FES Bridges Lecture

8/8/2019 FES Bridges Lecture

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BRIDGES BRIDGES

Maria F. Parra

February 7th 2009Revised by Florida Engineering Society, February 2009

SECME M-DCPS Division of Mathematics and Science Education


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H istory of Bridge Development

How Bridges Work

Basic Concepts

Types of Bridges

Concepts Associated withBridge Engineering

Truss Analysis

Tips for Building Bridges

Bridge Construction

Work PlanWork Plan


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700 A.D . Asia700 A.D . Asia

100 B .C. R omans100 B .C. R omans

Natural B ridgesNatural B ridges

Clapper Bridge

T ree trunk Stone

T he Arch Natural Cement

Rom an Arch Bridge

H istory of B ridge D evelopment H istory of B ridge D evelopment

Great St o ne Bridge in China

Low Bridge Shallow Arch

1300

A.D

.R

enaissance1300

A.D

.R

enaissance

Strength of Materials MathematicalT heories Development of Metal


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First Cast-Iron BridgeCoalbrookdale, England

18 00 A.D .18 00 A.D .

H istory of B ridge D evelopment H istory of B ridge D evelopment

Britannia T ubular Bridge

185 0 A.D .185 0 A.D .

Wrought Iron

T russ Bridges

Mechanics of Design

Suspension Bridges Use of Steel for the suspendingcables

19 00 A.D .19 00 A.D .

192 0 A.D .192 0 A.D .

PrestressedConcrete Steel

2 000 A.D .2 000 A.D .


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Every passing vehicle shakes the bridge up anddown, making waves that can travel athundreds of kilometers per hour. Luckily thebridge is designed to damp them out, just as it

is designed to ignore the efforts of the wind toturn it into a giant harp. A bridge is not a deadmass of metal and concrete: it has a life of itsown, and understanding its movements is asimportant as understanding the static forces.

H ow B ridges Work? H ow B ridges Work?


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Com pressi o n Tensi o n

B asic ConceptsB asic Concepts

Span - the distance between two bridgesupports, whether they are columns, towersor the wall of a canyon.

C om pressi o n - a force which acts tocompress or shorten the thing it is actingon.

Tensi o n - a force which acts to expand or lengthen the thing it is acting on.

Fo rce - any action that tends to maintain or alter the position of a structure


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Bea m - a rigid, usually horizontal, structural element

P ier - a vertical supporting structure, such as a pillar

Cantilever - a projecting structure supported only at one end,like a shelf bracket or a diving board

Beam

Pier

Lo ad - weight distribution throughout a structure


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Truss - a rigid frame composed of short, straight pieces joinedto form a series of triangles or other stable shapes

Stable - (adj.) ability to resist collapse and deformation;stability (n.) characteristic of a structure that is able to carry a

realistic load without collapsing or deforming significantly

Def o r m - to change shape


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To dissipate forces is to spread them out over a greater area,

so that no one spot has to bear the brunt of the concentratedforce.

To transfer forces is to move the forces from an area of weakness to an area of strength, an area designed to handlethe forces.


Buckling is what happens when the force of compression overcomes an object's ability tohandle compression. A mode of failurecharacterized generally by an unstablelateral deflection due to compressive actionon the structural element involved.

Snapping is what happens when tension overcomes anobject's ability to handle tension.


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T he type of bridge used depends on various features of the

obstacle. T he main feature that controls the bridge type is thesize of the obstacle. H ow far is it from one side to the other?T his is a major factor in determining what type of bridge to use.

T he biggest difference between the three is the distances theycan each cross in a single span.

T ypes of B ridgesT ypes of B ridgesBasic Types :

Beam Bridge Arch BridgeSuspension Bridge


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T ypes of B ridgesT ypes of B ridges

Beam

Bridge

Beam

Bridge

Consists of a horizontal beam supported at each end by piers.T he weight of the beam pushes straight down on the piers. T hefarther apart its piers, the weaker the beam becomes. T his iswhy beam bridges rarely span more than 250 feet.


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Fo rces

When something pushes down on the beam, the beambends. Its top edge is pushed together, and its bottomedge is pulled apart.


Beam

Bridge

Beam

Bridge


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T russ B ridgeT russ B ridge

Fo rces

Every bar in this cantilever bridge experiences either apushing or pulling force. T he bars rarely bend. T his is whycantilever bridges can span farther than beam bridges



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ArchB

ridges ArchB

ridgesT he arch has great natural strength. T housands of years ago,Romans built arches out of stone. Today, most arch bridgesare made of steel or concrete, and they can span up to 800feet.



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S uspension

Bridges

S uspension

Bridges

T his kind of bridges can span 2,000 to 7,000 feet -- way farther than any other type of bridge! Most suspension bridges have atruss system beneath the roadway to resist bending and

twisting.



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Fo rces

In all suspension bridges, the roadway hangs from massivesteel cables, which are draped over two towers and securedinto solid concrete blocks, called anchorages, on both ends of the bridge. T he cars push down on the roadway, but becausethe roadway is suspended, the cables transfer the load intocompression in the two towers. T he two towers support most of the bridge's weight.


S uspension

Bridges

S uspension

Bridges


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T he cable-stayed bridge, like the suspension bridge, supportsthe roadway with massive steel cables, but in a different way.T he cables run directly from the roadway up to a tower, forminga unique "A" shape.

Cable-stayed bridges are becoming the most popular bridgesfor medium-length spans (between 500 and 3,000 feet).


CableCable- - S tayed B ridgeS tayed B ridge


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To design a bridge like you need to take into account themany forces acting on it :

The pull of the earth on every part

The ground pushing up the supports

The resistance of the ground to the pull of the cables

The weight of every vehicle

Then there is the drag and lift produced by the wind

The turbulence as the air rushes past the towers

B asic math and science conceptsB asic math and science concepts

B ridge E ngineering B ridge E ngineering


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T ips for building a bridgeT ips for building a bridge

1 . Commitment - Dedication and attention to details. Be sure youunderstand the event rules before designing your prototype.

1 ) Draw your preliminary design

2) ALL joints should have absolutely flush surfaces beforeapplying glue.

G lue is not a "gap filler", it dooms the structure!

3) Structures are symmetric.

4) Most competitions require these structures to be weighed. Upto 20% of the structure's mass may be from over gluing.


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S tresses flow like water.

Where members come together there are stressconcentrations that can destroy your structure.

Here is a connection detail of one of the spaghettibridges.

T he I mportance of ConnectionsT he I mportance of Connections


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T acoma Narrows FailureT acoma Narrows Failure

O n November 7, 1 940,at approximately 11 :00

AM, the first TacomaNarrows suspension

bridge collapsed dueto wind-inducedvibrations. Situated onthe Tacoma Narrowsin Puget Sound, near the city of Tacoma,Washington, thebridge had only beenopen for traffic a few

months.

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FES Bridges Lecture