Bridges

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29 Νοε 2013 (πριν από 3 χρόνια και 4 μήνες)

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Bridges

Discover Engineering

ENGR 096


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)

Bridges


Beam bridge: spans up to 200 feet


Arch bridge: 1000 feet


Suspension bridge: 7000 feet



Difference comes from compression and
tension

Bridge Forces


Compression (squeeze force)


Too much compression (buckling)


Tension (pull force)


Too much tension (snapping)

Bridge Forces


Dissipation (spread out over greater area)


Arch bridge


Transfer (move force from area of weakness
to area of strength)


Suspension bridge



The Beam Bridge


Rigid horizontal structure resting on two piers


Weight of bridge and load supported by piers


The Beam Bridge


Usually concrete or steel beams


Taller beams can span longer distances (more
material to dissipate tension)


Tall beams are supported with a truss (adds
rigidity to existing beam)


Limited in size


Trusses

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!

Arch Bridge


Semicircular with abutments on each end


Arch diverts weight from deck to abutments


Compression: always under compression (no
tension)

Arch Bridges


Does not need additional supports or cables


Arches made of stone don’t even need
mortar


Suspension Bridge


Cables, ropes, chains suspend the deck from towers


Towers support majority of the weight


Compression


Pushes down on suspension bridge’s deck


Cables transfer compression to towers


Tension


Cables running between two anchorages under tension

Suspension Bridge

Suspension Bridge




Have supporting truss system

underneath

Suspension Bridge


A classic suspension bridge in New York City

Suspension Bridges


Two types:


Suspension (curved cables)


Cable
-
stayed (straight cables, no anchorages
required)

Cable
-
Stayed Bridge

Other Forces


Torsion (twisting force)


Eliminated in beam and arch bridges


Critical in suspension bridges


High winds


Minimized by deck
-
stiffening trusses

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 one’s 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

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

Lab


Build a bridge entirely out of uncooked spaghetti pasta and
glue. Your bridge is to span a distance of 8 inches and
withstand 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 of
bridge).


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


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

Example of how to load your bridge