The importance of shear resistance in building construction was discussed and the various options for strengthening a wall were demonstrated with model wall, following the guidelines in Seismic Sleuths. Each of the teachers was provided with the materials to construct a model wall for their classrooms. All of the craft sticks had been pre-drilled and bases had been constructed, so this basically amounted to connecting the craft sticks together with the nuts and bolts. One helpful modification to the original plans was the used of nylon-insert lock nuts rather than nuts and lock washers. After the tension at each joint was adjusted for a small amount of friction, he lock nuts remained fixed and did not require further adjustment.
A small shake table was used to further demonstrate the need for shear resistance in a wall. The shake table is powered by a variable-speed electric drill, so the frequency of the vibrations can be adjusted. Attached to the table are two button magnets about 5 inches apart. Four sticks are provided, each with a button magnet at each end. Using the sticks, a rectangular wall with a diagonal brace can be constructed, and this is the only arrangement of the sticks that will not fall down when the shake table is turned on to its highest speed. This setup was tested during the Tanana Valley State Fair last summer and was a big hit with the kids. It took some of them quite a while to figure out that the diagonal brace was necessary, but I'm sure that those who did figure it out will never forget.
An apparatus for generating longitudinal standing waves in a long spring was designed and built. It is powered by a saber saw which is firmly attached to a board about 1 foot wide and 3 feet long. One end of the spring is attached in place of the blade and the other end is attached to the far end of the board. When the saw is run, nodes and antibodies are clearly visible. A worksheet was provided to the teachers which students may use to learn about standing waves by plotting the sum of two waves as they pass each other traveling in opposite directions. After completing this graphical exercise it is clear why the spacing of the nodes is equal to one half of the wavelength. With this apparatus, one can show that if one loop of the spring located at a node is held in a fixed position, then there is no effect on the standing wave. If, however, two adjacent loops of the spring are held firmly, then the spring will only vibrate between that point and the motor.
A "string machine" was demonstrated. This apparatus was designed by Don Rathjen of the Exploratorium Teacher Institute, San Francisco, CA. It consists of two small dc motors that are held facing each other by arms made from PVC pipe. A string is attached from one motor's shaft to the other's. When the motors are powered, the string forms a loop, somewhat like the loop of a jump rope. The interesting thing is that if the loop is pinched in the center, then a pattern with one node can be created. It was possible, by adjusting the distance between the motors and their speed, to get on, two, or three stable nodes. A stroboscopic viewer was used to "stop" the motion of the string.
A building oscillation seismic simulation (BOSS) model from the Seismic Sleuths book was constructed for the teachers to observe and manipulate. The directions from Seismic Sleuths were followed, with the addition of four roller skate wheels to make it easier to vibrate the model. The long threaded rods are a bit vulnerable to bending, so one might want to use wooden dowels as described in the Exploratorium Science Snackbook "Resonator."
For background information, the short EERI Earthquake Basics Brief No. 1 on liquification was provided to each teacher. The liquefaction exercise from the Seismic Sleuths book was performed by teachers working in groups of two or three. Results were not as good as they might have been if better sand material were available. We tried with loess, which did not work very well, and with sand which was not too well sorted. The sand was better, and sand with a more uniform grain size might be better yet.
The program SEISMIC/ERUPTION by Alan Jones of SUNY Binghamton, New York, that dynamically displays earthquakes and volcanic eruptions on a computer-displayed map was shown to the PEPP teachers. They were provided with the URL location where the software can be obtained. The software includes a program that can convert a listing of earthquakes obtained from "finger" into the format needed for display. Alan's SEISMIC WAVES program was also demonstrated. This program shows the progression of seismic wave fronts as they pass through the earth and are converted and reflected.
A version of the "sliding brick on a bungee" earthquake generator has been built and was displayed to the teachers. In this model, a small winch is used to pull a 10 pound piece of volcanic rock along a strip of coarse sandpaper. The cord attached to the rock can be disconnected so that a 14 inch-long piece of bungee cord can be introduced between the rock and the cord to the winch. Without the bungee cord in place, the rock slides more-or-less continuously across the sandpaper as the winch handle is turned. However, when the bungee cord is included, then the rock does not move at all until the bungee cord has become quite stretched. When the rock does move, it generally slides a few inches at a time. It is explained that this is a model of elastic rebound; energy is stored in the bungee until the force on the rock exceeds static friction. Then the rock continues to slide until the force on it is less than the sliding friction. As an added element to this demonstration, a geophone is placed on the table and connected to an oscilloscope When the rock slides, a clearly defined 'earthquake' can be seen on the oscilloscope screen. A worksheet has been developed based on the relationship between magnitude and energy so that the magnitude of the sliding-rock earthquake can be computed. To do this, one measures the tension in the bungee cord before and after the rock slides, and the distance the rock slides. The average force times the distance gives the energy of the event, which can be converted to magnitude by using a table or graph provided. A small fish scale can be introduced into the line between the winch and the rock to measure the force on the rock. We have contacted Arlen Juels of Seistex '86, Inc. about obtaining used exploration geophones for use by teachers. He has expressed a willingness to provide used phones for the cost of shipping to teachers. Arlen can be contacted at email@example.com (303) 289-7001