Balsa Wood Tower
OBJECTIVES:
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.
Students will work in groups of two to design and build a balsa wood tower. The tower will be tested until failure to see how much weight it will hold.
INTRODUCTION
A tower is 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).
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 (San Joaquin Expanding Your Horizons, n.d.).
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
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.
Materials and Specifications
1. The tower must be constructed ONLY from the following materials:
-21 ft. of 1/8" x 1/8" balsa wood
-non-toxic glue
2. Equipment:
-one cardboard work area
-straight pins (to hold wood in place while the glue dries)
-Scissors
3. Specifications:
-Over-all tower size height: 12" (a 1/2" tolerance will be allowed without deductions.)
- The center of the tower must allow for a ½” vertical rod to be placed through it
4. The tower must support a target weight of 70lbs (32 Kg).
5. Limitations:
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 and glue laminating are not allowed.
Goal
Build a tower that holds a weight of 70lbs (32 Kg).
Having the greatest efficiency rating (Strength to Weight Ratio).
Stress Test Procedure
1. Check tower to see that it meets all specifications.
2. Compare tower to drawings.
3. Measure weight of tower and record in Kg.
4. Place tower in test area.
5. Place test block onto tower (1.2 Kg)
6. Carefully place weights, one at a time onto the test block
8. Continue until the tower (breaks).
9. Determine weight held (test block (1.2 Kg), weights (1 Kg = 2.2 lbs) )
10. Calculate efficiency using the following equalizing formula:
Strength to Weight Ratio = Failure Weight / Tower Weight
Engineering Physics – Balsa Wood Tower Names: ______
TIPS FOR STRUCTURE: The best advice for building 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.
3) Structures are symmetric. When building a bridge or a tower with two, three, 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) 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.
Procedure (4 steps):
1. Design: (1 Day)
1. Create a scaled sketch of a possible design. Sketch 3 different ideas – Sketch the side and top view of the towers, label the sketches, and show the scale. Provide a 1 paragraph rationale for each
2. For each sketch, find the total number of inches of material (each balsa wood stick comes in a 3 foot length, and has a mass of about 1.81 grams) with this information you can estimate the mass of your tower.
2. Building the Tower: (4 Days)
1. Using a preferred sketch as a blueprint, construct the sides of your tower. A well-constructed tower will hold more, so take your time and don’t be sloppy.
2. Once all the sides are built, you can put your tower together.
3. We will test your structure by applying weights to the top – ensure that all the sides are well connected and the top of the tower is as even as possible.
4. Weigh your tower once it’s complete.
3. Stress Test: (1 Day)
1. Place the tower over the vertical rod and place the test board onto the tower. This test board weighs 1.2 kg or 2.6 lbs.
2. Carefully place masses onto the test block until the structure fails.
3. Record the highest mass the structure held BEFORE it failed. The structure must hold the masses for at least 10 seconds or it will be considered failed.
4. Calculate the Strength to weight ratio of the tower.
4. Reflection on Redesign: (1 Day)
1. Write 2 paragraphs about your tower. How it held up to the stress. Discuss what you would do differently to the design to enable it to hold more weight.
2. Create another scaled sketch of the redesigned tower.
______
Rubric (45 pts):
Description Possible Received
Design/Engineering:
1. Sketches – showing a scale and total inches of balsa wood 5 pts. _____
2. Paragraph rational for each design 5 pts. _____
Stress Test: Weight Held:
a. 70 lbs + 25 pts.
b. 65 – 69 lbs 21 pts.
c. 60 – 64 lbs 16 pts.
d. 50 – 59 lbs 12 pts.
e. 40 – 49 lbs 8 pts.
f. 30 – 39lbs 5 pts.
g. 0 – 30 lbs 0 pts _____
2. Efficiency:
(Calculate Strength to Weight Ratio Here!) 5 pts _____
Mass of Tower: _____
Maximum Mass held: _____
Strength to Weight Ratio: ______
Bonus: Highest Strength to Weight Ratio of the Class: (+3 pts) _____
3. Redesign/Reflection:
a. Redesign and reflection 5 pts. _____
Total Score : _____