Tech Directions - May 2008 - (Page 19)

the arm. The load on Big Buster is gradually applied to a hanger rod that is suspended from the cantilevered arm. We add metric weights in 500 gram increments with students waiting 10 seconds between each weight application. We videotape the loading, and on the failure of the structure students calculate the structural efficiency of their towers and analyze the videotape in slow motion to explain how and where the ultimate failure occurred. The analysis must provide a failure diagram that includes force direction arrows showing which structural members they believe were under tension or compression. In the analysis, they must also use key words in the correct context. The key words include compression, tension, load, failure, diagonal, column, tie, strut, and structural efficiency. Typically, students find that failure results from inadequate joining or poor material selection. Finally, students use computers to design an Excel spreadsheet that Tower side is carefully placed inside Big Buster’s Plexiglas “sandwich,” then tested for its structural efficiency. strongest structure is not necessarily the most efficient. This realization often leads to a discussion about the most important criteria involved in designing a structure like a tower or bridge. Building Big Buster To build Big Buster, I bolted two pieces of Plexiglas 5/32" apart so that the twodimensional tower side can slide between them. Picture an ant farm and you get the idea. The Plexiglas pieces are 1/4" shorter than the tower design so the entire load goes on the tower side rather than on the testing equipment. The load arm is hinged and cantilevered over the Plexiglas loading area and has an eyebolt fastened near the load end. I selected 4" × 4" red oak for the arm, which measures 4" wide × 30" long. You can easily change any of the dimensions to suit your structure building needs. Just remember that the height of the Plexiglas “sandwich” must be 1/4" shorter than your tower side design to give the structure room to compress without placing the load on the Plexiglas. In my original design, I used a 5-gallon pail with water for the load. That method worked well enough but ended up being messy—by the end of testing, we had a small pond on the floor. I purchased slotted metric weights from a scientific supply catalog (www.sargentwelch.com) and we use up to 30 kilograms of weights. We also use both a triple beam balance and a digital balance that students use to weigh their structures. To make the tower sides, students use 1/8" square balsa sticks purchased from Balsa USA (www.balsausa.com). To join the balsa, we use white Elmer’s glue. My students mostly use mitered and butt joints, but a few opt for half-lap connections. For shaping, we make our own “emery” boards from strips of 100-grit sandpaper glued to tongue depressors. Nearly all of the tower building materials can be ordered from Pitsco (www.catalog.pitsco.com). My students visit a great Web site to understand structural forces written by Douglas Prime of Tufts University (www.prek-12engineering.org). Big Buster at work contains testing data from each class member. It must calculate the efficiency of each structure and sort the data to determine whose tower was the most efficient. Most students are surprised to learn that the Editor’s note: For a version of this project with the student procedure in step-by-step format and full-scale patterns, ready to copy and use, visit www.techdirections.com/projectsB. html to purchase “Cost-Effective Tower Building and Testing.” www.techdirections.com PRE-ENGINEERING 19 http://www.balsausa.com http://www.catalog.pitsco.com http://www.prek-12engineering.org http://www.techdirections.com/projectsB.html http://www.techdirections.com/projectsB.html http://www.sargentwelch.com http://www.techdirections.com

Table of Contents Feed for the Digital Edition of Tech Directions - May 2008

Tech Directions - May 2008 Contents Technically Speaking Direct from Washington The News Report Mastering Computers Technology's Past Technology Today Road to Success - Service Learning Enhances Tech Ed Experience Balsa Tower Walls Brave 'Big Buster' These Kids Are Really on the Ball - Inventors Competition Winners School Web Site of the Month Animator Career Exploration Tools Guide Index to Volume 67 More than Fun

Tech Directions - May 2008

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