Bridge Design Challenge
Year 9 Industrial Technology – Engineering
Turramurra High School
© Commonwealth of Australia 2009
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Bridge Design Challenge – Unit Summary
Week
Week 1
25 May
Unit 4: Towers and Design Challenge 2 (7 weeks)
Activities
 Select teams for Design Challenge 2 and discuss the
rules for the design challenge.
 Introduce unit of work and the concept of civil engineering.
 Introduce basic theory concerning civil structure.
 Compare bridges to other civil structures: buildings, roads, dams,
tunnels, towers.
Registration
 Describe scope of work carried out by a civil engineer.
 Select teams and discuss the rules for the design challenge.
 ICT: Planning. Use of Microsoft OneNote to help organise and
plan the project and final presentation report.
 Briefly revise link between force and stress.
 Introduce concept of “Factor of Safety”.
 Revise nature of forces – tensile, compressive, shear, torsion, live and
dead loadings:
 Watch Video “Structures”.
 Complete worksheet 30 from Unit 10 of Introducing Technology .
 Briefly discuss properties of common materials used in civil structure:
 Steel: cables, beams.
 Concrete: reinforced.
 Contrast the effect tension and compressive forces have on size and
bracing of members, columns and beams.
Week 2
 Introduce freehand sketching technique.
1 June
 Students brainstorm idea.
 Teams commence preliminary sketches for Tower
Design.
 Teacher reviews preliminary sketches.
© Commonwealth of Australia 2009
 Investigate alternate designs for common bridges design problems,
e.g. beam, arch, trussed, suspension and cable stay bridges.
 ICT: Research. Visit this web site and complete associated
exercises http://www.pbs.org/wgbh/buildingbig/bridge/index.html
 Students produce free hand sketches of preliminary designs.
 Students evaluate preliminary designs using a PMI chart.
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Week
Week 3
Unit 4: Towers and Design Challenge 2 (7 weeks)
Activities
 Finalise designs in preparation for construction.
 ICT: Research. Search the web for free truss analysis programs
such as West Point Bridge design Contest or go to
http://www.jhu.edu/virtlab/bridge/truss.htm
8 June
Registration
 ICT: Detail Drawings. Completed using CAD (either TurboCAD or
SketchUp).
Week 4
15 June
 Construction begins on designs for Design Challenge 2.
 Construction begins in the workshop. Stress safety.
Week 5
22 June
 Continue construction.
 Construction continues.
Week 6
29 June
 Carry out Design Challenge 2.
 ICT: Revision. Visit http://www.pbs.org/wgbh/nova/bridge/ and
complete the bridge challenge from this web site.
 Testing of bridge designs as per challenge rules.
Week 7
6 July
 Team Evaluations.
© Commonwealth of Australia 2009
 ICT: Presentation. Teams present their final evaluation using
PowerPoint (basic) or Presenter (extension) incorporating digital
photography and/or video.
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The challenge
Design a trussed bridge that can support a load of 45 N (4.5 kg) when it is placed on
the top of the truss.
The best design will be the one with best specific strength (strength to weight ration)
at this load.
Engineers are not always interested in designing the strongest bridge. They are
more interested in designing the best bridge to do the job.
An optimist says the glass is half full; the pessimist says the glass is
half empty; the engineer says the glass is too big.
A second challenge requires the bridge to be tested to destruction in order to
determine its overall specific strength.
Restrictions
 The bridge is to be made from 3 mm x 3 mm balsa wood.
 It is to be a truss design with no members doubled up or reinforced along their
entire length.
 Thin cardboard (e.g. the type of cardboard used for manila folders) can be used to
make gussets to reinforce the joints of the truss.
 The overall sizes of the truss are:
 maximum length: 200 mm (must be able to span 160 mm)
 maximum height: 100 mm
 maximum width: 100 mm
 minimum flat area on top of truss: 100 mm x 100 mm or equivalent area.
 There must be a clear passage way through the middle of the truss to allow a
70 mm x 70 mm piece of timber to pass through. This represents the space
required for the traffic to pass through.
 You will be given 4 mm x 900 mm lengths of balsa wood. This is the maximum
amount of material you may use.
© Commonwealth of
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Some useful information
Balsa wood strength
The compressive strength of balsa wood is approximately 15.5 MPa.
Formula:
Stress = Force / Area
or
Force = Stress x Area
Data:
Stress = 15.5 MPa
Area = 3 x 3 = 9 mm2
Therefore:
Force = 15.5 x 9 = 139.5 N
This is the theoretical maximum compressive force that can be applied to any one
member (about 14 kg).
When in tension balsa wood is stronger, but mainly because it doesn’t bend and
buckle under the tensile load, so members in tension can exceed this loading value.
Factor of safety
All civil structures are designed with a built-in factor of safety. This allows us to be
confident that our design will work and not collapse and injure people due to bad
workmanship, mistakes in design calculations and modifications to the original
design.
It is suggested you also build in a factor of safety for the same reasons, but
remember that the more conservative you are, the heavier your bridge will be, and
the lower your marks. On the other hand, if your bridge fails, the maximum mark you
can receive for the first part of the challenge is only 30 out of 60.
Designs where the members are more upright will use more material but the force in
each member will be less. By laying the members down, you will save material, but
possibly over-load the member causing it to break.
As well as being squashed and stretched, the two trusses may tilt over. You will
need to consider bracing them to prevent this.
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Possible approach to design and construct a truss
Step 1: Team allocation
Most engineers work in teams, with each team member given special duties
depending upon their special skills. There are lots of advantages of working in a
team but there are also some disadvantages.
Can you think of two advantages and two disadvantages of working in a team?
List them below:
Advantages
1. ________________________________________________________________________
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2. ________________________________________________________________________
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Disadvantages
1. ________________________________________________________________________
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2. ________________________________________________________________________
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It is time to establish the team members for this design challenge. For this exercise
you will be allocated to a team randomly. Teams will generally comprise two or three
members. Teams need to think about the role of each team member and how to
overcome any disadvantages there may be working in a team.
Step 2: Project Planning
Large projects, such as running the Olympics, require specialised teams of project
managers to make sure that everything is completed on schedule. Imagine how
embarrassing it would be if the Olympic stadium was not completed in time for the
opening ceremony (this happened in 1976 in Montreal!).
Many projects have milestones—dates that cannot be changed. For students,
usually the due date of an assignment can’t be changed, so this is an example of a
milestone. Have you ever stayed up late the night before an assignment is due to
finish it off? To help prevent a last minute rush you will be required to show you
have planned your time by setting mini milestones along the way.
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Use the Unit Summary at the start of this booklet as a guide.
Activity
Using Microsoft OneNote, Excel or Word, produce a time/action plan for the
seven-week project.
The Time/Action plan will list:
 tasks to be completed (these are the ‘actions’)
 who is going to be responsible for each action (others can help but one person
needs to be responsible for each action)
 the length of time it will take to complete each action (remember to set milestone
dates).
Step 3: Preliminary Designs
To be successful you need at least some understanding of forces, materials and
basic bridge design. Visit this website:
http://www.pbs.org/wgbh/buildingbig/bridge/index.html
and complete the associated exercises from your workbook.
Sketch some possible truss designs. Try to work out whether a member will be in
compression or tension. Members that are placed in compression will have to be
shorter than those placed in tension or will need to be braced part of the way along
to stop them from bending under the weight. Keep in mind at all times the
restrictions set out in the design challenge. You could be disqualified or lose marks if
you don’t obey the restrictions.
Step 4: Evaluation of preliminary designs plus an extension activity for the
mathematically-minded
If you wanted to get very technical you can now start to work out the force in each
member of the truss using the Method of Joints truss analysis.
Work out how much force is going to be placed in each of the top joints. Designs
based on a trapezium will have the weight shared along the top joints of the truss.
Next, work out the reactions on the left and right-hand sides of the truss. This
shouldn’t be too hard if your truss is symmetrical.
Starting at one of the more simple joints (probably at one of the reactions) carry out
a vector addition by placing the forces in each of the members tip to tail. Since the
truss is in equilibrium then the addition of the forces should add up to zero. That is,
where you start should be where you finish.
There are a number of free web-sites that help you build a bridge and carry out truss
analysis. Try trusss analysis programs such as West Point Bridge design Contest or
go to http://www.jhu.edu/virtlab/bridge/truss.htm
© Commonwealth of
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Evaluate your preliminary designs to decide on a final plan. Using a PMI (Plus,
Minus, Interesting) chart is a good way of evaluating preliminary designs.
Set up a chart or table like this one for recording: idea, plus, minus, interesting:
idea
plus (+)
minus (-)
interesting (i)
Apply three questions to the idea (the ‘idea’ can be a concept, issue, proposal,
suggestion or statement):
 what is positive about this? (plus)
 what is negative about this? (minus)
 what is interesting about this? (interesting)
Deal with each question one at time. You might want to allocate a time limit for each
question.
More pluses than minuses could indicate that the idea should receive further
attention. More minuses could indicate that the idea should not receive further
attention. If there are many ‘interesting’ points, this could indicate that further
research is needed before deciding whether to keep the idea or reject it.
Step 5: Detail plans
Draw up your design accurately full size on an A4 piece of paper. If you use
TurboCAD it will be easier to calculate the actual sizes because the program has a
function that enables you to accurately measure what you have drawn.
Start by drawing up envelopes or boxes for the front view, top and side views. This
will allow you to check the overall dimension to see that they compile with the
regulations of the challenge. Add in all the members.
Step 6: Construction
Using the full-size drawing as a guide cut out the members from balsa wood.
Glue the members of the two trusses together.
A simple method is to photocopy your full-size drawing twice and use it as a
template for your cutting. Then you can simply lay the cut pieces of balsa wood onto
the drawing when gluing the pieces together. You will need the other photocopy for
the second truss.
The joints of the truss may be reinforced with small paper or cardboard gussets. At
school you will be using a hot glue gun to join your truss together but you can also
use 5-minute araldite or liquid nails to join each member of the truss. Araldite is a
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strong two-part adhesive that has good gap-filling properties. Liquid Nails comes in
a caulking gun and also has good strength and gap-filling properties. PVA glue is
another good glue to use but it takes a fair while to dry, is not quite as strong as
araldite and won’t fill up all the gaps as well.
Note: glue should not be placed along the full length of any truss member.
Step 7: Construction continues
Once you have two identical trusses you will need to join them together. Think of
how long your joining pieces need to be and where you are going to put them to
comply with the width requirements.
There is a very real possibility that the trusses may fall over. Bracing may be
required to prevent this, but remember that a 70 mm x 70 mm piece of timber still
has to be able to pass through the middle.
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Performance scale
Design, construction and testing: 80%
Part A
You will receive a mark between 40 and 60 if your truss does manage to support the
load.
 The lightest bridge that can support the load will receive a mark of 60.
 The heaviest bridge will receive a mark of 40.
 A sliding scale will be used to determine marks between the lightest and heaviest
bridges.
Bridges that collapse before being fully loaded will receive a mark between 0 and
30.
 These bridges will receive 1 mark per 150 grams depending on the load at the
time of collapse.
Part B
The bridge will then be tested to destruction to determine a second mark out of 20
based on the overall strength to weight ratio of the bridge.
The bridge with the best strength to weight ratio (specific strength) will receive 20
marks and the bridge with the worst strength to weight ratio will receive zero marks.
Note: bridges that did not pass the first test can still do well in this part of the
competition.
Size penalties
There will be a tolerance of plus or minus 5 mm on the outside dimensions of the
bridge. Outside of this tolerance, 1 mark per mm will be deducted.
As it is critical that traffic can move freely through the truss, there will be no
tolerance on the 70 mm x 70 mm internal sizes. Outside of these measurements, 1
mark per mm will be deducted.
Testing date: Week 10 Term 2, 2009
Note: trusses not ready on the testing day will receive a penalty of 20% per day on
the assessment mark unless there is good reason for the delay.
© Commonwealth of
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Evaluation Report / Presentation: 20%
A presentation report must be completed in the week following the testing of the
bridge designs. Teams can treat this section as a group task or may decide to
allocate responsibility for its presentation to one person in the team.
 The presentation must cover:
 role of each person in the team
 examples of preliminary sketches of alternative designs produced by the team
 photos or sketches of alternate designs found on the Internet
 evaluation of the team’s performance (i.e. what would you change if you had to
do this task again? What were the advantages and disadvantages of working
collaboratively in a team?). Note: the evaluation is the most important part of
the report.
 The presentation is to be in the form of a short talk supported by a Microsoft
PowerPoint or Presenter file. Digital photo and/or video evidence of the design, its
testing as well as of each team member working on the project in some way must
support the report.
 Maximum presentation length is 4 minutes. This will be strictly adhered to, so that
all presentations can be completed within a reasonable timeframe during the
week.
 It is strongly recommended that information for the presentation report be
collected along the way. Teams that leave this section to the very end will find
they do not have access to the evidence required for the presentation.
 Presentation of the information is important. Teams should consider the audience
they will be presenting to and they are expected to use technical vocabulary
appropriate to the design challenge undertaken.
Note: All members of the team will receive the same mark unless it can be shown
that a team member did not contribute in any way to part or parts of the project.
© Commonwealth of
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Associated Theory Notes: Bridges
Bridges have existed since the start of recorded history. The simplest bridge was
probably a beam-type bridge formed by a felled tree across a creek. As humans
developed their skills, they were able to manipulate materials and their increasing
ingenuity and scientific knowledge allowed them to build bigger and stronger
structures for different uses.
In this exercise we will investigate bridge types, materials and construction
techniques. Then you will make a bridge from limited resources. The bridge must
traverse a given distance, and you will compete with other students to determine
whose structure can support a specified load, using the least amount of material.
Research
Go to: http://www.pbs.org/wgbh/buildingbig/bridge/index.html and look for
information on bridges.
1. Complete the table below.
Type of bridge
Illustration
Description/examples
Beam bridge
Truss bridge
Arch bridge
Suspension bridge
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2. Answer the following questions
From taking part in this challenge, what new information do you know about
bridges?
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Why did the Tacoma Narrows Bridge collapse?
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What could have been done during the design and construction stages to improve
this bridge and prevent its collapse?
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3. Research the bridges listed below and complete the table with information on
location, span, materials and date constructed for each one.
Bridge
Location
Span
Materials used
Date of
construct
(metres)
Akashi Kaikyo
Brooklyn
Firth of Forth
Garabit
Golden Gate
Iron Bridge
New River Gorge
Sunshine Skyway
Tacoma Narrows
Tower Bridge
4. Check out the forces lab section and complete this chart.
Force
Type of force
Technical Name
Description of the effect
Squeezing
Compression
Force pushing towards the centre of the beam.
Stretching
Bending
Sliding
Twisting
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Materials
How does changing the material a beam bridge is constructed from affect the
bridge’s reaction to loading?
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Shape
Look at the section on shapes. Which shape do you think can support the greatest
loading without losing its shape? Why and how could the other shapes be reinforced
to prevent deformation of their shape? Use an illustration to support your answer.
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Sydney Harbour Bridge
Australia’s most famous bridge is the Sydney Harbour Bridge. Use information from
these web sites to answer the questions below.
 http://www.cityofsydney.nsw.gov.au/AboutSydney/HistoryAndArchives/SydneyHis
tory/HistoricBuildings/SydneyHarbourBridge.asp
 http://www.australianexplorer.com/sydney_harbour_bridge.htm
 http://www.cultureandrecreation.gov.au/articles/harbourbridge/
 http://www.austehc.unimelb.edu.au/tia/426.html
Question
Answer
When was the Sydney Harbour Bridge
opened for public use?
________________________________________
What is the span and height of the Sydney
Harbour Bridge?
 Span:
________________________________________
 Height of roadway above water:
________________________________________
 Height from water level to top of arch:
________________________________________
How many rivets were used in the
construction of the Sydney Harbour
Bridge?
________________________________________
By what other name is Sydney Harbour
Bridge commonly known?
________________________________________
What is the function of the four pylons (two
on the north side and two on the south
side) on the Sydney Harbour Bridge?
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Question
How was the Sydney Harbour Bridge
tested before it was opened to the public?
Was this a good method of testing? If not,
why not?
Answer
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How is the roadway supported on the
Sydney Harbour Bridge?
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On the southern end (Sydney City end) of
the Sydney Harbour Bridge the roadway is
allowed to move horizontally. Why?
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Question
Answer
The Sydney Harbour Bridge is continuously
being painted. Apart from keeping the
bridge looking good, why is this necessary?
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During the construction of the Sydney
Harbour Bridge a number of people needed
to relocate and their homes were
destroyed. What compensation were these
people given? Would this situation be
different today? If yes, how would it differ?
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Millau Viaduct
Open the PowerPoint presentation in Student Free/Industrial Arts/Year 9
Engineering/Bridge Design/MillauViaduct.
The Millau Viaduct is now open. You may have seen it on a recent episode of Top
Gear.
Are there any other recent examples of spectacular bridge designs that you know
about? Describe them below.
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Materials Testing
The way the paddle pop sticks are laid down can affect the strength of the structure.
They can be either flat or on edge, which method is the strongest and why?
Describe a test you could conduct to check your answer.
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Paper can be used demonstrate vital information about the characteristics of
materials used in the construction of a civil structure such as a bridge. A flat sheet
paper has very little strength. However, if folded or rolled, its strength can be greatly
increased. Complete the following experiments which use a piece of A4 paper and
record the results below.
Experiment 1
Place a sheet of A4 paper between two blocks 250 mm apart. What happens? Does
the paper support its own weight?
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Take the sheet of paper and draw lines along its length to form 15mm strips.
Carefully fold along each line to form a ‘concertina’. Open the paper slightly and
place it between the blocks. What is the result now?
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Place the piece of plywood on the corrugated sheet and load it with a heavy weight
(a 1 kg mass). What happens now? What is the purpose of the plywood?
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Experiment 2
Carefully join the two short edges of a sheet of A4 paper to form a circular tube
ensuring that there are no wrinkles. Stand the tube vertically and place a piece of
plywood on top of it. Can the tube support the 1 kg mass?
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Cut the sheet of paper in half (lengthways) and repeat the procedure above to form
a tube but only half as long. Is this tube more successful at supporting the 1 kg
mass? Why?
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Does it make any difference if you fold the paper to make a square shape, instead of
a cylinder, to support the 1 kg mass?
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Revision
Go to the NOVA website http://www.pbs.org/wgbh/nova/bridge/ and try out their
bridge design challenge.
How would you compare your knowledge of forces and types of bridges now
compared to five weeks ago when you started this unit of work? Describe 5 things
that you have learned.
1. _______________________________________________________________________
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2. _______________________________________________________________________
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3. _______________________________________________________________________
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4. _______________________________________________________________________
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5. _______________________________________________________________________
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