝐼
A = Area in ha
𝐼 = critical intensity of rainfall in cm/hr Q in m3/see
iii) By Are-velocity measurement :
1st need – calculation of area :
i) Calculation of area :
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Area is calculated by direct observation by knowing the HFL at that site by previous rainfall cross
section along site i.e. 1) at bridge site 2) at 1 km d/s 3) at 1 km u/s and by knowing depth of catchment at
these section using instrument we can determine the area of catchement.
ii) Determination of velocits :
Velocity of flow can be determined by
a) By empirical formula :
V = C √𝑅𝑆
where R = Hydraulis radius =
=
S = slope of catchement
C = Chezy’s constant depends on catchment characteristics v in m/sec
b) By direct observations : i) using surface float
ii) using sub-surface float
iii) Using current meter
iv) Using velocity rod
Note :
i) In surface float we need to apply correction on calculated value. If velocity less than 0.9 m/sec then
mean velocity = 4/5 × surface velocity.
when velocity > 0.9 m/sec then
mean velocity =
× surface velocity
correction not required for sub-surface float, velocity rod or current meters.
ii) By sub – surface float :
iii) Velocity rod : nothing but a hanging cable which has weight at bottom attached.
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iv) Current meter : Kept at 0.6D and velocity is measured.
v) Also by taking velocities at depth 0.8D and 0.2D by sub-surface float and take average of these two.
∴𝑉
=
.
.
Then Q = A.V
Note : All above method does not gives a accurate result
∴ design flood is obtained by frequency analysis.
Note : For small bridges, flood discharge is obtained for 20 years of frequency and for major bridges it is
designed for 100 years.
Determination of Waterway :
Area through which water flows below bridge is known as ‘waterway’.
After determining flood discharge, we need to find waterway.
If ‘Q’ is design flood and V is the permissible velocity then waterway =
Where V depends on nature of bed
1) ∴ 𝐿 = 𝐶 𝑄 This is lacey’s formula.
It is used for large alluvial streams with undefined banks, the linear waterway is determined.
Where, L = linear waterway in ‘m’
Q = Discharge in m3/sec
C = constant ≈ 4.8 to 6
2) 𝐿 =
×
=
×
(
)
This formula is used for rigid boundary channel
h = head causing flood
ℎ = afflux depth
This waterway is divided into no. of spans by keeping view of economy.
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Determination of Afflux :

Heading up of water on upstream side of bridge is called ‘afflux’
Note : Generally afflux is desirable to keep to 150mm.

Due to reduced waterway of bridge site, velocity if flow increases, which coulses more scouring on
the
∴ for safe depth of foundation, determination of afflux is necessary.
Note :
1) Increased velocity due to afflux should be less than permissible velocity at bridge site.
2) Keep afflux as low as much possible to reduce depth of foundation and economy.
Reason to have min afflux :
1) Foundation can be provided at shallow depth.
2) Sufficient free board is available at bridge site.
3) Submersion of bridge. If allowance is not made for afflux, it is likely that the bridge floor will get
submerged during flood.
Need to find afflux :
1) For determining head room
2) For determining free board
3) For determining bank level
Determination of afflux by :
i) Marriman’s equation
both are empirical formulae
ii) Molesworth’s equation
i) Marriman’s formula :
ℎ =
.
−
Approach velocity = v = Natural velocity of flow at site in m/sec
A = Natural waterway under bridge in ‘m2’
𝐴 = Contracted area under bridge in ‘m2’
𝐴 = Enlarged area u/s of the bridge in ‘m2’
C = coeff of discharge which accommodates the contraction and frictional
lossed occurring as water passes by the piers.
= 0.75 + 0.35
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− 0.1/
Page 33
C = 0.7 for sharp and 0.9 for bes mouthed entry.
ii) Moles warth’s formula :
ℎ =
.
ℎ =
.
+
−1
.
+ 0.015
−1
𝑣 , A, Acare same as that of above
𝐸𝑥 : A bridge has a linear waterway of zoom, constructed across a stream whose natural waterway is 300m. If
avg. design discharge is 1500 m3/sec and avg. flood depth is 4m, calculate afflux under the bridge = ?
Q = 1500 m3/sec
A = 300×4 = 1200 m2
Ac = 200 × 4 = 800m2
∴ v = velocity of approach =
∴h =
=
.
.
.
+ 0.015
+ 0.015
=
= 1.25𝑚/𝑠𝑒𝑐
−1
−1
ℎ = (0.08 + 0.015) (1.25)
ℎ = 0.1275𝑚
Determination of economic span :
Economic span of bridge is such that overall cost of superstructure + substructure is min.
Note : If span length increases then cost of sub-structure decreases but cost of super-structure
increases and vice versa. ∴ for economic span, cost of super-structure is equal to the cost
of sub-structure.
Assumption for derivation :
i) Bridge consists of equal span
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ii) Bridge is an arch bridge or girder bridge
iii) Cost of abutment and it’s foundation is min
iv) Also cost of 1 pier and it’s foundation is constant
v) The cost of super structure per span varies directly as that of square of span
vi) Cost of substructure varies 𝛼 span
∴ cos of bridge = cost of super structure + cost of pier – cost of abutment
cost of superstructure = Nal2
a = constant of variation
N = no. of spans
𝑁= =
𝑐 = 𝑁. 𝑎𝑙 + (𝑁 − 1) 𝑃 + 2 𝑐𝑎𝑝
P = cost of 1 pier + foundation
C = Total cost of bridge
Cap = cost of abutment
∴ 𝑐 = 𝑁. 𝑎 ×
𝑐=
+ (𝑁 − 𝑄)𝑃 + 2 𝑐𝑎𝑝
+ (𝑁 − 1)𝑃 + 2 𝑐𝑎𝑝
for c to be min.
=0
∴ i.e. derivative of total cost w.r.t no. of span should be min
∴ 𝑝 = 𝑎𝑙
or 𝑙 =
∴ from equation we can say that,
Note : For economical span cost of 1 pier with it’s foundation. Should be equal to the cost of super
structure per span.
or cost of pier is equal to the half of the cost of two span which the pier supports or cost of sub-structure
= cost of super –structure.
Exceptions for economic span :
i) Dead load of super-structure : There is considerable increase in dead load of the bridge
with increase
in span. In this case we can not go for economical span.
ii) Foundations of piers : suitable foundation is not available where economical span pier need to be
situated.
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Determination of length of bridge :
After determining waterway and economic span we can determine length of bridge as :
𝐿 = 𝑁𝑙 + (𝑁 − 𝐿) b
𝐿 = length of bridge
N = no. of economic span
l = economic span
b = thickness of each pier
Grip Length :
In erodible soil, the depth of foundation of piers and abutments is kept more than the maximum scour
depth. This depth of foundation below maximum scour depth is called ‘grip length’.
i) for road bridges, Grip length > 𝑟𝑑 of maximum scour depth
ii) for railway bridges, Grip length > of maximum scour depth
Uses :
1) It protects the foundation from the scouring action of water.
2) It helps in resisting horizontal forces acting on pier or abutment due to lateral earth pressure.
Q. The approximate costs of 1 pier and 1 super structure span from a multiple span bridge for various lengths of
span are tabulated determine a economic span.
Span in ‘m’
Cost of 1 pier
Cost of 1 superstructure
10
25000
7000
15
28000
13815
20
32500
31000
25
33700
36000
30
34800
41400
we know cost of superstructure 𝛼 𝑙 = 𝑎𝑙
a = constant of variation, l = span
∴𝑎=
= for span of 10 m =
∴𝑎
𝑎
=
= 77.50,
𝑎
= 70, 𝑎
=
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=
= 57.6,
= 61.4
𝑎
=
= 46
Page 36
a) ∴ avg value of a =
.
.
.
= 62.5
Avg of cost 1 pier =
∴ economic span (l) =
= 30800
=
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.
= 22.2𝑚
Page 37
CHAPTER – 4
SCOURING
Definition :
The process of cutting and deeping of river bed due to action of water is called ‘scouring’
 But in erosion there is horizontal widening of the river.
 Scoure depth is not uniform even through in straight reach. More scouring is at the round nose of the pier.
Note : Erosion means horizontal widening of the bed where as scouring is vertical cutting.
Determination of Normal scour depth :
Note : Scour depth is always measured from HFL (High flood level)
Normal scour depth is the depth of water in the middle of the stream when it is carrying max flood
discharge.
Actually the scouring depth is determined by the sounding during the flood at bridge site otherwise it is
theoretically determined by using equation.
1) Scour depth of alluvial streams :
Case : 1 when linear waterway of the bridge is equal to the regime width :
In this case normal scour depth is equal to the regime depth given by
𝐷 = 0.473
/
-given by lacey’s
D = Normal ‘scour’ depth below HFL for regime section ‘m’.
Q in m3/sec, f = silt factor = 1.76 𝑑
d in ‘mm’ = avg size of bed material in ‘mm’
Case : 2 when linear waterway is less than the regime width :
𝐷𝟏 = 𝐷
.
𝐷𝟏 = Normal scour depth with contracted waterway in ‘m’
D = scour depth when L = W
W = width of the regime channel = wetted perimeter
W = 4.8(Q)1/2
Scour depth for quasi-alluvial streams :
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Quasi- alluvial stream means is that stream in which banks are rigid but bed easy to erodible.
Case : 1 :
When velocity of flow is known :
𝐷=
.
W = fixed width of stream in ‘m’
V = velocity of flow
Case : 2 :
when slope is known :
𝑄 = 𝑊. 𝑠 / 𝐷 /
Determination of maximum scour depth :
By IRC recommendations max scour depth is determined by as follows :
Note : Usually max scour depth occurs at bends, nose of piers on u/s. for safety of structure we need
to
determine max scour depth.
i) In case of bridge having straight reach and having single span, the max scour depth should be
taken as 1.5 times normal scour depth.
ii) for cross bridge and having multiple spans the scour depth = 2 × normal scour depth
iii) In case of contraction, 𝐷 = 𝐷
.
𝐷 = max scour depth
D = Normal scour depth
W = width of regime channel
L = waterway
Note :
1) max scour depth at nose of pier = 2 × normal scour depth
2) max scour depth is not constant which is obtained by relation,
𝐷
=𝐷×𝑅
where, R = constant, D = Normal scour depth
No.
Condition of flow
Value
of R
1.
For straight reach
1.27
2.
For moderate bent
1.5
3.
At sharp or serve bent
1.75
4.
At right angled bent
2.00
5.
At upstream noses of
2.75
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guide bank
Prevention of scouring :
1) Stream flow should be straight at site
2) River bed should be such that it should resist the scouring
3) No obstruction to the flow at site, which will creates currents and eddies. ∴ pier nose should be
rounded.
4) Sheet piles are provided at U/S and D/S of site to prevent scouring. Also provide large
foundation
stones.
Note :
Foundation level must be atleast 2m below the scour line for arched bridge and other bridges it should
be 1.2m below scour line.
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CHAPTER – 5
PIERS, ABUTMENT, APPROACHES
Piers :
They are intermediate supports of the superstructure. They must withstand high vertical loads and
large lateral loads.
Function is given before
Types :
1) Dimensions :
Height (H) : Pier top is kept 1 to 1.5m above HFL H is measured up to the support level of girder or the
springing point of arch.
Pier Batter : The sides of piers are either vertical or battered generally a batter of 1 in 12 to 1 in 24 is
provided.
Pier width : It should be adequate to accommodate the seats of 2 bearing with 15cm clearance.
Pier width at top should be equal to
be
𝑠𝑝𝑎𝑛 𝑙𝑒𝑛𝑔𝑡ℎ or
to of span length. Pier width at bottom should
of total height.
Length of pier : Equal to the width of bridge. It should to 1 − times the top width beyound the centre line of
the outer trusses or girders plus length of cut water and ease-water. This is to prevent diagonal shearing at the
ends.
Cut and ease water : The pier ends are shaped for easy passage of water. The end of U/S side is known
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as cut water and that on the down stream side is known as ease water. Semicircular cut waters facilitate
streamlined flow and reduce scour most effectively.
Used to prevent formation of eddies and whirls in water by ease water respectively.
Pier caps : Used to pier cap is provided for whole top width with projection of 7.5cm beyond its side.
Types :
1) Solid piers : Less obstruction to flow provided in rapid flow.
2) Open piers : Free passage of water through them.
3) Abutment piers : Used in arch
Abutment piers : Which has thicker section than the adjacent pier and
- Used to receive thrust from either side
- Used to facilitate construction of arch bridge in steps which reduces cost of form work.
4) Cellular piers : Saving of concrete
5) Straight column piers : Used where passage between pier is more like grade saperators etc.
Abutments : These are end supports of the superstructure. If serves as both pier as well as retaining wall.
Weep holes are provided to the body of abutments for design of retained earth.
Types :
Abutments without wing wall
Abutment with wing wall
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Wing walls : Used to 1) retain earth banks of the river
2) To retain earth bank of approaches then wing wall called ‘Return wing wall’.
Types of pier
Open pier : water opening provided in pier itself.
1) Multiple Bent : Often used on ground
2) Pile Bent : Often used on ground
3) Cylindrical pier : These consist of mild steel or cost iron cylinders which are filled with concrete.
4) Cylindrical piers : They are used for temporary work and for timber work.
Some of the special piers :
1) Separate piers : These may be low piers with steel columns above on concrete piers up to bridge
level.
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[about framed pier – it is used in rivers subjected to sudden floods near hills & forests would be
subjected to floating debris e.q. floating tree trunk etc.]
2) Abutment piers : In the case of multiple span arch bridges. Every third or fourth pier is designed as
an abutment to receive the thrust from either side.
Such a pier is thicker in section and is known as abutment pier objects.

The arches can be turned in sets, and thus heavy expenditure in centring is saved.

In any damage occure in any portion due to floods, the same will probably not extend further
than next abutment pier.
3) Cellular piers : Consist of two concentric R.C.C. shells connected by radial ribs and horizontal bands
at suited intervals. Saving inconcrete.
4) Framed piers : Pier reduced effective span length for girders.
Approaches : The approaches are the lengths of communication route at both ends of the bridge.

For high level bridge and culverts approaches are provided in embankment and culverts approaches are
provided in embankment and for low level bridges (i.e, sub-mersible) bridges and causeway approaches
are provided more than bridge width.

Width of approaches more than bridge width.

IRC Recomendates the approaches should be straight for a min length of 15m on eitherside of a bridge
and then provide horizontal curve with required super elevation and radius of curve beyond approaches
on either side of bridge.
Types : 1) Approaches runnings over extended portion of the main bridge.
Grade of PCC = M10 and RCC M15 for both pier and permissible tensile stress – for M 10 is 300 t/m2
and for M 15 – 500 t/m2
compressive
M10 – 1000 t/m3
M15 – 1500 t/m3
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CHAPTER – 6
FOUNDATION AND LOADING
Foundation :
Types :
1) Land foundation : (foundation under ground) ex : spread, raft, grillage etc.
2) Soil charged with water at shallow dept : (Soil + Water present there)
3) Under water foundations : (foundation in submerged ground or soil) ex. Pile, well, caisson
Suitability :
1) Pile foundation : Suitable where 1) soil is weak 2) scouring is more 3) heavy concentrated loads are
expertected to come 4) hard strata available in very high depth.
2) Well foundation : Suitable where sand is present in relatively high depth and possibility of more
scouring.
3) Caisson : Hard strata is at reasonable depth but water depth available is to high to construct a
foundation & dewatering is difficult.
Depth of foundation (As per I.R.C. code part (III)
 Below the HFL at 1.33D, D = max scoure depth below HFL + Grip length
o ∴ (i.e, length below the max scour depth)
 𝑙𝑔 > 𝑟𝑑 of max scour depth for road bridge.
 𝑙𝑔 > of max scour depth for railway bridge.
 It I s advantageous under the heavy loading case of heavy flood condition which gives more stability to
the bridge.
 Minimum depth of foundation below maximum scoure depth should be
 1.5 to 1.8 m fore piers and abutments with arches
 1.2 m for piers and abutment with other support
 For other bridge at least 1.5 to 1.8m deep below natural bed level of stream should be provided.
 In hard rock at least 0.3m below
 For other hard material at least 0.6m below.

Coffer dam : Temporary structure construct to devert or obstruct water to make space available for
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Page 45
construction. (Temporary)
Types :
1) Earth : Suitable where 1) Stream depth < 2.5m
2) Water velocity is low, i.e. 0.5 to 1 m/sec
3) River bed is sufficiently firm
2) Rockfill : 1) Made from rock, boulder in huge quantity
2) Percolation is more
3) Suitable upto 2m height
3) Rock fill crib : Filling crib with boulder gravel or rock crib : Frame work of horizontal and cross
beams laid in alternate layers.
4) Single wall
5) Double wall
6) Cellular : for upto 15 to 20m
Well foundations :

Deep well : also called punjab system (depth less than scour depth)

Shallow well : also called madras system (depth of well foundation more than scoure depth)
Well curb : For supporting wall steining and transfer of load of well below sub-soil which gives support
to structure above it at bottom (made up of R.C.C., prestress, timber etc.) having cutting edge at bottom.
Well steining : Concrete ring of a well foundation, also acts as a cofferdam during sinking of well and
acts as a cofferdam during sinking of well & act as structural member for supporting load coming on it.
Bottom seal : At bottom concrete provided used for load transfer from steinning to the soil below.
Top seal : After sand filling, gives base for well cap.
Sand filling : Transfer load from cap to the bottom seal.
Construction of well foundation
Short steps - 1) Sinking of foundation well
2) Providing bottom plug or seal of concrete
3) Sand filling 4th
4) Providing top plug
5) Providing cap
6) Construction of pier
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Problems in well sinking
1) Sand blowing : occurs during dewatering in sandy sail due to sudden change in arrangement of soil
layer
2) Fitting of well
3) Shifting of well
4) Obstacle skin friction to overcum
Shapes : 1) circular
2) Rectangular 3) Double D – Commonly used
4) Rectangular with D shape end 5) Dumb well 6) Twin circular 7) Twin hexagonal 8) Twin
octagonal – for large span bridges
Caisson : Permanent structure ( it is one of foundation type)
Type :
1) Box caisson – closed at bottom
2)
Open caisson – open at both top + bottom
3)
Pneumatic caisson : open at bottom & closed at top and sunk by means of compressed air.
Note : -
Open caisson consisting of RCC is known as ‘well foundation’ in India.
-
Box caisson is used where firm ground is available at depth below
-
Open caisson is used where depth of water is more
Note : This caisson may occure caisson dieseage due to compressed air and nitrogen intake with oxygen.
Loading on Bridges
1)
Bouyancy pressure : due to buoyancy, wt of bridge reduces
Note :

for design of submersible bridge, full buoyancy effect on super structure piers and
abutments is to be considered.

If member under consideration displaces water then reduction due to buoyancy is taken equal to the wt
of water displaced.

2)
Centrifugal forces : Considerted when bridge is on curved road
(I) Road Bridges : 𝐶 =
.
in KN acts normally point load contact or in KN/M length in case of
uniformly distribute live load.
W = in KN for wheal load or KN/M for Udl
N = Vehicle speed in kmph
R = Radius of curvature in ‘m’
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Note : This centrifugal force to be acting at height = 1200mm above level of carriage way and not to be
increase for imact effect
(II) Railway Bridges : 𝐶 =
C = Horizontal c.f. in KN/M length
.
W = in KN/M Udl
V = max speed in kmph
R = in ‘m’
Note : It acts at height of 1830mm above rail level for B.G. and it acts at height of 1450 mm for MG
MKS
𝐶=
C and W tones instead of KN in mks unit.
𝐶=
.
SI
1) Dead load : 1st D.L. is assumed from some imperical formulae and after finialization actual load is
calculated.
i) Unwin’s formula : 𝑊 =
in ‘tonnes’
P = Load to be carried in tones
L = Span in ‘m’ W = Wt of one span in tones excluding wt or cross girders florring
f = working stress in tones per cm2
𝑟=
ii) American formula for plate girder :
P = total live load in kg
𝑊=
L = span in ‘m’
D = depth of girder in ‘cm’
Span : Impact load 𝛼
Speed : V ↑ → ↑ Impact
Provision for impact load in design is taken equal to the fraction of live load stresses and that fraction is
termed as ‘impact factor’ or ‘ coefficient of Impact’.
Formulae :
1) Indian Roads congress :
a)
for IRC class A & B loading :
𝐼=
𝐼=
.
for CRCC bridges of spans less than 3m with max value 0.5)
.
for steel bridges of L ≤ 3m with max value 0.545
L = span in ‘m’
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b) For class AA loading
Tracked vehicle
Wheeled vehicle
For bridge,
effective span >
9m
RCC bridges
0.1 upto 40m span
0.25 for l = 12m
and
and 0.25 to 0.088
0.088
upto
and above 45m
for t betn 12m to
45m
Steel bridge
0.1 for all spans
0.25 for span upto
23m 0.25 to 0.154
span 23 to 45m
0.154, l ≥ 45m
for main girders having l < 9m
RCC
+
steel
bridges
0.25 for l upto 5m
0.25
linearly reduced to
0.1 for span 9m
a) RCC Arches : W = 16.4 CL
L = span in ‘m’
W = avg dead loag in kg/m2 including wt of wearing
surface and wt of fill above crown
b) RCC slab bridge : upto 6m span W = 425 + 148 L
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W = D.L. in kg/m2 , including wt of surface of 195 ‘ kg per m2 L = span in ‘m’
c) RCC slab and T beam bridges : W = 425 + 82L for span 6m to 15m
d) Deformation stresses : Only for steel bridges
- This is due to deflection of bridge i.e. bending stresses produced. It is taken as > [16% of L.L. and D.L.
stresses]
Note : Deformation stresses are ignored in case of prestressed ‘girder of steel’
Earth pressure : Use ‘coulomb theory’
-
Height of centre of earth pressure is taken at 0.42 of height of wall above the base instead of
0.333H
- Also bridges are designed for min earth pressure equivalent to the 1 exerted by a fluid weighing
4800 N/M3
𝑃=
P = Total
By Rankine’s used in Railway Bridge
p = Total earth pressure in ‘KN’
w = Unit wt of soil in ‘KN/m3’
h = Height of backfill
𝜙 = angle of repose
Erection stresses : Should be within the permissible limit
Impact load : Due to uneven surface o fraud or railway bridge
Note :
1) If depth of solid floor is more, effect of impact is less
2) In case of spandrel – filled arch bridges, the effect of impact is considerably reduced due to
absorbing power of filling
3) Footways are not designed for impact
Hammer blow action : More predominant in case of railway bridges than the road bridges.
Live load
IRC bridge loading is of following :
1)
IRC class ‘AA’ loading :
-
Provided within certain municipality limits. I certain existing or
contemplated industrial area, in other specific areas and along certain specified highways.
-
It should be checked for class ‘A’ loading. (For Temporary structure)
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2)
IRC class ‘A’ loading : [for permanent bridges & culverts] includes worst combination of loads. In this
load train is used for analysis which consists of driving vehicle and two trailers of specified axle loads and
spacing.
3)
IRC class ‘B’ loading : (for these are temporary bridges)
It includes same train as ‘A’ but reduced loads and tyre contact dimensions
4)
IRC class 70R loading – (permanent structure)
Tracked – 70 tonne
Wheel – 100 tonne with 7 axle
Loading for class AA :
Tracked – 700 KN or 70 tonne
Wheeled – 400 KN or 40 tonne with dual axle
1) Tracked vehicle : 70t i.e, 700 KN which is distributed equally on two wheel which are 2.9m end to
end.
∴ on 1 wheel load = 35t
2) Wheeled wheel : Total load for dual axle = 40t = 400KN and for single axle = 20t
max load of single wheel = 6.75 tonne clearance between kerb of load and vehicle on single lane 3.8 m
road = 30cm
Class A loading : Train with engine and two bogie length of loading train = 20.4m (end to end) and
spacing between successive trains > 18.4 m
total load for train for A loading = 67.6 tonnes
total load for train for B loading = 40.5 tonnes (vehicle is similar to class A)
Note :
1) Min clearance on 5.5m width load for A = 15cm
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2) For foot ways which are only for pedestrian and animal = 400 kg/N or 4 KN/m2 but when it
is crowded it designed for 5 KN/m2 or 500 kg/m2
3) Similar to class A with lesser load and adopted only for temporary bridges, like timber
70R : - R for revised, additional loading to be considere is plane of class AA loading.
-
For tracked wheel : load = 700 KN or 70 Tonnes with l = 7.52m spacing between successive
vehicle
> 30m
-
for wheeled : 100KN or 100 tonne with 7 axles, length 15.22 m
Longitudinal loading : acts along roadways at a height of 1200 mm above the road surface.
Earthquake load : fraction of total wt of structure is consider
seismic load – s = x, w x = seismic coeff.
W = wt ignoring buoyancy & uplift
x = 0 for zone I on hard soil & more for zone v.
Seismic load : It vertical to be consider then = 50% of horizontal seismic coefficient.
Temperature variation : only when restricted to change in lengh of bridge
𝑠𝑡𝑟𝑒𝑠𝑠 = 𝐸𝛼∆𝑡
Water pressure : 𝑝𝒘 = 𝑘 𝑟 ×
= 500 𝑘𝑣
𝑁/𝑚
for square ended pier = k = 1.5 for circular k = 0.66
Wind load : P = 𝑘𝑣
𝑁/𝑚
𝑝𝛼𝑣
Wind load assumed to act at H = 1500mm above road
For foundation : EL & WL does not act simulteniously increase bearing capacity by 25% due to E.L +
W.L.
Erection suspension bridge : 1) Erection of tower 2) Erection of suspenders 3) Erection of cat walkes 4)
Erection of stiffening trusses 5) Erection of flooring system – in this sequence erection of suspension
bridge is done.
Posting of bridges : It is process of restriction on the use of bridge so as not to disturb its safe load
carrying capacity
1) Load limit posting – for limiting loads
Advance warning – at 200m
Load restriction sign – at 60m
2) Speed posting
Rating : Process of assessing the safe load carrying capacity is known as ‘Rating of old bridges.
Equipments used :
1) Magnetic particle detector : Use of magnetic material to detect defect.
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2) Radio graphic equipment : Uses x-rays or gamma rays.
3) ultra sonic testing equipments :
Rebuilding bridges : 1) Damage 2) Exccessive maintenance 3) obselescence 4) weathering
Use : 1) epoxy resin injection
2) External prestressing
3) Externally bonded steel plates
4) Groouting, shortereting
Testing and strengthening : Testing usually, carried by use of ‘deflectometer’.
Safe bearing capacity work out 1) correlation method
2) load testing
3) Theoretical method – with help of charts, formulas etc
Strengthing of bridges :
Note : Old bridges are strengthened.
Strengthing of bridge sub-structue like piers is carried out by removing weak part, creating cavity in
that and then filling that cavity with rich concrete or by pressure grouting of cement slurry.
Strengthing of super-structure :
1) For continuous bridges : Strengthened by keeping view that adjacent span does not become weak.
2) Masonry arch bridges : by constructing new R.C.C. arch on the previous arch.
3) R.C.C. slab bridges : Place a slab on existing old slab.
4) Steel bridges : by encasement in concrete or by providing extra new steel.
5) Suspension bridges : by providing extra cables with fasterners.
Generally strengthening of bridge can be done in following cases :
When
1) Corrosion of reinforcement
2) Displacement of decks
3) Flexural and shear cracks are there
4) Movement of abutments
5) Sinking of wells
6) Tilting of piers
7) Topping of bearing etc
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CHAPTER : 7
BRIDGE, BEARING
Bearing : Placed at top of abutment or pier for free expansion contraction and deflection of
superstructure.
Function :
1)
Distribute load
2)
Longitudinal expansion and contraction due to tempreture
3)
To allow angular movement due to deflection
4)
Allow vertical movement due to sinking
5)
Transfer horizontal forces due to application of breakes etc
6)
Keep compressive forces within limits
Note : In bridges – 1 bearing is kept fixed and other free.
Types :
1) Fixed bearing : Which does not allow longitudinal movement or deflection of girder known as fixed
bearing
2) Expansion or free bearing : The bearings which allow horizontal lonigitudinal movement or also
called free bearing
Types of fixed Bearing
1) Shallow or fixed bearing
- Simplest type
- Used for steel girder bridge
- Rectangular steel plate fixed at bottom of flange of main girder which is fixed to top of pier by nuts
and bolts.
- Suitable for steel bridges upto 12m span
- Suitable for steel girder bridge
2) Deep-cast base bearing
- Made of steel
- In this case deep cast is attachment to the under side of main girder upto 12 to 20m
- Used in steel girder bridge
- Distribute load over abutment to avoid concentration of load on top of pier
3) Rocker bearing
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 Used for angular movement of girder
 Bearing in which rocker pin is provided between top shoe and bottom shoe
 Suitable for span more than 20m
 It behave like a hinged pin transmit pressure centrally
 Pin is designed for shear + bearing steel girder
4) Knuckle bearing :
 In this bottom of top shoe and top of bottom shoe is provided with semi-circular shapes
 Used in span over 20m
 Used to allow angular moment which is fixed to top shoe i.e., for angular moment of top shoe is possible
 Eliminates rocker pin

Types of free of expansion bearings
1) Sliding plate bearing :

A sole plate with bearing allow horizontal movement of bridge.

Bed plate is fixed on top of pier and on that sliding plate with bearing with slotted hole.

upto 12 to 20m span
Deep cast base curved plate :

Here slote plate is fixed to the bottom of main plate girder and a curved bed plate is used on this
sole plate is rest.

Allow free angular movement of main girder and expansion also

Upto 12 to 20m span
Rocker bearing with curved base :
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
Curved base connected to abutment or pier

Type of rocker bearing in which bottom shoe is curved in circle

Allow expansion with reduced horizontal force
1) Rocker and roller bearing

Span > 20m
Bearing for RCC bridges
1) For slab bearing

Used for paper sandwidtened between bridge slab and capping slab of the pier or abutment.

Tar paper is served layer used to avoid bonding of two concrete layer.

Tar paper is served layer used to avoid bonding of two concrete layer.
2) Fixed end bearing : Which do not allow free expansion of slab
dia of pipe culvert > 300 mm (always)
elastometric bearing is expansion type bearing
Note Points :
1) Class AA loading :
i) Wheeled vehicle = 40 tonnes = 400 KN
ii) Tracked vehicle = 70 tonnes = 700 KN
2) Class 70 R loading :
i) Wheeled vehicle = weight 100 tonnes = 1000KN
ii) Tracked = 7000 KN
= 70 tonnes
3) As per IRC, minimum clear distance between kerb and vehicle on a single lane road of 3.8m = 0.3m
4) Simplified new loading standard for the design of highway bridges in India, as per mr. P.V. Thomas is
= 10
.
5) Minimum width of carriage way for single lane vehicular traffic bridge = 4.25m
6) Min width of carriage way for two lane = 7.5m and if there is multispan bridge then per lane = 3.5m
7) Min width for pedestrian traffic = 2.5m
8) Min footpath = 1.5m
9) Min median = 1.2m
10) Min cycle truck = 2m
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11) No live load is considered to be acting on bridge, when wind velocity at bridge site > 130 km/hr
12) Inspection for bridges are i) Indepth inspection
ii) Routine inspection
iii) Special inspection – It is carried out after 3 to 5 year
13) Concrete used for pier cap = m20
14) Economic span for steel truss bridge = 3m
15) Economic span for RCC bridge = 1.5m
16) Min. free board for arch type bridge = 300 mm
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