Key concepts
Physics
Engineering
Earthquakes

Introduction
Earthquakes can cause serious damage to buildings and be dangerous to people. But some of the world’s most populous cities are in earthquake-prone regions. How can engineers keep the millions of people in those cities safe? Find out how you can use science to save lives in this fun activity!

Background
Earth’s tectonic plates are continuously moving. Usually this movement is very slow—so slow that we can barely notice it. This slow movement may eventually cause cracks to form in buildings over time but will usually not cause sudden, unexpected damage. Sometimes, however, the plates get stuck for awhile and then rapidly become “unstuck”—kind of like when you try to open a stuck window when at first it doesn’t move at all and then suddenly moves very fast. We call this sudden movement of the ground an earthquake. These events can cause buildings to wobble and even collapse, posing a serious danger to the people inside or near them.

Engineers have developed various methods to make buildings more earthquake-resistant. For example, some buildings are slightly flexible so they can wobble back and forth a bit without breaking. Some very tall buildings have a heavy weight at the top called a tuned mass damper that helps cancel out the vibrations from a quake. Other buildings have isolation bearings, also referred to as a base isolation system. As the name implies, these bearings help isolate the base of a building from the ground, allowing it to move independently during an earthquake—that is, as the ground moves back and forth, the structure does not move with it. The concept is similar to shock absorbers on a car. When you ride over a big speed bump or pothole in the road, the shock absorbers help prevent the vibration from being transferred to the passengers in the car.

Engineers test their earthquake-resistant designs on model buildings using shake tables—special tables that shake back and forth to simulate an earthquake. In this project you will use a simple hand-powered shake table to demonstrate how isolation bearings can help prevent a building from shaking back and forth with the ground during an earthquake.

Materials

• Small cardboard box
• Piece of corrugated cardboard, larger than the base of the box
• Scissors
• Tape
• Several round markers or crayons
• Pen or pencil
• Piece of paper
• Flat desk or table next to a wall
• Ruler

Preparation

• Carefully cut two or three small squares from the piece of cardboard, and tape them together.
• Tape the cardboard squares to the wall about one foot above your desk or table.
• Tape the top edge of the piece of paper to the cardboard squares so the paper hangs down with a gap between it and the wall. Allow the paper to hang freely—Do not tape its other edges to the wall.
• Use the pen and ruler to make a bold mark in the middle of one lower edge of the cardboard box.
• Tape the pen to the top of the cardboard box, so its point hangs over the edge.

Procedure

• Place the cardboard box in front of the piece of paper you attached to the wall, with the point of the pen lightly touching the paper.
• Tape the ruler to the table in front of the box.
• Practice shaking the box side to side a few times.
• The pen should press against the paper hard enough that it draws a horizontal line when you shake the box. If it does not draw a line, try adjusting the distance of the box from the wall or try a different writing instrument (such as a different pen, or a pencil or marker) to see if that works better.
• Using the ruler and the center mark you drew on the box, try to consistently shake the box back and forth the same distance (for example, by one centimeter in each direction). Keeping this distance and the speed at which you shake the box constant is important for your tests.
• Now, try placing the box on top of several round markers, on top of a piece of cardboard. The markers should be perpendicular to the wall.
• Shake the piece of cardboard back and forth, the same distance and speed you shook the box itself. What do you notice about the box's movement? Does the mark the pen makes on the paper change?
• Extra: You just demonstrated a very simple base isolation system, but it has room for improvement. For example, you might notice it is hard to keep the box centered and it tends to drift to one side and fall off the markers. Can you design a system to keep the box centered and make it return to its original location or prevent it from sliding off the edge of the cardboard? For example, try using rubber bands attached to the cardboard base and sides of the building or build padded stoppers with cotton balls on the cardboard base.
• Extra: The background section mentions some other ways to make a building earthquake-resistant. Try building an earthquake-resistant tower. (For example, with building toys such as LEGO or K'Nex or with craft materials such as Popsicle sticks and glue or wooden skewers and Styrofoam balls.) Can you make the tower flexible so it bends but does not break during a simulated earthquake? Can you add a tuned mass damper to your tower to decrease how much it vibrates? (Search online for videos showing how tuned mass dampers work.)
• Extra: Even with a ruler, consistently shaking your building by hand can be difficult. You can build a simple rubber band–powered shake table that allows you to do more controlled, repeatable shaking. See the “More to explore” section for more information.

Observations and results
You should notice the cardboard box does not move back and forth as much when you place it on the markers. This is because the round markers allow the box and the ground to slide back and forth with respect to each other. When you shake the cardboard, it moves back and forth under the box—but this motion is not completely transferred to the box. (Some motion is still transferred due to friction in the rollers.) This simulates a base isolation system that lets a building move side to side independently of the ground.

Real-life base isolation systems are more complicated than this. As mentioned in the “extra” step above, you might have noticed that it was hard to get your building to return to exactly where it started. It might tend to drift off to one side and fall off the markers. Obviously that would not be acceptable with a real building—you need it to stay in the same place! Real base isolation systems contain springs, which generate a restoring force to pull the structure back toward the middle when it moves to either side. This introduces a new problem, however—springs oscillate (think: a Slinky bouncing back and forth after you pull on one end). So, base isolation systems also include dampers, or elements that use friction to diminish the oscillations. You might be able to build a better base isolation system using household materials. But be careful—strong springs (such as tightly-pulled rubber bands) with too little friction can actually make the oscillations worse!

More to explore
Earthquake Protector: Shake Table Crash Testing, from Benchmark for Structural Vibration Control
Earthquake-Proof Engineering for Skyscrapers, from Scientific American
Set Your Table for a Sweet and Sticky Earthquake Shake, from Science Buddies
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies