Transcript Bridges and Forces - Frost Middle School

Bridges & Forces
How Forces Affect Different Types of Bridges
Forces on a Beam Bridge
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Simplest design (girder bridge)
Compression on top of the beam
Tension on bottom of beam
Middle part not much of either forces
Tension & Compression
•If add enough weight the top surface of the beam would
buckle
•The bottom would snap
•Add truss lattice to dissipate the tension and compression
•The force spreads through the truss
Forces on an Arch Bridge
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The arches allow the forces to Dissipate or transfer
(Transferring force- the spread out evenly over a greater
area)
Design allows to move stress from an area of weakness to
an area of strength
Arch bridges are able to span greater distances than beam
or suspension
Forces on an Arch Bridge
•Tension and compression are present in all bridges
•Buckling occurs when compression overcomes an object’s
ability to endure that force
•Snapping is what happens when tension surpasses an objects
ability to handle the lengthening force
Forces on a Truss Bridge
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A truss bridge is a beam bridge with a triangular
structure either above the bridge called Through
Truss or below the bridge called Deck Truss
Compression affects the top of the beam
Tension affects the bottom of the beam
A truss structure has the ability to dissipate a load
through the truss triangle’s rigid structure
Transfers the load from one point to wider area
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Forces on a Suspension Bridge
In a suspension bridge, the roadway is suspended by
cables from two tall towers
The towers support the majority of the weight as
compression pushes down on the suspension bridge’s deck
and then travels up the cables
Transfer compression to the towers
Forces on a Suspension Bridge
The towers then dissipate the
compression directly into the Earth
• The supporting cables receive the
bridge’s tension forces
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Forces on a Suspension Bridge
•The cables run horizontally between the two flung
anchorages
•Anchorages are solid rock or massive concrete blocks in
which the bridge is grounded
•Tensional force passes to the anchorages and into the ground
•Have a deck truss beneath the bridge which helps to stiffen
the deck and reduce the tendency of the roadway to sway
and ripple
•Span 2,000-7,000ft (610-2,134m)
•anchorage 
Forces on a Cable Stayed Bridge
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Cables attached from different points to a single point on
the tower
Basic design in 16th century
Europe- after WWII
Forces on a Cable Stayed Bridge
•Span– 500 – 2,800ft (152-853m)
•Lower cost than suspension bridge
•Less steel cable, faster to build, more precast concrete
sections
Forces on a Cable Stayed Bridge
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Don’t require anchorages nor do they need two towers
The cables run from the roadway up to a single tower that
alone bears the weight
It absorbs and deals with compressional forces
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Torsion
Especially in suspension bridges
Torsion occurs when strong winds cause the suspended
roadway to rotate and twist like a rolling wave
Washington’s Tacoma Narrows Bridge disaster 1940
Arch and truss bridges are protected from this force
Torsion
•Suspension bridge engineers use deck truss to protect the
bridge from torsion
•In long spans use aerodynamic truss structures and
diagonal suspender cables to mitigate the effects of torsion
Shear & Resonance
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Shear stress occurs when two fastened structures (or two
parts of a single structure) are forced in opposite
directions
If unchecked can rip the bridge materials in half
Shear & Resonance
•Resonance is the vibration as in
a snowball rolling down a hill
and becoming an avalanche
•Begins small and grows big
•A stimulus in harmony of natural vibration
of bridge
•Vibration can increase in the form of waves
Shear & Resonance
•Example- Tacoma Narrows Bridge in Washington,
1940
•Like singer shattering a glass
•Engineers create dampeners in the design to
interrupt the waves
•Create sections overlapping which change the
frequency of the waves and prevents waves from
building up
Tacoma Narrows Bridge
Galloping Girder