A lesson from FEMA/AGU earthquake curriculum for teachers of grades 7-12.
Unit 4.3
The BOSS Model:
BUILDING OSCILLATION SEISMIC SIMULATION
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Rationale
During an earthquake, buildings oscillate. If the frequency
of this oscillation is close to the natural frequency of the building,
resonance may cause severe damage, The BOSS model allows
students to observe the phenomenon of resonance.
Focus Question
Why do buildings of different heights respond differently in an
earthquake?
Objectives
STUDENTS WILL:
1. Predict how a structure will react to vibrations (oscillations)
of different frequencies.
2. Perform an experiment to establish the relationship between
the height of a structure and its natural frequency.
3. Describe the phenomenon of resonance.
Materials
for each small group
Teacher Preparation
Build the BOSS model by following the directions on Master
4.3a. Practice with your model until you've got a feel for
each frequency and you can get any of the rod assemblies to resonate. One
technique is to use a firm push, then watch the number you want
an wiggle the base very lightly at its natural frequency
to get resonance.
A. Introduction
Find out what students already know about the concepts of amplitude,
frequency, and resonance. If they are not familiar with these
terms, introduce them by building on what students already know
from other areas. They may know for example, that resonance
and frequency are used in describing the tone of musical instruments
and the quality of sound produced by different recording techniques
and players. The phenomenon of resonance also accounts for
laser light and for the color of the sky.
B. Lesson Development
1. Direct students' attention to the BOSS model, and explain its
name. Ask students to predict which numbered rod assembly will
oscillate the most when you wiggle the base.
2. Oscillate the BOSS model so that some rod assembly resonates other that the one most students predicted. Let the students predict again. Again make the rod resonate for an assembly they did not predict. Finish this demonstration after several tries by making the rod resonate for the assembly most of the students did predict. Invite discussion.
3. Relate the blocks and rods to buildings of various heights in an earthquake. Ask students if they think buildings would oscillate like this in an earthquake. (They always do, and in some earthquakes the effect is especially pronounced. In the 1985 Mexico City earthquake, the ground shaking resonated with the natural frequencies of 8-to10-story buildings. The effect was severe damage to medium height buildings that had the same frequency as the ground shaking and resonated with it. Higher and lower buildings were hardly damaged.) Use the BOSS model as a visual aid when describing this event.
4. Divide students into seismic engineering teams and distribute one copy of Master 4.3b worksheet, to each group. Tell students that they will take turns performing an experiment with the model, recording their data, and providing the answers asked for on the data sheet. Give these directions:
a. Hold the base stationary, pull the wooden number 1 out several centimeters to the side, and release it. As the rod oscillates, use a stopwatch to measure the time for 10 oscillations. Record this number.
b. Practice until you can get almost the same swing each time, then repeat the measurement four times. Calculate the average time. Record it. Repeat this procedure for the other three numbers.
c. Measure the height of each assembly from the base to the top, and record it.
d. Plot height versus natural frequency on the graph provided, (Students should come up with a hyperbola, a curve representing an inverse relationship in which, as the the height of the structure increases, its natural frequency decreases.)
e. Ask the class: From what you have learned, do the earthquakes with the highest numbers on the Richter Scale always do the most damage? ( To illustrate the relationship of frequency and resonance, use the example of someone pushing a child in a swing. The person pushes a little at a time, over time, and soon the swing goes very high without a big push. Each small push is at the right frequency. Similarly, a building may vibrate with a great amplitude without big earthquake vibrations because the smaller vibrations came at the structure's natural frequency.)
5. Ask the seismic engineering teams to share and discuss
their results. Again point out the connection between the
experimental results and the way real buildings resonate. Other
things being equal, taller buildings have lower natural frequencies
than short buildings.
C. Conclusion
Review the terms and concepts introduced in this lesson. Explain
that seismic waves caused by earthquakes produce oscillations,
or vibrations, in materials with many different frequencies. Every
object has a natural rate of vibration that scientists call its
natural frequency. The natural frequency of a building depends
on its physical characteristics, including the design and the
building material. Resonance is a buildup of amplitude in
a physical system that occurs when the frequency of an applied
oscillatory force is close to the natural frequency of the system.
In the case of an earthquake, the ground shaking may be at the
same frequency as the natural frequency of a building. Each
vibration in the ground may come at or dangerously close to the
natural frequency of the structure.
Ask the class to hypothesize what would happen when buildings of two different heights, standing next to each other, resonate from an earthquake. Wiggle the BOSS model so that assemblies 2 and 3 vibrate greatly, and let students see how buildings hammer together during powerful earthquakes. If you have some images of this effect from actual earthquakes, show them now.
Entice students to further investigation by leaving them with
the question: "How could you add structural elements to reduce
resonance in a building?"
Created By Dave Love, Chris Durand and John Van Der Kamp
last modified: 25 April, 2022