problem definition: design a bridge to span a given distance while supporting a maximum load using a...

Problem Definition: Design a Bridge to span a given distance while supporting a

maximum load using a minimum materials.

Bridge Project

Project GoalsThe goals for this project are for students to:• Learn the design process.• See practical applications for trigonometry, physics,

and engineering statics (force analysis).• Improve skills associated with collecting data and

drawing meaningful conclusions.• Experience the usefulness of prototype testing.• Recognize the necessity of good communication

skills for engineers by completing memos, reports, drawings, and presentations.

Introduction to Bridge

• Bridge Type

–Truss Bridge

–Arch bridge

–Suspension Bridge

–Cantilever BridgeReference web sites:http://pghbridges.com/basics.htmhttp://www.pbs.org/wgbh/buildingbig/bridge/basics.htmlhttp://science.howstuffworks.com/bridge.htm

Truss Bridge

• Truss design is to support the bridge deck

• The truss may have ompression or tension

• The joint of truss is important

Force Analysis (Truss)• Loads members in tension and compression.

• Members are pinned at joints (Moment = 0).

• Triangles provide stability and strength.

•Top members in Compression.

•Bottom members in Tension.

Hint - Imagine entire truss wrapped in rubber skin, so you have a large rubber box. When you bend it, where would the wrinkles be? They indicate compression.

Force Analysis (Beam) • Ways to strengthen members in bending.

– Decrease overall length (deflections).– Cross section design (moment of inertia)– Use stronger materials (elastic modulus).

Tension

Compression

Steel

=

Axis of bending

vs.

Truss

Deck

Arch bridge

• Appear mostly in Ancient time

• New arch bridge is modified to reduce the material

Function of Arch structure

• Puts members in compression.

• Need horizontal support at abutments.

Abutment

Suspension Bridge• Replace the

Beam with cable

• Reduce the need for the Pier , Girder and Truss

Suspension• Puts members in tension.

• Carries weight up to the top of the towers.

• Good for long spans.

Tower Design

Cantilever Bridge

• No support at the bridge it self

• The material must be very strong

• Or the structure must be different

Box Girder Bridge

• Box structure reduce the weight and material

• Increase the strength on top and bottom to resist compression and tension

Project Constraints

Size: See above & 100 stick limit per bridge

Shape: Original ideas encouraged!

Strength: Must support a minimum of 15 lbs.

Constraints:Building Materials

• 100 Wood tongue depressors (6”)

• Glue guns and glue sticks and wood glue

• String

Scoring Equation

Score = Load at Failure(performance) Weight of Bridge(cost)

As engineers, you want to maximize the load held using the least amount of material.

Testing Procedure

2” dowel

2” x 6” thin plate

testing jig

Project Break-down• Project Intro (Problem Definition)• Component Strength Tests (Information Gathering)• Individual Brainstorming of Ideas (Idea Generation)• Group Prototype Brainstorming (Idea generation)• Prototype Selection (Idea selection)• Full-Scale Prototype Construction (Implementation)• Prototype Testing (Information Gathering)• Engineering Analysis w/ software (Information Gathering)• Redesign (Idea generation)• Final bridge construction (Implementation)• Final Test Competition! (Information Gathering)

Project Schedule

Timeline:• Prototype Design, Build, Test – 1

week• Final Design, Build, Test – 2 weeks• Presentation and report – 1 to 2

classes after Final Test

Grading

• Design and testing– Bridge Ideas -10%– Components test Memo-5%– Prototype bridge performance-5% – Draft Report-5%

• Project report – Bridge Final Presentation-10%– Competition-5%

Engineering Fundamentals

• Mechanics of Materials • Construction Methods &

Hints!

What is the easiest way to break a tongue depressor?• Pull?• Push?• Twist?• Shear?• Bend?

Engineering terms - tension, compression, torsion, shear, & bending

Principle of Scissors

Bending! Thus bridge design Do’s & Don’ts:

• DON’Ts – avoid bending bridge members when

possible.– avoid compressing long bridge members -

causes buckling (a kind of bending).

• DO’s– load members in tension and compression

(short) when possible.– brace bending members when possible.

Bridge Type we have learned

• Truss Bridge• Arch Bridge• Suspension Bridge• Cantilever Bridge

– Box Girder Bridge

Quiz

• Which orientation of a beam is stronger under bending and why?

• Arch members are in T or C?

• Label members in T and C

I

Ivs.

Axis of bending

C C

T T T

How can you improve your bridge design?

• Incorporate truss structure (triangles).• Design a 3-D structure from the start!• Use short members in compression.• Use string for tension members.(Reduce

material and weight)• Avoid overloading joints.• Strengthen base supports and load point.

Component Test-Compression and buckle Test-1

• Compress your spaghetti until it start to buckle(When you don’t feel you need to apply additional force but the structure still keep bending)

• Hold(use you hand) the center point to see how much force you need to increase to have buckling

• Hold another two point to see how much force you need to increase to reach buckling

Component Test-Compression and buckle Test-2

• Cut the spaghetti in half and try again

• Cut the spaghetti in half again and try again

• Record all the force and Test Situation

Tension Test on the joint

• Specimen preparation – Use four depressor

• Two as a group. Glue them together

• Drill one hole on the each group

• Overlap two group according to

test procedure and glue them together

• Use hook to hold the structure

• Start tension and compression test by

force gauge and scale

• Do unit conversion if necessary

• Take your record home and make excel plot

and report

Fatigue Test

• Bend Paper clip and count how many cycle it is needed to break it!

• Test 5 Paper clip

Torsion Test

• Twist a chalk and see how it break

• Explain why it happen(explain in Components test Memo )