Construction: Balsa Wood Tower
1. Learn basic engineering principles
2. Identify basic shapes that form structures
3. Design a tower for structural strength, that will hold weight
4. Use the student made design to construct a tower using given materials
5. Calculate efficiency rating to determine best designed tower
Students will design and build a balsa wood tower. The tower will be tested to see how much weight the tower will hold. The student then figures out the efficiency rating for their tower.
If you look up the definition of a tower, you will find the following; "An exceptionally tall building or part of a building or an exceptionally tall structure used for some functional purpose. In the past, towers were usually used to house bells (bell towers), for observation (watch towers) or for signaling (light houses).
Perhaps the earliest record of a tower comes from the Bible where the story of the tower of Babel is told. Other notable towers include the Tower of Pisa and Eiffel Tower, two completely different types of towers. Today there are many more types of towers which are used for a wide variety of functions. A few examples are transmission line towers, radar towers, radio and TV broadcasting towers, even towers for suspension bridges.
BASIC ENGINEERING PRINCIPLES AND DEFINITIONS
Tension: the action of being stretched to stiffness
Compression: the action of being pressed or squeezed
Weight: a load with a certain heaviness, a specific force due to gravity
Support: to hold up or serve as a foundation
Tensile Strength: the greatest stress a substance can bear without tearing apart
Butt Joint: A technique in which two members are joined by simply butting them together. The butt joint is the simplest joint to make since it merely involves cutting the members to the appropriate length and butting them together. It is also the weakest because unless some form of reinforcement is used it relies upon glue alone to hold it together. Because the orientation of the members usually present only end grain to long grain gluing surface, the resulting joint is inherently weak.
Lap Joint: In woodworking, a lap joint describes a technique for joining two pieces of material by overlapping them. Usually, no material is removed from either of the members to be joined, resulting in a joint which is the combined thickness of the two members.
BASIC STRUCTURAL SHAPES
triangles - rigidity of the triangular shape can support weight securely
squares - can support weight, but can twist and collapse if not supported
polygons - like squares, can support some weight, but will twist and collapse if not supported.
TIPS FOR STRUCTURE COMPETITION
The best advice for building winning structures starts with commitment. Your dedication and attention to detail is the winning combination. The following list of tips will help you succeed but only if you are dedicated to the project. Be sure you understand the event rules before designing your prototype.
1) Draw your preliminary design with full wood outlines. This should help you to decide whether to use butt joints or lap joints.
2) ALL joints should have absolutely flush surfaces before applying glue. Anytime glue is used as a "gap filler", it dooms the structure! Cut the wood precisely and carefully sand the part so that it fits flush. Then, number the part and use it as a template to make numbered duplicates in assembly sequence (i.e.: two for bridges, four for towers).
3) Structures are symmetric. When building a bridge or a tower with two or four sides, build the two primary sides one on top of the other. Once the first side is built, cover it with wax paper and build the second side directly on top of the first. This helps insure the structure's symmetric integrity.
4) Most competitions require these structures to be weighed. Up to 20% of the structure's mass may be from over gluing. Adhesives do not work better when they are drooled all over the structure. Use the adhesives sparingly where any more than a translucent, moist surface becomes wasted, excess mass.
5) When building a balsa wood structure, pre-test all the strips for tensile strength before assembly. A simple deflection test works best. Anchor half of the strip on a surface and ballast the free end using clay on a bent straight pin. Use a simple gauge to categorize the strips. Group similar tensile strengths together. The strips that deflect the least are the strongest. Use those for the longest pieces. The ones that defect the most use as the shortest pieces. When the structure is finished, it should have a relatively consistent load carrying capability.
COMPETITION OBJECTIVE:to determine which tower has the greatest EFFICIENCY.
1. The tower must be constructed ONLY from the following materials:
-21 ft. of 1/8" x 1/8" balsa wood
-one cardboard work area
-wax paper to cover the drawing during construction
-straight pins (to hold wood in place while the glue dries)
-one X-acto knife
A. Over-all tower size
height: 12" (a 1/4" tolerance will be allowed without deductions.)
width: 3" maximum
B. The center of the tower must provide passage for a 11/4" test rod. Towers that do not allow passage will not be tested.
4. Plans for the tower must be completed in two views:
- at least four sketches BEFORE you begin your final design
- a drawing must be submitted with the tower.
- the tower must support a 11"x11"x3/4 test block at the top of the tower
A. Only balsawood and non-toxic glue will be used.
B. Wood cannot be treated in any way to change its strength or appearance.
C. Only the amount and type of materials specified will be used in the tower.
D. Wood pieces may be bonded together with glue ONLY at joints.
E. Splitting or laminating are not allowed.
1. Deductions of 20% of the greatest weight held in the event are given for:
a. Failure to submit a drawing.
b. Illegal lamination.
c. Excessive use of glue (smears or spots that are larger than 3/8" in width/ diameter or structural elements that are coated with glue).
d. the tower is 1/4" less than or 1/4" greater than the specified height.
2. Structures will not be tested if:
a. Any materials other than those specified are used.
b. Center is blocked.
c. Test block will not rest on top.
d. Tower is 1" less than or 1" greater than the specified height.
e. More than two (2) rule violations are present
Have an efficiency rating of at least 2.
A rating of 3 or better is considered "outstanding".
STRESS TEST A TOWER
A. Tools and Materials
1. Completed tower (allow overnight drying time)
2. Testing equipment (provided by Instructor)
a. Testing block
b. Weights (various sizes)
d. Tower support device
B. Procedure (NOTE: To qualify for testing, the tower must meet all specifications given in previous hand-outs.)
1. Check tower to see that it meets all specifications.
2. Compare tower to drawings.
3. Measure weight of tower and record.
4. Place tower on support system.
5. Place testing block on top of tower.
6. Place weight on testing block in even, increasing measurements by sliding them on top of the testing block, then carefully lowering the testing block to rest on the tower.
8. Continue until the tower (breaks).
9. Determine weight held (test block, weights)
10. Calculate efficiency using the following equalizing formula:
Efficiency = Failure Weight / Tower Weight
11. Clean up test area.