TE Activity: Tower O' Power
Contributed by: Integrated Teaching and Learning Program, College of Engineering, University of Colorado at Boulder
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Figure 1. Students ready to test their CAD tower click for copyright |
Summary |
Engineering Connection |
| Engineers often revise a design in a computer modeling program many times before finalizing a project. Many CAD (computer-aided design) programs can be used in conjunction with other programs or equipment to mathematically replicate the effects of loads on a structure or to machine a prototype's parts from a variety of materials. Engineers must balance many different constraints and parameters when designing a part or structure. A solid bar will almost always be stronger than an I-beam of the same size, but it will also be a lot heavier and more expensive. Oftentimes, engineers will redesign to make it lighter and/or more cost effective without sacrificing much of its strength. |
Contents
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| Grade Level: 9 (9-12) | Group Size: 2 |
| Time Required: 300 minutes Note: This is an engineering project that covers several days of class time and includes a field trip to a local college or manufacturing lab with a universal testing machine. |
Activity Dependency  :None |
Expendable Cost Per Group
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US$ 7 |
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| Keywords: stress, stress concentration, Computer-Aided Design (CAD), tower, compression, tension, manufacturing, laser cutter, strength, weight, truss, design, cross supports, materials | |
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Related Curriculum
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| subject areas | Geometry Measurement Physical Science |
Pre-Req Knowledge (Return to Contents)
Basic 2-Dimensional Drafting Skills with any CAD program
Learning Objectives (Return to Contents)
After this activity, students should be able to:
- Know how to calculate stress.
- Define a stress concentration.
- Describe what geometrical shapes influence higher stress concentrations.
- Use calculations to determine a strength vs. weight ratio of a prototyped tower.
- Explain why engineers do not always use solid pieces of materials in design.
Materials List (Return to Contents)
Each group needs:
- Access to a computer with basic CAD program
- 10" x 22" x 1/8" sheet of acrylic plastic
To share with the entire class:
- A variety of plastic adhesives for students to use when assembling their towers (available at hardware or hobby stores)
- Several small files for smoothing rough edges of plastic
- Access to a laser cutter
- Access to a college or manufacturing lab for compression testing
- Enough safety glasses for ½ the class to wear at once (can likely borrow from the testing laboratory)
Introduction/Motivation (Return to Contents)
Have you ever drawn a really cool design but had no way of building it exactly how you drew it? With today's new manufacturing machinery, it is easier to go from drawing to reality. Engineers can design parts or prototypes of parts on their computer using CAD (computer aided drawing) programs. Such programs include SolidWorks®, AutoCAD®, Inventor® and Ideas®. Once complete, the engineers take their design to a machine and simply press 'Print' to build their design out of different materials. CNC (computer numerically controlled) machines can manufacture solid 3-D designs out of plastic or metals. Rapid prototyping machines, also known as stereolithography machines, can create just about any design imaginable out of plastic. Finally, laser cutters can cut intricate 2-D designs out of plastic. So, don't worry, even if you are not the greatest artist with a pencil, you can always draw a straight line or a perfect circle with the computer to help you out.
Stress (σ) is defined as force divided by area and has units of Pascals or psi. One Pascal is a Newton per square meter and 1 psi is a pound per square inch. Normally, stress is distributed evenly throughout a material. However, sometimes stress can build up and intensify in certain areas. This is known as a stress concentration and occurs at holes and other sudden changes in geometry. A general rule is that sharper, more sudden edges cause higher stress concentrations (Figure 2 illustrates this principle). Circles have lower stress concentrations than rectangles, and cracks have the highest stress concentrations of all. This is why materials usually break shortly after cracks are formed. Coincidentally, cracks are usually formed at areas of high stress concentrations.
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Figure 2. Stress Concentration. A solid block and blocks with a hole, square, and crack in them. Lines represent the stress field in the blocks. Stress concentrations are in the areas where the lines build up next to one another. You can see the stress concentrations are the greatest for the square and crack. click for copyright |
Think about a crack in a car windshield. Cracks in windshields usually start when a high stress concentration is applied to the window, such as a pebble or rock from the road hitting the windshield. Then, as a crack is formed, it weakens the windshield and eventually breaks. If you catch the initial crack in the windshield in time, you can drill a little circle around the end of the crack and keep the windshield from breaking or cracking further.
Oftentimes engineers cut holes or odd geometries into materials even though they will cause stress concentrations. Many times this is due to the need to reduce material cost or weight, or even insert something through the hole. If material is removed carefully, the strength of the material will not be drastically affected. Engineers aim for high strength:weight or strength:cost ratios. Remember, smooth-rounded edges will always minimize stress concentrations compared to jagged-sharp edges.
In designing tall buildings or long bridges, engineers use truss supports to prevent twisting. Trusses are bars or beams to help increase the stability of the structure. In simple structures, truss supports are most effective at a 45° angle from the horizon. When two truss supports cross, they are often times called cross supports because they form a 90° angle just like a cross.
Engineers utilize today's new prototyping techniques to quickly see if their designs have any mistakes that they may have overlooked on paper or on the computer. Using CNC (computer numerically controlled) machines, rapid prototyping machines and laser cutters, engineers can quickly produce a scaled replica of their design and test where or not stress concentrations will actually be a problem, or if their truss system is properly designed.
For this project, we are going to work as engineers and design prototype towers out of acrylic plastic. We are going to design our towers in a CAD program and test the strength:weight ratio of our towers in a manufacturing lab, using a universal testing machine. Let's see if we can use what we have just learned about stress concentrations to design a small Tower O' Power that is really strong.
Fun Fact: Martial artists use the truss support principle in self-defense by keeping their elbows and knees at 45° from full extension when blocking an attack. This helps stabilize their body with the same principles of stabilizing a building.
Procedure (Return to Contents)
Before the Activity
- Make sure students have access to computers with a CAD program
- Order one sheet of 10"x22"x1/8" acrylic for each group. Place order with an acrylic plastic distributor such as:
- Colorado Plastic Products - (303) 443-9271
- Interstate Plastics (www.interstateplastics.com)
With the Students
Part I: Tower Design
- Introduce the students to the ideas of stress, stress concentrations and truss supports from the activity introduction.
- Explain the following rules of the Tower O' Power Competition to the students:
- Their tower must be 12" tall.
- Their tower's cross-section must fit within a 4"x4" square, though their tower does not necessarily have to be square.
- The pieces of their tower will be laser cut from 1/8" thick acrylic.
- Each piece of acrylic is 10" x 22".
- They can use any adhesive they want to construct their tower. (Note: This is a critical part of the design, and can cause failure if poor adhesives are used.)
- Finally, most importantly, the wining tower will be decided by which tower has the highest strength:weight ratio. (Strength will equal the maximum load the tower holds, and weight will equal the weight of the tower). (Hint: The winning tower will not necessarily hold the most load.)
- Give the students a deadline for when CAD drawing deadlines are due (1-2 weeks).
- Explain the CAD drawing specifications:
- Drawings must be 2-Dimensional. (The laser cutter will only cut by moving the laser left and right.)
- Drawings must have all tower pieces fit into a 10" x 22" rectangle.
- Drawings must be saved as the .DXF file format (Verify with whomever is doing the laser cutting if any other file formats are acceptable.)
Part II: Laser Cutting & Design Deadline
- If you have not already done so, pick up acrylic sheets from plastics distributor.
- Collect all designs as .DXF files (or other acceptable files) from students, and save them on a CD ROM or USB Flash-Drive.
- Take students' designs and acrylic sheets to be laser cut. (Note: The process of laser cutting is relatively quick, but cutting many towers is time consuming. On average, each tower should take about 20 minutes to cut. Make sure to plan adequate time for this process.) Some helpful hints:
- Although increasing the laser's speed and power will decrease cutting time, a good cut will not be accomplished and some pieces will not be fully cut if the speed is increased too much.
- One or two practice runs may be a good idea to optimize the laser's settings and save time in the long run.
Part III: Tower Construction
- Once all towers are cut, give the students their designed tower pieces to put together (see Figure 3).
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Figure 3. Students building their acrylic tower click for copyright |
- Encourage students to use a file to smooth out any sharp edges or burrs caused by the laser cutting.
- Remind students that they can use any adhesive or glue they choose, and that it may have a large effect on their tower's performance.
- Setup a field trip for tower testing and a lab tour at a local college or manufacturing center. (Arrange the students to come in preferably early in the morning when the college and labs are less crowded.)
Part IV: Field Trip & Tower Testing Day
- Review engineering lab safety issues with students.
- Split the students into two groups.
- Group 1 will go to an engineering lab to test their towers. (Arrange for a lab technician to operate compression tester.)
- Group 2 can take a tour of the facilities. (Arrange for a tour guide or extra chaperone.)
- In the testing group, have each group weigh their tower on a scale and then place their tower in a universal testing machine for compression testing. Make sure to label and record each tower's weight.
- Instruct each group member to put on safety glasses.
- Have the lab technician run and record each compression test on the universal compression machine. Note: Inform the technician that on average, towers will hold 500-4000lbs. This will help determine the rate of loading of the towers. (Hint: Slower loading is more suspenseful and fun, but keep time in mind.) Figure 4 illustrates a Tower during testing.
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Figure 4. An acrylic tower under compression at the University of Colorado smash lab. click for copyright |
- Have students record the maximum load held by the tower for the strength:weight ratio calculation. Some towers may exhibit a decrease in load before breaking.
- After all of the towers have been tested, help clean up the group's broken pieces of plastic and return and safety equipment borrowed from the lab (i.e., safety glasses).
- Have groups 1 and 2 switch and repeat steps 3-7.
Part V: Design Review
- Back in the classroom, have the students calculate their strength:weight ratio or the strength divided by the weight of the tower. (Strength will equal the maximum load the tower holds, and weight will equal the weight of the tower.)
- Have students compare the strength:weigth ratio of their towers with the class and determine which tower had the highest strength:weight. Did the winning tower hold the most load?
- Discuss with the students what they would do if they had a chance to redesign and test their towers again. Remind them that engineers often go through several iterations of a design before they complete a project.
Safety Issues (Return to Contents)
Construction
- Sharp edges and burrs can be caused from the laser cutting process. Inform students to be careful and inspect their received cut pieces for these defects. They can simply be removed with a nail file.
- Make sure students use caution with certain adhesives or glue. Hot glue can easily and quickly burn students, and some epoxies are harmful to inhale.
Field Trip
- Make sure students DO NOT wear open-toe shoes to the field trip, as they will not be allowed into the engineering laboratories.
- Safety glasses will be required in the lab, make sure there will be enough for every student.
- Remind students that they will be removed from the lab if they are caught misbehaving.
Troubleshooting Tips (Return to Contents)
Remind students to be creative and design their tower with minimizing weight in mind. Also, enforce the concept of smoothing out sharp corners from their design. This can be done with simple 'Round' or 'Fillet' commands in CAD.
Choosing the right adhesive or glue can be critical. Hot glue can be dangerous and may dry too quickly, preventing accurate assembly of the tower pieces. However, Elmer's glue is non-toxic but is weak in bonding plastic and takes too long to dry. Have the students research the best glue for the application. (Hint: In the past, Gorilla Glue® helped one team with an extremely light tower win their competition.)
Assessment (Return to Contents)
Pre-Activity Assessment
Brainstorming: As a class, have the students engage in open discussion. Remind students that in brainstorming, no idea or suggestion is "silly." All ideas should be respectfully heard. Take an uncritical position, encourage wild ideas and discourage criticism of ideas. Have them raise their hands to respond. Write their ideas on the board. Ask the students:
- Why is it that planes and ships have oval windows instead of the normal square windows you see in your home or school? (Actual answer discussed in Post-Activity assessment.)
Activity Embedded Assessment
Question/Answer: Ask the students and discuss as a class:
- What has the highest stress concentration, a circle, square or crack? (Answer: A crack. A general rule is that sharper, more sudden edges cause higher stress concentrations. This is why materials usually break shortly after cracks are formed.)
- Coincidentally, what leads to cracks? (Answer: High stress concentrations lead to cracks.)
Post-Activity Assessment
What's the Stress? Draw a 2" x 2" square bar that is subjected to 100 lbs of force (as drawn in Figure 5). Ask the students to determine the stress experienced by the bar. Is the bar in tension or compression? (Answer: Stress = Force / Area = 100 lbs / (2"x2") = 25 psi in tension.)
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Figure 5. Tension or compression? click for copyright |
Engineering Problem Solving: In the mid 50's, Great Britain's De Havilland's Comet airliner was dominating the world market until a series of mysterious crashes ruined the company. Figure 6 shows a comparison of the Comet and today's airplanes. What happened? (Answer: The square windows of the Comet caused a high enough stress concentration to create a crack. After a few years of flying, the repeated pressurization of the cabin caused a crack to form, grow, and eventually crash the plane. This is why we see oval windows in planes today.)
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Figure 6. Stress Concentration: square vs. oval. click for copyright |
Activity Extensions (Return to Contents)
More Tower Power: When testing the towers, the lab technician should be able to record the force exerted by the tower as it is compressed. This data can be saved and opened in Microsoft Excel®. Have the students plot the Force of the Tower vs. Compression (units should be lbs vs. inches or Newtons vs. mm). Engineers often use a normalized version of these plots (i.e., stress vs. strain) when testing material properties. (Note: Consult the lab technician beforehand to make sure she can save and copy the data for the students.)
Related Curriculum: Another activity on stress and strain can be found on www.TeachEngineering.com, under Mechanics Mania unit, Stressed and Strained Lesson, Breaking Beams activity.
Activity Scaling (Return to Contents)
For upper grades or for more advanced students, have them create 3-D assemblies of their towers in CAD. The students or teacher can critique these 3-D assemblies and supply suggestions on improving the design.
Also, more advanced students could do the More Tower Power activity extension above and add key points of interest on their plots (i.e. maximum load, compression to failure, or any other points of interest they can think of).
References (Return to Contents)
Callister, William D. Jr., Material Science and Engineering: An Introduction, 7th Edition, John Wiley & Sons: New York, NY, 1996.
Public Broadcast System, Chasing the Sun, "Planes, Jet Age: DEHAVILLAND COMET," http://www.pbs.org/kcet/chasingthesun/planes/comet.html - accessed August 16, 2006.
Contributors
Christopher M. Yakacki, Malinda Schaefer Zarske, Diana Wiant, Janet YowellCopyright
© 2006 by Regents of the University of ColoradoThe contents of this digital library curriculum were developed under a grant from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education and National Science Foundation GK-12 grant no. 0226322. However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.
Supporting Program (Return to Contents)
Integrated Teaching and Learning Program, College of Engineering, University of Colorado at BoulderLast Modified: September 26, 2008