BRIDGESBRIDGES

Bridge BasicsBridge Basics BBridge is a ridge is a structure built to span a valley, road, built to span a valley, road,

river, body of water, or any other physical river, body of water, or any other physical obstacle. obstacle.

Designed to carry their own weight, (dead load) Designed to carry their own weight, (dead load) and to carry people and traffic, (live load) and to and to carry people and traffic, (live load) and to resist natural forces, such as wind and resist natural forces, such as wind and earthquakes.earthquakes. Must consider effects of contraction and/or Must consider effects of contraction and/or

Expansion due to temperature changesExpansion due to temperature changes

Designs of bridges will vary depending on the Designs of bridges will vary depending on the function of the bridge and the nature of the function of the bridge and the nature of the area area where the bridge is to be constructed. where the bridge is to be constructed.

Bridge BasicsBridge Basics Bridges have been used since the dawn of Bridges have been used since the dawn of

humankindhumankind Bridges are based on one or more of three basic Bridges are based on one or more of three basic

structures that are derived from forms found in structures that are derived from forms found in nature.nature. Beam: a log fallen across a streamBeam: a log fallen across a stream Arch: natural rock formationsArch: natural rock formations Suspension: from a hanging vineSuspension: from a hanging vine

Though traditionally constructed from wood, Though traditionally constructed from wood, stone, concrete, or steel, new bridges made from stone, concrete, or steel, new bridges made from high-performance composite materials such has high-performance composite materials such has reinforced plastic are emergingreinforced plastic are emerging

Bridge Terminology Bridge Terminology Abutment-Abutment- A substructure element supporting each A substructure element supporting each

end of a single span or the extreme ends of a multi-span end of a single span or the extreme ends of a multi-span superstructure and, in general, retaining or supporting superstructure and, in general, retaining or supporting the approach embankment.the approach embankment.

Deck-Deck- The roadway portion of a bridge that directly The roadway portion of a bridge that directly supports vehicular and pedestrian traffic.supports vehicular and pedestrian traffic.

Span-Span- The distance between the main supports The distance between the main supports of a bridge.of a bridge. The length normally measured when describing the The length normally measured when describing the

size of the bridgesize of the bridge Simple Span-Simple Span- a plank across a stream a plank across a stream Continuous Span-Continuous Span- supported along its length by piers and supported along its length by piers and

abutmentsabutments

Cofferdam-Cofferdam- A watertight structure allowing underwater A watertight structure allowing underwater foundations to be built in the dry.foundations to be built in the dry.

Bridge TerminologyBridge Terminology

Bridge ForcesBridge Forces

Four types of forces act on a bridge, either Four types of forces act on a bridge, either separately or in combination.separately or in combination. TensionTension CompressionCompression TorsionTorsion ShearShear

http://www.pbs.org/wgbh/buildingbig/lab/forces.htmlhttp://www.pbs.org/wgbh/buildingbig/lab/forces.html

BridgesBridges

Beam Bridge (Girder)Beam Bridge (Girder) The simplest form of a bridgeThe simplest form of a bridge A rigid horizontal structure that is resting on two piers, A rigid horizontal structure that is resting on two piers,

one at each end. one at each end. The weight of the bridge and any traffic on it is directly The weight of the bridge and any traffic on it is directly

supported by the piers. The weight is traveling directly supported by the piers. The weight is traveling directly downward. downward.

Mostly built from steel and concrete beams, or Mostly built from steel and concrete beams, or girders.girders.

The size of the beam, and in particular the height of the beam, controls the distance that the beam can span.

Truss BridgesTruss Bridges Truss bridges are a type Truss bridges are a type

of beam bridge made up of beam bridge made up of many small beams of many small beams attached together in attached together in triangular configuration to triangular configuration to support a large amount of support a large amount of weight and span great weight and span great distances. distances.

They function by They function by compression and tension compression and tension forces and not by bending forces and not by bending forces.forces.

They are identified They are identified according to the way the according to the way the chords are arranged.chords are arranged.

How Truss Bridges WorkHow Truss Bridges Work

A truss is an A truss is an interconnected interconnected framework of beams framework of beams designed to hold designed to hold something up. something up.

Trusses don’t bend, they Trusses don’t bend, they get pulled apart and get pulled apart and pushed together. pushed together.

However once the weight However once the weight is increased the bridge is increased the bridge will stag in the middle. will stag in the middle. This is due to the This is due to the individual beams individual beams expanding and expanding and contracting due to the contracting due to the weight.weight.

Trusses must be stable, Trusses must be stable, and not able freely in any and not able freely in any direction in order for direction in order for them to work. them to work.

The beams have to be The beams have to be placed carefully in the placed carefully in the right angles and in equal right angles and in equal distances away from distances away from each other so they can each other so they can distribute the weight distribute the weight equally.equally.

They are usually They are usually supported at the ends by supported at the ends by abutments and abutments and sometimes in the middle sometimes in the middle by piers. by piers.

Arch BridgesArch Bridges Arch bridges are one of the Arch bridges are one of the

oldest types of bridges and oldest types of bridges and have great natural strength. have great natural strength.

The load on the roadway of this The load on the roadway of this bridge is carried by the arch. bridge is carried by the arch. The arch is supported at each The arch is supported at each end by a support called an end by a support called an abutmentabutment

The arch is held together by The arch is held together by the main piece in the middle the main piece in the middle which is called the “keystone”which is called the “keystone”

The roadway can be placed The roadway can be placed above or below the arch.above or below the arch.

Arch BridgesArch Bridges

Arch Bridges consist of compression and tensionArch Bridges consist of compression and tension.

CompressionCompression Arch bridges are Arch bridges are

continuously under continuously under compression. The force compression. The force of compression is pushed of compression is pushed along the curve of the along the curve of the arch toward abutments.arch toward abutments.

TensionTension The greater the degree of The greater the degree of

curvature (the larger the curvature (the larger the semicircle of the arch), the semicircle of the arch), the greater the effects of tension greater the effects of tension on the underside. on the underside.

The natural curve of the arch The natural curve of the arch and its ability to dissipate the and its ability to dissipate the force outward greatly force outward greatly reduces the effects of reduces the effects of tension on the underside of tension on the underside of the arch. the arch.

Suspension BridgesSuspension Bridges Suspension BridgeSuspension Bridge

Tall towers on both sides of the Tall towers on both sides of the roadway support the main cables. roadway support the main cables.

The cables run the entire length of The cables run the entire length of the suspension bridge and are the suspension bridge and are anchored in the concrete abutments anchored in the concrete abutments at each end.at each end. Smaller cables are suspended from Smaller cables are suspended from

the main cables and these cables the main cables and these cables support the roadway.support the roadway.

Almost all suspension bridges Almost all suspension bridges have, in addition to the cables, a have, in addition to the cables, a supporting truss system beneath supporting truss system beneath the bridge deck (This helps to the bridge deck (This helps to stiffen the deck and reduce the stiffen the deck and reduce the tendency of the roadway to sway tendency of the roadway to sway and ripple. and ripple.

Suspension BridgesSuspension Bridges

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

They are ideal for covering They are ideal for covering busy waterways.busy waterways.

This type of bridge is the This type of bridge is the only practical type suitable only practical type suitable for very long spans or when for very long spans or when it would be hazardous to it would be hazardous to maritime traffic to add maritime traffic to add central supports.central supports.

Cable-Stayed BridgesCable-Stayed Bridges Similar to the suspension bridge Similar to the suspension bridge

but the difference lies in how the but the difference lies in how the cables are connected to the cables are connected to the towers towers

This bridge supports the This bridge supports the roadway by cables that run from roadway by cables that run from towers to the actual roadwaytowers to the actual roadway

Cable-stayed require less cable, Cable-stayed require less cable, can be constructed out of can be constructed out of identical pre-cast concrete identical pre-cast concrete sections, and are faster to build.sections, and are faster to build.

Cost-effective bridge that is Cost-effective bridge that is undeniably beautiful. undeniably beautiful.

A cable stayed bridge is a A cable stayed bridge is a bridge with one or more bridge with one or more pillars. pillars.

The two types of cable-stayed The two types of cable-stayed bridges are bridges are parallel parallel attachmentattachment design and design and radial radial attachmentattachment design. design.

In a In a parallel attachment parallel attachment design the cables are design the cables are attached at different heights attached at different heights along the tower and are along the tower and are parallel to one another parallel to one another

in a in a radial attachment radial attachment design design the cables are attached at a the cables are attached at a single point at the top of the single point at the top of the tower and on several places tower and on several places on the road.on the road.

Parallel Attachment Design

Radial Attachment Design

Cable-Stayed Bridges

Cantilever BridgesCantilever Bridges A structure or beam that is unsupported at one end A structure or beam that is unsupported at one end

but supported at the other, like diving boards but supported at the other, like diving boards A cantilever does not reach all the way across the A cantilever does not reach all the way across the

bridge so they are connected in the middle by a part bridge so they are connected in the middle by a part called the suspension spancalled the suspension span

The suspended span may be built off-site and lifted The suspended span may be built off-site and lifted into place, or constructed in place using special into place, or constructed in place using special traveling supports. traveling supports.

Cantilever BridgeCantilever Bridge

Forth Railway Bridge Forth Railway Bridge The Forth Railway Bridge (or The Forth Railway Bridge (or

Firth of Forth Bridge) is made Firth of Forth Bridge) is made of a pair of cantilever arms that of a pair of cantilever arms that extend out from two main extend out from two main towers.towers.

The beams are supported by The beams are supported by diagonal steel tubes projecting diagonal steel tubes projecting from the top and bottom of the from the top and bottom of the towers. towers.

These spans support a central These spans support a central suspended span. Some suspended span. Some structural members of the structural members of the bridge are as large as 12 feet in bridge are as large as 12 feet in diameter.diameter.

Cantilever BridgesCantilever Bridges

Movable BridgesMovable Bridges•This bridge is usually used to span canals and rivers that carry heavy boat traffic

•Usually powered by electric motors

•In the past they were powered by steam engines

•original movable bridges were the drawbridges of medieval times, raised by ropes and pulleys

• There are three main types:

1.Bascule

2.Vertical lift

3. Swing

Bascule Bridge or DrawbridgeBascule Bridge or Drawbridge

•Used for short distances

•Have two movable spans the rise upward, opening in the middle

•When open the weight is supported by the stationary section of the bridge

Vertical-lift BridgeVertical-lift Bridge

Used for longer distancesUsed for longer distances Straight bridge, held between two towersStraight bridge, held between two towers Lifted by steel ropes, attached to counterweightsLifted by steel ropes, attached to counterweights

-as the counterweights go -as the counterweights go downdown the bridge goes the bridge goes up up and vise-versaand vise-versa..

Operate in an elevator like fashionOperate in an elevator like fashion

Swing BridgesSwing Bridges

Mounted on a central pierMounted on a central pier

The central pier allows the bridge to rotate to The central pier allows the bridge to rotate to the sidethe side

Uncommonly used because the central pier is Uncommonly used because the central pier is located in the area where boats like to travellocated in the area where boats like to travel

Moveable BridgesMoveable Bridges

Bascule Bridge

Lift Bridge

Swing Bridge

Understanding BridgesUnderstanding Bridges

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