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 There are three major types of bridges: The beam bridge The arch bridge The suspension bridge The biggest difference between the three is the distances they can cross in a single span. A span is the distance between two bridge supports, whether they are columns, towers or the wall of a canyon. A modern beam bridge, for instance, is likely to span a distance of up to 200 feet (60 meters), while a modern arch can safely span up to 800 or 1,000 feet (240 to 300 m). A suspension bridge, the pinnacle of bridge technology, is capable of spanning up to 7,000 feet (2,100 m). What allows an arch bridge to span greater distances than a beam bridge, or a suspension bridge to span a distance seven times that of an arch bridge? The answer lies in how each bridge type deals with two important forces called compression and tension: Compression is a force that acts to compress or shorten the thing it is acting on. Tension is a force that acts to expand or lengthen the thing it is acting on. A simple, everyday example of compression and tension is a spring. When we press down, or push the two ends of the spring together, we compress it. The force of compression shortens the spring. When we pull up, or pull apart the two ends, we create tension in the spring. The force of tension lengthens the spring. Compression and tension are present in all bridges, and it's the job of the bridge design to handle these forces without buckling or snapping. Buckling is what happens when the force of compression overcomes an object's ability to handle compression, and snapping is what happens when the force of tension overcomes an object's ability to handle tension. The best way to deal with these forces is to either dissipate them or transfer them. To dissipate force is to spread it out over a greater area, so that no one spot has to bear the brunt of the concentrated force. To transfer force is to move it from an area of weakness to an area of strength, an area designed to handle the force. An arch bridge is a good example of dissipation, while a suspension bridge is a good example of transference. 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) = Ways to strengthen members in bending. vs. Ways to strengthen members in bending. Decrease overall length (deflections). Cross section design (moment of inertia) Use stronger materials (elastic modulus). Compression = Axis of bending Tension Steel

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 Prototype test: 10/10 Final design test 10/19

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: 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? Arch members are in T or C? Label members in T and C I vs. I 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 )