|
Publications: SRL: EduQuakes |
EduQuakes
May/June 2007
Educational Shake Tables
Creating and destroying miniature buildings and other structures is an enjoyable way to spend an hour or two. If watching toy-size skyscrapers shake, crumble, and even disintegrate becomes tiresome, it’s always possible to experiment with bricks in saturated sand. While such activities may seem unusual for adults, these experiments, when conducted on a shake table, are a remarkably effective and popular way to illustrate earthquake hazards such as resonance and liquefaction. On a more constructive level, one can easily illustrate ways to reduce or avoid earthquake damage, such as shear-wall strengthening or base isolation. All these demonstrations require a shake table capable of strong motions. Although at first this may seem to be an expensive constraint, creative educators have devised and used a wide variety of shake tables, and a solution is available for almost any budget and educational environment.
The use of shake tables to demonstrate earthquake hazards largely has been pioneered by the earthquake engineering community, which possesses considerable experience in this area. A future “EduQuakes” article will discuss educational shake tables and related activities developed by the University Consortium on Instructional Shake Tables (UCIST). Yet the natural link between earthquakes and geology makes these demonstrations highly relevant to earth science classes, both at the K–12 and university levels. Physical science classes also will benefit by seeing graphic examples of phenomena such as resonance. In this column we will provide a brief sampling of some current efforts and results, including a list of resources.
The first step is to obtain a shake table capable of high accelerations. The term “shake table” is used in a generic and all-encompassing fashion. The simplest version is a wooden board on rollers that can be shaken by hand. A slightly more sophisticated variant uses rubber bands to create a simple suspension for a board on a bed of marbles or wooden dowels. These basic, homebuilt shake tables, although limited in motion, are simple, cheap, and safe. The next step up is a mechanically shaken board. One way to set it in motion is to “drive” the board with a variable-speed electric drill, which allows easy (though somewhat noisy) modulation of the shaking. As always, an adult should carefully supervise the use of power tools, with safety foremost in mind (e.g., you’ll want to avoid catching fingers or hair in high-speed spinning devices). Running an electric drill for long periods may lead to overheating, and demonstrations of unexpected postseismic fire are not advised.
Basic shake tables can be used as a demonstration tool for lectures. The results are graphic, engaging, and entertaining. Shake-table demonstrations are also well-suited for laboratory exercises and student projects, both in college and at the secondary and middle-school level. Investigations include testing the response of structures to various types of ground motion or measuring the susceptibility of soils to liquefaction.
One favorite hands-on exercise is the “building contest.” In these experiments, students are provided with a limited set of materials and asked to create a miniature earthquake-resistant structure. Usually, some constraints are placed on design to ensure both a certain amount of conformity and potential weakness. For example, the structure might be a three-story building, a bridge, or a water tower. Potential building materials might include cardboard and tape, wooden sticks and magnets, sticks and marshmallows, and uncooked pasta. At the end of the exercise the “buildings” are tested, often to failure, on a model shake table. The purpose is not so much to discover the next Frank Lloyd Wright but to display the catastrophic effect of earthquakes on buildings. Designs are frequently creative and when shaken may display failure modes similar to those that occur in “real” buildings. “Soft” stories collapse, resonance sets in, and vital structural members detach, all of which the students watch with great amusement. The correspondence between damage to the models and real earthquake damage can be enhanced by displaying pictures of buildings damaged or destroyed by earthquakes. Students generally respond positively to this type of lesson. At the IRIS (Incorporated Research Institutions for Seismology) annual workshop for high school teachers, the building contest is always ranked highly and attendees seem to enjoy the exercise greatly. The final “shake-off ” often draws a crowd from the hallway as well as participants in the individual workshop.
A drawback to these simple shake tables is that the shaking cannot be easily compared with a real earthquake, which is necessary for projects that aim beyond the level of a simple demonstration. One solution is to calibrate the motion with an acceleration sensor. Simple, lightweight acceleration sensors can be purchased, along with display software suitable for a PC, for less than $100—although using these requires some basic technical skills (and time). Placing one sensor on the board and one at the top of the building shows amplification and resonance effects. A more sophisticated solution is a computer-controlled shake table such as the ones developed by the UCIST project, which allow calibrated reproduction of earthquake ground motion. A wide variety of exercises have been developed as part of this effort.
| Some educational shake-table resources: | |
| http://www.thetech.org/education/downloads/dconline/BuildingForTheBigOne.pdf | Earthquake hazard lesson plans for grades 4–8 |
| http://www.fema.gov/plan/prevent/earthquake/pdf/fema-253-intro.pdf | Seismic sleuths—a variety of earthquake hazard lessons for grades 7–12 |
| http://cive.seas.wustl.edu/wusceel/ucist | The UCIST Web site with description of shake table and many related activities (all levels) |
| http://web.ics.purdue.edu/~braile/edumod/building/building.htm | Examples of a building contest and acceleration sensors |
| http://peer.berkeley.edu | Shake-table videos; graduate education modules |
| http://www.curee.org | Videos of large-scale tests and online educational modules |
| http://www.nees.org/ | Additional earthquake engineering resources |
| http://webshaker.ucsd.edu/ | An online “webshaker” |
One difficulty with basic shake-table demonstrations is that they emphasize the dangers of collapsing structures but do not address hazards posed by poorly secured interior items (water heaters, bookshelves, etc.). However, several science museums such as the Tech Museum of Innovation in San Jose, California, or the California Science Center in Los Angeles possess large-scale shaking exhibits that allow visitors to “feel” an earthquake. Various vendors also sell mobile units that are capable of vibrating portable rooms. For people who have never experienced a strong or even moderate earthquake, a simulation of an earthquake can be eye-opening. Another effective way to convey indoor hazards is to watch videos of large-scale tests, which is enough to inspire one to go home and secure everything firmly or to live in a tent. Both the Pacific Earthquake Engineering Research Center (PEER) and the Consortium of Universities for Research in Earthquake Engineering (CUREE) have online videos and related online material available.
Finally, the “webshaker” (http://webshaker.ucsd.edu/) allows educational access to a research-quality shake table via the Internet and produces graphs of the resulting displacements and accelerations. The site contains a considerable amount of other resources related to shake tables, including an energetic online video of a K–6 “shake-off ” and a library of online “earthquake sounds.”
Model shake tables remain an effective way to blend earth science with seismology and engineering. Based on our experience with K–12 educators, considerably more potential exists, especially if an effective but low-cost shake table could be developed.
As always, we encourage comments and discussions.
Rob Mellors
San Diego State University
rmellors [at] geology.sdsu.edu
Robert de Groot
Southern California Earthquake Center
degroot [at] usc.edu
[Back]
Posted: 10 May 2007