Critical Thinking

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Enhancing Thinking Skills in Science
ContextLesson 6

• Critical Thinking
• in Engineering Process

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Introduction to the different types of bridges

• beam bridge
• suspension bridge
• arch bridge

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Why are there different types of bridges?

• The major difference between the three types of
  bridges is the distance that they can cover in a
  single span. Span is the difference between two
  supports. Each of the different types of bridges
  holds weight in different ways.

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Beam Bridge

• A beam bridge is basically a rigid horizontal
  structure that rest on two supports, one located
  at each end of the bridge as shown in Figure (a).
  A simple beam bridge is flat across and supported
  by two ends.

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What happens to the bridge when a load is put on
the bridge?

• When the load pushes down on the beam the top
  edge is pushed together or compressed, while the
  bottom of the beam is stretched or is under
  tension.

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What factor(s) should you consider when you build
a beam bridge?

• The beam must be strong enough so that it doesn't
  bend under its own weight and the added weight of
  the traffic crossing it.

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Arch Bridge

• An arch bridge is composed of a curved structure
  with abutments on each end.

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What happens to the bridge when a load is put on
the bridge?

• The weight of the load is carried outward along
  the curve of the arch to the abutments at each
  end of the arch. The abutments also keep the end
  of the bridge from spreading outward.

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What factor(s) should you consider when you build
an arch bridge?

• The arch bridge is always under compression
  because the weight of the deck is pushed outward
  along the curve of the arch towards the
  abutments. The rise in the form of the curved
  arch causes the vertical load to have a
  horizontal thrust.

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Suspension Bridge

• The suspension bridge literally suspends the
  roadbed from huge cables, which extends from one
  end of the bridge to the other. The cables are
  attached to two tall towers and are secured at
  each end by anchorages.

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What happens to the bridge when a load is put on
the bridge?

• The cable carries the weight on a suspended
  bridge to the anchorages that are imbedded in
  solid rock or massive concrete blocks. The cables
  are spread over a large area in order to evenly
  distribute the load inside the anchorages to
  prevent the cables from breaking free.

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What factor(s) should you consider when you build
a suspension bridge?

• The anchorages help to stabilise the bridge.
  Therefore, the tower must be embedded to earth
  firmly. In such a way, the cables transfer the
  forces to the towers which carry the forces
  directly into the earth where they are firmly
  imbedded.

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Scenario

• The class is divided into groups of 4 or 5. You
  are required to design and build a bridge made of
  drinking straws. The bridge will cross a gap of
  50cm. The width of the bridge must not be less
  than 20cm. The total mass of the bridge must not
  exceed 50 g. The strength of the bridges would
  be tested by hanging loads from the middle of the
  bridges.

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Activity 1

• Draft a design

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Activity 1

• Now, you have 10 minutes to discuss with your
  group members how to design the bridge. Draft
  the steps you come across. Also, sketch a
  picture of the bridge you intend to build.

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Activity 2

• Introduction to the steps suggested by
  Isidro-Cloudas and Cassis, Glenn in engineering
  process

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Activity 2

• The steps involved in Engineering Process are
  written on 6 cardboards. You are required to
  design a bridge. Arrange the cardboards in
  correct order. Try to answer the following
  questions.

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Activity 2

• Question
• What is the sequence of Engineering Process to
  design the bridge?

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Activity 2

• Answer
• Identify the Problem
• Determine the Constraints
• Brainstorm the preliminary design and choose the
  best design
• Analysis of Design
• Design Refinement
• Implementation Plan

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Activity 2

• Question
• What is/are the task(s) performed in each step
  of Engineering Process?

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Activity 2

• Answer
• Identify the Problem
• Before you begin working, the problem or task
  that is going to be undertaken must be known.

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Activity 2

• Answer
• Determine the Constraints
• Constraints are the limitations that must be
  considered before you begin designing your
  bridge.

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Activity 2

• Answer
• Brainstorm the preliminary design and choose the
  best design
• Once both the problem(s) or task have been
  determined and the constraints have been
  identified, the group needs to think of as many
  ways as possible to solve the problem. Even
  though all of the ideas may not be good ones,
  they may inspire another idea that may lead to a
  solution to the problem(s).
• Once all reasonable ideas are listed and the
  sketches are drawn, the group should choose the
  best two or three ideas for further development.
  The rough sketches should be converted to scaled
  or measured drawings.

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Activity 2

• Answer
• Analysis of Design
• The designs are studied based on their merit in
  relationship to strength, cost, market appeal,
  and manufacturability. A decision should be made
  at this point on which design to use or rather to
  begin a new design.

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Activity 2

• Answer
• Design Refinement
• Each design team should attempt to rectify the
  problems by making improvements in the design.

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Activity 2

• Answer
• Implementation Plan
• Once the final design has been approved, it must
  be translated from an idea on paper to the real
  thing.

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Activity 3

• Redesign the bridge

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Activity 3

• According to steps of Engineering Process, draw a
  completed design of the bridge you intend to
  build. THREE views of the bridge should be
  included. You may consider the materials
  provided and the following rubrics to draw the
  diagram

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Activity 3

• Materials provided
• 80 drinking straws, 1 sticky tape, 10 paper
  clips, 1 pair of scissors, 1 roll of strings,
  some A4 papers.

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Activity 3

• Points can be earned by the following rules.
• The drawing is neat and legible.
• The three views are labelled.
• The bridge span is at least 30 cm long.
• The roadbed is at least 5 cm wide.
• The distance between the bridge supports is at
  least 10 cm apart.

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Activity 3

• Question
• Compare the steps you drafted with the
  Engineering Process. Discuss among yourselves
  which steps you need to reconsider.

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Activity 3

• Question
• What are the constraints when you build a bridge?

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Activity 3

• Answers
• Weight, length and width of the bridge,
  materials and other requirements of the bridge
  itself.

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Activity 4

• Bridge Building Contest

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Activity 4

• Build a bridge according to your completed
  design. You may change your design if there is a
  serious problem in your design. You may refer to
  the following rubrics for the design of the
  bridge. The strength of the bridges would be
  tested by hanging loads from the middle of the
  bridges until it begins to buckle.

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Activity 4

• At the end of this activity, you are going to
  evaluate the designs of different groups and
  choose
• a. the most cost-effective design (materials
  used vs strength)
• b. the strongest bridge design

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Activity 4

• Points can be earned by the following rules.
• The team adhered to the design.
• The model is neatly done.
• The model is built with materials supplied.
• The bridge span is at least 30 cm long.
• The roadbed is at least 5 cm wide.
• The distance between the bridge supports is at
  least 10 cm apart.

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Activity 5

• Professionals judgment

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Activity 5

• An expert or professional is invited to give you
  comments so that you know which aspect of the
  bridge design needs improvements.