Balsa Bridge Assignment

Balsa Bridge Assignment In this assignment, you are to design and build a balsa bridge using the materials provided. The bridge will be tested to the point of failure using the described loading apparatus, and a score for your bridge is determined using the scoring formula. Design Criteria: 1. The balsa wood materials provided to you and a glue of your choice are the only permissible building materials. The wood pieces provided are 1/8” x 1/8” x 42”. You may cut them in whatever way you need. You may use other clamps, jigs, etc to aid your construction, but they cannot be a part of the finished product. You receive 10 sticks of balsa wood and one sheet of flat balsa approximately 2”x9”. 2. FINISHED DIMENSIONS: The finished bridge shall have a maximum transverse dimension of 2". The finished bridge shall be designed to carry load over an open span of 12" (maximum longitudinal dimension of 14"). Bridge height is not limited, but the deck shall allow for the loading apparatus to be placed on it (this is especially important if you have anything above the bridge deck). The finished mass of the bridge cannot exceed 30.0grams. 3. Joint design criteria: The bridge cannot have any laminated joints, except when connecting to the flat sheets as gusset plates. Contact on the gusset plate is limited to a length of ¼” of any single stick member. Gusset plates are allowed a maximum area of 0.25 square inches (for example, ½” x ½” = ¼ sq. in.). Flat sheets cannot be laminated (one sheet thickness only). Joints must be neatly glued, without excess glue coating the joint. 4. Bearing: The bridge must be supported vertically at the ends of the 12" span. Intermediate supports are not allowed, and the bridge cannot use other parts of the loading frame for support. 5. Finished bridge mass: The finished mass of the bridge cannot exceed the amount stated in finished dimensions. The finished bridge shall have its mass measured prior to loading the structure. This mass shall be used in the evaluation formula. 6. Loading apparatus: The loading apparatus shall have an axle separation of 2", and will be placed in the center of the bridge. The loading apparatus is in the soils lab if you need to inspect it. 7. Loading procedure: A bucket shall be suspended from the loading apparatus, and the designer(s) shall add ballast to the bucket until the bridge fails. The load carried by the bridge (loading apparatus + bucket + ballast) shall be used in the evaluation formula. Dimensions of the loading frame and loading car are provided in this document. The loading mass can be hung over the top groove in the loading car, or hung through the center of the car. 8. Evaluation formula: The score for the bridge shall be determined by dividing the load carried by the mass of the finished bridge. The highest score is the winning bridge. Tip: The score for your bridge is based on the efficiency of your design, not on achieving the largest load carried. 9. Peer evaluation: Students shall also vote for a "best design" prior to bridge testing. Sample Load and Score Calculation Sheet Builder(s) Name:________________ Bridge mass (mB):___________ Load carried at failure (P):________ Score = =________________

Balsa Bridge Assignment This assignment is worth a total of 100 Points, broken down as follows: 1. Bridge loading score (50 points). Bridge Loading Score Points Awarded 5 0 to 199 200 to 399

10

400 to 599

15

600 to 699

20

700 to 799

25

800 to 899

30

900 to 999

35

1000 to 1099

40

1100 to 1199

45

1200 and higher

50

2. Design score average awarded by peers (20 points). The score awarded to your bridge by your peers is rated on a 0-10 scale. The average score awarded by your peers (the score assigned by yourself is not counted) is multiplied by 2, producing your score out of 20 points. 3. Judges evaluation Rubric (30 points). Category Bridge Mass Uses only approved materials Bridge dimensions Jointing (no excess glue allowed)

Maximum Score 5 10 10 5

Builders name(s): ________________________ Bridge adheres to mass requirements Bridge uses only approved construction materials Bridge adheres to dimension criteria (width =2” max and length does not exceed 14”) Award 5 points for length, 5 for width Jointing – Bridge cannot have excess glue exposed on the surface of the joint.

0 or 5 0 or 10 0 5 10 0 or 5

Balsa Bridge Assignment

Loading Frame – Opening is 12” x 2.5”

Loading “Car” – Axle Separation is 2”