Grant #:
DGE058532
Bridge Builders
A comprehensive unit covering the engineering and design process of a model
bridge system.
Author(s)
Michael Starr
Subjects
Physics
Grade level
11-12
Duration
25, 60 minute periods
Rationale (How this relates to engineering and the STEMcinnnati theme)
Bridges have been essential for transportation and moving of people and goods
throughout the history of man. Means of crossing waterways have been
practiced since the dawn of civilization, as water is essential to life. The advent
of bridges allowed easy access for people on opposite sides of rivers and
waterways to conduct commerce, exchange of ideas and many other activities
that have accelerated the development of technology in the human race. The
process of working with each other to accomplish a task aimed at improving the
living of humans is the essence of engineering. By working in groups to design
the most structurally efficient, lightest and cost effective bridge the students will
be engaging in the most fundamental of ideas of the engineering profession.
Activity Summary
1. Catches
a. Day 1 – Show PBS series ‘Building Big’ episode on bridges.
b. Days 2 and 3 – Teacher will stand on a structure built of straws
c. Day 4 – Show the difference between compression and tension
using a spring.
d. Day 5 – Introduce competition.
e. Days 6 – 13 – Guest speaker to discuss design and construction of
bridges
f. Day 15 - 25 – Give the students their build materials they have
‘ordered’
2. Planned activities
a. Day 1 – Watch the PBS video and fill in the worksheet with blanks
and questions based on content of the video
b. Day 2 and 3 – Student individual activity will be to build a wireframe cube using toothpicks cut to specified dimensions and
measure how much load it can take using something like nuts as
the load (counting how many each structure holds). Next they will
build a structure with two cross beams (trusses) for added support
and test the strength again. Finally they will compete in a
competition where they build their own structure and see who has
the lightest structure that holds the most weight (I would expect the
students to max out the amount of load that can be applied, the tiebreaker is lightest structure, i.e. least amount of material). They will
also be completing the pre-test at this point which will have
questions about which members they think are in tension or
compression for a particular situation as well as advanced
questions about structures which they should not be able to answer
at this time (all the way up to calculating the load in each member
of a simple truss or triangle system).
c. Day 4 – Present students with the letter for the bridge competition.
Discuss different truss styles and what it means to be in
compression and tension. Use the simple truss example where
one structure the diagonal trusses go 90 degrees to the other
example and explain why in one situation the cross-members are in
tension and the other they are in compression. Explain why the
members in tension is the better scenario.
d. Day 5 – Students will be filling out a worksheet with the competition
rules on it with blanks at key points. Students will complete this
individually and fill in the blanks as the instructor or different
students take turns reading the rules to the class. If students are
reading the rules the instructor must help them fill in the blanks
when they come to them in the course of reading. Conduct a
question and answer session for the students on the bridge
competition rules so students are clear on the rules. Additional
rules for the lesson will include keeping a group engineering
notebook complete with technical notes, design and development
ideas as well as calculations for cost analysis of their materials (i.e.
balsa wood, doll rods and adhesive of their choice).
e. Days 6 – 13 – Try to get a structural engineering professor to come
in from a local university to discuss different types of structural
ideas and design ideas for bridges with the students. Also a good
opportunity for discussing opportunities for choosing engineering as
a profession. Students will conduct guided planning in groups of 3
where they design their bridges using the help of instructors and
software, which they will be introduced to after the discussion from
the engineering professor.
f. Day 14 – Students will be presenting their design to the class along
with the amount of materials they are requiring and a cost analysis
based on the values applied to each of the build materials.
g. Day 15 – 25 – Students will build their bridges to the designs they
have come up with.
Activity
Day 1
Catch and Activity – Video with
questions to answer
Days 2 & 3
Catch – Teacher stand on structure
built of straws
Activity and Pre-test – Students will
build the structures of a wireframe cube
and a trussed cube and answer
questions at the end
Building – Students will build their own
structure out of straws and compete
against each other
Testing their structures
Day 4
Catch – Present students with the letter
for the Bridge competition so they will
be introduced to the competition at this
point.
Pre-test:
Diagonals in tension or
compression
Tension verses compression activity
using straws – Includes worksheet
Discussion of the differences between
tension
and
compression
using
examples
Post-test: Same question on diagonals
Day 5
Catch – Pass out the rules for the
competition
Read the rules with blanked out key
items with the class having students fill
in the blanks as they go.
Question and Answer session on
competition rules
Discussion of cost analysis portion and
engineering notebook requirements
Assemble students into groups of 3
and have them work on the computer
to build trusses in the computer
program and apply loads recording
answers to questions
Days 6 -13
Catch – Dr. Miller in to talk about
bridge design and design theory
Planning – Students will begin planning
their bridges in their groups, recording
Time
60 minutes
2 minutes
40 minutes
40 minutes
38 minutes
5 minutes
5 minutes
15 minutes
30 minutes
5 minutes
1 minute
15 minutes
10 minutes
10 minutes
24 minutes
60 minutes
420 minutes
their
engineering
journals
and
performing cost analysis calculations
Day 14
Student groups will present their design
ideas to the class along with their cost
analysis and an engineering report
Day 15 – 24
Catch – Students will be given their
build materials
Building – Student groups will
assemble their bridges
Post test – Will be asked the same
questions from the activity they
performed on days 2 and 3
Day 25
Competition Day!
60 minutes
5 minutes
525 minutes
10 minutes
Depends on the competition organizers
Objectives
Upon completion of this lesson, students will be able to:
1. Identify members of a truss system as being in compression or tension
and it is possible for some members to experience no load.
2. Use trial and error for improving the design of a supportive structure using
both physical and computer models.
3. Work as a group on an engineering project and complete a bridge
structure from design conception to completion for testing.
Standards
Ohio Standard: Mathematics
Geometry and Spatial Sense:
11-12 Benchmark A: Use trigonometric relationships to verify and determine
solutions in problem situations.
Mathematical Process:
11-12 Benchmark I: Communicate mathematical ideas orally and in writing with
a clear purpose and appropriate for a specific audience.
Ohio Standard: Science
Physical Sciences:
11-12 Benchmark D: Apply principles of forces and motion to mathematically
analyze, describe and predict the net effects on objects or systems.
Scientific Inquiry:
11-12 Benchmark A: Make appropriate choices when designing and participating
in scientific investigations by using cognitive and manipulative skills when
collecting data and formulating conclusions from the data
Background knowledge
Students will have been introduced to the concept of forces involved in motion
situations as well as static situations. They will be familiar with vector
representation of forces. They will have been exposed to the concepts of tension
and compression.
Materials Required
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Worksheets for the various activities
Toothpicks
Masking tape
Scissors
3x5 index cards
Plastic cups
Graduated cylinders
Water
Drafting Kit – including T-square, 12” architectural scale, 45-45-90
Triangle, 30-60-90 Triangle, Exacto Knife, 6” protractor, erasing shield, 6”
compass, 6” divider, mini lead pointer, French curve
0.7 mm mechanical drafting pencil
Balsa wood ¼” x ¼” cross section
Poplar wood ¼” x ¼” cross section
Poplar ¼” diameter dowel rods
Poplar 1/16” diameter dowel rods
Balsa wood strips
Glue (super glue, gorilla glue, epoxy, etc… whatever the students think
about using)
Matchbox car
Activities
Present the stimulus material (activity):
Day 1:
Hand out the work sheet on the video to be shown. Show the students the
‘Building Big: Bridges’ video from the PBS series. As they watch they will fill out
the answers to the questions on the worksheet. Hand in the completed work
sheet for grading.
Days 2-3:
Students will be given straws and masking tape along with a worksheet with two
drawings on it. They are to construct the first drawing as seen below using the
masking tape and toothpicks, cut to the dimensions as illustrated on the
worksheet.
Students will then place a 3x5 note card on top of their structure and a plastic
cup on the card. This must be done near a sink and not at their desks as it will
get very wet. They will fill a graduated cylinder with water and begin slowly
pouring water in the cup, keeping track using the graduated cylinder, how much
they have poured. Once their structure falls over they are to record the amount
of water their structure held.
Next they will make a new structure using their straws with cross members on
opposite sides of the structure going in the same direction and repeat the loading
experiment. The structure should look like the following:
After loading they should see that the amount of load increased because of the
added stability of the truss system. Next they will be charged with maximizing
the load capacity by designing their own cube structure capable of carrying the
most weight. This will be a competition type thing and the student that makes the
lightest structure that carries the most weight will receive a candy bar or
something… This whole activity should take only two days. They will have the
competition during the second half of the second day. After this activity they will
be given the pre-test. This happens before the only discussion they have heard
concerning different bridge structures and loading has been in the video they
watched. The instructor has not yet talked about truss systems and how loads
are distributed.
Day 4:
Present the letter to the students for the bridge competition. Discuss the truss
style to reinforce the idea of compression and tension in truss systems. Show
the example of the truss diagonals where one is in compression and the other is
in tension and get them to infer that the member in tension is better than the
member in compression. Perhaps an activity is in order here. One where they
test the strength of a straw in compression verses its strength in tension (this part
is up to the instructor). A simple hand pressure experiment will show them that in
compression the member tend to buckle whereas in tension it takes a lot more
stress to do anything. Ask them how they could improve the members in
compression. See the figure for what to do on this. Use these figures to show
how load in truss systems is distributed either in compression or tension and
further discuss how tension and compression can change as the load moves
along the bridge.
Load
Diagonals are in compression
Load
Diagonals are in tension
So now the students will have an idea of where to start with truss systems. Next
discuss the necessity of a fixed node to one end and a horizontal or vertical
rolling node on the other to allow for flexing of the structure and even distribution
of force throughout the fixture by allowing it to flex. Ask if they know how this will
be achieved in the competition (i.e. neither end will be fixed and can flex
horizontally if need be).
Day 5
Present the competition rules with blanked out words to the students and read
through it with them having them fill in the blanks as they read it. Discuss the
rules in more detail clarifying points of confusion and having an answer question
session with the students regarding the rules. Discuss the additional aspects
required for the physics class involving an engineering notebook complete with
technical notes, design and development journal, structural analysis, and cost
analysis. Explain how much the balsa wood will cost and other types of wood
that can be used and strategies such as laminating and types of glue used (pass
out the handout with material costs). Assign cost values to each item, types of
wood, types of glue, etc. Have computers on this day and show them the link to
the truss designing software. Take the rest of the day and probably into the next
on how to use the software to design 5 examples of truss systems as covered in
the discussion from the previous day (see the handout). Then have them apply
50 N load to the middle of the bottom part of the structure and see what happens.
Answer questions about members in tension and in compression and figure out
how they could improve the design. Could throw some examples in where
members experience no load and have them realize that they could cheapen the
cost of the bridge and lighten it by removing that member without compromising
the structure. This will probably spill into day 6 where you will let them play with
the software on their own and start designing their own bridges and testing them
in the software environment.
Days 6-13:
Try to get a structural engineering professor to come in from a local university to
discuss different types of structural ideas and design ideas for bridges with the
students. Also a good opportunity for discussing opportunities for choosing
engineering as a profession. Students will conduct guided planning in groups of
3 where they design their bridges using the help of instructors and software,
which they will be introduced to after the discussion from the engineering
professor.
Day 14
Students present their designs to the class in terms of what type of materials they
are using, type of structure and how they are going to put it all together. Will also
present why they have chosen the design they have chosen based on their own
research in the classroom. Their presentations should include some drawings
and a cost analysis sheet that they hand in to the teacher.
Days 15 – 25:
Start the build process. Students will be given their needed amount of materials
and begin to build and keep a log of the building process. What appears to be
working and what is causing trouble? Will also have to keep track of actual
amount of material used i.e. if cut some away and do not use the cut piece have
to count it as an additional cost. Also screw-ups will be counted so they can see
how close they are to their initial budget proposal. Build all the way to the
competition then hand in their engineering notebooks and journals for evaluation.
Place high at the competition!
Eliciting the desired behavior/review:
Performing the activities should provide an understanding in students as to how
triangles can be used to measure the size of objects.
Assessment of Student Learning
Assessment will be based on scores on pre-assessment as compared to post
assessment
Assessment of the Activity
Distribution of the STEP activity assessment form to the students and to the
teacher.