Overview:
Students should design and build the most efficient free standing structure (tower) to fit over an opening in a testing platform. The structure should be constructed entirely out of flat toothpicks and glue and must be able to support a test block at the top of the structure. Students should be given minimum height, base span (the minimum distance needed to fit over the opening in the test platform), and test block dimensions. The structure will be tested on its efficiency--the load supported compared to the tower's mass.

Specifics:

• Introduce this activity by investigating towers, bridges, houses and other interesting architecture. Many of the concepts associated with bridges can be used in tower building. Discuss the difference between tension and compression forces on structures. The major emphasis in this preactivity is to emphasize the use of trusses (triangles) in the design and construction. The original Tacoma Narrows Bridge (Galloping Gertie) is a great example of engineering failure due to the lack of triangles.
  (WWW Resources: Galloping Gertie and Other Bridge Failures)
  

• Introduce the toothpick tower project. Explain that the project goal is efficiency, not pure strength. Efficiency is the comparison of the tower's load held to the mass of the tower. The minimum dimensions of the tower must be the following:
  
  • Height = 25 cm
    
  • Base Span = The tower must stand over a 15 cm by 15 cm square hole in the testing platform.
    
  • The tower must support a wooden test block that is 5 cm by 5 cm at the top of the tower.

• Explain the testing procedure. The first step of the testing procedure is to find the mass of the tower using a triple beam balance (or equivalent). The tower will be tested by placing a block on the top of the tower. The testing block will be attached to an eye bolt and chain. The eye bolt and chain will hang through the tower and a five gallon bucket will be attached to the chain. The tower will be tested by pouring sand into the bucket until the tower breaks(students should wear safety glasses while testing the tower). The test block and chain are considered part of the load and should be measured with the bucket.
  
  
• Following the preliminary investigation and introduction, students should sit down with a ruler, some toothpicks, and graph paper. With these materials, the students should begin formulating their design. It is important to emphasize the value of trusses again at this point of the activity.
  
  
• When the students have formulated a plan (number of sides the tower will have and a general truss pattern) they should make a full scale drawing of one side of the tower.
  
  

	Students should begin construction by placing a piece of wax paper over their diagram and gluing toothpicks together until the side is completed. This process should be repeated until all of their sides are completed. (Note: It is helpful to keep a triple beam balance in the classroom so students can monitor the mass of the tower as it is being constructed.)
	The next step is to upright the tower. To accomplish this, students can stand the sides up and twist tie them together. Once the sides are twist tied together, they should be glued. When the glue is dry, remove the twist ties and test the tower.

The objective of this project is to expose students to concepts of equilibrium/stability, forces (tension and compression), center of gravity, and Newton's Third Law of Motion.

Timeline:

It is difficult to give an exact time limit on this project. The amount of time depends on how quickly students work and how much time is used in studying other structures before construction begins. An average time span for this project is 10 to 15 class days.

Materials:

Construction Materials

• Flat toothpicks
  
• White glue or wood glue (see helpful hints about glues)
  
• Graph paper (for designs)
  
• Triple beam balance
  
• Wax Paper
  
• Scissors (Used to cut toothpicks)
  
  

• 1-2 Five gallon buckets
  
• Sand
  
• Test Platform (See helpful hints)
  
• Test Block
  
• Scale (For determining the mass of the sand and bucket)
  
• Safety glasses
  
• Triple beam balance
  

The assessment portion of this project will be based on three items:

1. Quality and originality of design-A small portion of the student's grade should be based on the overall quality of the finished tower. The quality of the original diagram should be a portion of this section. This will reward students who invest more time in planning, drawing, and building the project.
   
   
2. Performance Score-Each student's tower will be tested according to an efficiency. The efficiency is a comparison of the load held to the mass of the tower. Use the following formula to calculate the efficiency:
   E=LOAD HELD(GRAMS)/MASS OF TOWER (GRAMS)
   (Note: This is a unitless measurement) The scale used to assess the students performance should reflect the teachers individual expectations. The following is an example of a possible scale:
   D 0-150
   C 151-300
   B 301-450
   A >450
   
   A spreadsheet is useful to calculate the efficiency of each tower. [Sample Excel Spreadsheet]

3. The paper-Each student's paper should be graded based on the quality of their explanation of his/her tower. Specific questions about science behind the structure should be included in the paper. (Sample paper template.)
   
   

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