Adg1 18-8-11

Bridges

• Bridge provides passage over valley for a road, a railway, pedestrians, a canal or a pipe line

Tree fallen across a stream was perhaps the first beam type of bridge

Natural rock arch formed by erosion of loose soil is first type of arch bridge

Creepers hanging from tree to tree allowing monkeys to cross from one branch to another is suspension bridge

About 250 BC Chinese built Stone Arch Bridge

About 200 BC Romans constructed Stone Arch Bridge

Eighteenth century witnessed Wooden Bridge

The first Iron Bridge was built in 1779 in England

The first Cantilever Bridge was constructed in 1857 in Germany

The first RCC bridge was constructed in 1871 in England

The first Prestress Concrete bridge was constructed in France

The first PSC Bridge in India in 1954 is Palar Bridge near Changleput

The main supporting member in a bridge

– Simply Supported Beam

– Cantilever

– An Arch

– A Cable

Components of Bridge

i. Decking (consisting of slab, girders, trusses)

ii. Under the girder

iii. Abutment and piers

iv. Foundations for abutment and piers

v. River training works, rivetment – bed aprons

vi. Approaches to bridge

vii. Hand rails, Guard stones

Bridges – Types

Culverts & Box Culverts : For small crossings of 3 to 4 m span culverts are constructed

A culvert have piers on either side and a RC slab resting on it

T-Beam Bridge

RCC Girders with deck slab

Commonly used for small spans from 10m to 15 m

Arched Bridge

When concrete is not developed, Masonry Arched Bridges are very popular upto 6 m to 8 m spans

Bow String Girder Bridge

It is an adoption of two hinged arch. For longer spans upto 30 m, RCC bow string girder bridge is under use in early nineteenth century

Prestressed Bow String Bridge

With prestressing concrete, these RCC bow string girders were in vogue.

In the early 1980s, the prestressed bow string girder bridge have been again in use.

The third railway bridge at Rajahmundry on the river Godavari is a prestressed bow string girder bridge

Open Spandrel Arch Bridge

To concrete two hill locks and if good foundations are available open spandrel arch upto 30 m is a good choice

Prestressed Concrete Bridge

With prestressed concrete, bridge construction was revolutionised. Bridging longer spans could be done with ease

Prestressed beams are cast and will be launched on piers

Widthwise setions of 0.5m thickness are cast on the ground. They will be lifted and placed one behind the other and it will be prestressed

Prestressed Concrete Bridge

Suspension Bridge (Cable Stayed Bridge)

For very long spans 200 to 500 m, suspension bridges are very suitable. Cable will be suspended and the deck of the bridge will be suspended from the cable

Balanced Cantilever Bridge Continuous spans give lesser bending moments – longer spans can be usedUnyielding support required – Any settlement give rise to reversal of stressesBalanced cantilever bridges have the continuous effect but yielding of supports is not a disadvantageSpans from 35 m to 60 m can be achieved

Howrah bridge at Kolkata is a double

cantilever bridge with main span 457 m.

(Third longest in the world). It is fabricated

with steel.

Truss Bridges (Steel Bridges)

Economical for a span of 100 to 200 m

Railways used to prefer only steel bridges earlier

The steel bridge is with steel trusses either Pratt type (N-Type) or Warren Type or K-Type made up of channels, angles and plates and used to be riveted or welded at the joints

Main Girder

Main Girder

Cross

Girder

Top

Bracing

There are two types of steel bridges

(i) Deck Type and (ii) Through Type

One deck type the vehicles move on top of the girder

In through type, vehicles move through the girder

In through type, vehicles move through the girder

Bridges are aligned in three ways

Square Alignment : Road way and the river are perpendicular

Skew Alignment : Road way and the river are not perpendicular

Curved Alignment : The road way on the river is curved

Loads on Bridges

Dead Loads : Weight of the structure including foundations

Live Loads : Loads caused by vehicles

IRC has given – Class AA, Class A,

Class B

Railways have their own loading

For foot bridges, live load is taken as uniformly distributed

70 R – In addition, Class 70R loading is recommended. It is same as Class AA of 70 tonnes tracked vehicle but the contact length of the track is 4.57 meters

The wheeled vehicle is 15.22 meters long, has 7 axels, total load 100 tonnes

Class AA

CLASS A

Impact Loads

Movement of load is dynamic action

Due to this, the load will cause higher stresses

than when it is stationary

Wind Load

Wind exert pressure

Wind pressure increase non-linearly with

height

Longitudinal Forces (Traction)

Forces caused due to tractive effort of wheels

Breaking effect due to application of brakes

Centrifugal Forces

When bridge is in curve, centrifugal forces due to

movement of vehicles

Horizontal Forces (Due to water current)

Any part of bridge submerged in water, horizontal

forces due to water

Buoyancy

When the part of bridge or whole structure under

submersion of water, the buoyancy forces to be

considered

Earth Pressure

Mainly abutments and wings are to be retained

earth

The earth pressure due to the back fill have to be

calculated

Temperature effects

The effect of rise or fall in temperature have to be

considered and provision to be made in stresses

for these temperature effects

Seismic Forces

The effects of earthquakes on the structure to be

calculated and the structure to be designed

BED BLOCK

Bed block rests on pier and

monolithic with pier

It is slightly wider and longer

than pier

Bed block will be with M20

concrete with nominal

reinforcements

Hitherto girders rest on bed

block. Now a days, pedestals

are provided on the bed block

to facilitate operation of jacks

Pedestals

PIERS & ABUTMENTS

Piers and abutments are sub-structures

Intermediate supports for girder are called Piers

The end piers are abutments

Piers are constructed with masonry, concrete

Piers for river bridges are provided with cut waters to

facilitate stream line flow

Piers may be solid, cellular, trestle, hammer head

Abutment is one terminal structure of a

bridge

In addition to weight of girder, abutment

have to support the embankment that

approach the bridge. It also protects the

embankment from scour

Abutments are also constructed by masonry,

concrete, reinforced concrete

The abutment have to be designed for load from the bridge, earth pressure and live load on the embankment, adjacent to abutment

Wing walls may be solid wall or flying wing

walls

Immediately behind abutment i.e. just when

entering the bridge, an approach slab is

provided

Cause ways & Submergible Bridges

A cause way is a rigid raised platform in the bed.

When floods are of short duration i.e. 6 to 8 hours

once in a way, this is preferred.

Sometimes, pipes are provided to dispose of lean

flows in the river

A pucca bridge on the river allows traffic for most

of the time

During heavy floods, once in a way, the bridge will

be submerged. This is called submergible bridge.

Military Bridges

Generally temporary and sometimes permanent

Construction and dismantling in a short time

Two types

i. Fixed Bridge – Bailey Bridge

ii. Floating Bridge – Pontoon Bridge

Bailey Bridge

It is a steel bridge Steel panels, bracings, stringers will be brought

and will be assembled It will rest on grillage type of girders and cribs

Pontoon Bridge

It consists of a number of pneumatic pontoons or punts across the river at regular intervals

Trussed beams and transverse flanks are placed

Rigid Framed Bridge

It is an RCC continuous portal

It may be of slab type or may be a rib type

In the rib one, the slab will be connected continuously on the ribs

Bearings

Bearings transmit the loads from superstructure to pier

and also permits certain movements (longitudinal

movement and rotation)

For slabs, no special bearings are required – a kraft paper

under the slab and rounding of the edges will suffice

For submersible bridges, the slab have to be

anchored to the pier to resist uplift pressure

Bearings for Girders – Types

i. Plate Bearings

ii. Steel Roller Bearings

iii. RC Rocker Expansion Bearing

iv. Elastomaric Bearing

Girder

Pier

Mild Steel Roller cum Rocker Bearing - Expansion

Pier

Girder

Elastomeric Bearing

FOUNDATIONS

Foundations are important part of bridge

There are two types of foundations

i. Shallow Foundations

ii. Deep Foundations

Shallow foundations by open excavation

practicable up to maximum 5.0 m depth

For larger depths and for work under water shoring

with sheet piles or cofferdams to facilitate

excavation

If foundations on rock – a minimum depth of 0.6 m below

ground level – Rock to be benched – Dowel bars of 38 mm

diameter at 0.8 m to be provided to anchor

Deep foundations :

Pile Foundations – Fraction piles

Bearing Piles

Fraction pile transfer the load by skin friction i.e. by

friction along the embedded length

Piles also classified as Cast-in-situ and Precast

Piles may be timber, steel, concrete

For design of foundations

i. In a river the scour depth

ii. The grip length – minimum is 1/3 rd of scour

depth

iii. Soil pressures at the base

A group of piles will be driven and on that a R.C.C. cap

will be provided and on that the pier will be resting

Well foundations :

Well foundations are commonly adopted

Well foundation consists of

i. Well curb – consists of cutting edge

ii. Stearing – may be with masonry / concrete

iii.Bottom plug – concreting of bottom of well

iv. Sand filling

v. Top plug – concreting of top

vi. Well cap

Well foundations

Shape of well may be

i. Circular

ii. Double D

iii.Square

iv. Rectangular

Construction of Bridges – Surveys - Requirements

Site survey at the bridge site (topographic surveys) to locate the bridge site and approaches. As far as possible a straight reach is possible

Sub-soil investigations to design the foundations

Traffic survey to fix the number of lanes and footpaths

Maximum discharge of the river to fix the length of the bridge

Maximum water level and free board (vertical clearance)