CeAnn Chalker

* This presentation was prepared using draft rules. There may be some changes in the final copy of the rules. The rules which will be in your published Rules Manual will be the official rules.

* Students will design and build the most efficient bridge wooden structure (i.e. – lightest that holds the most weight up to 15 kg.)

* Only 1 structure entered per team * No Impound * Event Supervisor provides all assessment devices * Testing apparatus, hardware, sand, bucket stabilization sticks

* Testing maximum load 15 kg * Students must wear proper eye protection (ANSI Z87+) * teams given a warning to obtain proper eye protection

* Bridge is a single structure * Built by Team members prior to Check-in * Made of wood bonded by adhesive * No other materials included - rule 3.h. * Adhesive defined in the rules – rule 3.j.

* Unlimited laminations by students is allowed * No limit on the cross section size of individual pieces of wood

* Bridge must be able to: * Sit upon 2 Test Supports at either end of the bridge * Support a Loading Block at the center of the Bridge * Span an opening of: * Div. C – 45.0 cm * Div. B – 35.0 cm

* The Bridge must be at least 5.0 cm wide (outside dimension) along it’s entire span at any height * The Bridge may not extend below the top surface of the Test Supports prior to testing.

* Accommodate a Loading Block – * 5.0cm x 5.0cm x 2.0cm * ¼ inch diameter center hole * ¼ threaded eyebolt (max. 4”) * 1” nominal eye outside diameter * ¼ wing nut * Loading Block must be supported – * In the center of the Bridge span

* Provided by the Event Supervisor * Solid, rigid, smooth, low-friction surface * At least 55.0 cm long x 32.0 cm wide * Center opening approx cm x 20.0 cm

* Lines marked on Test Base * Centerline dividing width in half * Clear Span - Parallel lines across the width * Div. B – 17.5 cm from Centerline * Div. C – 22.5 cm from Centerline

* The Bearing Zones are the test base surfaces wider than the Clear Span lines * The Test Supports must rest in the Bearing Zones

Centerline Div. B – Clear Span 17.5 cm either side of Centerline Div. C – Clear Span 22.5 cm either side of Centerline Bearing Zones Div. B Div. C

* Two identical supports * At least 3.0 cm x 3.0 cm x 15.0 cm long * Made of non compressible material when loaded * Provided by Event Supervisor * All teams use the same Test Supports

*Note – 3 errors in this picture 1 – Adult should not be setting the chain & bucket 2 – Testing supports are not under the bridge 3 – Adult should have goggles on!

* Only Students are to handle their Bridges throughout measurement, setting up, and testing * No alterations, substitutions, or repairs are allowed to the Bridge after check-in

* Suspended through the Loading Block * A ¼” threaded bolt * Chain * S-hooks * Approx. 5 gallon plastic bucket

* Students may adjust the structure until they begin loading the sand * Structures tested with sand or sand like material * Up to maximum 15 kg * Teams are given 5 minutes to load the sand into the bucket (New time limit this year!)

Bucket Stabilization Sticks * Direct contact of the bucket by team members is not allowed after loading begins * Students may only stabilize the bucket using the Event Supervisor provided bucket stabilization sticks * Example – * 2 dowels with spring style door stops on the end

* When maximum load is supported (15 kg) * When failure of the structure occurs * The inability of the Bridge to carry any additional load * Any part of the load is supported by anything other than the Bridge (incidental contact between the chain and the device is not failure) * When 5 minute test time elapses

* Event Supervisor must verify the combined mass is at least kg and no more than kg prior to testing * Load Supported includes – * Loading block * Eyebolt * Washer(s) * Wing nut * Bucket * Sand * Not pieces of the Bridge!

* Highest Score wins * Structural Efficiency * = Load Supported (grams)/Mass of the Structure (grams) * Ties * 1 – Lowest Bridge Mass * 2 – Shortest Bridge height prior to loading

Teams ranked by the highest score w/in each Tier * Tier 1 – Bridges meeting all Construction Parameters and no Competition Violations * Tier 2 – Bridges with one or more Construction Violations and no Competition Violations * Tier 3 – Bridges with one or more Competition Violations or both Construction & Competition Violations * Tier 4 – Bridges unable to be loaded for any reason (including goggle violations) are ranked by lowest mass

* * * Search bridge, truss designs

* Research online –Bridges, & Trusses * Student drawn rough designs * Discuss what might work

* Efficiency = Mass Held/Mass of Structure * Examples - * 20 g structure holds all 15 kg 15000/20 = 750 * 15 g structure holds 12 kg 12000/15 = 800

* Draw designs on gridded paper * Draw the thickness of the wood pieces * Square and Level * Mirror Sides/Matching Sides

* Measurements are within specs to the rules * Bigger is always better than too small * Tape to building board (that can take pins) * Cover plans with – * Clear packing tape, plastic wrap, wax paper

* Balsa * Highest strength to weight ratio * Better tensile (pulling apart) strength than compression strength * Very easy to work with * Less expensive than other woods * Poplar, Bass, Spruce * Heavier and stronger

* Pick your Glue with care! * Use your Glue modestly! * Glue weight is a place to cut down on overall structure weight!

* Wood Glue - Dilute with water or rubbing alcohol (1:1) * Longer to dry but doesn’t make the wood brittle * More flexible, moves with the wood * Super Glue with Accelerator – * quick but can dry out the wood * Rigid when dry

* Tension - the pulling force exerted by a string, cable, chain, or similar solid object on another object * Compression - a pushing force

* Balsa’s Tensile strength is much greater than it’s Compression strength

* One of the strongest * Use as often as possible * Strengthens compression pieces by adding stiffness * Flaw – only as strong as the face of the wood!

* Not strong for tension members * Under Tension will pull apart * Under Compression will stay together

* Stronger than Butt Joint * Less strength than a Lap Joint * Difficult to build

* Combine a Butt Joint with a Lap Joint * Lap another piece of wood at the joint * Strong in both tension and compression

* Diagonal Pieces & Cross Bracing are important! * Prevents structure from torqueing/twisting * Adds additional strength * If the Cross Braces cross (make an X), Glue them at the X Glue here

Right Triangles in Design Slants Face Inward

Right Triangles in Design Slates Face Away from Center

Tough to Build!