This project gives the students a chance to design a simple engineering structure which must support a mass which is subjected to dynamic loads using materials that are limited in strength. Three sheets of common 8 1/2in by 11in Xerox paper and two feet of cellophane office tape are to be provided, along with a 12in by 12in base made of cardboard, plywood, or Masonite. The structures must be designed to hold a single raw egg 18in above the platform. Each structure is attached to a wooden pallet to which are tied four lengths of rope, one on each side. The "earthquake" is provided by four students pulling back and forth more and more vigorously until each structure fails. In general, students at the ropes should be those with structures on the pallet. Everybody should be given a chance at the ropes.
Instructor obtains pallet (free from local building supply companies), rope, clamps, bases (almost any rigid material will work), paper, scissors, rubber bands and tape.
*** First one hour meeting ***
Rules of project explained to class.
Large class: teams of 2 to 4 people assigned by instructor, one structure per team. Small class: individual structures
Paper, tape, scissors passed out, students design and build structures.
Eggs installed.
Structures mounted on pallet, shaken to failure.
Instructor discusses failure modes (see page 2) as they occur.
*** Second one hour meeting ***
continue with 6. and 7. until finished
Instructor presents 20 - 30 minutes introduction to the properties and behavior of simple structures and sheet materials (see below).
Students design and build improved model, some work at home OK.
** Third one hour meeting ***
continue with 9. as necessary
Structures tested ( and ranked according to failure sequence if grading is desired). "Special Awards" may be given to lowest ranking structures for "most original", "most unusual", "most creative", etc. to avoid creating losers.
Instructor summarizes, mentions that this is a simple example of the engineering design process, iteration, materials utilization, etc.
Material in sheet form resists pulling, or tension, very well (demonstrate with piece of paper) but is not so good for compression, or pushing, kinds of loads (demonstrate again). This type of failure is known as "buckling". To improve compression behavior, sheets can be stiffened by folding (demonstrate) or rolling (demonstrate). Sheet materials are used a lot for lightweight structures like aircraft. The very same principles as we've seen here apply and must be taken into account by many types of engineers (like civil, aerospace, and mechanical engineers for example) working on a particular structural design.
Structures are often composed of separate parts which support different kinds of loads. For example (use one student project), on this structure, these bands mainly support tension (they can't really support compression) while the central part here supports mostly compression (see how it failed?). On some of the structures like this other one here, the center part had to also support some bending loads (see how this one failed?) Bending loads put tension on one side and compression on the other. If we break a pencil, see how the wood splinters on the tension side. A thin walled structure made of sheet material will probably fail in compression on the other side. The amount of this bending load depends on the distance of the load from the supported end, the farther out the load, the more bending load is created. So projects like this one may failure early due to too much bending load. In this project, the lateral load is produced by the effects of Newton's second law (F=ma) on the egg as it is accelerated back and forth.
The damaging forces on structures from external vibrations can sometimes be reduced by making a more flexible connection with the source of vibrations. In our case, a more flexible support for the egg could reduce the lateral force acting on the egg simply because the egg would accelerate more slowly than with a stiff support and thus reduce F=ma. However, if the exciting vibration happens to be the same as the natural vibration frequency of the structure and support system, very dangerous large vibrations can develop. As a result, this kind of isolation must be done very carefully to keep structures from self-destructing! Usually, the natural frequency of the structure and support system is kept at least three times lower than the excitation frequency.
This project was developed by Dr. David Jenkins.