NOVEMBER 2002 VOL. 62 NO. 3 - International Technology and ...

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IDSATaut, Light & Flexible:Visiting Compression, Tension,& ModularityA single module design, wellconceived and constructed, is generallysuperior to multiple modules ofmediocre design or craft.R. Buckminster Fuller was a masterdesigner and builder of light, flexiblestructures and objects. Undoubtedlythe most ubiquitous design to emergefrom his energetic mind was thegeodesic dome. These lightweightRoger L. Funkstructures have been employed aroundthe world from the extremes ofAntarctica to mundane work on thefarm. While the success of thisbuilding system is legend, one of hisother important, but lesser knowndesigns, is the tensegrity structure.Perhaps less useful than the geodesicdome, this structural concept serves toillustrate a wide variety of forces anddesign issues present in almost anyobject or structure. The term tensegritycomes from the word combinationof tension and integrity. One of theobvious features of the tensegritystructure is the coupling and balancingof compression and tension forces. Atthe same time, it has a powerful visualelegance that is reinforced by aneconomy of material and the dynamicsof internal and external forces.We are surrounded in both thenatural and constructed worlds withthese structural forces that must beunderstood in order to function asknowledgeable designers. These elementalforces of compression and tensionare often viewed assuming thatcompression members are generallycolumnar and rigid while tensionmembers are frequently seen as flexibleand cable-like, albeit they may also berigid. Coupled with tension and compressionare the ancillary forces oftorsion, bending, and shear that arealso of critical importance in theunderstanding of designing structures.The project outlined in this article notonly serves to illustrate the basic compression/tensionequation but will alsoprovide an understanding of the qualitiesof torsion and bending forces.In introducing this project to yourstudents, it would be useful to ask that16THE TECHNOLOGY TEACHER • November 2002
IDSAthey engage in research on the fivebasic forces and have them discussthese in a group setting. You mightinvite the instructor of physics to jointhe discussion. As an entry point, sucha discussion could center around somecommon and obvious objects that areoften used as tension or compressionmembers in their application. Forexample, rope, cable, cord, string,thread, and wire are typical materialsapplied as tension elements. Another,less obvious, example could be a flexiblemetal tape measure. Its inherentflexibility allows it to roll up into asmall case, but it also bends easily(usually just when you want it to stayrigid), it twists easily (torsion), itshears easily (with metal snips), and itcollapses easily under compression.However, it is unusually strong in tension.In all likelihood it could supportseveral hundred pounds were it to beused as a cable—in tension.In contrast, a common concreteblock is very strong in compression,and four such blocks can easily supportan automobile. However, if theblocks were to be incorporated into ahoisting chain for an automobile, theywould fail quickly as tension membersand behave equally badly in shear,torsion, and bending.As you review other materials anddiscuss them with your students interms of the forces being examined, itwould be useful to test some samplematerials relative to the five basicforces. At the same time, you couldintroduce the two materials to be usedin this project—plastic drinking strawsand common sewing thread. Have thestudents handle them in order to feelhow the two materials respond to theforces under discussion. To furtherassist the students in developing anappreciation of the materials, alongwith a lesson on structural configuration,ask the students to do the following:First, cut (shear) a straw tovarious lengths and discover the differencelength makes in the stiffness of acompression member. The variouslengths will also illustrate the resistancethat shorter structural members haverelative to four of the five basic forces.Which four?Next, instruct the students to cutfour lengths of straws (they can beequal or unequal or even four wholestraws) and pin the straws at their endsto form a square or rectangular structure.The direction of the pins shouldbe perpendicular to the cross section ofthe straws and parallel with oneanother. Next, ask the students torepeat the exercise but with only threestraws in order to form a triangle.Then, ask them to feel the differencein the two structures relative to stabilitywhen a lateral load is applied.(Note: the four-member structure willcollapse as a parallelogram very easilywhile the three-member structure willresist collapse until a member fails.)Finally, have them run a piece ofthread through the center of a fulllength straw and leave about 18 incheshanging out of one end while securelyfixing the other end of the thread tothe opposite end of the straw. Theninvite them to hold the end of thestraw, where the thread exits, in theirhand and pull the thread parallel withthe straw to determine which memberfails first—the thread by breaking orthe straw by bending?The Problem Statement: Design astructure that is either vertical (atower) or horizontal (a cantilever) thatmoves from a base not more than twofeet square. The materials of constructionwill be only plastic drinkingstraws and standard weight sewingthread. This is to be a tensegrity structurecomposed of modular elementsconstructed of straws with the individualmodules connected viaNovember 2002 • THE TECHNOLOGY TEACHER 17
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