introduction to bridges

A bridge is a structure that permits us to cross over an obstacle. Suppose we lay a plank across a brook. If we have a bridge. If the plank is thin in

relation to its length , it will sag. In fact, if it is too long, it will collapse. This shows that we

must consider the weight of a bridge itself---the “dead load”. If we stand at the middle of the plank, it sags even more. So, we must provide for the weight of whatever our bridge is designed to carry--- the “live load”. Also, when you walk across the plank, it bounces under you, illustrating the effect of a “moving live load”. Finally, there is a “wind load”. A strong wind pushes against the sides, lifts the deck, shakes the whole structure. Certainly the force of the wind is something to keep in mind. Bridges help us to connect to each other and the world. There are four main types of physical bridges. They are arch, beam, suspension and cable-stayed.

Functions : aqueduct, viaduct, highway, pedestrian etc. Materials of construction : reinforced concrete, pre-

stressed concrete, steel, composite, timber etc. Form of superstructure : slab, beam, truss, arch,

suspension, cable-stayed etc. Inter-span relation : simple, continuous, cantilever. Method of construction : pin-connected, riveted, welded

etc. Span : short, medium, long, right, skew, curved. Type of service and duration of use : permanent,

temporary bridge, military

SUSPENSION BRIDGE

COMPARISION

Advantages over other bridge types

Longer main spans

A suspension bridge can be made out of simple materials such as wood and common wire rope.

Less material may be required than other bridge types, even at spans they can achieve, leading to a reduced construction cost

Except for installation of the initial temporary cables, little or no access from below is required during construction, for example allowing a waterway to remain open while the bridge is built above

May be better to withstand earthquake movements than heavier and more rigid bridges

Disadvantages over other bridge typesConsiderable stiffness may be required to prevent the bridge deck vibrating under high winds

The relatively low deck stiffness compared to other (non-suspension) types of bridges makes it more difficult to carry heavy rail traffic where high concentrated live loads occur

The two basic parts are: Substructure - includes the piers, the abutments and

the foundations. Superstructure - consists of the deck structure itself,

which support the direct loads due to traffic and all the other permanent and variable leads to which the structure is subjected.

The connection between the substructure and the superstructure is usually made through bearings. However, rigid connections between the piers (and sometimes the abutments) may be adopted, particularly in frame bridges with tall (flexible) piers.

Bridge is a structure which provides a passage over a gap without closing the opening which is beneath that gap. The passage may be due to railway , roadway , canal & natural river etc. Initially the naturally available materials such as stone and timber were extensively used for bridges but now days artificial materials such as cement concrete & steel are utilized more in the construction of bridge.

During the king “harshavardhna” or even before him india appears to have a good highway system & such highways had a number of bridge.

“firoze shah” who ruled the delhi in mid 14th century built a number of canal & bridges.

“portuguese” in 16th and 17th century built many old arch masonary bridges in “goa”.

One of oldest stone slab bridge still in use across the river “cauvery” at “srirangapatnam” bulit by “tippu sultan”.

A number of cable stayed bridges has been built in india in past two decades. The major one is “vidhyasagar sethu” across “hooghly” at “kolkata” & “nalini bridge” on river “jamuna” at “allahabad”.

Inidian railways build a number of large steel arch bridge in “j & k”. BRO has erected a cable stayed bridge in early part of this millenium

which is claimed to be only bridge of the type at highest altitude in the world at the time of construction.

700 A.D. Asia700 A.D. Asia

1,304 years 1,304 years agoago

100 B.C. Romans100 B.C. Romans2,104 years ago2,104 years ago

Clapper Bridge

Tree trunkStone

Arch design evenly distributesstressesNatural concrete made from mud and straw

Roman Arch Bridge

Great Stone Bridge in China

Low bridgeShallow archAllows boatsand water to passthrough

Truss Bridges

Mechanics of DesignWood

Suspension Bridges

Use of steel in suspending cables

1900 1900

1920 1920

Prestressed ConcreteSteel

2000 2000

MASONARY ARCH BRIDGE Probably first category of bridge to be involved. Aesthetically superior to slab bridges. Consist of a arch shape slab supported on two apposite wall

& it was adopted earlier for small of 3 to 15m in masonry & extended up to 519m in steel & 305m in concrete has been built in the world.

Beam Bridge When something pushes down on the beam, the beam bends.

Its top edge is pushed together, and its bottom edge is pulled apart.

Beam bridges are horizontal beams supported at each end by substructure units and can be either simply supported when the beams only connect across a single span, or continuous when the beams are connected across two or more spans.

PIPE CULVERT Consist of a pipe barrel under the embankment with

protection works at the entry and exit. It is suitable for cross drainage flow on relatively flat terrain

& in this discharge is limited & it has negligible maintenance

Hydraulically short conduit which conveys stream flow through a roadway embankment or past some other type of flow obstruction

SLAB BRIDGE Simplest type of construction. Adopted for small bridges and culverts. Span is between 10-20m. Concrete slab cast monolithically over longitudinal girder. No. of longitudinal girders depends upon the width of road

PLATE GIRDER BRIDGE Span ranges 10 to 60m. Can be extending up to 250m in

continuous construction

TRUSS BRIDGE Span 30 to 375m in simply supported

case. Span 30 to 550m in cantilever

combination case

All beams in a truss bridge are straight. Trusses are comprised of many small beams that together can support a large amount of weight and span great distances.

SUSPENSSION BRIDGE Made up of high tensile steel cables strung in

form of a centenary to which the deck is attached by steel suspenders which are made up of steel rods/members/cables.

Deck can be of timber , concrete or steel spanning across the stiffening girders transmitting loads to suspenders.

Forces

In all suspension bridges, the roadway hangs from massive steel cables, which are draped over two towers and secured into solid concrete blocks, called anchorages, on both ends of the bridge. The cars push down on the roadway, but because the roadway is suspended, the cables transfer the load into compression in the two towers. The two towers support most of the bridge's weight.

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 higher.

Similar to suspension bridge except that there will be no suspenders in the cable stayed bridges .

A number of cables are stretched from support tower directly connected the decking.

The cable stayed bridge is newer than the other types of bridge. Large upright steel supports are used to transmit the load into the ground.

A cantilever bridge is a bridge built using cantilevers: structures that project horizontally into space, supported on only one end.

Cantilever bridges are built using cantilevers horizontal beams supported on only one end. Most cantilever bridges use a pair of continuous spans that extend from opposite sides of the supporting piers to meet at the center of the obstacle the bridge crosses.

The arch has great natural strength. Thousands of years ago, Romans built arches out of stone. Today, most arch bridges are made of steel or concrete, and they can span up to 800 feet.

Arch bridges have abutments at

each end. The weight of the bridge is thrust into the abutments at

either side.

Forces

The arch is squeezed together, and this squeezing force is carried outward along the curve to the supports at each end. The supports, called abutments, push back on the arch and prevent the ends of the arch from spreading apart.

A movable bridge, is a bridge that moves to allow passage usually for boats or barges.

Based upon type of structural arrangement

I-girder bridge Plate girder bridge truss girder bridge Suspension bridge

Based upon structural action or nature of superstructure action

Simply supported span bridge Continuous span bridge Cantilever bridge Arch bridge Rigid frame bridge

Based upon type of connections

Riveted bridge Welded bridge Bolted bridge Pinned bridge

Based upon floor action

Deck type bridge Through type bridge Semi –through type bridge or pony

bridge Double deck bridge-used in rail cum

road bridge

Based upon movement of structural parts of the bridge

Fixed(permanent) bridge Movable bridge: Can opened either

horizontally or vertically so as allow the river or channel traffic to pass

Based upon purpose of bridge

Road bridge Railway bridge Pedestrian bridge

Based upon loading

IRC class AA loading bridge IRC class A loading bridge IRC class B bridge

.

Base upon span length

Culvert – up to span length 6m Minor bridge – up to span length 6 to

30m. Major bridge – up to span length over

30m

700 A.D. Asia700 A.D. Asia

100 B.C. Romans100 B.C. Romans

Natural BridgesNatural Bridges

Clapper BridgeTree trunk

Stone

The ArchNatural Cement

Roman Arch Bridge

History of Bridge DevelopmentHistory of Bridge Development

Great Stone Bridge in China

Low BridgeShallow Arch

1300 A.D. Renaissance1300 A.D. Renaissance

Strength of MaterialsMathematical TheoriesDevelopment of Metal

First Cast-Iron Bridge

Coalbrookdale, England

1800 A.D.1800 A.D.

History of Bridge DevelopmentHistory of Bridge Development

Britannia Tubular Bridge

1850 A.D.1850 A.D.

Wrought Iron

Truss BridgesMechanics of Design

Suspension BridgesUse of Steel for the suspending cables

1900 A.D.1900 A.D.

1920 A.D.1920 A.D.

Prestressed ConcreteSteel

2000 A.D.2000 A.D.

• The most common and basic type

• Typical spans : 10m to 200m

• Truss is a simple skeletal structure.

• Typical span lengths are 40m to 500m.

Beam Bridge

Basic Beam Bridge Actual Beam Bridge

A beam or "girder" bridge is the simplest and most inexpensive kind of bridge.

In its most basic form, a beam bridge consists of a horizontal beam that is supported at each end by piers. The weight of the beam pushes straight down on the piers.

The beam itself must be strong so that it doesn't bend under its own weight and the added weight of crossing traffic.

When a load pushes down on the beam, the beam's top edge is pushed together (compression) while the bottom edge is stretched (tension).

Basic Arch Bridge Actual Arch Bridge

Bixby Creek Bridge, Monterey, CA

Arch bridges are one of the oldest types of bridges and have great natural strength.

Instead of pushing straight down, the weight of an arch bridge is carried outward along the curve of the arch to the supports at each end.

These supports, called the abutments, carry the load and keep the ends of the bridge from spreading out.

Basic Suspension Bridge

Aesthetic, light, and strong, suspension bridges can span distances from 2,000 to 7,000 feet -- far longer than any other kind of bridge.

They also tend to be the most expensive to build.

True to its name, a suspension bridge suspends the roadway from huge main cables, which extend from one end of the bridge to the other.

These cables rest on top of high towers and are secured at each end by anchorages.

The towers enable the main cables to be draped over long distances.

Most of the weight of the bridge is carried by the cables to the anchorages, which are imbedded in either solid rock or massive concrete blocks.

Inside the anchorages, the cables are spread over a large area to evenly distribute the load and to prevent the cables from breaking free.

Basic Cable-stayed Bridge

Cable-stayed bridges may look similar to suspensions bridges -- both have roadways that hang from cables and both have towers.

Two bridges support the load of the roadway in very different ways.

The difference lies in how the cables are connected to the towers. In suspension bridges, the cables ride freely across the towers, transmitting the load to the anchorages at either end.

In cable-stayeded bridges, the cables are attached to the towers, which alone bear the load.

The cables can be attached to the roadway in a variety of ways.

In a radial pattern, cables extend from several points on the road to a single point at the top of the tower.

In a parallel pattern, cables are attached at different heights along the tower, running parallel to one other.