Bridges

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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 and tension

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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 (more
  material 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 need mortar

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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
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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

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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 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.

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Example of how to load your bridge