Bridge Project Problem Definition Design a structure to

Bridge Project Problem Definition: Design a structure to span a given distance while supporting a maximum load using a minimum of 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.

Project Constraints Size: See above & 100 stick limit per bridge Shape: Original ideas encouraged! Strength: Must support a minimum of 15 lbs.

Building Materials • Wood tongue depressors (6”) • Glue guns and glue sticks • String

Scoring Equation Score = Load at Failure(lbs) Weight of Bridge (lbs) 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 Individual Brainstorming of Ideas Component Strength Tests Group Prototype Brainstorming Prototype Selection Full-Scale Prototype Construction Prototype Testing Engineering Analysis w/ software Redesign Final bridge construction Final Test Competition! DESIGN PROCESS Problem Definition Idea Generation Information Gathering Idea Selection Implementation

Project Schedule Timeline: • Prototype Testing – 2 weeks • Final Testing – 1 week • Presentation and report – 1 to 2 classes after final test

Grading 20% Group Final Bridge Report 5% Group Oral Presentation 25% Bridge Project + HW assignments (bridge ideas, component test memo, bridge analysis assignment, etc. . ) + Review handout for additional information.

Engineering Fundamentals • Mechanics of Materials • Bridge Examples • 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

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.

Let’s examine some existing bridge designs and see…. • • • Beam Arch Suspension Truss Pre-Stressed Box Girder

Beam vs. • Ways to strengthen members in bending. – Cross section design (moment of inertia) – Use stronger materials (elastic modulus). – Decrease overall length (deflections). – Use reinforcement. Compression = Axis of bending Steel Tension

Arch • Puts members in compression. • Need horizontal support at abutments. Abutment

Suspension • Puts members in tension. • Carries weight up to the top of the towers. • Good for long spans.

Truss • Loads members in tension and compression. • Members are pinned a 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.

Quiz • What orientation of a tongue depressor allows it to be strongest under bending? • Arch members are in T or C? • Label members in T and C (by inspection)? C T T

Quiz Axis of bending I • Which orientation of a beam is stronger under bending and why? vs. I • Is this member in tension or compression? T

Pre-Stressed Box Girder • Pre-fabricated off site. • Steel wires (in tension) run through base of box sections. Why? Road deck Steel wires Pier

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. Avoid overloading joints. Strengthen base supports and load point. Maximize moment of inertia of cross-section.