In this demonstration, students will understand how energy affects three idealized building models, of varying height. This will be accomplished by introducing a source of energy into the foundation of the model(either by shaking the foundation or using a rubber mallet, for example), and then observing the natural frequencies of each model. This energy will represent stored energy released in the event of an earthquake.
During an earthquake buildings oscillate, but not all buildings respond to an earthquake equally. If the frequency of oscillation of the ground is close to the natural frequency of the building, resonance (high amplitude continued oscillation) may cause severe damage. Small building are more affected, or shaken, by high- frequency waves (short and frequent). For example, a small boat sailing in the ocean will not be greatly affected by a low-frequency swell where the waves are far apart. On the other hand several small waves in quick succession can overturn, or capsize, the boat. In much the same way, a small building experiences more shaking by high- frequency earthquake waves. Large structures or high rise buildings are more affected by low-frequency, or slow shaking. For instance, an ocean liner will experience little disturbance by short waves in quick succession. However, a low-frequency swell will significantly affect the ship. Similarly, a skyscraper will sustain greater shaking by long-period earthquake waves than by the shorter waves.
-Taken from Teachable Moment resources for Haitiproduced by IRIS and the University of Portland
The above images represent a physical building model and idealized building model respectively.
Earthquake Engineering Component
Civil Engineers and architects design buildings and bridges to provide us with safe structures. One of their challenges is designing it to safely withstand natural disasters like earthquakes. The ground motion cased by earthquakes can oscillate large structure. If the natural frequency of a structure matches the oscillating frequency of the earthquake, then the structure could oscillate uncontrollably. One famous example of this is the Tacoma Narrows Bridge (http://www.youtube.com/watch?v=v7GvGqJquCU amp;feature=related and http://www.youtube.com/watch?v=j-zczJXSxnw amp;NR=1). The natural frequency is a function of the structures size, weight (mass) and stiffness. Engineers and architects use building codes that help them specify the appropriate conditions to avoid these conditions. These codes are based on testing and evaluation of models to predict the performance of these structures under dynamic conditions like earthquakes. However, sometimes these codes force the designers to overdesign their structures, which can be expensive. New experimental methods can provide us with opportunities to better understand how systems behave in various conditions.Systems can be mathematically modeled and used to predict how it will perform under various dynamic conditions. For example, civil engineers can model building structures that can be used in a computational analysis.
Learning Objectives and Standards
- Understand that an energy source introduced into a system, will cause structures of varying properties to behave differently
Links to the National Science Standards and to individual State Science Standards are available by using this link:Understand that natural frequency of a building varies based on building properties such as height
- Understand building stiffness
- Understand that building response in the event of an earthquake depends on the frequency content of the earthquake as well as natural frequency of the building
- A strong foundation to mount wires in (this can be wood or pages in a book)
- Three wires of the same properties, but three different lengths
- Three identical masses
- Assemble model building as demonstrated below
- Explain to students that this represents three idealized model buildings of varying helght
- Explain the students that frequency is defined as cycles per second.
- Introduce energy into the system by shaking the foundation at different frequencies and try to find the natural frequency in each individual model by shaking the foundation such that only one model is moving at a time
- Introduce energy into the system by using a rubber mallet, or a quick pound of the fist (why are the models shaking at different frequencies?)
Links and Resources
Three narrated demonstrations of seismic resonance and why certain buildings fall during earthquakes. Dr. John C. Lahr,USGS emeritus seismologist, demonstrates the concept using three different methods.
Teachable Moment resources for Haiti produced by IRIS and the University of Portland
See attachments for 30 second videos.
Researchers should cite this work as follows:
Tenille Denise Medley (2011), "Demonstrating Frequency using a "Lollipop Model"," http://nees.org/resources/3438.