Student’s Declaration

We hereby declare that the work being presented in this report entitled

RETRACTABLE BRIDGE is an authentic record of our own work

carried out under the supervision of Dr. K.V. OJHA

The matter embodied in this report has not been submitted by us for

the award of any other degree.

Dated:……………..

Manish Uttam (0703240030)………………..

Mayur Goyal (0703240031)…………………..

Rajat Singhal (0703240042)…………………..

Prashant Khokhar(0703240039)……………

Shashank Gupta (0703240049)……………

Deepak Kumar (2703240001)………………..

This is to certify that the above statement made by the candidates is

correct to best of my knowledge.

……………………. ……………………..

Prof. S.S.S. Govil Dr. K.V.OJHA

HOD Associate Professor

Mechanical Engineering Mechanical Engineering

Acknowledgement

We take this project report as an opportunity to thank all those people

who contributed in this project towards its fulfillment.

We wish to express our sincere gratitude to Dr.K.V.Ojha under whose

guidance and cooperation we carried our project work.

Whole hearted thanks to Prof. S.S.S. Govil, HOD - Mechanical

Engineering for providing us an opportunity to work on this title and

guiding us towards the right path for its success.

Lastly we thankful to all the faculty members of Mechanical

Engineering Department for providing their precious time in answering

our queries and giving us valuable information on the title.

Manish Uttam(0703240030)………………………

Mayur Kumar Goyal(0703240031)………………

Prashant Khokhar(0703240039) ………………

Rajat Kumar Singhal (0703240042)……………...

Shashank Gupta(0703240049)………………….

Deepak Kumar(2703240001)…………………….

2

Table Of Contents

Page no.

Title page

Declaration i

Acknowledgement ii

List of figures iv

Abstract v

1. Chapter-1 6-17

1.1 Introduction 6

1.2 Objective 12

1.3 Methodology 17

2. Chapter-2 18-40

2.1 Mechanism used 18

2.2 Fabrication 20

2.3 Retractable bridge component 21

2.4 Working 22

2.5 Material 30

2.6 Project Gantt chart 40

Chapter-3 41-42

3.1 Result 41

3.2 Conclusion and Recommendation 42

Chapter-4

4.1 References 433

List Of Figures

Figure no. Description Page no.

1.1 The forth rail bridge 6

1.2 Double decked bridge 10

1.3 Bridge’s part 11

1.4 Lifting bridge 13

1.5 Retractable bridge 14

1.6 Thrust bridge 16

1.7 Methodology 17

2.1 Rack and pinion mechanism 18

2.2 Moving centre tube 19

2.3 Fabrication flow chart 20

2.4 Motor and gear assembly 22

2.5 Channel system 23

2.6 Tube casing 24

2.7 Step down transformer 25

2.8 Sliding switch 26

2.9 DC motor 27

2.10 Motor assembly 27

2.11 PCB circuit 29

2.12 Prestressed concrete 34

2.13 Gantt chart 40

3.1 Final model 41

4

Abstract

Retractable bridge are required for proper movement of ships and known as

THRUST BRIDGE. These bridges can move in perpendicular and may also in

parallel direction of their axis. A moveable bridge is a bridge that moves to allow

passage for (usually) boats or barges. The principal disadvantage is that the traffic

on the bridge must be halted when it is opened for passages. For seldom used

railroad bridges over busy channels the bridge may be left open and then closed

for train passages.

The four main factors are used in describing a bridge. By combining these terms

one may give a general description of most bridge types.

span (simple, continuous, cantilever),

Material (stone, concrete, metal, etc.),

placement of the travel surface in relation to the structure (deck, pony,

through),

Form (beam, arch, truss etc.)

Bridge will be divided in some parts. Either one or two part will perform motion.

Moveable parts, first they will slightly lift up from their original position and then

sliding will takes place. Thus this mechanism will provide a gap for proper

movement of ship.

The model of retractable bridge was fabricated with the use of IC’s, motor, gear

and sensors. The model is also tested by moving some object in front of

retractable bridge and found working satisfactorily.

5

Chapter-1

Introduction

“A bridge is a structure built to span physical obstacles such as a body of water,

valley, or road, for the purpose of providing passage over the obstacle”. Designs

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

where the bridge is constructed, the material used to make it and the funds

available to build it. The first bridges were made by nature itself — as simple as a

log fallen across a stream or stones in the river. The first bridges made by

humans were probably spans of cut wooden logs or planks and eventually stones,

using a simple support and crossbeam arrangement. Some early Americans used

trees or bamboo poles to cross small caverns or wells to get from one place to

another.

.

Figure 1.1:The Forth Rail Bridge

6

Types Of Bridges

There are six main types of bridges:

Beam bridges

Cantilever bridges

Arch bridges

Suspension bridges

Cable-stayed bridges

Truss bridges.

Beam bridge

Beam bridges are 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. The earliest beam bridges were simple

logs that sat across streams and similar simple structures. In modern times, beam

bridges are large box steel girder bridges. Weight on top of the beam pushes

straight down on the abutments at either end of the bridge. They are made up

mostly of wood or metal. Beam bridges typically do not exceed 250 feet (76 m)

long. The longer the bridge, the weaker. The world's longest beam bridge is Lake

Pontchartrain Causeway in southern Louisiana in the United States, at 23.83 miles

(38.35 km), with individual spans of 56 feet (17 m).

Cantilever bridge

Cantilever bridges are built using cantilevers—horizontal beams that are

supported on only one end. Most cantilever bridges use a pair of continuous spans

extending from opposite sides of the supporting piers, meeting at the center of the

obstacle to be crossed. Cantilever bridges are constructed using much the same

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

7

forces through the bridge. The largest cantilever bridge is the 549-metre (1,801 ft)

Quebec Bridge in Quebec, Canada.

Arch bridge

Arch bridges have 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. Dubai in the United Arab Emirates is

currently building the Sheikh Rashid bin Saeed Crossing which is scheduled for

completion in 2012. When completed, it will be the largest arch bridge in the world.

Suspension bridge

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. The

longest suspension bridge in the world is the 12,826 feet (3,909 m) Akashi Kaikyo

Bridge in Japan.[14] See simple suspension bridge, stressed ribbon bridge,

underspanned suspension bridge, suspended-deck suspension bridge, and self-

anchored suspension bridge.

Cable-stayed bridge

Cable-stayed bridges, like 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.[15] The first known cable-stayed bridge was designed in

1784 by C.T. Loescher.[16] The longest cable-stayed bridge is the Sutong Bridge

over the Yangtze River in China.

8

Movable bridge

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.

Double-decked bridge

Double-decked or double-decker bridges have two levels, such as the San

Francisco – Oakland Bay Bridge, with two road levels. Tsing Ma Bridge and Kap

Shui Mun Bridge in Hong Kong have six lanes on their upper decks, and on their

lower decks there are two lanes and a pair of tracks for MTR metro trains.

Likewise, in Toronto, the Prince Edward Viaduct has four lanes of motor traffic on

its upper deck and a pair of tracks for the Bloor–Danforth subway line. Some

double-decker bridges only use one level for street traffic; the Washington Avenue

Bridge in Minneapolis reserves its lower level for automobile traffic and its upper

level for pedestrian and bicycle traffic (predominantly students at the University of

Minnesota).

Robert Stephenson's High Level Bridge across the River Tyne in Newcastle upon

Tyne, completed in 1849, is an early example of a double-deck bridge. The upper

level carries a railway, and the lower level is used for road traffic. Another example

is Craigavon Bridge in Derry, Northern Ireland. The Oresund Bridge between

Copenhagen and Malmö consists of a four-lane highway on the upper level and a

pair of railway tracks at the lower level.

The George Washington Bridge between New Jersey and New York has two

roadway levels. It was built with only the upper roadway as traffic demands did not

require more capacity. A truss work between the roadway levels provides stiffness

to the roadways and reduced movement of the upper level when installed. Tower

Bridge is different example of a double-decker bridge, with the central section

consisting of a low level bascule span and a high level footbridge.

9

Old Yamuna Bridge(Delhi) or Bridge No. 249 in technical railway parlance, was

constructed in 1866 by the East India Railway at a cost of £16,16,335. It was built

with a total length of 2,640 feet and consisted of 12 spans of 202.5 feet each. With

the completion of this bridge, two principal cities of North India, Kolkata and Delhi,

were connected by the Railways; this being the last link of the trunk line on this

route. In 1913, this was converted into a double line by adding down line girders of

12 spans of 202 feet each and 2 end spans of 42 feet to the bridge. For the

movement of road traffic, two road bridges were provided below the lines. The

entry of trains into Delhi Junction Railway Station, in such close proximity to the

Red Fort, never ceases to impress the rail traveller, reminding all that after the

Uprising of 1857, Delhi was a fortified city. The old Yamuna Bridge has an

identical twin, a bridge further downstream at Naini on the Allahabad —

Mughalsarai section of the now North Central Railways.

Figure1.2: Double Decked Bridge

10

Terms used in Bridge:

1.Abutment

2.Pier

3.Overpass:

4.Underpass

5. Deck

Figure1.3: Bridge’s parts

11

Objective

The main objective is:

To prepare a model of Retractable Bridge by rack and pinion mechanism.

To check its feasibility.

Study the advantages of Retractable bridge over other moveable bridge.

12

Moveable bridge

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.

Figure1.4: Lifting Bridge

13

Types of moveable bridge:

• Draw bridge

• Folding bridge

• Curling bridge

• Lift bridge

• Retractable bridge

• Swing bridge

• Submersible bridge

Figure1.5: Retractable Bridge

14

Brief Description Of Movable Bridge

Draw bridge: A drawbridge is a type of movable bridge typically associated with

the entrance of a castle surrounded by a moat. The term is often used to describe

all different types of movable bridges, like bascule bridges and lift bridges

Folding bridge: A folding bridge is a type of movable bridge.An example of a

folding bridge is the Hörnbrücke (Hoernbridge) in the city of Kiel in the German

state of Schleswig-Holstein. It is a three-segment bascule bridge that folds in the

shape of the letter N.

Lift bridge: A vertical-lift bridge or lift bridge is a type of movable bridge in which

a span rises vertically while remaining parallel with the deck. The vertical lift offers

several benefits over other movable bridges such as the bascule and swing-span

bridge. Generally speaking they cost less to build for longer moveable spans.[1]

The counterweights in a vertical lift are only required to be equal to the weight of

the deck, whereas bascule bridge counterweights must weigh several times as

much as the span being lifted. As a result, heavier materials can be used in the

deck, and so this type of bridge is especially suited for heavy railroad use.

Swing bridge: A swing bridge is a movable bridge that has as its primary

structural support a vertical locating pin and support ring, usually at or near to its

center of gravity, about which the turning span can then pivot horizontally as

shown in the animated illustration to the right. Small swing bridges as found over

canals may be pivoted only at one end, opening as would a gate, but require

substantial underground structure to support the pivot.

Submersible bridge: A submersible bridge is a type of movable bridge that

lowers the bridge deck below the water level to permit waterborne traffic to use the

waterway. This differs from a lift bridge or table bridge, which operate by raising

the roadway. Two submersible bridges exist across the Corinth Canal, one at each

end, in Isthmia and Corinth.

15

Retractable bridge:

A retractable bridge is a type of movable bridge in which the deck can be slide

backwards to open a gap for crossing traffic, usually a ship on a waterway.This

type is sometimes referred to as a ”Thrust Bridge”. Retractable bridges date back

to medieval times. Due to the large dedicated area required for this type of bridge,

this design is not common. A retractable design may be considered when the

maximum horizontal clearance is required (for example over a canal).

Several examples exist in New York City, (e.g., Carroll Street Bridge (built 1889) in

Brooklyn, Borden Avenue Bridge in Queens). A recent example can be found at

Queen Alexandra Dock in Cardiff, Wales, where the bridge is jacked upwards

before being rolled on wheels. Helix Bridge at Paddington Basin, London is a more

unusual example of the type, consisting of a glass shell supported in a helical steel

frame, which rotates as it retracts. The Summer Street Bridge over Fort Point

Channel in Boston is another variant type. This bridge is oriented northwest-

southeast, with the NW-bound lanes of traffic retracting diagonally to the north,

and the SE-bound lanes retracting diagonally to the west.

Figure1.6: Thrust Bridge

16

Methodology

Figure1.7: Methodology

17

1

2

3

4

5

Litreture survey

Concept generation

Selection of appropriate mechanism

Fabrication

Future plans

Chapter-2

Details of Project Report Work

Mechanism used (RACK AND PINION)

A rack and pinion is a type of linear actuator that comprises a pair of gears which

convert rotational motion into linear motion. The circular pinion engages teeth on a

linear "gear" bar–the rack. Rotational motion applied to the pinion will cause the

rack to move to the side, up to the limit of its travel. For example, in a rack railway,

the rotation of a pinion mounted on a locomotive or a railcar engages a rack

between the rails and pulls a train along a steep slope.The rack and pinion

arrangement is commonly found in the steering mechanism of cars or other

wheeled, steered vehicles. This arrangement provides a lesser mechanical

advantage than other mechanisms such as recirculating ball, but much less

backlash and greater feedback, or steering "feel".

Figure2.1: Rack and Pinion feeding mechanism

18

• The Rack and Pinion mechanism converts rotational motion into linear

motion and vice-versa.

• Less effort required.

• Much lesser Backlesh.

Specification of Rack Pinion assembly used

Length of rack assembly 140MM

Length of rack used 95MM

No of teeth 68

Pinion diameter 8MM

Length of pinion with rod 65MM

Without rod 50MM

Length cover assembly 90MM

Figure2.2: Moving centre tube

19

Fabrication : Fabrication was started with formulation of a flow chart depicting

all the steps to be taken to manufacture to complete mechanism. Flow chart is

shown below:

Figure2.3: Flow chart

20

Fabrication of rack and pinion

Fabrication of housing

Installation of pinions

Installation of assembly components

Complete assembly

Retractable bridge component:

• Gear arrangement

• Channel system

• Moving Centre tube

• Span

• Support

• Transformer

• Sliding Switch

• DC Gear Motor

• PCB board

21

Working of each component

Gear arrangement: We used rack and pinion gear system under the channel to

move centre shaft to back and forth. Here this rack and pinion converting rotary

motion of shaft of gear motor into linear motion of centre shaft. This gear

arrangement is situated under the channel and beside the center shaft.

Figure2.4: Motor and Gear assembly

Channel system: There two channel system is used. These channels provide

the support to move the span(retractable span) over it. Our span do sliding motion

over this channel with the help of bearings. Length of one channel, we used ,is 7

inch. These channel are situated over the support.

22

Figure 2.5. Channel system

Moving centre tube: This is main and center part of our bridge,it provides both

support and motion to retracted span. Basically it is a micriscope’s tube which we

used in our project to provide the linear motion to span. This tube is length of

130mm and of 18mm diameter.

23

Figure2.6:Tube casing

Span: Our total bridge length is 24 inch, in which we total gap is open of 4inch

and remaining we have as fixed span length of 10 inch each. Cardboard is used

to make span of both type,fixed and moving.

Support: Support is also made of cardboard. And height of bridge is decided by

these support. Generally bridge’s span is at the height of one third of total height of

support. This is very necessary to keep this fact in mind for factor of safety point of

view that span should be at one third of total height of support othervice bridge will

not be able to bear the vibration and many natural activities. Our bridge support’s

height is about 10 inch.

Transformer: A transformer is a device that transfers electrical energy from one

circuit to another through inductively coupled conductors—the transformer's coils.

A varying current in the first or primary winding creates a varying magnetic flux in

the transformer's core and thus a varying magnetic field through the secondary

24

winding. This varying magnetic field induces a varying electromotive force (EMF)

or "voltage" in the secondary winding. This effect is called mutual induction.

Figure 2.7: Step down Transformer

If a load is connected to the secondary, an electric current will flow in the

secondary winding and electrical energy will be transferred from the primary circuit

through the transformer to the load. In an ideal transformer, the induced voltage in

the secondary winding (Vs) is in proportion to the primary voltage (Vp), and is given

by the ratio of the number of turns in the secondary (Ns) to the number of turns in

the primary (Np) as follows:

By appropriate selection of the ratio of turns, a transformer thus allows an

alternating current (AC) voltage to be "stepped up" by making Ns greater than Np,

or "stepped down" by making Ns less than Np

Here we used step down transformer,which reduces 230 volt input voltage to 12

volt output voltage.

25

Sliding switch: We used a slide switch. Main function of this slide switch here is

to reverse the direction of current by changing the position of wire on PCB, this

helps in back and forth motion of sliding span.

Figure2.8: Sliding switch

DC GEAR MOTOR:

A DC motor is an electric motor that runs on direct current (DC) electricity. An

electric motor converts electrical energy into mechanical energy. Most electric

motors operate through interacting magnetic fields and current-carrying

conductors to generate force, although electrostatic motors use electrostatic

forces. The reverse process, producing electrical energy from mechanical energy,

is done by generators such as an alternator or a dynamo. Many types of electric

motors can be run as generators and vice versa. For example, a starter/generator

for a gas turbine or traction motors used on vehicles often perform both tasks.

Electric motors and generators are commonly referred to as electric machines.

26

Figure2.9: DC motor

Electric motors are found in applications as diverse as industrial fans, blowers and

pumps, machine tools, household appliances, power tools, and disk drives.Here

we used a motor having 60rpm in output.

Figure 2.10: Model having DC motor

27

Printed circuit board (PCB): A printed circuit board, or PCB, is used to

mechanically support and electrically connect electronic components using

conductive pathways, tracks or signal traces etched from copper sheets laminated

onto a non-conductive substrate. It is also referred to as printed wiring board

(PWB) or etched wiring board. A PCB populated with electronic components is a

printed circuit assembly (PCA), also known as a printed circuit board assembly

(PCBA). Printed circuit boards are used in virtually all but the simplest

commercially-produced electronic devices. Conducting layers are typically made of

thin copper foil. Insulating layers dielectric are typically laminated together with

epoxy resin prepreg. The board is typically coated with a solder mask that is green

in color. Other colors that are normally available are blue, black, white and red.

There are quite a few different dielectrics that can be chosen to provide different

insulating values depending on the requirements of the circuit. Some of these

dielectrics are polytetrafluoroethylene (Teflon), FR-4, FR-1, CEM-1 or CEM-3. Well

known prepreg materials used in the PCB industry are FR-2 (Phenolic cotton

paper), FR-3 (Cotton paper and epoxy), FR-4 (Woven glass and epoxy), FR-5

(Woven glass and epoxy), FR-6 (Matte glass and polyester), G-10 (Woven glass

and epoxy), CEM-1 (Cotton paper and epoxy), CEM-2 (Cotton paper and epoxy),

CEM-3 (Woven glass and epoxy), CEM-4 (Woven glass and epoxy), CEM-5

(Woven glass and polyester). Thermal expansion is an important consideration

especially with BGA and naked die technologies, and glass fiber offers the best

dimensional stability.

FR-4 is by far the most common material used today. The board with copper on it

is called "copper-clad laminate".

Copper foil thickness can be specified in ounces per square foot or micrometres.

One ounce per square foot is 1.344 mils or 34 micrometres

28

Infrared Sensor:

Figure2.11: PCBcircuit

WORKING OF THE INFRARED SENSOR

In this IR detector and transmitter circuit the IC 555 is working under

MONOSTABLE mode. The pin 2 i.e. trigger pin and when grounded via IR

receiver, the pin 3 output is low. As soon as the IR light beam transmitted is

obstructed, a momentary pulse actuates the relay output (or LED). The IR

transmitter is simple series connected resistor network from battery. The timing

capacitor connected to pin 6 and 7 to ground. The time can varied as per

requirement by changing the R value.

29

Material

The traditional building materials for bridges are stone, timber and steel, and more

recently reinforced and prestressed concrete. For special elements aluminium and

its alloys and some types of plastics are used. These materials have different

qualities of strength, workability, durability and resistance against corrosion. They

differ also in their structure, texture and colour or in the possibilities of surface

treatment with differing texture and colour.

For bridges one should use that material which results in the best bridge regarding

shape, technical quality, economics and compatibility with the environment.

Natural Stone

The great old bridges of the Etruscans, the Romans, the Fratres Pontifices of the

Middle Ages (since about 1100) and of later master builders were built with stone

masonry. The arches and piers have lasted for thousands of years when hard

stone was used and the foundations constructed on firm ground. With stone one

can build bridges which are both beautiful, durable and of large span (up to 150

m). Unfortunately, stone bridges have become very expensive, if considered solely

from the point of view of construction costs. Over a long period, however, stone

bridges, which are well designed and well built, might perhaps turn out be the

cheapest, because they are long-lasting and need almost no maintenance over

centuries unless attacked by extreme air pollution. Stone is nowadays usually

confined to the surfaces, the stones being preset or fixed as facing for abutments,

piers or arches. Of course, sound weather-resisting stone must be chosen, and

fundamental rock like granite, gneiss, porphyry, diabas or crystallized limestone

are especially suitable. Caution is necessary with sandstones, as only silicious

sandstone is durable.

In Western Germany basalt-lava from the Eifel Mountains is popular. In choosing

the stone one should respect any local experience gained from old buildings and

bridges. Stone is worked upon in different ways, depending upon the direction of

30

the natural strata occurring in the quarry and on the requirements in the bridge

Very different effects can be produced with stone by the choice of the type of

masonry, the height of the courses, the proportion of the stones (length to height),

the arrangement of the joints, the surface treatment etc., and especially the overall

scale.

The choice of colours of the stone is also relevant. Granite of a uniform grey colour

and sawn surface can look as dull as simple plain concrete. A harmonious mixture

of different colours and slightly embossed surfaces can look very lively, even when

the masonry areas are extensive. Surfaces can also be enlivened by bright or dark

joint-filling. The sizes of the stone blocks and the roughness of their surfaces must

be harmonized with the size of the structure, the abutments, the piers etc. Coarse

embossing does not suit a small pier only 1 m thick and 5 m high, but large sized

ashlar masonry is suitable for large arch bridges such as the Saalebrucke Jena or

the Lahntalbrucke Limburg. Granite masonry was preferred for piers of bridges

across the River Rhine, because it resists erosion by sandy water much better

than the hardest concrete.

Artificial Stones, Clinker and Brick

Amongst the artificial stones, clinker and hard-burned brick are used in bridges

both as liners and for bearing vaults. They were often used in northern Germany,

the Netherlands, Belgium and Denmark, because there is no suitable natural stone

available. The warm colours of clinker or brick blend happily into the landscape.

Also in an urban environment, they are preferable to plain concrete, if brick is the

regional construction material.

The sizes of these stones are standardized, and one can only choose between

different types of joint arrangements. Small differences in colour and a pleasing

treatment of the joints can embellish the surfaces. Finally, one can also use split

concrete blocks for facing. If the concrete is made with colourful aggregates, which

break when being split, then masonry-work produced with these artificial blocks

31

can also look good - similar to masonry of natural conglomerates, which are in fact

nothing else but natural concrete.

Reinforced and Prestressed Concrete

Concrete is an all-round construction material. Almost every building contains

some concrete, but its questionable application in certain buildings-for example in

its use in the style of brutalism - has brought it into discredit. Its dull grey colour

has contributed to the fact that the word concrete has become a synonym for ugly.

In the field of bridges, concrete deserves a more favourable judgement. Not all

concrete bridges have turned out to be beauties, but pleasing bridges can be built

with concrete if one knows the art. Concrete is poured into forms as a stiff but

workable mix, and it can be given any shape; this is an advantage and a danger.

The construction of good durable concrete requires special know-how - which the

bridge engineer is assumed to have.

Good concrete attains high compressive strength and resistance against most

natural attacks though not against de-icing saltwater, or CO2 and SO2 in polluted

air. However, its tensile strength is low, and the use of concrete alone is therefore

limited to structures which are only subject to compressive stresses. But tensile

stresses also occur in abutments and piers due to earth pressure, wind, breaking

forces and to internal temperature gradients.

To resist these tensile forces, steel bars must be embedded in the concrete, the

so-called reinforcing bars, and this has lead to the development of reinforced

concrete. The steel bars only really come into play after the concrete cracks under

tensile stresses. If the reinforcing bars are correctly designed and placed, then

these cracks remain as fine "hair cracks" and are harmless. A second method of

resisting tensile forces in concrete structures is by prestressing.

The zones of concrete girders which are under tensile stress due to loads or other

actions are first put under compression - are pre-compressed - so that the tensile

32

forces must first reduce these compressive stresses before actual tensile stresses

come into being. This pre-compression is obtained by tensioning high strength

steel bars or wire bundles, which are in ducts inside the concrete girder.

Tensioning elongates the steel bars and they are anchored in this state at the

ends of he girder, transferring this tensioning force as a compressive force onto

the girder. These girders, prestressed with 'active steel" (prestressing steel) are in

addition reinforced with "passive steel" (non-stressed steel bars) for various

reasons. Prestressed concrete revolutionized the design and construction of

bridges in the fifties. With prestressed concrete, beams could be made more

slender and span considerably greater distances than with reinforced concrete.

Prestressed concrete

If correctly designed - also has a high fatigue strength under the heaviest traffic

loads. Prestressed concrete bridges soon became much cheaper than steel

bridges, and they need almost no maintenance - again assuming that they are well

designed and constructed and not exposed to de-icing salt. So as from the fifties

prestressed concrete came well to the fore in the design of bridges.

33

Figure 2.12: Prestressed concrete

All types of structures can be built with reinforced and prestressed concrete:

columns, piers, walls, slabs, beams, arches, frames, even suspended structures

and of course shells and folded plates. In bridge building, concrete beams and

arches predominate. The shaping of concrete is usually governed by the wish to

use formwork which is simple to make. Plain surfaces, parallel edges and constant

thickness are preferred. This gives a stiff appearance to concrete bridges, and

avoiding this is one task of good aesthetic design.

The extra cost for one-way curved surfaces, for tapering piers, for varying depth of

beams or arch ribs is as a rule comparatively small. Therefore one should not

hesitate to choose such divergences from the most primitive and simple forms in

34

order to improve appearance.

There is one great disadvantage to concrete as it emerges from the forms: the

inexpressive, dull grey colour of the cement skin. The surfaces frequently show

stains, irregular streaks from placing the concrete in varying layers, and pores or

even cavities from deficient compaction, which ire then patched more or less

successfully. These deficiencies have lead to a widespread aversion to concrete,

As well as to efforts for improvement. Some of the methods used to achieve a

good concrete finish in buildings, like profiles and patterns on the formwork, ribs or

accentuated timber veins etc are not generally suitable for bridges.

The best effect is obtained by bush hammering as was usual between 1934 and

1945 for the bridges of the German autobahn system. The concrete coating of the

rein- forcement is increased by 10 to 15 mm, so that a thin layer together with the

cement skin can be taken off by fine or coarse bush hammering. The aggregate is

then exposed with its structure and colour.

The protection of the embedded steel is not damaged, because the exterior

cement skin is in any case the worst part of concrete. It is very porous, because

mixing water collects at the forms by vibrating the concrete, and it is the porosity of

the cement skin which makes it so susceptible to collecting the dirt of polluted air.

With bush hammering one can adapt the degree of roughness to the size of the

surfaces. Piers of viaducts, for example, were chiselled very roughly, taking off

pieces 20 to 30 mm in depth by oblique chisel work.

The colour can be favourably influenced by the choice of coloured aggregates like

red porphyry or yellow limestone. Such surfaces age as well as natural stone

masonry, and they retain their texture over a long period of time. The cement skin

can also be washed off by special means after the concrete has hardened - such

"exposed aggregate" surfaces can look pleasing, depending on the colour and

size of the aggregates. Bush hammering was given up after about 1950 due to the

high labour cost. At that time suitable machines were not yet available, but with

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modern machinery this treatment should now be taken up again to embellish

concrete surfaces.

Another possibility is colouring the concrete it has been well developed during the

last decade. By the use of mineral colour pigments natural warm tones can be

attained - earthy colours with tones of ochre, reddish-brown sepia. umber, greyish-

green, slate-grey. Dark toned piers of a viaduct often look better in the landscape

than with a light grey colour. Bright coloured concrete-with white cement-can for

example be chosen to emphasize a fascia beam.

Fritz Leonhardt has often recommended the painting of bridges in the same way

that steel bridges are painted for corrosion protection. At the same time the dreary

grey of normal concrete is converted into a harmonious colourful statement. For

painting, soft colours should again be chosen and not bright loud colours. Before

painting, the porous cement skin must be removed, so that the paint will not peel

off later.

Mineral colours, especially those with fluor- or silicious compounds, can also give

an additional protection to the concrete. The colourfilm must be hygroscopic, so

that it does not prevent the change of moisture content in the concrete. If the

choice of colour and type of paint is based on the most up-to-date information,

then these paints can last long and keep their colour like the paintwork of many old

houses and churches, particularly in the Alps, which is often more than 200 years

old and still beautiful. Colour painting of concrete bridges has already been used in

several places. A most striking example is that of the long bridges along the

riverbanks in Brisbane, Australia.

Steel and Aluminium

Amongst bridge materials steel has the highest and most favourable strength

qualities, and it is therefore suitable for the most daring bridges with the longest

spans. Normal building steel has compressive and tensile strengths of 370

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N/mm2, about ten times the compressive strength of a medium concrete and a

hundred times its tensile strength. A special merit of steel is its ductility due to

which it deforms considerably before it breaks, because it begins to yield above a

certain stress level. This yield strength is used as the first term in standard quality

terms.

For bridges high strength steel is often preferred. The higher the strength, the

smaller the proportional difference between the yield strength and the tensile

strength, and this means that high strength steels are not as ductile as those with

normal strength.

Nor does fatigue strength rise in proportion to the tensile strength. It is therefore

necessary to have a profound knowledge of the behaviour of these special steels

before using them. For building purposes, steel is fabricated in the form of plates

(6 to 80mm thick) by means of rolling when red hot. For bearings and some other

items, cast steel is used. For members under tension only, like ropes or cables,

there are special steels, processed in different ways which allow us to build bold

suspension or cable-stayed bridges.

The high strengths of steel allow small cross-sections of beams or girders and

therefore a low dead load of the structure. It was thus possible to develop the light-

weight "orthotropic plate" steel decks for roadways, which have now become

common with an asphalt wearing course, 60 to 80 mm thick.

The pioneers of this orthotropic plate construction called it by the less mysterious

and less scientific name "stiffened steel slabs". Plain steel plate, stiffened by cells

or ribs, forms the chord of both the transverse cross girders and the longitudinal

main-girders. Simultaneously it acts as a wind girder. This bridge deck owes its

successful application mainly to mechanized welding, which is now in general use

and which has greatly influenced the design of steel bridges. So plate girder

construction now prevails, in which large thin steel plates must be stiffened against

buckling. Previously, vertical stiffeners were placed by preference on the outer

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faces; longitudinal stiffeners were then arranged on the inside.

Today all stiffeners are placed on this inside so as to achieve a smooth outer

surface allowing no accumulation of dust or dirt deposits that retain humidity and

promote corrosion - the "Achilles heel" of steel structures. Modern steel girder

bridges now hardly differ from prestressed concrete bridges in their external

appearance - except perhaps in their colour. This is perhaps regrettable, because

stiffeners on the outside enliven the plate-faces, give scale and make the girder

look less heavy. In addition to plate girders, trusses also take full advantage of the

material properties of steel. Very delicate looking bridges can be built by joining

slender steel sections together to form a truss. Again welding has improved the

potential for good form, because hollow sections can be fabricated and joined

without the use of big gusset plates. In this way smooth looking trusses arise

without the "unrest" which occurs by joining two or four profiles of rolled section

with lattice or plates. Steel must be protected against corrosion and this is usually

done by applying a protective paint to the bare steel surface. Painting of normal

steels is technically necessary and can be used for colour design of the bridge.

The choice of colours is an important feature for achieving good appearance.

There are steels which do not corrode in a normal environment (the stainless

steels V2A and V4A to DIN 17440), but are so expensive that they are used only

for components that are either particularly susceptible to the attacks of corrosion

or that are very inaccessible. From the USA came Tentor steel, alloyed with

copper, its 'first corrosion layer being said to protect it against further corrosion.

This protective rust has a warm sepia-toned colour which looks fine in open

country. This type of protection, however, does not last in polluted air and the

corrosion continues. For steel bridges, good use should be made of the technical

necessity of protecting the steel with paint to improve appearance and to achieve

harmonious integration of the structure within the landscape.

Aluminium was occasionally used for bridges and the same form was used as for

steel girders. Aluminium profiles are fabricated by the extrusion process which

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allows many varied hollow shapes to be formed, so that aluminum structures can

be more elegant than those of steel. Aluminium profiles are popular for bridge

parapets because they need no protective paint.

Timber :

Timber has favourable qualities of strength for resisting compression, tension and

bending. Rough tree trunks or sawn timber beams have been used since primitive

times for beam bridges; raking frames and arches soon allowed larger spans. The

Swiss carpenters, the brothers Grubennann reached a 100 m span with the timber

bridge across the River Rhine near Schaffhausen. Timber should be protected

against rain and therefore covered bridges with a roof and sidewalls with windows

evolved, and many of these are rightly preserved in the Alpine countries, testifying

to the high standard of their craftsmanship. Many now only serve pedestrians.

Recently timber bridges have been given a new impetus by glue technology which

allows larger cross-sections and larger lengths of beams to be made than grow

naturally. Moreover timber can now be better protected against weather and insect

attack. So new possibilities have arisen or the choice of structure, for its shaping

and for the size. Large timber trusses and even folded space trusses have been

built using steel gusset plates for jointing the members. Timber bridges, however,

have limits of span and carrying capacity, confining them mainly to bridges for

pedestrians or for secondary roads.

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Project Gantt chart

TASKS15 -30

1-31

1-30

20-31

1-28

1-10 11-20

21-31

1-10

11-20

Sept Oct Nov Jan Feb Mar Mar Mar Apr Apr

Team selection

Literature survey

Problem Formulation

Project synopsis

Concept generation

Sketching

Finalising rough designMaterial selection

Designing

Motion simulation

Bridge assembly

Fabrication

Report Submision

Future plans

Figure2.13:Gantt chart

Chapter-3

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3.1 Result

The fabricated Retractable bridge assembly with rack and pinion mechanism has

the following specification:

• Total size 24*18 inch

• Gap open 4 inch

• Moving center tube 18mm (diameter)

• Moving center tube 130mm (length)

• Outer tube casing outer diameter 34 mm

• Connecting screws 1.5mm-5mm

• DC gear motor 60 rpm(in o/p)

• Transformer 12 volt (step down)

• Slide channel 7 inch

Figure3.1:Final model

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3.2 Conclusion and Recommendation for further studies

Thus we can conclude that a retractable bridge model gives following advantages

over other bridges:

• Can be used for “Double Decker” bridge.

• Simple mechanism.

• Possibility to make upper pass over the bridge.

• No need to move both span like in “Draw Bridge”.

• Speed of span can be controlled easily.

Following recommendation for further studies are given below:

• Bearing Mechanism can be used.

• Alternatively, Hydraulic Piston Mechanism can also be used.

• Pulley Mechanism can also be used in movable bridge design.

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References

1. “Production Technology” by “R.K.JAIN”.

2. “Manufacturing Process” by “K.M.MOEED”.

3. “Machine Design” by “R.S.KHURMI”.

4. “Electrical Engineering” by “SANJAY SHARMA”.

5. “http://www.nireland.com/bridgeman/Bridging%20Materials.htm”.

6. “http://en.wikipedia.org/wiki/Moveable_bridge”.

7. “http://en.wikipedia.org/wiki/Retractable_bridge”.

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