BRIDGE ENGINEERING FOR ALL CIVIL STUDENTS

INTRODUCTION:

Bridge is a man made structure built to avoid physical obstacles without closing the way underneath such as river, valley, road, canal, railway, etc.

PURPOSE:

The purpose for construct a bridge is providing passage over an obstacle.

HISTORY:

The first bridge is made by humans were probably spans of cut wooden logs, planks, eventually stones using simple supported or crossbeams arrangement.

The Romans were built arch bridge and aquaducts by using cement which is reduced in the variation of strength in natural stones. 

PARTS:

Bridge is generally divided into 3 parts:-

1. Foundation
2. Sub-structure
3. Super-structure

NOTE:-Foundation is the lowest part of the bridge structure,which bears the load from the sub structure.

Sub-structure is the middle part of the bridge which consist piers, abutments and wing wall components . The components up to the level of bearing is called substructure. Sub structure is transfer the load from the super structure to the foundation.

Super-structure is that part of bridge structure which is directly carrying the load or traffic. It includes the deck slab, beam, arch, cable stayed, girders, hand rail, parapet wall, etc. The components above the level of bearing is called super structure.

COMPONENTS:

• PIER:-  Pier is an vertical load bearing member such as an intermediate support for adjacent ends of two bridge spans. OR
• Pier is a vertical supporting structure providing in the intermediate span.
•  ABUTMENTS:- Ends of the superstructure is known as abutments. OR
• Abutments are a vertical supporting structure providing on the ends of the span which are rigidly connected, such as pillars.
• BED BLOCK:- It is the block on the top of the pier/abutment which receives load from bearing and disperses to the pier.
• BECK FILL:- The material used to fill the space at the back of abutment is known as back fill. Stone, gravel, sand, etc. are used for back filling. it should be well compacted.
• KERB:- It is the raised portion of the edge of carriage way on both sides. It checks the vehicles going out of the carriage way. width and height of the kerb 600 mm and 225 mm, respectively. The roadside slope of kerb is 1 in 8 up to a height of 200 mm and the top portion is curved.              
• WING WALL:They are remaining walls constructed to retain the earthwork of the approach embankment behind abutments. 
• CURTAIN WALL:The floor provided between the masonry walls below the river bed is known as curtain wall.
• BEARINGS:-Bearings are the devices used in the long bridges to avoid development of high stresses in the main girders due to temperature changes and deflections. Bearing is the part of the structure ,which transfers the load from superstructure to the sub structure in such manner that bearing stresses induced in sub structure are within the permissible limit.
• SPAN:-The distance between any two bridge supports such as columns, walls, towers, etc. are known as span.
• DECK:-Deck is the floor of the bridge which directly carrying the traffic load.
• GIRDER/BEAM:-Girder is a beam support or support beam used in the construction. It is the main horizontal beam of a structure which supports smaller beams.Girder is the part of the super structure which is under bending along the span. It is load bearing part which supports the deck.
• APPROACH SLAB:The slab which is provided to join road with the bridge is called approach slab.

DIAGRAM:

CLASSIFICATION:

1. On the basis of purpose:

• road bridges
• railway bridges 
• foot bridges
• aquaducts(for carrying canal water)
• viaducts(for taking roads across valleys)

2. On the basis of alignment:

• square bridge(if it is at right angles to obstacle)
• skew bridge(if it is not at right angles to obstacle)

3. On the basis of life period:

• permanent
• temporary(temporary bridges are built during military operations, during project execution or in rescue operations)

4. On the basis of span:

• culverts(less than 6 m)
• minor bridges(6 m to 60 m)
• major bridges(more than 60 m)
• long span bridges(more than 120 m)

5. On the basis of position of high flood level:

• submersible(flow of water above bridge deck level permitted during heavy rains)
• non submersible(all permanent bridges having deck level above the high flood level)

6. On the basis of navigation:

• movable bridges(movable-swing, movable-lift, movable-bascule)
• fixed bridges

7. On the basis of location of bridge floor:

• deck
• semi-through
• through type

8. On the basis of superstructure:

girder bridge

• portal frame bridge 
• truss bridge
• cantilever bridge
• arch bridge
• suspension bridge
• cable-stayed bridge
• turnover bridge

9. On the basis of materials:

• timber bridge
• masonry bridge
• R.C.C bridge
• pre stressed concrete bridge
• steel bridge

10. On the basis of connection:

• pinned
• rivet
• weld

TYPES OF BRIDGES:

1. ARCH BRIDGE:- 
   

• Arch bridge have abutments on both sides.
• The weight of the bridge is thrust into the abutments at the either sides.
• The first arch bridge were built by GREEKS.
• These bridges use such as one or more hinges, depending on what kind of forces/loads, stresses they must endure.
• Arch bridge has great natural strength.
• thousands of years, ROMANS were built arch bridge with the help of stones. Today most of arch bridges were built by steel and concrete.
• They have span up to 800 ft.
• They are often choose for their strength and appearance.

FORCES:-

The compressible forces are created by load and transferred down through the arches and resisted by the supports/abutments at its base. Abutment supports prevents arch spreading under load.

ADVANTAGES:-

They are very strong and can be built from a wide range of materials.

DISADVANTAGES:-

Limited span unless multiple uses of arch and uneconomical use of materials.

2. GIRDER BRIDGE:- 

• Beam/girder bridges is the oldest and simplest bridge design consisting of vertical piers and horizontal beam.
• They are suitable only for shorter spans but can be used for larger spans by adding additional piers.

FORCES:-

As the bridge is loaded by traffic for example- the beam bends which causes the top surface to be compressed and the bottom surface to be stressed or put it in tension.

ADVANTAGES:-

They are easy to built and inexpensive as compared to other type of bridges, so they are very common.

DISADVANTAGES:-

They have limited span and do not allow large vehicles and boats to pass underneath.

3.TRUSS BRIDGE:-

• Truss bridges consist a assembly of triangles.
• Truss bridge is commonly made from a series of straight steel bars.
• Truss bridge structure is built up of jointing together length of materials to form an open framework-based mainly on the triangles because of their rigidity.
• They are very strong and can supports heavy loads.

FORCES:-

As the bridge is loaded by traffic for example- the beam bends which causes the top surface to be compressed and the bottom surface to be stressed or put it in tension.

ADVANTAGES:-

They are very strong and make an efficient use of materials.

DIADVANTAGES:-

They are very complex to construct and need a high level of maintenance.

4.CANTILEVER BRIDGE:-

• Cantilever bridge are based on the structures that projects horizontally into space, and supports are only at one end.

FORCES:-

If two cantilever projects are out from the central pier the forces are balanced.

ADVANTAGES:-

More easily constructed at difficult crossings.

DIADVANTAGES:-

More complex to maintain.

5.SUSPENSION BRIDGE:-

• Suspension bridge is the type of bridge in which deck is hung below the suspension cables on vertical suspenders. The basic structural components are stiffening girders, cables, towers, trusses and the anchorages for the cables at each end of the bridge.

FORCES:-

Any load applied on the bridge is transformed into the tension in the main cables which have to firmly anchored to resist it.

ADVANTAGES:-

Strong and span can long distance such as across river.

DISADVANTAGES:-

Expensive and complex to built.

6.CABLE-STAYED BRIDGE:-

• cable stayed bridges are similar to the suspension bridge but infact that they are quite different in their principles and in their constructions.
• There are two major classes of cable stayed bridges:-
• fan type(which are more efficient)
• parallel type(which allow more space for the fixings)

FORCES:-

As the traffic loaded on the roadways. The cables are attached to the roadways and transfer the load to the towers and put them in the compression. Tension is constantly acting on the cables which are stretched because the cables are attached to the roadways.

ADVANTAGES:-

Good for medium spans, greater stiffness than suspension bridges, can be constructed by cantilevering out from the tower, horizontal forces are balanced so large ground anchored are not required.

DISADVANTAGES:-

More expensive than other type of bridges, except suspension bridge.

7.TURNOVER BRIDGE:-

• Turnover bridge/ change-line bridge/ roving bridge is a bridge over a canal constructed to allow a horse towing boat to cross canal when the towpath changes sides.

SELECTION OF A BRIDGE SITE:-

• Stream should be straight.
• Stream should be narrow with well defined and firm banks.
• There should not be whirls and cross currents.
• There should be high banks above flood level on each side.
• The site should be at reasonable proximity to the direct alignment of road.
• The site should be geologically sound.
• As far as possible it should not need river training works and avoid excessive under water construction works.
• The approach should be free from obstacles.
• Absence of sharp curves in the approaches.

REQUIREMENT OF AN IDEAL BRIDGE:-

• It should be economical
• it should serve intended functions with safety and convenience
• It should give aesthetic elegant look

SCOUR DEPTH:- 

• The scour may be defined as the removal of materials from the beds and banks of the streams during the passage of flood discharge.
• Scouring may be defined as the process due to which the particles of the soil or rock around the periphery of the abutments/piers of the bridge gets eroded or removed over a certain depth is called scour depth.
• Scouring usually occurs when the velocity of the flowing water increases or crosses the limiting value.

                              Dsm = 1.34(Db^2/Ksf)^0.33

where, Dsm - mean depth of scour below H.F.L(in m)

Db- discharge m^3/s per m width

{total design discharge/effective linear water way}

Ksf - silt factor

AFFLUX:-

• Afflux is increase in water level that can occur upstream of the structure that creates an obstruction in the flow.
• Afflux is defined as the heading up of water when they hit any obstruction. In bridges the water hit at upstream side, hence water level rises.
• Afflux increases with increasing flow rates and with an increasing degree of obstruction.
• Elevated water levels upstream of bridges and culverts are a potential source of flood risk.

ECONOMICAL SPAN OF BRIDGE:-

The most economical span length is that for which the cost of super structure equals the cost of sub structure.

 cost of substructure=cost of superstructure