Shake It Up! Engineering for Seismic Waves

In certain areas of the world, earthquakes are a serious concern. Civil and structural engineers who focus on designing buildings, bridges, roads and other infrastructure for earthquake-prone areas must understand seismic waves and how to construct structures that are able to withstand the forces from the powerful ground motions of the Earth. For testing purposes, engineers design shake tables to simulate (or re-enact) the seismic waves produced by earthquakes and verify the stability and survivability of their structures.

After this activity, students should be able to:

• Explain the four different types of seismic waves produced by earthquakes.
• Describe the purpose of shake tables and how engineers use them.

Students should be able to measure length with a ruler, and have an understanding of seismic waves (as provided in the associated lesson).

How many different kinds of waves can you think of? (Listen to student suggestions and add others. For example, electromagnetic [light, radio], sound, ocean [water], seismic, pressure, compression, standing and sine waves.) No matter what kind of wave, what do they have in common? (Draw a wave on the board and identify its parts.) That's right: amplitude, wavelength, crest, trough, frequency.

What types of waves do we associate with earthquakes? That's right, seismic waves. Seismic waves are waves that move through the Earth, and are typically created by earthquakes. For all seismic waves, the amplitude or intensity of the wave is dependent on three things:

1. The depth at which the earthquake took place (the closer to the surface, the greater the amplitude of the wave)
2. The intensity of the earthquake (earthquakes with higher Richter scale ratings produce more intense seismic waves)
3. The composition of the Earth's crust

The people who work in "earthquake engineering" focus on protecting us and the natural and human-built environments from earthquakes. They want to limit our risk of death and damage from earthquakes. How can we possibly make sure that our school or stadium or a skyscraper or a freeway overpass will not collapse in a big earthquake? Well, engineers create shake tables to test the ability of buildings and other structures to withstand the seismic waves produced by earthquakes. To do this, they carefully design and construct shake tables that can accurately re-enact the ground motion of the Earth during earthquakes. Sometimes they test full-size buildings and sometimes they test small-scale model buildings or components. Some shake tables are large enough to put a real-size building on; others are smaller, even tabletop size. By doing this, engineers can test materials, designs, and construction methods to develop building codes and best practices that provide people living in earthquake-prone areas with safe and survivable surroundings.

Engineers must understand everything about the various seismic waves produced during earthquakes and how they cause the Earth to move. Who can tell me the four types of seismic waves that engineers need to simulate? They are:

1. P-waves (or primary waves, a type of body wave)
2. S-waves (or secondary waves, a type of body wave)
3. Love waves (a type of surface wave)
4. Rayleigh waves (a type of surface wave)

What do you know about these different types of seismic waves? How are they different from each other? P-waves and S-waves are body waves, which travel through the body of the Earth. P-waves are the fastest of all the seismic waves and can travel through any medium, although they move through solids faster than through liquids and gases. P-waves vibrate the parallel to Earth or in the direction of their propagation. They are similar to a compression wave moving through a slinky. S-waves are the second fastest type of seismic waves, and they can only move through solids. S-waves are transverse or shear waves and move the Earth perpendicular to the direction of propagation. Both P-waves and S-waves are types of body waves and travel through the interior of the Earth.

Love waves and Rayleigh waves are surface waves, which travel along the surface of the ground. In general, surface waves are slower than body waves—and more destructive. Love waves cause a horizontal shifting of the Earth perpendicular to the wave propagation. Rayleigh waves are a type of sinusoidal wave and move like ocean waves. They are produced by the interaction of P-waves and S-waves. Rayleigh waves are the slowest of all the seismic waves with a speed approximately equal to 3 km/second.

Smart design and testing make buildings resistant to the seismic wave movement of earthquakes. A properly engineered structure does not necessarily have to be extremely strong or expensive, but it must be correctly and intelligently designed to withstand the seismic effects while sustaining an acceptable level of damage. What are your ideas? Let's create our own shaker tables and model buildings to test them.

Before the Activity

• Gather materials and make copies of the Shake It Up! Activity Worksheet (PDF).
• Build the teacher's shake table that uses a variable speed drill to create the shaking movement. Or, if you do not want to build the drill-powered shake table, at the end, have the class select the best shake table from all the groups and use that one for the earthquake challenge.

With the Students

1. Show students the available materials. Point out that this project follows the steps of the engineering design process: understand the need (requirements, objective), brainstorm different design solutions, select the most promising design, plan (strategy, drawings, measurements, materials), create and test, and improve to make the best solution possible.
2. Identify a few design requirements and constraints:

• For the shake tables, a wooden board must serve as a base that can move around to simulate the ground movement during an earthquake. The goal is to create shake tables that move in ways that resemble the different types of seismic waves. For example, movement could be the back and forth motion of a P-wave or a more destructive rocking type movement representing a surface wave. Also design a way to control the shake table from outside of the box (so your hands are not in the box where the model building will be located).
• The model buildings must be made only from toothpicks and marshmallows, and be at least one-foot (.3 m) tall. (Consider imposing a materials limitation to make the project more challenging.) Its base will be taped to the wooden board for testing.

3. Hand out the worksheets to students. Give them time to independently design and draw their shake tables and buildings, as instructed on page 1 of the worksheet. If possible, assign this as homework the night before so students have a chance to develop their own ideas before coming together in teams to determine the most promising designs.

(Note to teacher: If students need more clarification of the movement generated by the four seismic wave types, refer to the PowerPoint presentation in the associated lesson. Tips: It is a challenge to make the student shake tables really replicate all four wave movements; it is easiest to focus on the P-waves and surface waves. To replicate P-waves, students must find ways to move the board back and forth along a horizontal plane. To replicate surface waves, students must find ways to move the board in a more up-and-down, lopsided fashion. Placing marbles under a board enable it to slide back and forth, and using different-sized marbles generates a more "surface wave" type motion. To control the shake tables, it helps to cut access holes in the sides of the box, and/or adhere string or Popsicle sticks to the box and its components.)

4. Divide the class into groups of three or four students each.
5. Ask students in each group to brainstorm ideas, starting by sharing their individual ideas. Have each team choose one design to construct for its shake table. In the spirit of true brainstorming, encourage teams to combine and compromise their ideas to come up with creative solutions. (Review brainstorming guidelines at http://www.cs.unb.ca/profs/fritz/cs3503/storm35.htm.)

Students learn about the distinctly different movements of various types of seismic waves as they build shake tables from shoeboxes to test the strength of model buildings.CopyrightCopyright 2010 Carleigh Samson, ITL Program, University of Colorado Boulder

6. Provide students with materials and give them time to construct their shake tables—a minimum of 30 minutes for construction is suggested.
7. Once shake tables are completed, have groups brainstorm ideas for their model building structures that use only mini marshmallows and toothpicks as the materials. Require that the buildings be at least one-foot (.3 m) tall. Have teams each agree upon a final design that they will construct.
8. Give students time to construct their model buildings, and then use their own shake tables to test and modify (improve) the designs. Point out that the testing-improving-testing process is an important part of the bigger engineering design process. That's how weaknesses are discovered and problems solved—before you have an actual earthquake! Emphasize that in the upcoming earthquake challenge they will have only one chance to put their final building designs through a "real earthquake" test to see if they survive, so they must be certain that their buildings are survivable. What works? What doesn't? What could be improved? Test, test, test!
9. Earthquake Challenge: Once teams have one-foot tall structures and are satisfied with their stability and robustness, put the structures through a one-minute simulated earthquake challenge in which every team uses the same shaker table—either the teacher's shake table (that uses a variable speed drill to shake the table), or the best of the teams' shake tables, as agreed-upon by the class.

Students testing model buildings on a shake table that simulates the movement of an earthquake.CopyrightCopyright 2010 Carleigh Samson, ITL Program, University of Colorado Boulder

10. Have one student use a stopwatch to time how long each building survives the earthquake simulation. Remind groups to be ready to record the length of time their buildings lasted, the end building heights, as well as observations about how the building structures behaved under the shaking conditions. Have students watch all team tests to gather observations that they will use to finish the worksheet questions.

• Failure: If the building collapses or any part of the building besides its base touches the shake table, consider it failed, and note the time and stop the shake table. The building is not earthquake-safe for people. Once the shake table is off, measure the height of the building.
• Success: If the building survives for a full minute and is still one-foot tall, consider it a success—the group has engineered a solution to the challenge and is "hired" to design real buildings for their community. Record measurements and observations.

11. Have students complete the concluding worksheet questions, incorporating what they learned from observing their own and other groups' model building behavior under seismic stress. Have them draw conclusions about the relationship between the appearance of the structure and its building strategies, and its performance. If time permits, lead a class discussion using the concluding questions (see the Investigating Questions section) so students can hear each others' opinions and ideas.

Pre-Activity Assessment

Design Section of the Worksheet : As either pre-activity homework or the first task of the activity, assign students to complete their own designs for shake tables and model building structures, including drawings, measurements, material specifications and explanations of how the designs function. Require that students describe which types of seismic waves their shake tables will produce and how those types of seismic waves move the Earth.

Activity Embedded Assessment

Observations and Questioning: During the activity, move around the classroom to observe students and ask them questions about what they are doing to determine how well they understand the activity. Ask individual students to explain what the group is working on, their strategies, what type of seismic waves their shake table creates, etc.

Post-Activity Assessment

Conclusion Section of the Worksheet and Class Discussion: Review students' answers to the Shake It Up! Worksheet (PDF)questions to gain an understanding of why they think certain structures performed better than others. See whether or not students thought the ability of their shake tables to accurately represent seismic waves helped in the evolution of their building designs. Explore the questions in a class discussion format so that students can hear each others' opinions and ideas (see the Investigating Questions section).

• Review safety precautions for using glue guns and box cutters.

Make sure students test, revise and improve the integrity of their structures using their own shake tables. If this is not emphasized, students may just build a shake table and a structure, and move directly to the "real earthquake" without the learning that comes from the testing/redesign cycle.

For good descriptions and drawings of seismic waves types, see Michigan Tech's UPSeis web page at http://www.geo.mtu.edu/UPSeis/waves.html

Find more information on earthquakes, earthquake engineering, earthquake shaking table, Love wave, P-wave, Rayleigh wave, S-wave, seismic wave, and wave at https://en.wikipedia.org/wiki/Main_Page

Show students a 43-second video showing a comparative test of two 12-floor model towers under earthquake simulation, one with seismic base isolation in place and one without. See Earthquake Protector: Shake Table Crash Testing at https://www.youtube.com/watch?v=kzVvd4Dk6sw&feature=related

World's Largest Earthquake Shake Table Test in Japan. Simpson Strong-Tie Company, Inc. Accessed April 20, 2011. (Article and a five-minute video show a full-scale seven-story wood-framed condominium tower being tested on world's largest shake table in July 2009, where it survived a 7.5 magnitude earthquake simulation with minor damage) http://www.strongtie.com/about/research/capstone.html?source=hpnav