Term Project: Balsa Wood Bridge Part 1

Engineering Design ProcessTurning 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• Analysis• Design• Test• Development• Sales• Research• Management• Consulting• Teaching

In your project you will

• “Research”• Design• Analyze• Build• Test

• And learn about the engineering design process

The Engineering Design Process

1. Customer Needor Opportunity

2. Problem Definition/ Specifications

3. Data and Information Collection

4. Development of Alternative Designs

5. Evaluation of Designs/ Selection of Optimal Design

6. Implementation of Optimal Design

you want to build a bridge

Span > 30cmMass < 30gmade of Balsa etc.R

ules

• browse web resources• look at Seattle bridges• study rules etc.

Deliverable 1

Analyze Designs (software)Deliverable 2

Build and Test Deliverable 3

Note: th

is last

one

is not

quite

true!

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 logisticsNote: 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

To start click on bridge truss

Read the instructionsThen …

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 RulesYour bridge MUST have• One fixed node• One horizontal rolling node• Loads (at least one)• And (most importantly) …

𝑀+3=2𝑁M= # of members N= # of nodes

(including the support nodes)

How many members?

M = 7

How many nodes?

N = 5

M + 3 = 2 N7 + 3 = 2 x 5

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 tofixed 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 BridgeStep 3: Add members

Modeling a BridgeHow many members?

How many nodes?

M + 3 = 2 N14 + 3 = 2 x 917 = 18 (oops)

Modeling a BridgeAdd a load … … or better two

Error Message

Modeling a Bridge – Correct versionHow many members?

How many nodes?

M + 3 = 2 N15 + 3 = 2 x 9

18 = 18

Modeling a Bridge – Correct versionClick 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

• 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!