2015 BALSA BRIDGE BUILDING CONTEST
RULES AND REGULATIONS
Construction Guidelines
1. You must first design your bridge on paper (SketchUp).
2. You must build your bridge according to your design.
3. Materials used in the construction of the bridge shall consist only of commercially available rectangular balsa stock and glue.
4. The total mass of the bridge plus glue must not exceed 60.0 g.
5. The bridge shall contain no element wider than 1/2" nor thicker than 1/4“. Note: Two or more elements, each separately meeting this requirement, may be laminated together to construct members exceeding these dimensions.
6. The bridge shall be "free standing".
7. The minimum length is 16”, the maximum length is 18”.
8. The minimum width is 2”
9. The bridge needs to support a 2” x 2” plate on the deck of the bridge to support the added weight on the bridge. The center of this plate has a hook on it that will need to pass through the bridge floor.
Bridge Testing
1. The bridge pedestals shall be placed on level surfaces separated by approximately 14”. These surfaces shall be level with respect to each other.
2. The standard test frame will be placed on the roadway over the centre of the bridge span. Depending on the bridge design, the load applied to the bridge shall either be suspended from a single 1/2 " diameter rod placed in the centre slot of the test frame, or from two such rods placed in the outer slots. Where either option will work, the judge(s) shall decide on the method to be used.
3. A container shall be suspended from the load-supporting frame. To this container (which may be pre-weighted with steel weights as warranted in the opinion of the judge(s)), water and/or steel weights shall be added at a slow, steady rate, until either an audible cracking sound together with visual evidence indicates the failure of some structural member or glue joint of the bridge, or until a suitable reference point on the roadway at the centre of the span has been lowered by more than 2.0 cm. A competitor may not participate in the addition of weight to his/her own bridge. All decisions of the judge(s) are final.
4. The total mass of the test frame, container, hanging devices and container contents shall be recorded as the competitor's score.
Bridge Building Tip Sheet
· Use a small balsa wood saw (about $3) instead of an exacto knife to make cuts
· Cut small notches to connect bridge components
· Use a basic carpenter's glue
· Yellow glues contain aliphatic resin, used in the majority of winning bridges
· Fewer pieces mean fewer problems
· Keep pedestals (feet) simple
· Clamp glued pieces for about half an hour (use protective strips to avoid damaging the balsa). If you don't have any clamps you can use clothes pegs. C-type clamps are available quite inexpensively at Canadian Tire
· Design for strength at the load application point
· Construct roadway of thin, narrow strips of balsa
· Don't glue down ends of road ways…they usually bend upwards under load
· Roadway must support a small Hot Wheels-type car
· Most bridges bend inwards (as viewed from one end); consequently they require horizontal bracing
· Write your team number prominently on a main horizontal beam of your bridge and on both sides of the bridge
· Double check that a 16”-long board will fit between the pedestals (feet) of your bridge
· Do not cover your bridge with any material. Glue should be used only to join components
· Use light sandpaper (number 150 or higher) to gently clean your bridge and remove excess glue
· Some common mistakes:
o Using a single sheet of balsa for roadway (solution: cut into strips)
o Making outside width 2” instead of inside dimension
o Making overall length less than 16” instead of span (between pedestals)
o Not allowing room for bolt(s) on test frame
o Your bridge will lose weight as it dries (common sense???)
Design and plan your structure
Start simple! Draw out your designs first thinking about what will happen when you place weight on the top of the structure. When building, take the time to do things right, don't try to rush, be precise, don't be sloppy with the glue bottle! Think about what size and weight of wood you will use for each piece.
Try to get your structure as close to the weight limit as you can. The more wood you can put in it, the more it should be able to hold; however, this does not mean that the structure with the most wood will hold the most.
Deciding how wide to make a structure is a difficult task. You must consider that the wider the structure, the more it will weigh with the extra bracing.However, if a structure is too narrow, it may lose some stability. Also, do not push the height limit to the maximum. The shorter the structure the better.
Remember: Minimum of 8"! The shorter the structure the less you have to brace, and the wood that is not used saves weight.
Balsa wood's weight
Two 36" sticks of balsa wood 1/8" square may weigh as much as 4 grams, or as little as .5 grams. Also, balsa wood absorbs water from the air. Dry wood weighs less, wet wood weighs more. Think about ways you could get the structure to lose some of the excess water.
Balsa wood grain patterns
Look extremely close at a stick of balsa wood: Do the grain lines run parallel with the edges of the stick, or do they cut diagonally across the stick? Believe it or not, but this does make a substantial difference in how much weight a structure can hold.
Adhesives/Glues
Explore different options: there are wood glues (aliphatic resins), super glues (cyanoacrylates), model cements, and epoxies. Try gluing two pieces of balsa wood together with the different types of glue, then try breaking them apart with your hands. Can you tell a difference? Look at closely at where it broke, did the glue give way, or did the wood itself break? Read warning labels on all products. Check to see what materials the glue is recommended for. A thick glue will bridge tiny gaps, a runny easily absorbent glue will partially soak into the wood.
Keep accurate records
Keep drawings of the structures you built. Jot down notes about how much it held, what type of glue you used, how much it weighed, the sizes of wood used, or anything else that might have had some effect on the way it held up or broke down.