Identify and describe the forces that act on very large bridges'

1
Objectives

• Identify and describe the forces that act on very
  large bridges.
• Explain how truss bridges counteract the forces
  acting on structures.
• Describe the three major types of bridges.
• Calculate the efficiency of a structure.
• Create a structural model, test a design, and
  optimize a design.
• Maintain a journal for an engineering-design
  project.
• Contribute to a group endeavor by offering useful
  ideas, supporting the efforts of others, and
  focusing on the task.

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The Big Idea

• Construction is the systematic process of
  erecting structures to meet human needs and
  desires. It reflects cultural norms,
  environmental conditions, and the requirements of
  enterprises and institutions.

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Purpose of Lesson

• To familiarize students with bridge design and
  construction, including an understanding of the
  forces acting on structures.

4
Engagement

• Everyone has seen a bridge, and its almost as
  likely that youve traveled over one today.
• If youve ever laid a plank or log down over a
  stream to keep from getting wet, youve even
  constructed a bridge.
• Bridges are a natural part of everyday life.
• A bridge provides passage over some sort of
  obstacle a river, a valley, a road, railroad
  tracks, etc.
• Think about the longest and highest bridge you
  have ever crossed.

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Engagement

• The highest bridge in the world can be found in
  the Ladakh valley between the Dras and Suru
  rivers in the Himalayan mountains.
• The valley lies at an altitude of about
  5,602meters (m) (18,379 feet ft) above sea
  level on the India side of Kashmir.
• Called a Bailey Bridge, it is only 30 m (98 ft)
  long, and was built by the Indian Army in August
  1982.

Example of a Bailey Bridge being installed
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Engagement

• The bridge that stands highest over water, is the
  Royal Gorge Bridge over the Arkansas River in
  Colorado.
• Built in 1929 for 350,000, it spans 321 m and is
  1,053 ft above the water.

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Engagement

• The largest bridge in the world is the 13.27
  kilometers (km) (8.25 miles) long Trans Bay
• Bridge, which links San Francisco to Oakland. It
  was built in 1936 at a cost of 77 million.

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Engagement

• The longest bridge in the world is the
  Pontchartrain bridge in New Orleans, with a total
  length of 38.6 km (24 miles). It was completed in
  1956.

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Engagement

• The most expensive bridge is the
  Seto-Ohashi-Kojima bridge in Japan. At 13.22 km
  (8.21miles) long, it was built in 1988 at a cost
  of 8.3 billion.

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Engagement

• The worlds largest natural bridge is the Rainbow
  Bridge, tucked away among the rugged, isolated
  canyons at the base of Navajo Mountain, Utah.
• It is a natural wonder. From its base to the top
  of the arch, it reaches 88.4 m (290 ft)nearly
  the height of the Statue of Libertyand spans
  83.8 m (275 ft) across the river. The top of the
  arch is 12.8 m (42 ft) thick and 10 m (33 ft)
  wide.

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Exploration

• Identify the forces that might act on very large
  bridges.

http//www.pbs.org/wgbh/buildingbig/
You have 10 minutes to find this information.
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Exploration

• Who can lift this shoe in the air only by
  pulling down on the string?

• This basic suspension bridge design can be
  applied using other materials to build larger,
  stronger bridges.

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Exploration

• With the materials provided, design and construct
  the suspension bridge seen below.

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Exploration

• Experiment with attaching the cables from the
  bridge deck only to the tops of the towers,
  instead of extending them back down to the
  surface at the ends of the bridge.

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Exploration

• Experiment with attaching the cables from the
  bridge deck only to the tops of the towers,
  instead of extending them back down to the
  surface at the ends of the bridge.

How strong the bridge is this way and why.
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Exploration

• Adding the cables to the straw bridge and
  anchoring the cables on both sides significantly
  increases the load that the bridge can support.
• A suspension bridges cables and towers transmit
  the dead load of the bridge deck and the live
  load of traffic to the massive anchor blocks at
  each end of the bridge.
• The tension in the cables leading up from the
  bridge deck is balanced by the tension in the
  cables leading to the anchor blocks, as well as
  the compression in the towers.
• The anchor blocks must be massive enough to
  resist the tension in the cables caused by the
  weight of the bridge deck.

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Explanation

• There are three major types of bridges (beam
  bridges, arch bridges, and suspension bridges).
• The biggest difference between the three is the
  distances they can cross in a single span.
• A span is the distance between two bridge
  supports, whether they are columns, towers, or
  the wall of a canyon.
• A modern beam bridge, for instance, is likely to
  span a distance of up to 200 feet (60 meters),
  while a modern arch can safely span up to 800 or
  1,000 ft (240 to 300 m).
• A suspension bridge, the pinnacle of bridge
  technology, is capable of spanning up to 7,000 ft
  (2,100 m).

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Explanation
What allows an arch bridge to span greater
distances than a beam bridge, or a suspension
bridge to span a distance seven times that of an
arch bridge?
Each bridge type deals with two important forces
called compression and tension.
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Explanation

• Compression is a force that acts to compress or
  shorten the thing it is acting on.
• Tension is a force that acts to expand or
  lengthen the thing it is acting on.

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Explanation

• When we press down, or push the two ends of the
  spring together, we compress it.
• The force of compression shortens the spring.
• When we pull up, or pull apart the two ends, we
  create tension in the spring.
• The force of tension lengthens the spring.
• Compression and tension are present in all
  bridges, and its the job of the bridge design to
  handle these forces without buckling or snapping.
• Buckling is what happens when the force of
  compression overcomes an objects ability to
  handle compression, and snapping is what happens
  when the force of tension overcomes an objects
  ability to handle tension.

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Explanation

• The best way to deal with these forces is to
  either dissipate them or transfer them.
• To dissipate force is to spread it out over a
  greater area so that no one spot has to bear the
  brunt of the concentrated force.
• To transfer force is to move it from an area of
  weakness to an area of strengthan area designed
  to handle the force.
• An arch bridge is a good example of dissipation,
  while a suspension bridge is a good example of
  transference.
• Bridge efficiency is determined by dividing the
  load the bridge can withstand by the weight of
  the bridge structure.

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Explanation

• A single beam spanning any distance experiences
  compression and tension.
• The very top of the beam experiences the most
  compression, and the very bottom of the beam
  experiences the most tension.
• The middle of the beam experiences very little
  compression or tension.
• If the beam were designed so that there was more
  material on the top and bottom, and less in the
  middle, it would be better able to handle the
  forces of compression and tension. (For this
  reason, I-beams are more rigid than simple
  rectangular beams.)

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Explanation

• A truss system takes this concept one step
  further. Think of one side of a truss bridge as a
  single beam. The center of the beam is made up of
  the diagonal members of the truss, while the top
  and bottom of the truss represent the top and
  bottom of the beam.
• Looking at a truss in this way, we can see that
  the top and bottom of the beam contain more
  material than its center (corrugated cardboard is
  very stiff for this reason).

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Explanation

• Another reason why a truss is more rigid than a
  single beam A truss has the ability to dissipate
  a load through the truss work.
• The design of a truss, which is usually a variant
  of a triangle, creates both a very rigid
  structure and one that transfers the load from a
  single point to a considerably wider area.

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The RULES...
1. Materials used in the construction of the
bridge shall consist only of commercially
available rectangular 1/8 x 1/8 balsa stock, 5
cm wide 30 cm long piece of construction paper
(for road bed) and white or wood glue. 2. The
total mass of the bridge plus glue must not
exceed 30.0 g. 3. The bridge shall contain no
element wider than .65 cm (1/4) nor thicker than
0.65 cm (1/4 "). Two or more elements may be
laminated together to construct members. 4. The
bridge shall allow a 5.0 cm x 5.0 cm x 40 cm
stick to pass through without touching the
structure. 5. The bridge shall be "free
standing". 6. An approximately level, smooth
roadway surface 2cm above the bottom of the
bridge that is at least 30.0 cm in length, shall
be provided, across which a small metal car (e.g.
the black car demonstrated) will roll when given
a single light push of the hand. This roadway
shall have a minimum width of 5.0 cm and shall
allow a 5.0 cm x 5.0 cm x 40 cm stick to pass
freely along its extent. Note the roadway
materials must conform to rule (3). 7. No
fastening mechanism except mechanical interlock
of the balsa pieces or commercial glue is
permitted.
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For Your Consideration
2 cm
30 cm

• A bridge has two sides
• A bridge has a road deck
• A bridge has a top keeping the sides apart

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Extension

• Working in groups create a structural model, test
  the design, and optimize the design of a truss
  bridge.
• http//bridgecontest.usma.edu/
• Maintain a journal documenting the design process.