Term Project:
Balsa Wood Bridge Part 1
Engineering Design Process
Turning Ideas Into Reality
Read section 2.2,
pp. 37-47
What is the difference between
an optimist, a pessimist and an
engineer?
Optimist: the glass is half full
Pessimist: the glass is half empty
Engineer: the glass has a 50%
overall inefficiency
Engineering Job Functions
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Analysis
Design
Test
Development
Sales
Research
Management
Consulting
Teaching
In your project you will
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“Research”
Design
Analyze
Build
Test
• And learn about the engineering
design process
3. Data and
Information Collection
Rules
2. Problem Definition/
Specifications
you want to build
a bridge
Span > 30cm
Mass < 30g
made of Balsa etc.
• browse web resources
• look at Seattle bridges
• study rules etc.
4. Development of
Alternative Designs
Deliverable 1
5. Evaluation of Designs/
Selection of Optimal Design
Analyze Designs (software)
Deliverable 2
6. Implementation of
Optimal Design
Build and Test
Deliverable 3
The Engineering Design Process
1. Customer Need
or Opportunity
Cautionary Notes:
1. Technically our last step (Testing) would NOT be
the implementation.
• It would still be part of step 5, finding the optimal
design.
• We would test and then evaluate the results to
optimize.
2. There are different versions of the Engineering
Design Process. The one presented here is just one of
them.
Example: What went Wrong?
The Tacoma Narrows Bridge Disaster
Example: Engineering Design in Action
Mars Rover Curiosity
in class: 0:00-0:50 and 9:45-16:00
at home: watch the rest (pretty amazing)
What is left today?
• rules
• some physics without math
• deliverables
• links
• forming teams
• materials and logistics
Note: the second part of the presentation will
cover the software and questions.
Term Project:
Balsa Wood Bridge Part 2
Let’s Start the Modeling Software …
You can find the software on the project tab of our website:
http://seattlecentral.edu/faculty/rheller/Engr110/classproject.html
and clicking on Modeling Software
Or you can go directly to:
http://www.jhu.edu/~virtlab/bridge/truss.htm
How to find the Modeling Software
Click on
Class Project
How to find the Modeling Software
Click on
Modeling Software
How to find the Modeling Software
Read the instructions
Then …
To start click on
bridge truss
Bridge
Terminology
Your bridge will have
• Two trusses
• Joints or Nodes
• Members
• One fixed node
• One horizontal rolling node
• Loads (at least one)
What does the software do?
For a given LOAD
the software will calculate the FORCE
(compressive or tensile) in each
member.
The software can only do this if you follow the
RULES
Bridge Rules
Your bridge MUST have
• One fixed node
• One horizontal rolling node
• Loads (at least one)
• And (most importantly) …
𝑀 + 3 = 2𝑁
M= # of members
N= # of nodes
(including the support nodes)
How many members?
M=7
How many nodes?
N=5
M+3=2N
7+3=2x5
Why?
• To find out take STATICS, Engr214 (offered in
fall)
• Statically determinate system
Modeling a Bridge
Step 1: Start with NODES
Modeling a Bridge
Step 2: Convert one node to
fixed node and
one to horizontal, rolling node
Note on fixed and rolling nodes
• You have to first create a node.
• Then you select ‘fixed node’ and click on the
node you want to be fixed.
• You cannot click on fixed node directly.
Modeling a Bridge
Step 3: Add members
Modeling a Bridge
How many members?
How many nodes?
M+3=2N
14 + 3 = 2 x 9
17 = 18 (oops)
Modeling a Bridge
Add a load …
… or better two
Error Message
Modeling a Bridge – Correct version
How many members?
How many nodes?
M+3=2N
15 + 3 = 2 x 9
18 = 18
Modeling a Bridge – Correct version
Click on Calculate
Then click anywhere on the grid …
Modeling a Bridge
What does this mean?
Find the total load:
Find the member under the highest compression:
Find the member under the highest tension:
What are you looking for?
• Note: the loads are relative. Doubling the load
will double all forces.
• For a given load, you want the maximum
compressive and tensile forces to be as small
as possible.
• If the material behaves better under tension,
then your tensile forces can be a little higher.
Apply your load realistically.
Let’s Form Teams
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Find a team member.
Make sure you have matching schedules.
Exchange email and phone info.
When you have a partner, come to me and I
will write down your names.
Have fun!