Bridge Project Problem Definition Design a Bridge to

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 Reference web sites: http: //pghbridges. com/basics. htm http: //www. pbs. org/wgbh/buildingbig/bridge/basics. html http: //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) 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

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 Arch reduce the bending on the member !

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 testing jig 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? 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 • • 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 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 )