Diploma Civil Project

7/27/2019 Diploma Civil Project

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CONTENTS

Abstract1. Introduction1.1 Activities Involved

1.2 Definition Of A Bridge1.3 History Of A Bridge

2. Bridges-Types2.1 Classification Of Bridges

2.1.1 Function2.1.2 Material Of Construction2.1.3 Form

2.1.4 Inter-Span Relations

2.1.5 Position Of The Bridge To The Superstructure2.1.6 Method Of Connections2.1.7 Method Of Clearance

2.1.8 Length Of Bridge2.1.9 Degree Of Redundancy

2.1.10 Type Of Service

2.2 Types Of Bridges2.2.1Beam Bridges2.2.2 Cantilever Bridges

2.2.3 Arch Bridges2.2.4 Suspension Bridges2.2.5 Cable-Stayed Bridges And

2.2.6 Movable Bridges

2.2.7 Truss Bridges2.2.1.1 Beam Bridges2.3 Need For Investigation2.4 Economic Range Of Span Lengths For Different

Types Of Structures2.5 Selection Of Bridge Site

3. Standard Specifications3.1 Standard Specification For Road Bridges

3.2 Loads To Be Considered In A Design3.2.1 Dead Load3.2.2 Live Load3.2.3 Impact

3.2.4 Wind Load

3.2.5 Longitudinal Forces3.2.6 Dynamic Load3.3 Indian Road Congress Bridge Code

3.4 Bridge Loading Standards3.4.1 Irc Class Aa Loading

3.4.2 Irc Class70 R Loading3.4.3 Irc Class A Loading

3.4.4 Irc Class B Loading3.5 Width Of Carriageway

3.6 Clearances4. Components Of A Bridge

4.1 Type Of Foundations4.1.1 Shallow Foundations

4.1.2 Deep Foundations4.1.2.1well Foundations:4.1.2.2 Pile Foundations

4.2 Piers4.3 Abutments

4.4 Bearings4.5 Super Structure

4.5.1 General Arrangement Of Girders In Super Structure4.5.1.1 Girder And Slab Type4.5.1.2 Girder Slab And Diaphragm Type

4.5.1.3 Girder, Slab And Cross Beam Type
http://en.wikipedia.org/wiki/Beam_bridgehttp://en.wikipedia.org/wiki/Cantilever_bridgehttp://en.wikipedia.org/wiki/Arch_bridgehttp://en.wikipedia.org/wiki/Suspension_bridgehttp://en.wikipedia.org/wiki/Cable-stayed_bridgehttp://en.wikipedia.org/wiki/Movable_bridgehttp://en.wikipedia.org/wiki/Movable_bridgehttp://en.wikipedia.org/wiki/Cable-stayed_bridgehttp://en.wikipedia.org/wiki/Suspension_bridgehttp://en.wikipedia.org/wiki/Arch_bridgehttp://en.wikipedia.org/wiki/Cantilever_bridgehttp://en.wikipedia.org/wiki/Beam_bridge


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4.5.2 Types Of Prestressing And Its Proper Use5. Design Considerations For Different Bridge5.1 Concrete Bridges

5.2 Design Criteria For Railway Bridges

5.2.1. Steel Used In Concrete5.2.2. Web Thickness5.3 Design Criteria For Road Bridges

5.3.1 Design Procedures For Bridge Superstructure5.3.1.1 Introduction5.3.1.2 Design Procedure For Railway Bridges:5.3.2.1 Steel Girders

5.3.2.2 Concrete Girders5.3.3 Road Bridge Design

5.3.3.1 Approach To Design5.4 Design Of Concrete Road Bridges

5.4.1 Design Of The Longitudinal Girders5.4.1.1 Courbons Method5.4.1.2. Henry-Jaegers Method

5.4.1.3. Marcie Little Method

5.5 Design Features Of The Pier5.6 Design Features Of The Abutment

6. Well Foundations6.1 Introduction

6.2 Comparision With Pile Foundations6.3 Well Types And Their Sutability6.3.1circular Well

6.3.2double D Well

6.3.3double Octagonal Well6.3.4rectangular Well6.3.5twin Circular Well6.3.6wells With Multiple Dredge Holes

7. Prestressed Concrete

7.1 Definition7.2 Pretensioned Concrete7.3 Post Tensioned Concrete

7.3.1 Bonded Post Tensioned Concrete

7.3.2 Unbounded Post Tensioned Concrete7.3.3 Procedure For Tensioning And Transfer7.4 Advantages Of Prestressed Concrete7.5 Terminology

7.6 Applications8. Misclaneous Items Of Work8.1 Material To Be Used8.1.1 Concrete

8.1.2 Under Water Concreting

8.1.2.1. Tremie8.1.2.2. Direct Placement With Pumps

8.1.2.3. Drop Bottom Bucket

8.2 Steel8.3 Future Prestressing Arrangements

8.4 High Performance Concrete8.5 Anti Corrosive Treatment

8.6 Erection Scheme Of Girders


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ABSTRACT

This project is about a precast prestressed girder bridge located at Borduskuru in Andhra

Pradesh, which involved in study of foundations adopted, piers, abutments and bearings. The study

also includes the precasting of girders their launching and launching of launching system along with

the methods of prestressing and prestressing techniques.

This report also attempts to explain the design procedure of some components of this bridge structure.

The practical problems encountered along with their solutions are also illustrated.


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1. INTRODUCTION

A bridge project from its conception to completion involves various stages of planning, design,

approval/sanction, tendering and execution. Also inspections, maintenance and repairs are continuing activities

for enhancing the service life of the structure.

1.1. ACTIVITIES INVOVED:

A bridge project is required to carry out survey for the bridge location and collect requisite preliminary survey

data that is required for bridge planning and design. Generally 2-3 cross sections at prospective sites are taken

and the bridge length is decided for the purpose of preparing stage-I estimate needed for obtaining Approval.

Depending on site conditions, particularly the foundation conditions (which could be a guess/ interpolation at

this stage) the type of bridge viz. P.S.C., R.C.C., high level, submersible etc. is decided.

For bridges having span more than 60m, detailed estimate is required to be submitted to Government for

obtaining administrative approval. It is, therefore, necessary that site is to be finalized by the Engineer. So that

detailed soil explorations as may be necessary could be done.

The detailed proposal is then prepared by Engineer. The detailed proposal would generally mean giving

sufficient details for preparation of estimate after working out the stability of structures i.e. piers and abutments

and deciding the tentative dimensions for superstructure and other components along with specifications.


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1.2 DEFINITION OF A BRIDGE:

A bridge is a structurebuilt to spanphysical obstacles such as abody of water, valley, orroad, for the

purpose of providing passage over the obstacle.

Designs of bridges vary depending on the function of the bridge, the nature of the terrain where the bridge

is constructed, the material used to make it and the funds available to build it.

1.3 HISTORY OF A BRIDGE:

The greatest bridge builders of antiquity were the ancient Romans. The Romans built arch bridges and

aqueducts that could stand in conditions that would damage or destroy earlier designs. Some stand today.

Rope bridges, a simple type of suspension bridge, were used by the Inca civilization in the Andes mountains of

South America, just prior to European colonization in the 16th century.

During the 18th century there were many innovations in the design of timber bridges by Hans Ulrich, Johannes

Grubenmann, and others.

With the Industrial Revolution in the 19th century, truss systems of wrought iron were developed for largerbridges, but iron did not have the tensile strength to support large loads.

With the advent of steel, which has a high tensile strength, much larger bridges were built, many using the ideas

ofGustave Eiffel.

In 1927 weldingpioneerStefan Bryadesigned the first welded road bridge in the world, which was later builtacross the riverSudwia Maurzycenearowicz, Polandin 1929.

1.4 IMPORTANCE OF BRIDGE:

Bridges have always figured prominently in human history. Cities have sprung up at a bridgehead or where at

first a river could be forded at any time of the year.

Examples: London, Oxford, Cambridge and Innsbruck.

Bridges add beauty to the cities.

Examples: the bridges across the river seine in Paris and the bridges across the river Thames in London.

They enhance the vitality if the cities and aid the social, cultural and economic improvements of the areas

around them. Great battles have been fought for cities and their bridges. The mobility of an army at war is often

affected by the availability or otherwise of the bridges to across rivers. That is why military training puts special

emphasis on learning how to destroy bridges during combat and while retreating and how to build new ones

quickly while advancing.

2. BRIDGES-TYPES

2.1 CLASSIFICATION OF BRIDGES:

Function Material of construction Form Inter-span relations Position of the bridge to the superstructure Method of connections
http://en.wikipedia.org/wiki/Structurehttp://en.wikipedia.org/wiki/Structurehttp://en.wikipedia.org/wiki/Span_(architecture)http://en.wikipedia.org/wiki/Body_of_waterhttp://en.wikipedia.org/wiki/Body_of_waterhttp://en.wikipedia.org/wiki/Body_of_waterhttp://en.wikipedia.org/wiki/Valleyhttp://en.wikipedia.org/wiki/Roadhttp://en.wikipedia.org/wiki/Roman_Engineeringhttp://en.wikipedia.org/wiki/Arch_bridgeshttp://en.wikipedia.org/wiki/Aqueducthttp://en.wikipedia.org/wiki/Rope_bridgehttp://en.wikipedia.org/wiki/Incahttp://en.wikipedia.org/wiki/Andeshttp://en.wikipedia.org/w/index.php?title=Hans_Ulrich&action=edit&redlink=1http://en.wikipedia.org/wiki/Johannes_Grubenmannhttp://en.wikipedia.org/wiki/Johannes_Grubenmannhttp://en.wikipedia.org/wiki/Industrial_Revolutionhttp://en.wikipedia.org/wiki/Trusshttp://en.wikipedia.org/wiki/Wrought_ironhttp://en.wikipedia.org/wiki/Tensile_strengthhttp://en.wikipedia.org/wiki/Gustave_Eiffelhttp://en.wikipedia.org/wiki/Weldinghttp://en.wikipedia.org/wiki/Stefan_Bry%C5%82ahttp://en.wikipedia.org/wiki/Stefan_Bry%C5%82ahttp://en.wikipedia.org/wiki/Stefan_Bry%C5%82ahttp://pl.wikipedia.org/wiki/S%C5%82udwia_(rzeka)http://pl.wikipedia.org/wiki/S%C5%82udwia_(rzeka)http://pl.wikipedia.org/wiki/S%C5%82udwia_(rzeka)http://en.wikipedia.org/wiki/%C5%81owicz,_Polandhttp://en.wikipedia.org/wiki/%C5%81owicz,_Polandhttp://en.wikipedia.org/wiki/%C5%81owicz,_Polandhttp://en.wikipedia.org/wiki/%C5%81owicz,_Polandhttp://pl.wikipedia.org/wiki/S%C5%82udwia_(rzeka)http://en.wikipedia.org/wiki/Stefan_Bry%C5%82ahttp://en.wikipedia.org/wiki/Weldinghttp://en.wikipedia.org/wiki/Gustave_Eiffelhttp://en.wikipedia.org/wiki/Tensile_strengthhttp://en.wikipedia.org/wiki/Wrought_ironhttp://en.wikipedia.org/wiki/Trusshttp://en.wikipedia.org/wiki/Industrial_Revolutionhttp://en.wikipedia.org/wiki/Johannes_Grubenmannhttp://en.wikipedia.org/wiki/Johannes_Grubenmannhttp://en.wikipedia.org/w/index.php?title=Hans_Ulrich&action=edit&redlink=1http://en.wikipedia.org/wiki/Andeshttp://en.wikipedia.org/wiki/Incahttp://en.wikipedia.org/wiki/Rope_bridgehttp://en.wikipedia.org/wiki/Aqueducthttp://en.wikipedia.org/wiki/Arch_bridgeshttp://en.wikipedia.org/wiki/Roman_Engineeringhttp://en.wikipedia.org/wiki/Roadhttp://en.wikipedia.org/wiki/Valleyhttp://en.wikipedia.org/wiki/Body_of_waterhttp://en.wikipedia.org/wiki/Span_(architecture)http://en.wikipedia.org/wiki/Structure


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Method of clearance Length of bridge Degree of redundancy Type of service

Bridges may classify in many ways as below:

2.1.1. FUNCTION: According to function as aqueduct (canal over a river), viaduct (road or railway over a

valley), pedestrian, highway, railway, road-cum-rail or a pipeline.

2.1.2.MATERIAL OF CONSTRUCTION: According to the material of construction of the superstructure as

timber, masonry, iron, steel, reinforced concrete, pre-stressed concrete, composite or Aluminium Bridge.

2.1.3.FORM: According to the form or type of the superstructure as slab, beam, truss, arch, cable stayed or

suspension bridge.

2.1.4. INTER -SPAN RELATIONS: According to the inter-span relations as simple continuous or cantilever

bridge.2.1.5. POSITION OF THE BRIDGE TO THE SUPERSTRUCTURE: According to the position of the

bridge to the superstructure as deck, though, half-through or suspended bridge.

2.1.6. METHOD OF CONNECTIONS: According to the method of connections of the different parts of the

superstructure, particularly for the steel construction as pin connected, riveted or welded bridge.

2.1.7. METHOD OF CLEARANCE: According to the method of clearance for the

navigation as high-level, movable-bascule, movable-swing and transporter bridge.

2.1.8. LENGTH OF BRIDGE: According to the length of bridge as culvert (60m) or a long span bridge when the main span of the major bridge is above 120m.

2.1.9. DEGREE OF REDUNDANCY: According to the degree of redundancy as determinate or indeterminate

bridge.

2.1.10. TYPE OF SERVICE: According to the anticipated type of service and duration of use as permanent,

temporary, military bridge.

2.2. TYPES OF BRIDGES:

There are six main types of bridges: Beam bridges, Cantilever bridges, Arch bridges, Suspension bridges, Cable-stayed bridges and Truss bridges

2.2.1 Beam bridgesare horizontal beams supported at each end by abutments, hence their structural name of

simply supported. When there is more than one span the intermediate supports are known as piers.

2.2.2 Cantilever bridges are built using cantilevers horizontal beams supported on only one end. Most

cantilever bridges use a pair ofcontinuous spans that extend from opposite sides of the supporting piers to meet

at the center of the obstacle the bridge crosses. Cantilever bridges are constructed using much the same

materials & techniques as beam bridges. The difference comes in the action of the forces through the bridge.

2.2.3 Arch bridgeshave abutments at each end. The earliest known arch bridges were built by the Greeks, and

include the Arkadiko Bridge. The weight of the bridge is thrust into the abutments at either side.
http://en.wikipedia.org/wiki/Beam_bridgehttp://en.wikipedia.org/wiki/Cantilever_bridgehttp://en.wikipedia.org/wiki/Arch_bridgehttp://en.wikipedia.org/wiki/Suspension_bridgehttp://en.wikipedia.org/wiki/Cable-stayed_bridgehttp://en.wikipedia.org/wiki/Beam_bridgehttp://en.wikipedia.org/wiki/Beam_bridgehttp://en.wikipedia.org/wiki/Simply_supportedhttp://en.wikipedia.org/wiki/Bridge_pierhttp://en.wikipedia.org/wiki/Cantilever_bridgehttp://en.wikipedia.org/wiki/Cantilever_bridgehttp://en.wikipedia.org/wiki/Cantileverhttp://en.wikipedia.org/wiki/Continuous_spanhttp://en.wikipedia.org/wiki/Arch_bridgehttp://en.wikipedia.org/wiki/Arch_bridgehttp://en.wikipedia.org/wiki/Abutmentshttp://en.wikipedia.org/wiki/Arkadiko_Bridgehttp://en.wikipedia.org/wiki/Abutmentshttp://en.wikipedia.org/wiki/Abutmentshttp://en.wikipedia.org/wiki/Arkadiko_Bridgehttp://en.wikipedia.org/wiki/Abutmentshttp://en.wikipedia.org/wiki/Arch_bridgehttp://en.wikipedia.org/wiki/Continuous_spanhttp://en.wikipedia.org/wiki/Cantileverhttp://en.wikipedia.org/wiki/Cantilever_bridgehttp://en.wikipedia.org/wiki/Bridge_pierhttp://en.wikipedia.org/wiki/Simply_supportedhttp://en.wikipedia.org/wiki/Beam_bridgehttp://en.wikipedia.org/wiki/Cable-stayed_bridgehttp://en.wikipedia.org/wiki/Suspension_bridgehttp://en.wikipedia.org/wiki/Arch_bridgehttp://en.wikipedia.org/wiki/Cantilever_bridgehttp://en.wikipedia.org/wiki/Beam_bridge


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2.2.4 Suspension bridges are suspended from cables. The earliest suspension bridges were made of ropes or

vines covered with pieces of bamboo. In modern bridges, the cables hang from towers that are attached to

caissons or cofferdams. The caissons or cofferdams are implanted deep into the floor of a lake or river.

2.2.5 Cable-stayed bridgeslike suspension bridges are held up by cables. However, in a cable-stayed bridge,

less cable is required and the towers holding the cables are proportionately shorter.

2.2.6 Movable bridges are designed to move out of the way of boats or other kinds of traffic, which would

otherwise be too tall to fit. These are generally electrically powered.

2.2.7 A truss bridge is a bridge composed of connected elements (typically straight) which may be stressed

from tension, compression, or sometimes both in response to dynamic loads. Truss bridges are one of the oldest

types of modern bridges.

2.2.1.1 Beam bridges:

Beam bridges are the most simple of structural forms being supported by an abutment at each end of the deck.

No moments are transferred through the support hence their structural type is known as simply supported.

The simplest beam bridge could be a slab ofstone, or a plankofwood laid across a stream. Bridges designed for

modern infrastructure will usually be constructed of steel orreinforced concrete, or a combination of both. The

concrete used can either be reinforced, prestressed orpost-tensioned.

Types of construction could include having many beams side by side with a deck across the top of them, to a

main beam either side supporting a deck between them. The main beams could be I-beams, trusses, orbox

girders. They could be half-through, or braced across the top to create a through bridge.

2.3 NEED FOR INVESTIGATION:

Before a bridge can be built at a particular site, it is essential to consider many factors, such as the need for a

bridge, the present and the future traffic stream characteristics subsoil conditions, alternative sites, aesthetics and

cost.

The aim of the investigation is to select a suitable site at which a bridge can be built economically, at the

sometime satisfying the demands of traffic, the stream, safety and the aesthetics. The investigation for a major

bridge project should cover studies on technical feasibility and economic considerations and should result in an

investigation report. The success of the final design will depend on the thoroughness of the information

furnished by the officer in charge of the investigation.

2.4 SELECTION OF BRIDGE SITE:

This is particularly so in case of bridges in urban areas and flyovers. For river bridges in rural areas, usually a

wider choice may be available.The characteristics of an ideal site for a bridge across a river are:

i. A straight reach of the river.ii. Steady river flow without serious whirls and cross currentsiii. A narrow channel with firm banksiv. Suitable high banks above high flood level on each sidev. Rock or other hard in erodible strata close to the river bed levelvi. Economical approaches which should not be very high or liable to flank attacks of the river during

floods; the approaches should be free from obstacles such as hills, frequent drainage crossings,

sacred places, graveyards or built up areas or troublesome land acquisition

vii. Proximity to a direct alignment of the road to be connectedviii.

Absence of sharp curves in the approaches
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ix. Absence of expensive river training works andx. Avoidance of excessive underwater construction

2.5ECONOMIC RANGE OF SPAN LENGTHS FOR DIFFERENT TYPES OF

STRUCTURES

Apart from the estimated cost based on schedule of rates, costs as quoted during tendering may be used for

constantly updating the cost analysis data. The ranges of span length within which a particular type of

superstructure can be economical along with other considerations Rice type of foundation etc. are given be low:-

R.C.C. single or multiple boxes 1.5 to 15 m

Simply supported RCC slabs 3 to 10 m

Simply supported RCC T beam 10 to 24 m

Simply supported PSC girder bridges 25 to 45 m

Simply supported RCC voided slabs 10 lo 15 m

Simply supported/continuous PSC voided slabs 15 to 30 m

Continuous RCC voided slabs 10 to 20 m

25 to 50 m

RCC box sections simply supported / Balancedcantilever continuous

35 to 75 mPSC box sections; simply supported / Balanced

cantilever

75 to 150 mPSC cantilever construction / continuousCable stayed bridges 100 to 800 m

Suspension bridges 300 to 1500 m

3. STANDARD SPECIFICATIONS

3.1 STANDARD SPECIFICATION FOR ROAD BRIDGES:

Standard specifications and code of practice have been evolved by the concerned government agencies and

professional institutions, based on years of observation, research and development. The purpose of the codes is

to ensure adequate safety and afford protection against legal liability arising out of failures due to no fault of thedesigner. Since the public roads and railways in India are owned and controlled by the government the bridges

built on them should follow the instructions follow specifications laid down by the respective authorities.

All highways bridges have to be built in accordance with the Indian Road Congress (IRC) CODES, besides

specifications prescribed by the Ministry of Surface Transport (Roads Wing), Government of India

(MOST).Similarly Indian Railway Standard (IRS) Bridge rules should be followed for the design of railway

bridges.

3.2 LOADS TO BE CONSIDERED IN A DESIGN:

3.2.1 DEAD LOAD:

The dead load consists of the weight of superstructure in any fixed support by the member.


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3.2.2 LIVE LOAD:

Class AA and a loading are adopted in the design. The standards are adopted as per IRC recommendations. As

the pedestrian traffic is very less, design load of kerb is sufficient.

3.2.3 IMPACT:

To take in to account the higher stresses is caused by the dynamic forces of the moving load, an impact

allowance should be made. The standards are adopted as per IRC recommendations.

3.2.4 WIND LOAD:

These forces are considered to act horizontally and in such a direction as to cause the maximum stresses in the

member under consideration. The area to be considered on which the wind force is assumed to act is, the area of

the structure as seen in the elevation including the floor system less the area of perforations. The wind loads are

adopted as per IRC recommendations.

3.2.5 LONGITUDINAL FORCES:

Tractive effort caused due to the acceleration of the driving wheels.

Braking effect caused due to the application of brakes to the wheels.

Resistance to the movement of bearings is due to temperature changes.

3.2.6 DYNAMIC LOAD:

The force exerted on a bridge as a result of unusual environmental factors, such as earthquakes or strong gusts

of wind.

3.3 INDIAN ROAD CONGRESS BRIDGE CODE:

The Indian Road Congress (IRC) Bridge code as available now consists of eight sections as below:

a) Section IGeneral features of designb) Section II- Loads and Stressesc) Section III-Cement concrete ( plain &reinforced)d) Section IV- Brick, stone and block masonrye) Section V- Steel road bridgesf) Section VI- Composite constructiong) Section VII- Foundations and substructureh) Section IX- Bearings

3.4 Bridge Loading Standards:

Bridge loading standards in many countries were first formulated to regulate heavy military vehicles and were

generally specified by local authorities. The loadings often considered of steam rollers and some form of

traction engines.

The earliest specifications of highway bridge loadings originated from the need to transport heavy military

vehicles in U.K and Europe.

The first loading standards in India was published by the Indian roads congress in 1958 and subsequently

reprinted in 1962 and 1963.The I.R.C 6 code has revised to include the combination of loads forces and

permissible stresses in fourth revision published in 2000.


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I.R.C. evolved different standard live loads. In terms of train tracked vehicle and wheeled vehicle with standard

axle loads and spacing.

I.R.C class AA loading:

Only adopted for bridges which are within certain municipal limits, in certain industrial areas and on certain

specified highways.

Bridges designed for class AA loading are to be checked for class A loading because under certainconditions class A loading causes heavier stresses than class AA loading.

I.R.C. class A loading (IRC standard loading):

Adopted for permanent bridges other than those specified under class AA loading.

I.R.C. class B loading (IRC light loading):

Used for temporary bridges.

I.R.C. class 70R loading (IRC heavy loading):

This has been evolved to confirm to required standard loading of defence authorities. This is to be used in place

of class AA loading. This government prescribed class 70R loading for bridges on national highways.

According to present practice, it is necessary to compute the maximum live load bending moment for three

different conditions of loading, and then adopt for design the greatest of three values. The computation of live

load bending moment only one loading condition need be considered namely

Class AA wheeled vehicle span up to 4m.

Class AA tracked vehicle span exceed 4m.

If shear is desired to be computed, class AA wheeled vehicle considered span up to 6m and tracked vehicle

beyond 6m for single lane bridge. However, for 2 Lane Bridge the shear due to class AA wheeled vehicle

controls the design for all spans from 1m to 8m.

The design moment for distribution is taken as 0.3 of Live load +0.2 Dead load moment.

The ministry of surface transport government of India, referred here in as most, has published a set of plans for

3.0m to 10.0m span reinforced deck slab.

3.4.1. IRC Class AA Loading

Two different types of vehicles were specified under this category grouped as tracked and wheeled vehicles withloadings of 700 kN and 400 kN respectively.

All the bridges located on National Highways and State Highways have to be designed for this heavy loading.

These loadings are also adopted for bridges located within certain specified municipal localities and along

specified Highways.

Alternatively, another type of loading designated as Class70R is specified instead of Class AA loading.

3.4.2. IRC Class70 R Loading

IRC 70 R Loading consists of following three types of vehicles.

Tracked vehicle of total load700 KN with two tracks each weighing 350Kn


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Wheeled vehicle comprising 4 wheels, each with a load of 100 kN totalling 400kN Wheeled vehicle with a train of vehicles on seven axles with a total load of 1000kN.

The various categories of loads are to be separately considered and worst effect has to be considered in design.

Only one lane of Class70R or Class AA load is considered whereas both the lanes are assumed to be occupied

by Class A loading if that gives the worst effect.

3.4.3 IRC Class A loading

IRC Class A type loading consists of a wheel load train comprising a truck with trailers of specified axle spacing

and loads as shown in fig. The heavy duty trucks with two trailers transmits load from 8axle varying from a

minimum of 27kN to a maximum of 114kN.The Class a loading is a 554 KN train of wheeled vehicles on eight

axles. Impact has to be allowed as per the formulae recommended in the IRC:6-2000.This type of loading is

recommended for all roads on which permanent bridges and culverts are constructed.

3.4.4 IRC Class B loading

Class B type of loading is similar to Class A loading except that the axle loads are comparatively of lesser

magnitude. The axle loads of Class B are a 332kN train of wheeled vehicle on eight axles as shown in fig.

3.5 WIDTH OF CARRIAGEWAY:

The width of carriage way required will depend on the intensity and volume of traffic anticipated to use the

bridge. The width of carriage way is expresses in terms of traffic lanes, each lane meaning the width required to

accommodate one train of class A vehicles.

The minimum width of carriageway for a one-lane bridge is: 4.25m

The minimum width of carriageway for a two-lane bridge is: 7.5m

For every additional lane, a minimum of 3.5m must be allowed. Three- lane bridges should not constructed, as

these will be conducive to the occurrence of accidents.

In case of a wide bridge, it is desirable to provide a central verge of at least 1.2m width in order to separate thetwo opposing lines of traffic.

From consideration of safety and effective utilization of carriage way it is desirable to provide footpath of at

least 1.5m width on either side of the carriageway for all bridges.

3.6 CLEARANCES:

The horizontal and vertical clearances required for highway traffic are given in fig., below wherein the

maximum width and depth of a moving vehicle are assumed as 3300mm and 4500mm respectively.

4. COMPONENTS OF A BRIDGE:

The main of a bridge structure are:

Decking, consisting of deck slab, girders, trusses etc.; Bearings for the decking; Abutments and piers; Foundations for the abutments and piers; River training works, like revetment for slopes for embankment at abutments, and aprons at

river bed level;

Approaches to the bridge to connect bridge proper to the roads on either side and Handrails, parapets and guard stones

Some of the components of a typical bridge are shown below:


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The components above the level of bearings are grouped as superstructure. While the parts below the bearings level are classed as the substructure. The portion below the bed level of a river bridge is called the foundation.

The components below the bearing and above the foundation are often referred

Sub-structure.

4.1 Type of foundations:

The subsoil characteristics obtained at a particularsite and consequently "file" type of foundations feasible, is

one of the major"considerations- in selector of type of structure and span arrangement as already mentioned:

4.1.1 Shallow foundations: Isolated open foundations are feasible where an SBC of about 15t/m2

or more is

available at shallow depths with in-redouble substratum. Here again, open excavation is feasible only up to a

depth of 3 to 4 m where the subsoil is porous and water table is high. In cases, where the SBC is still less and

where ~ smaller spans arc economical from other considerations, raft foundations or box structures with floor'protection arid curtain walls are the other options.

4.1.2 Deep foundations : Where suitable founding strata is available at a depth of 6 m or more with substantial

depth of standing water, highly pervious substratum and large' scour depth'/it may be "advisable to go for deep

foundation like (a) well, or (b) piles.

4.1.2.1 Well foundations: This is one of the most popular 'types of deep foundations in our Country, due

various reasons like its simplicity, requirement of very little of equipment's for' its execution, adaptability

to different subsoil conditions and difficult site conditions like deep standing water and large depths to

good founding strata. Caissons are an adaptation of well foundations to sites with deep standing water"

4.1.2.2 Pile foundations: Pile foundations are another type of deep foundations which are suited for

adoption in the following situations:-Availability' of good founding strata below large deep soft soil Need to

have very deep foundations beyond the limit of pneumatic operations usually depth beyond 35 meters or so. In

some cases of, strata underlying deep standing water and the strata being very hard not permitting easy

sinking of wells orbased on economic factors deciding the use of piles as compared to wells. However, pile

foundations are not preferred within the flood zone of the river with deep scour.

Classification of piles

(a) Precast driven piles

(b) Driven cast-in-situ piles

(c) Bored cast-in-situ piles

(d) Bored recast piles and

(e) Driven steel piles


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fig 4.1: components of bridge structure


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4.2 PIERS:

Piers are structures located at the ends of bridge spans at intermediate points between the abutments. The

function of the piers is two-fold to transfer the vertical loads to the foundation, and to resist all horizontal forces

and transverse forces acting on the bridge. Being one of the most visible components of a bridge, the pierscontribute to the aesthetic appearance of the structure.

The general shape and features of the pier depend to a large extent on the type, size and dimensions of the super

structure and also the environment in which the pier is located.

Fig 4.2: Pier

4.3 ABUTMENTS:

An abutment is the substructure which supports one terminals of the superstructure of a bridge and laterally

supports the embankment which serves as an approach to die bridge. It consists of generally three structural

elements.

a) The Brest wall, which directly supports the dead and live loads of the superstructure, and retains thefilling of the embankment in its rear.

b) The wing wall. Which act as extensions of the breast wall in retaining the fill, not taking loads from thesuperstructure

c) The back wall, which is small retaining wall just behind the bridge seat.

Fig 4.3-Abutment


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4.4 BEARINGS:

4.4.1 Bearings are vital components of a bridge which while allowing of longitudinal and/or transverse rotations

and/ or movements of the superstructure with respect to the substructure (thus relieving stresses due to

expansion and contraction), effectively transfer loads and forces from superstructure to substructure. Adequate

care shall be exercised in selecting the right type of bearings based on the guidelines given below:

(a) For solid/voided- slab superstructure resting on unyielding supports, no bearings arc generally provided if

the span length is less than 10m.

The top of piers/abutments caps are however rubbed smooth with carborandum stone.

(b)For girder and slab spans more than 10m length and resting on unyielding supports, neoprene bearings may

be considered. For spans larger than 25m roller and rocker bearings or PTFE bearings could be considered.

(d)For very large spans and where multidirectional freedom of movement and rotation are to be allowed

provision of pot bearings may be considered.

4.4.2 The design of metallic bearings and neoprene bearingsshall be in conformity with IRC: 8: Parts I &

II respectively.

4.4.3 In case of roller-cum-rocker bearings only full circular rollers are to beprovided.


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4.4.4 In order to cater for any possible relative undue movement of bearings over the abutment resulting in girder

ends jamming against the dirt wall preferably a larger gap may be provided between the girder end and the dirt wall.

4.4.5 All bearings assemblies shall be installed in accordance with. The instructions contained in the codes

and specifications and on the approved drawings. In particular the following important points shall not be

lost sight of:

(a)All bearings shall be set truly level so as to have full and even seating. Thin mortar pads (not exceeding 12mm)

may be used to meet this requirement.

(b)The bottoms of girders resting on the bearing shall be plane and truly horizontal.

(c)In case of rockers and roller bearings, necessary adjustment for temperature at the time of placement, shrinkage,

creep and elastic shortening shall be made, such that the line of bearing is as central as possible on the bearing

plates at the normal temperature taken in design.

(d) For elastomeric bearing pads, the concrete surface shall be level such that the variation is not more than

1.5mm from a straight edge placed in any direction across the area.

(e) For spans in grade, the bearings shall be placed horizontal by using sole plates or suitably designed R.C.C.

pedestals.

(f)Bearings of different sizes must not be placed next to each other to support a span.

(g)Installation of multiple bearings one behind the other on a single line of

Support is not permitted.

(h)The bearings shall be so protected while concreting the deck in situ that there is no flow of mortar or any

other extraneous matter into the bearing assembly and particularly on to the bearing surfaces. The protection shall

be

such that it can be dismantled after the construction is over without disturbing the bearing assembly.

4.5. Superstructure:

(i) It is the superstructure of a bridge that directly supports the traffic and facilitates its smooth uninterrupted

passage over natural/manmade barriers like rivers, creeks, railways, roads etc. by transmitting the loads and

forces coming over it to the foundation through the bearings and substructure.

(ii) The minimum functional requirement of superstructure are specified in IRC:5 and IRC: 21. In case of box

girder superstructure, the minimum clearheight inside the box girders shall be 1.5 m to facilitate inspection.

(iii)Aesthetics will be one of the major considerations while deciding oil the type of superstructure of a

bridge keeping in view the criteria mentioned therein.

(iv)Consistent with economy and local availability of the materials, labour and technology for a particular type

of superstructure selection may have to be made out of the following material options:

(a) Masonry

(b)Reinforcedcementconcrete

(c)Pre-stressedconcrete

(d) Steel

(e)Composite construction


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(v) Reinforced cement concrete superstructure: These are the most popular type of superstructure in the present day

which may take the form of solid slab, voided slab. T-beam and slab, box girder, rigid frame, arch, balanced

cantilever or bow-string girder.

Fig 4.4-Superstructure ( girder and slab type)

4.5.1 GENERAL ARRANGEMENT OF GIRDERS IN SUPER STRUCTURE:

Typical arrangements of RCC as well as PSC girder and slab type .It will be found that the main reinforcement

becomes heavy and for long spans becomes inconvenient for placement. The alternative arrangement is to provide

for box girders in which case a single box for both lanes or twin boxes for two lanes can be provided. Recent

long-span girders have been designed with a single box per pair of lanes also.

The typical arrangements for box for a two- lane bridge are indicated. There are three different ways of providing

the beams and slabs. These arrangements are equally applicable if the RCC T- beam is replaced by prestressed

concrete girders. As will be seen in the arrangement the three different arrangements for T-beam girders will have

differing effects on distribution of loads on slab as well as between girders.

4.5.1.1. Girder and slab type:

In this, the deck slab is supported on and cast monolithically with the longitudinal girders and no cross beam is

provided. This has the disadvantage of providing no torsional rigidity and there will be always the danger of the

girders tending to separate at the bottom level. They tend to tilt, particularly at bearings, and cause uneven loading

across the bottom bearing area. The slab is designed as a one-way continuous slab spanning between the

longitudinal girders.

4.5.1.2. Girder slab and diaphragm type:


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In this arrangement also, the slab is supported on and cast monolithically with longitudinal girders. However,

diaphragms are provided to connect the girders at the supports and on one or more location within the length of

span. Since these diaphragms do not extend up to the slab, the slab design is similar to the one mentioned in (a).The girders, however, are rendered more rigid by the diaphragms, and the distribution of load between the girders

, becomes more uniform.

4.5.1.3. Girder, slab and cross beam type:

In this, the diaphragms are replaced by cross beams provided at the ends and one or more intermediate locations

making any least three. They are cast monolithically with the deck slab. In this case, the deck slab is thus

supported on all four sides and hence it can be designed as a two- way slab. The cross beams provide still better

stiffness than diaphragms, and this hence results in a still better distribution of the loads among the longitudinal

girders in multiple-lane bridges. This also provides the advantage of reducing the number of longitudinal beams as

spacing can be increased without the fear of the need to have a deeper slab since the slab will be designed as

supported on all four sides.

Some experiments conducted by Prof. Victor at IIT Madras on one-sixth micro concrete model of a bridge 20,three- span girder bridge for these types gave the following conclusions:(1) The deflection of superstructure of type (b) and (c) were only 74 per cent and 63 per cent , respectively, of the

deflection for type (a)

(2) The transverse load distribution between the girders was better with type (b) and best with type (c); and

(3) The ultimate load-carrying capacity for the combined superstructure of types (b) and (c) were 132 per cent and

162 per cent, respectively, of the capacity for type (a).The only disadvantage in type (c) is the complicationinvolved in fixing form work and tying reinforcements. The current Indian practice is to use the type (b) or (c)

with one cross beam on each support and at least three cross beams in between for long spans. The spacing of

cross beams or diaphragms is generally kept not more than 1.5 times the spacing of the longitudinal girders. A

few more typical arrangements of beams and boxes below the slabs for RCC/PSC bridges are indicated.

4.5.2 TYPES OF PRESTRESSING AND ITS PROPER USE

Basically two types of prestressing i.e. pre tensioned and post tensioned are applied in bridge engineering.

Generally pretensioning is very rarely used in the state because of its limitations like proximity and availability of

plant, size of member, number of units etc.

Post tensioning system is mainly used in the state. Various systems of prestressing are (a) Freyssinet, (b) Magnel-

Blaton, (c) Gifford- Udall system


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Fig 4.5(a)-Freyssinet system

Fig 4.5(b) - Freyssinet system


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Fig 4.6-Gifford - Udall system

Many of the post tensioning devices are covered by patents. In case of Freyssinet system, cable with a fixed

number of wires e.g. 12-5f or 12-7f or 19-7f are used. The sheathing as specified in IRC: 18-2000 is generally

(CRCA) Mild Steel of bright metal finish or corrugated High Density Polyethylene (HDPE)


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5. DESIGN CONSIDERATIONS FOR BRIDGES

5.1 CONCRETE BRIDGES:

Reinforced concrete and pre stressed concrete have been found most suited for the construction of highway

bridges, the former for small and medium spans and the latter for long spans .Reinforced concrete has been used

on railways up to 10m span and pre-stressed concrete up to 24m in India but up to 35m in many countries.

Reinforced concrete even in form of open web type of girders is being tried in longer railway spans in Japan.

They have used continuous deck type spans up to 105m.There is however, reluctance on the part of Indian

railway engineers to adopt reinforced and pre-stressed concrete for longer spans on railways, due to the heavy

dead load to be dealt with the comparatively longer construction time and difficulty in maintaining adequate

quality control at the site of construction. They are also difficult to be replaced under traffic when the loading

conditions alter or major damages are caused due to derailments and the super-structure requires to be changed

The various codes referred to for design of the concrete bridges and bridges elements are:

1. IRS Code for concrete and pre stressed composite bridges on railways;

2. IRC 21-2000, standard specification and code of practice for road bridges, section 3, cement concrete (plain

and reinforced);

3. IS: 456-1964 Indian standard specification and code of practice for plain and reinforced concrete;

4. IS: 432-1966, Indian standard specification for mild and medium tensile bars and hard drawn wire for

concrete mix for cement;

5. IS: 1139-1959, Indian standard specification for hot rolled mild steel and medium tensile deformed bars for

concrete reinforcement;

6. IRC: 18-2000, design criteria for pre stressed concrete road bridges (post-tensioned);and

7. IS: 1786-1966, Indian standard specification for cold twisted steel bars for concrete reinforcement-tensile

steel deformed bars for concrete reinforcement.

5.2 DESIGN CRITERIA FOR RAILWAY BRIDGES

Ordinary Concrete with nominal mix by volume is used in bed blocks, column footing, foundation and mass

concrete works where the standard of specification and workmanship are likely to be lower. The maximum

permissible stresses in concrete for various mixes Controlled concrete is used in all girder parts, particularly in

super-structure slabs and girders, precast piles and for all prestressed concrete work.


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The minimum quantity of cement to be used for controlled concrete on the railways according to the IRS

concrete code is 325 kg/m3of concrete. IRC stipulates 360 kg/m

3for major bridges.

When the mix design and testing is done, the relationships used for arriving at various strengths are:

Fc=

28 days works test strength on cubes of size 150 mm in kg/cm2

fc= 28 days works test strength on cubes of size 150 mm in N/mm2

Cylinder strength=Cube strength*0.8

Works test strength:

Preliminary test strength=1.25 to 1.33

The various proportions for permissible stresses used are:

Direct compression = 0.26Fc or 0.26fc

Compression due to bending = 0.34FC or 0.34fc

Shear (as inclined tension) = 0.034FC or 0.034fc

Where shear reinforcement is used, four times the shear above is permissible.

Bond average for anchorage = 0.04Fc or 0.04fc

Bond-local = 1.75 times average, i.e., 0.07FC or 0.07fc

Bearing pressure on plain concrete-average on full area= 0.20Fc OR 0.20fc

Bearing pressure on plain concrete-average on an area less than one-third of full area=

0.30Fc or fc

Tensile stress in bending for plain concrete is same as permissible for shear stress

5.2.1. STEEL USED IN CONCRETE:


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The modulus of elasticity for steel to be used in prestressed concrete work is as follows.

Plain drawn wires 1.96105

N/mm2

(2106

kg/cm2)

Heat treated alloy bars 1.71105 N/mm2 (1.75106 kg/cm2)

Concrete 5630fc N/mm2

or

18000Fc kg/m2

Permissible stress in other steel bars used in all RCC and PSC works

In prestressed concrete, the concrete used should have fc not less than 41.1 N/mm2 for pretensioning and not less

than 34.3 N/mm2

for post-tensioning. The quantity of cement used for prestressed concrete should preferably be

equal to 530 kg/m3

the minimum being 380 kg/m3

for post-tensioning. The compaction and vibration should be

such that the density of the concrete is not less than 2400 kg/m3.

The ultimate strength of concrete at transfer should not be less than (2/3) Fc used for design.

Modular ratio is taken as 276/3 fc N/mm2(or 2812/3 FC kg/cm

2).


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Minimum cover and spacing for reinforcement:

The higher of the two alternatives mentioned will apply ( stands for diameter of bar).

Each end 25mm or 2

Longitudinal bars in column 38mm or

For columns of size 20 cm and 25mm or

UnderLongitudinal bars in beams 25mm or

Bars in slabs 13mm or

Any others 13mm or

Foundation footings 50mm

For structures submerged in water 75mm from surface or ends

Minimum distance between bars:

1. Horizontal if diameters are equal ora. of largest bars or

b. Nominal maximum size of aggregate +6 mm2. Vertical spacing between two horizontal layers < 13mm.3. Pitch of main slabs >300mm or > twice effective depth.4. Pitch of distribution bars in slabs > 600mm or > 4 times effective depth.

5.2.2 Web thickness:

Minimum diaphragm thickness should not be less than the web thickness of the girders connected the diaphragm

should be designed to resist 3% of the total compressive force carried by both the girders and provided both at

the bottom and top of the deformed bars with nominal reinforcement in the middle portion. In addition, the end

diaphragms in prestressed concrete girders should take the stress that may be induced due to different cracking

at the ends of the girders.

The Column reinforcement should not be less than 0.8 per cent of the cross- section. When lapping is required,

the maximum area should be restricted to 4 per cent of the area of cross- section. The minimum diameter of the

main reinforcement in the column will be 13mm. In addition, a minimum 0.3 per cent of the area should beprovided near the face which is subject to tension when the column is to be provided with tension reinforcement

also.

5.3 Design Criteria for road bridges:

IRC21-2000 applies to design of road bridges in concrete. Nominal mix concrete is not included for use inroad bridges. Material specifications and permissible stresses to be used for the concrete and steel generally

follow the provisions in relevant IS codes IS: 456, IS: 432, IS: 1139, IS: 1566, IS: 1786 subject to some minorchanges. Minimum cement content for major bridges is 360 kg/cum and maximum 540 kg/cum. It specifies

different minimum grades for culverts and major bridges. For bridges in severe exposure conditions one gradehigher concrete is to be used.


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For calculating stresses in section a modular ratio of 10 may be adopted. Permissible shear stress without

stirrups varies with the percentage of steel provided from 0.18Mpa to 2.5Mpa for M20 concrete and 0.15%

reinforcement for M40 and 3.0% for above grades of concrete.

Minimum cover to be provided for the reinforcement depends on the exposure conditions also. In moderate

conditions of exposure, minimum cover from any exposed surface shall be 40 mm and in conditions obsevere

exposure, it shall be 50 mm. In conditions of alternate wetting and drying the code requires provision of 75 mm

cover. Minimum size of bar to be used is 8 mm and in columns, minimum size of longitudinal bar is 12 mm.

The code also prohibits maximum diameter as 40 mm or a section of equivalent area, except in special

circumstances.

Fig 4.7-Cross section across diaphragm wallCross girders monolithic with the deck slab should be

provided at bearings and may be provided in intermediate locations according to design requirements. Minimum

thickness shall not be less than that of deck slab and it should extend at least three- fourths depth of main beams.

They are designed with reinforcement equal to approximately 0.50% of gross area at the bottom and 0.25% of

gross area of steel in top. Nominal two legged stirrups of 12mm diameter at 150mm centers are provided.


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5.3.1 DESIGN PROCEDURES FOR BRIDGE SUPERSTRUCTURE:

5.3.1.1 Introduction:

The design procedures for railway and road bridges primarily differ in consideration of loading. In general,

EUDL tables are available for design of not only main beams but also floor systems for railway bridges. On the

other hand, the highway bridge design takes into consideration individual disposition of the wheel loads of the

vehicles. The procedure is briefly dealt with individually for these two in this section.

5. 3.2 DESIGN PROCEDURE FOR RAILWAY BRIDGES:

5.3.2.1 Steel Girders

(a)Deck-type bridges: Generally, deck-type bridges are designed with two girders carrying a track. Some

principles of spacing of girders have already been indicated in subsection. The track is carried over the girders

generally using timber or steel sleepers which are connected to the top flanges of the girders by means of the

girders by means of hook bolts or other bolts. The sleepers can be designed to carry the loads coming through

the two rails as concentrated loads and for all standard spans up to 91.44 m and issued drawings to show general

arrangements as well as details of members and joints.

5.3.2.2 Concrete Girders

As mentioned earlier, so far as the design of long-span concrete girders for railway loading is concerned, thedetermination of the forces becomes simple since each track is carried by a pair of girders, spanned by the deck

slab as they are assumed to act only as stiffeners to the girders. End diaphragms are designed to take up

secondary forces that will be induced due to differential prestressing in girders.

The shorter due concrete spans are provided with one pair of girders per track or a number of T beam and slabs

placed side by side. In the matter case, the distribution of load between the girders is decided by using one of the

standard methods evolved and mentioned subsection.

5.3.3 ROAD BRIDGE DESIGN

5.3.3.1 Approach to design

Since concrete girders are mostly used for road bridges, only the design procedures for

concrete bridge are indicated here. Each component of the girder has to be designed separately by working out

the worst effect on the component by the most severe pattern of placement of vehicles adopted for the particular

class of loading. In general, as indicated, road bridges are designed for IRC class AA loading and also checked

for class A loading for the number of lanes can be occupied by class A load also.


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Once the worst loading moments and shear forces are determined for the severe conditions of loading on each

component, the design boils down to a problem of structural engineering. The section is designed by the trial

and error method starting with an assumed section and verifying if resultant stresses are within permissible

limits mentioned in respective IRC Codes for RCC and PSC and IS 456 as the case may be. For short spans up

to 6m, flat RCC slabs are adopted. Alternative arrangements of using precast PSC slabs are indicated

IRC has issued standard drawings for standard span slabs and beams. They contain full details and can be

adopted directly.

Normally, the minimum width for standard road bridge is far two lanes which, even without taking into

consideration the footpath, are 7.5 m. It is inconceivable to provide such a wide slab over two girders and where

the lanes are more, more number of girders are to be provided. With availability of computers for design,

stresses are computed using Finite Element Method.

5.4 DESIGN OF CONCRETE ROAD BRIDGES

(a)Design of Deck Slab: This first depends on the method of dispersion of wheel load and effective width of slab

to be considered for working out moments and shear. The methods used for this are based on Pigeauds methodor Westerguards method. Generally, Pigeauds method is used in India. It has three provisions:

1. Determination of effective width of slab for a single concentrated load over a slab simply supported attwo ends;

2. Determination of effective width of slab for a single concentrated load placed on a cantilever slab; and3. Determination of effective area over which the concentrated load is dispersed and coefficients to be

used for working out moments in either direction when slab is supported on four sides

For (1) effective width e is given by

Where l = effective span in case of simply supported slab and clear span in case of continuous slabs

x= distance of centre of gravity of load from the near support

W= width of concentration of load .i.e. Width of tyre or track at road surface in a direction perpendicular to

span, plus twice thickness of wearing coat.

k= a constant depending on l/l where l is the width of the slab and is tabulated in Annexure 14.5. For (2), i.e. in the case of the cantilever slab, the effective width e=1.2x+w. Knowing e and the load plusimpact, BM for unit width of slab can be calculated.

The dispersion of load on slab supported on all four sides will be shown

x=a in direction Lb in direction B

Knowing U and V , the coefficients m1and m2 are read from pairs of graphs provided by Pigeaud for values

corresponding to U/B and V/L.

M1= moment in short span = (m1+m2) P

M2= moment in long span =(m1+m2) P

= value of Pigeauds ratio, taken as 0.15 for RCC

This method has following limitations.

1. It applies to loads placed at centers. Since a number of loads will come on a panel and only one may beat centre, some approximations will have to be made while considering the effect of non-central loads.

2. Where V/L is small, the values of m1 and m2 tend to become less accurate.


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3. This method is most useful when k is more than0.55.The curves useful for design by this method are

available in many textbooks. The curves have been evolved for different values of K, i.e. the ratio of

the short span to the long span of the slab varying from 0.4 to 1.0. Readers more interested in the

method may refer to Victors Essentials of bridge Engineering where the full set of curves isreproduced. For precast slabs, the width of each slab is taken as the effective width.

Otherwise, the design of the slab is like any two way RCC slab reinforcement. The portion beyond the girder isdesigned as a cantilever for taking generally one track or line of wheels and or foot path loading plus parapet

loading.

5.4.1 DESIGN OF THE LONGITUDINAL GIRDERS

For the computation of the bending moments due to live load, the distribution of the live load between the

various longitudinal girders has to be first determined .When there are only two girders, the reactions can be

worked out assuming the deck slab as unyielding and by determining the worst placement. When three or more

girders are provided, the load distribution is estimated by using any one of the following three methods.

(a) Courbons method(b)

Henry-jaegar method

(c) Morice and Little version of Guyon and Massonnet methodThese three methods are briefly described below.

5.4.1.1 COURBONS METHOD: This is the simplest of the three methods in application. It requires no

reference to any tables or charts and also is applicable to majority of modern T-beam bridges. This method,

however, has certain limitations, as it is applicable only to cases where:

(1) The ratio of span to width of deck is more than 2 but less than 4;(2) The longitudinal beams are interconnected by at least five cross beams/diaphragms, symmetrically

spaced; and

(3) The depth of cross girders/diaphragms is not less than 0.75 of the depth of main girders. If theseconditions are satisfied, for a system of wheel (live) loads across a cross section under the loads,

the proportion of the load carried by a girder is given by

Ri= PIi/Ii(1+ Ii/Ii di^2*edi)

Where P= sum of loads at the section

Ii= moment of inertia of the girder

e=eccentricity of the loads with respect to axis of the bridge

di= distance of the girders under consideration from axis of the bridge

5.4.1.2 Henry-Jaegar method: This method assumes that all the cross beams can be replaced by a uniform,

continuous, transverse medium of equivalent stiffness. In the absence of cross beams, it takes into account the

stiffness of the slab over its entire length.

The distribution of the loads between the girders is based on three dimensional parameters as given below:

A=12/^4*(L/h)^3*nEIT/EIE=^2/2n(h/L)CJ/EIT where cross beams existAnd

F= LEIT When there is no cross beam

c=EI1/EI2

Where L= Span length of bridge

h= spacing of longitudinal beams

n= number of cross beams

EI= Flexural rigidity of one longitudinal girder

CJ= Torsional rigidity of one longitudinal girder


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EIT= Flexural rigidity of one cross beam

EI1 and EI2 are flexural rigidities of outer and inner longitudinal beams if they are different.

However, normally these will be equal particularly in RCC T- beam bridges. In a bridge with three or four

longitudinal with a number of cross beams F is taken as .

The distribution coefficients are given in a graphical form with parameter A as abscissa and moment coefficientm as ordinate. Different sets of graphs exits for F=0 and F= and for different number of girders in the system.For intermediate values of F the coefficient is interpolated using the formula

mF= m0+(m-m0)FA/3+FA

Graphs are available for system of 3 or more girders. Further information and graphs can be had from The

Analysis of Grid Frameworks and related structures.

5.4.1.3 Marcie Little Method: The method also calls for the use of standard graphs evolved for moment of

coefficient. It applies the orthotropic plate theory to concrete bridge systems, based on the approach first

suggested by Guyon neglecting torsion and later extended by Massonnet including torsion. Complete details of

the method are described along with graphs in the Concrete Bridge Design by R E Rowe. Some of the graphs

are reproduced also by Victor. Only the basic principle is given below for an appreciation of the method.

The distribution of loads between longitudinal girders is correlated to the differential deflection between the

longitudinal girders at a section where load are applied which can be as indicated.

For arriving at various factors, the girders and position of loads are divided

If the longitudinal girders are spaced at p, the effective width of deck is mp which is equated to 2b and b is

divided into four equal parts of considering reference stations for the coefficients and assumed load position.

The span L is equated to 2a.

The distribution coefficient is given by k0=k0 + (k1-k0) where is torsional rigidity parameter of the bridgedeck. Values of k1 and k0 are given in separate sets of graphs for each reference station 0, b/4, -b/4, etc., the

abscissa representing and ordinate giving the k0 or k1 value.k0=value is for =0

k1= value is for =1, and

is parameter giving flexural properties of the bridge deck as a whole.

Values of and are arrived at a follows.

=b/2a(i/j)^0.25

= a(i0+j0)/2Ej

and

i= I/p i.e. longitudinal moment of inertia (MI) of equivalent deck per unit width, I being MI of each girder and pbeing their transverse spacing.

J= transverse MI of equivalent deck/unit length

= J/q, J being MI of each transverse diaphragm or cross girder and q being their spacing

E= Youngs modulus of material of deck

G= modulus of rigidity of material of deck

I0= I0/P, i.e. longitudinal torsional stiffness per unit length

J0=J0/q , i.e. transverse torsional stiffness per unit length


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I0 and J0 are torsional stiffness factors of each longitudinal beam and cross beam/diaphragm respectively. The

graphs are available in the reference quoted above and have been reproduced by Victor also. In all the above

methods, the distribution factor for each beam and proportion of load is worked out for each beam. The

proportion of loads is worked out for the worst transverse position of each set of axles first. For applying the

morice little method a system of tabulation is required to arrive at the worst effect. Then, the worst position of

load system longitudinally for producing maximum BM and shear over the length is determined and then themaximum BM and load in intermediate beams and end beams are calculated.

5.5 DESIGN FEATURES OF THE PIER:

PIERS:

Piers are of:

Solid piers Single column piers Cellular piers Trestle piers Hammer head piersSolid and cellular piers for river bridges should be provided with semi-circular cutwaters to facilitate

streamlined flow and to reduce scour. Solid piers can be of mass concrete or of masonry for heights up to about

6m and spans up to about 20m. It is permissible to use stone masonry for the exposed portions and to fill the

interior with lean concrete. The stone layers should be properly bonded with the interior with bond stones.

Single column piers are increasingly used in urban elevated highway applications, and also for river

crossings with a skew alignment. In an urban setting, single column piers provide an open and free-flowing

perception to the motorists using the road below. Such piers when used for a skew bridge across a river results

in least obstruction to passage of flood below the bridge.

Cellular, trestle, hammer head and single column types use reinforced concrete and suitable for heights

above 6m and span over 20m. The cellular type permits saving in the quantity of concrete, but usually requires

difficult shuttering and additional labour in placing the reinforcements. The thickness of the walls should not be

less than 300mm. The lateral reinforcement of walls should not be less than 300mm. the lateral reinforcement of

walls should be 0.3% of the sectional area of the wall of the pier, and the quantity should be distributed as 60%

on the outer face and 40% on the inner face.

The trestle type consists of columns with a bent cap at the top. For all trestles, as in flyovers and the

elevated roads, connecting diaphragms between the columns may also be provided. The hammer head type

provides slender sub-structure and is normally suitable for the elevated roadways. When used for a river bridge

eg. Jawahar setu across Sone River at Dehri, this design leads to minimum restriction of the waterway. The

construction procedure should be arranged such that the construction joints are minimized. Simple geometry of

the pier leads to reduce construction costs.

The top width of the pier depends on the size of the bearing plates on which the super structure rests. It

is usually kept at a minimum of 600mm more than the out-to-out dimension of the bearing plates, measured

along the longitudinal axis of super structure. The length of the pier at the top should not be less than 1.2m in

excess of the out-to-out dimension of the bearing plates measured perpendicular to the axis of the super

structure. The bearing plates are so dimensioned that the bearing stress due to dead and live loads does not

exceed 4.2 MPa.

Other innovative designs for piers to suit urban site requirements includes H-shaped.


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Piers flaring at the top provide wider base at the top pier for stability of the deck and limited use of

space at the base of the pier at the ground level. The bottom width is pier usually larger than the top width so as

to restrict the net stresses within the permissible values. It is normally sufficient to provide a batter of 1 in 25on

all sides for the portion of the pier. In the case of river bridges, the portion pier located between wind andwater, that is the portion of the masonry surface which lies between the extreme high and extreme low water, is

particularly vulnerable to deterioration and hence needs special attention.

Reinforced concrete framed types of piers have been used in recent years. The main advantage in their

use is due to reduced effective span lengths for girders on either side of the center line of the pier leading to

economic in the cost of super structure. Reinforced concrete framed piers of V shaped supporting a shortlength of reinforced concrete decking have been used successfully in conjunction with suspended spans of pre-

stressed concrete for bridges in hilly areas.

The top width of pier depends on the size of the bearing plates on-which the super structure rests. It is

usually kept at a minimum 600mm more than more than the out-to-out dimension of the bearing plates,

measured along the longitudinal axis of super structure. The length of the pier at the top should not be less than

1.2m in excess of the out-to-out dimension of the bearing plates measured perpendicular to the axis of the super

structure.

The bottom width is pier usually larger than the top width so as to restrict the net stresses within the

permissible values. It is normally sufficient to provide a batter of 1 in 25 on all sides for the portion of the pier

between the bottom of the bed block and the top of the well.

The loads to be considered in the design of pier are

I. Dead load of super structure and the pier itself.II. Live loads of traffic passing over the bridge. The effect of eccentric loading due to the live load

occurring on one span only should be considered.

III. Impact effect of live load.IV. Effect of wind on moving loads and on the superstructure.V. Force due to wave action, if applicable.

VI. Longitudinal force due to the tractive effort of vehicles.VII. Longitudinal force due to breaking of vehicles.

VIII. Longitudinal force due to resistance in bearings.5.6 DESIGN FEATURES OF THE ABUTMENT

An abutment is the substructure which supports one terminals of the superstructure of a bridge and laterally

supports the embankment which serves as an approach to die bridge. It consists of generally three structural

elements.

c) The Brest wall, which directly supports the dead and live loads of the superstructure, and retains thefilling of the embankment in its rear.

d) The wing wall. Which act as extensions of the breast wall in retaining the fill, not taking loads from thesuperstructure

e) The back wall, which is small retaining wall just behind the bridge seat.In abutment design, the forces considered are:

i) Dead load due to superstructure.ii) Live load on the superstructure.iii) Self-weight of the abutment.iv) Longitudinal forces due to tractive effort and braking effort and due to temperature

variation.


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v) Thrust on the abutment due to retained earth and effect of live loads on the fill at the

rear end of the abutment.

It is important on abutment construction to replace the fill material carefully and to arrange for its proper

drainage. A good drainage system is secured by placing rock fill immediately behind the abutment and proper

drain pipes at the bottom.

6. WELL FOUNDATIONS

6.1 Introduction:

Well foundations had their origin in India and have been used for hundreds of years for providing deep

foundations below the spring water level for important buildings and structures. The technique of sinking

masonry wells for drinking water is very ancient and even today small drinking water wells are constructed all

over the country using the same methods as were prevalent centuries ago. Well foundations were used for

the first time for important irrigation structures on the Ganga canal including solani aqueduct at Roorkee (India),

which were constructed in the middle of the nineteenth century. With the advent of Railways in

India,construction of a large number of bridges across major rivers became necessary and it was

recognized very soon that much bigger and deeper well foundations were required for their piers and abutments.

Fig 6.1-well foundations

6.2 Comparison with Pile Foundation

i) Well foundations provide a solid and massive foundation for heavy loads as against a cluster of piles which

are slender and weak individually and are liable to get damaged when hit by floating trees or boulders rolling

on the river bed in case of bridge piers.

ii) Wells have a large cross sectional area and the bearing capacity of soil for this area is much greater than

that of the same soil at the same depth forbearing piles of small cross-section.


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iii) Well foundations can be provided up to any depth if only open sinking is involved and upto a depth of

33.5m if pneumatic sinking is required to be done. Pile foundations are generally economical up to a depth of

18m and in some cases for depths up to 27m.

iv) Piles cannot be driven through soil having boulders. Logs of wood which are very often found buried even

at great depths also obstruct a pile. It is possible to sink a well after overcoming these obstructions.

v) The size of well foundations cannot be reduced indefinitely as the dredge hole must be

enough to enable a grab to work and the steining must have the thickness necessary to

provide the required sinking effort. It is, therefore, not economical to use well foundations for very

small loads and pile foundations are more suitable for them.

vi) Wells are hollow at the center and most of the material is at the periphery. This Provides a large section

modulus with the minimum cross-sectional area. They can resist large horizontal forces and can also take

vertical loads even when the unsupported length is large. The section modulus ofindividual piles in a

cluster is small and cannot carry large horizontal force or vertical loads when the unsupported length is

considerable as in case of bridge piers and abutments in scourable riverbeds.

vii) The bearing capacity of a pile is generally uncertain. In most cases, it is not possible to determine the exact

strata through which each individual pile has passed. It cannot be said with confidence in the case of bearing

piles if they have gone and rested on the strata taken into account while designing them or if they are resting

only on an isolated boulder.

In case of wells sunk by dewatering or pneumatic sinking, it is possible to visually examine the strata

through which sinking is done in its natural state and the material on which they are finally founded. Even

when sinking is done by dredging, the dredged material gives a fairly good idea of the strata through which the

well is sunk. Drilled piles and caisson piles also have this advantage over the driven piles.

viii) Masonry in the steining wells is done under dry conditions and the quality ofmasonry

Or concrete is much better than in case of cast in situ piles for which concreting is done below theground level and in many cases below the water level, where it cannot be inspected. Even in case of precast

piles, the concrete is subjected to a lot of hammering and damage to it cannot be ruled out.

ix) In case of wells rising of the well steining and sinking are done in stages and a decision about the

foundation level can be taken as the workprogresses piles and the strata conditions become known. In

case of precast piles, a decision about the depth has to be taken in advance. If the bearing capacity of the

piles at the design depth is found to be less than the calculated value after testing, it may become

necessary to redesign the foundation and the piles of short length already cast may have to be rejected or

additional number of piles may have to be provided in each cluster. On the other hand if the stratum is too

hard, it may not be possible to sink them to the design depth and the piles may have to be cut which is

costly and wasteful. This does not apply to cast in situ piles.

6.3 Well Types and Their Suitability:

The followings are the different types of well in common use in Indian Railways as well as roadways.

The advantages and disadvantages of each type have also been discussed as below:

6.3.1Circular well

This type of well is used most commonly and the main points in its favour are its strength. Simplicity in

construction and ease in sinking. It requires only one dredger for sinking and its weight per sq. metre of

surface is the highest due to which the sinking effort for this well is also high. The distance of the

cutting edge from the dredge hole is uniform all over and the chances of tilting are the minimum for this

type of well. The well is generally adopted for piers of single track railway bridges and those of


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bridges on narrow roads. When the piers are very long the size of circular wells becomes unduly large, which

makes them costly and disadvantageous hydraulically also as they cause excessive obstruction to the flow of

water.

Nine metres is generally considered as the maximum diameter of circular wells. Allowing cantilever of one

metre on either side the maximum length of the pier resting on this type of well is about 11 metres.

6.3.2Double D well

This type of well is most common for the piers and abutments of bridges which are too long to be

accommodated on circular well. The shape is simple and it is easy to sink this type of well also. The

dimensions of the well are so determined that the length and the width of the dredge holes are

almost equal. It is also recommended by some engineers that the overall length of the well should not

be more than double the width. The disadvantage of this type of well is that considerable bending

moments are caused in the steining due to the difference in the earth pressure from outside

and water pressure from inside which result in vertical cracks in the steining

particularly in the straight portions where join the partition wall.

6.3.3Double Octagonal Well

These types of wells are free from the shortcoming of double D-well. Blind corners are eliminated and

bending stresses in the steining are also reduced considerably. They, however, offer greater resistance

against sinking on account of the increased surface area. Masonry in steining is also more difficult

than in case of double D wells.

6.3.4 Rectangular Well

These types of foundations are generally adopted for bridge foundations having shallow depths. They

can be adopted very conveniently where the bridge is designed for open foundations and a change of well

foundations becomes necessary during the course of construction on account of adverse conditions such as

excessive in flow of water and silt into the excavation.

6.3.5 Twin circular well

This type of foundation consists of two independent circular wells placed very close to each other with a

common well cap. It is necessary to sink these wells simultaneously to ensure that the cutting edges are

almost at the same level all the time. The wells have a tendency to tilt towards each other during the course

of sinking on account of the fact that the sand between them becomes loose and does not offer as much

resistance against sinking as on the other sides. If the depth of sinking is small say up to 6 or 7 metres, the

clear space between the two wells may be kept 0.6 to 1 m to avoid tilting. For greater depth of sinkingspacing of 2 to 3 meters may be necessary. Since it is necessary to sink these wells simultaneously it is

obligatory to have two sets of equipment for well sinking and in this respect they do not

offer any advantage over double D or double octagonal wells. They are, however, advantageous where the

length of the pier is considerable and the sizes of the double D or octagonal wells become unduly

large to accommodate the pier. If, however, the soil is weak, the larger size of double D or double

octagonal wells may be required to keep the bearingpressure on the soil within limits.

Twin circular wells are advantageous only when the depths of sinking is small and the foundation material

is soft rock or kankar or some other soil capable of taking fairly high loads. Design of well caps for the

twin circular wells also requires special care. Allowance is made for relative settlement of the two wells

and this adds to its cost. The possibility of development of cracks in the pier due to relative settlement


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cannot be ruled out inspite of the heavy design of the cap except where the wells are founded on rock or

other incompressible soils.

6.3.6 Wells with Multiple Dredge Holes

For piers and abutments of very large sizes, wells with multiple dredge holes are used. Wells of this type are

not common in India. Wells of this type were, however, used for the towers of Howrah Bridge. The size

of these wells is 24.8m x 55m and there are 21 dredge holes in each of them, In the United States wells of

this type are more common. The overall dimension of the largest well are 60.5m x 29.6m and they

support the piers of San Francisco Okland Bridge. Each well has 55 square dredge holes of 5.2m x 5.2m

size.


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Fig 6.2-shapes of well foundations


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7. PRESTRESSED CONCRETE

7.1 DEFINITION:

Prestressed concrete is a method for overcoming concrete's natural weakness in tension. It can be used to produce

beams, floors orbridges with a longer span than is practical with ordinary reinforced concrete. Prestressing

tendons (generally of high tensile steel cable or rods) are used to provide a clamping load which produces a

compressive stress that balances the tensile stress that the concrete compression member would otherwise

experience due to a bending load.

7.2 PRETENSIONED CONCRETE:

Pre-tensioned concrete is cast around already tensioned tendons. This method produces a good bond between thetendon and the concrete, which both protects the tendon from corrosion and allows for direct transfer of tension.

The cured concrete adheres and bonds to the bars and when the tension is released it is transferred to the concrete

as compression by static friction .However, it requires stout anchoring points between which the tendon is to be

stretched and the tendons are usually in a straight line. Thus, most pre-tensioned concrete elements are

prefabricated in a factory and must be transported to the construction site, which limits their size. Pre-tensioned

elements may be balcony elements, or a method of lintels, floor slab, beams or foundation piles. An innovative

bridge construction method using pre-stressing is described in Stressed Ribbon Bridge.

7.3 POST TENSIONED CONCRETE:

7.3.1 BONDED POST TENSIONED CONCRETE:

Fig 7.1-pre stressing cables along with reinforcement for Igirder
http://en.wikipedia.org/wiki/Concretehttp://en.wikipedia.org/wiki/Tension_%28physics%29http://en.wikipedia.org/wiki/Beam_%28structure%29http://en.wikipedia.org/wiki/Floorhttp://en.wikipedia.org/wiki/Bridgehttp://en.wikipedia.org/wiki/Span_%28architecture%29http://en.wikipedia.org/wiki/Reinforced_concretehttp://en.wikipedia.org/wiki/Tension_%28mechanics%29http://en.wikipedia.org/wiki/Steelhttp://en.wikipedia.org/wiki/Cablehttp://en.wikipedia.org/wiki/Rodshttp://en.wikipedia.org/wiki/Compressive_stresshttp://en.wikipedia.org/wiki/Tensile_stresshttp://en.wikipedia.org/wiki/Compression_memberhttp://en.wikipedia.org/wiki/File:Prestressed_concrete_en.svghttp://en.wikipedia.org/wiki/File:Prestressed_concrete_en.svghttp://en.wikipedia.org/wiki/Compression_memberhttp://en.wikipedia.org/wiki/Tensile_stresshttp://en.wikipedia.org/wiki/Compressive_stresshttp://en.wikipedia.org/wiki/Rodshttp://en.wikipedia.org/wiki/Cablehttp://en.wikipedia.org/wiki/Steelhttp://en.wikipedia.org/wiki/Tension_%28mechanics%29http://en.wikipedia.org/wiki/Reinforced_concretehttp://en.wikipedia.org/wiki/Span_%28architecture%29http://en.wikipedia.org/wiki/Bridgehttp://en.wikipedia.org/wiki/Floorhttp://en.wikipedia.org/wiki/Beam_%28structure%29http://en.wikipedia.org/wiki/Tension_%28physics%29http://en.wikipedia.org/wiki/Concrete


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Bonded post tensioned concrete is descriptive term for a method of applying compression after pouring concrete

and the curing process. The concrete is cast around plastic, steel or aluminum curved duct, to follow the area

where otherwise tension wou

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