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Bridges

Discover Engineering

ENGR 096


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Bridges

Three main types of bridges:

Beam bridge Arch bridge

Suspension bridge

Difference between the three is the distance

crossed in single span

Span: distance between two bridge supports

(columns, towers, wall of canyon)


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Bridges

Beam bridge: spans up to 200 feet

Arch bridge: 1000 feet Suspension bridge: 7000 feet

Difference comes from compression andtension


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Bridge Forces

Compression (squeeze force)

Too much compression (buckling) Tension (pull force)

Too much tension (snapping)


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Bridge Forces

Dissipation (spread out over greater area)

Arch bridge Transfer (move force from area of weakness

to area of strength)

Suspension bridge


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The Beam Bridge

Rigid horizontal structure resting on two piers

Weight of bridge and load supported by piers


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The Beam Bridge

Usually concrete or steel beams

Taller beams can span longer distances (morematerial to dissipate tension)

Tall beams are supported with a truss (adds

rigidity to existing beam)

Limited in size


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Trusses


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I-Beam

Top of beam experiences most compression

Bottom of beam experiences most tension Middle of beam experiences very little

compression or tension

Best design is beam with more material on

top and bottom than the middle (I-beams)

Works for trusses too!


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Arch Bridge

Semicircular with abutments on each end

Arch diverts weight from deck to abutments Compression: always under compression (no

tension)


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Arch Bridges

Does not need additional supports or cables

Arches made of stone dont even needmortar


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Suspension Bridge

Cables, ropes, chains suspend the deck from towers

Towers support majority of the weight

Compression

Pushes down on suspension bridges deck

Cables transfer compression to towers

Tension

Cables running between two anchorages under tension


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Suspension Bridge


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Suspension Bridge

Have supporting truss system

underneath


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Suspension Bridge

A classic suspension bridge in New York City


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Suspension Bridges

Two types:

Suspension (curved cables) Cable-stayed (straight cables, no anchorages

required)


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Cable-Stayed Bridge


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Other Forces

Torsion (twisting force)

Eliminated in beam and arch bridges Critical in suspension bridges

High winds

Minimized by deck-stiffening trusses


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Resonance

A vibration in something caused by external force

that is in harmony with natural vibration

Similar to making constant waves in a swimming pool or

maintaining ones oscillation on a swing

Check out what resonance did to this bridge in Washington

state back in 1940 (YouTube Tacome Narrows Bridge link)

Dampeners: Designed to interrupt resonant waves

Overlapping plates create friction to offset frequency of

waves
http://www.youtube.com/watch?v=3mclp9QmCGshttp://www.youtube.com/watch?v=3mclp9QmCGs


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Weather

Hardest to combat

Rain, ice, wind, and salt can bring a bridge down Design progression: iron replaced wood, steel

replaced iron

Each new design addresses some past failure

Preventative maintenance


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Lab

Build a bridge entirely out of uncooked spaghetti pasta andglue. Your bridge is to span a distance of 8 inches andwithstand the most amount of weight as possible

Record the weight of your bridge. Place your bridge on two piers spaced 8 inches apart and find

the maximum load that your bridge can support

Record the final weight that your bridge was able to support.Find your load to weight ratio (Load divided by weight ofbridge).

Turn in your ratio and a photo/video of your bridge in action tothe Discussion Board by Thursday, November 13.

Remember to use knowledge learned from the lecture. Beamand suspension bridges work the best for this project. Hint: usea truss system.


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Example of how to load your bridge

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