Transcript Document 7126341

Bridge Project
Problem Definition: Design a Bridge to span a
given distance while supporting a maximum
load using a minimum materials.
Project Goals
The 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 Bridge
Truss Bridge
• Truss design is
to support the
bridge deck
• The truss may
have ompression
or tension
• The joint of truss
is important
Truss(force analysis)
• 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.
Beam
vs.
• Ways to strengthen members in bending.
– Decrease overall length (deflections).
– Cross section design (moment of inertia)
– Use stronger materials (elastic modulus).
Compression
=
Steel
Axis of
bending
Tension
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.
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
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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?
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Pull?
Push?
Twist?
Shear?
Bend?
Principle of Scissors
Engineering terms - tension, compression, torsion,
shear, & bending
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
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Truss Bridge
Arch Bridge
Suspension Bridge
Cantilever Bridge
– Box Girder Bridge
Quiz
Axis of
bending
• Which orientation of a beam is stronger
under bending and why?
vs.
I
I
• Arch members are in T or C?
C
• Label members in T and C
T
C
T
T
How can you improve your
bridge design?
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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 )