Have you ever wondered how tall skyscrapers can stand up so impressively to the force of gravity? But what about more violent forces, such as those produced by earthquakes? A well-planned and tested design, when combined with the right materials, can keep a building intact through all sorts of shakes and quakes.
Once the tallest buildings in the world, the Petronas Towers in Kuala Lumpur, the capital of Malaysia, stand at an amazing 452 meters tall. Because Malaysia is in an area that experiences frequent earthquake activity, the towers had to be designed to withstand the lateral shaking force that is experienced during a quake. Can you, as an amateur architect and engineer, design a structure that can withstand lateral movements similar to that of an earthquake?
Architects and engineers must design buildings to withstand a variety of forces, some stronger than others, from many sources: gravity, people inside, weight of building materials, weather and environmental impacts. If the design is stable, these forces will not weaken the structure or cause it to collapse.
Lateral shaking is the force that can cause the most damage to a building during an earthquake. As you might have guessed from its name, this force usually occurs in a direction parallel to the ground. Designing a building to have lateral resistance is helpful not only for preventing quake damage, but also from other lateral forces, such as wind. Engineers can test how well a building will hold up to lateral force by placing a model of it on a "shake table," which moves horizontally to replicate the stresses created by an earthquake.
• LEGO bricks
• Large, flat LEGO baseplate
• Metric ruler
• Three-ring binder (an old one that is okay to take apart)
• Four small rubber balls (each the same size, about 2.5 centimeters in diameter)
• Two rubber bands (each about eight centimeters or longer when flattened and doubled on itself)
• Carefully cut the front and back covers off of the three-ring binder with scissors. (This might be a good task for an adult.)
• Place the two binder covers on top of one another and "rubber band" them together by stretching a rubber band around each end, about 2.5 centimeters from the edge of the boards.
• Insert the rubber balls between the boards at each corner, placing them about five centimeters in from the edges.
• Attach a large, flat LEGO plate to the top by slipping the plate underneath the rubber bands. Your "shake table" is now ready to shake some towers!
• Practice creating a lateral shaking movement with the shake table by pulling its top layer horizontally out of alignment and then letting it go.
• Gently try pulling the top layer as far out of alignment as you feel comfortable with (and without damaging the shake table) then measure the distance of displacement, which is the horizontal distance between the top and bottom layers.
• Build four or more LEGO towers of increasing height on a nearby surface. Use the same base size and shape for each tower, so that the size of the towers' footprints are the same, and only their heights vary. What are the heights of the different towers?
• One at a time, starting with the shortest tower and progressing to the taller ones, secure each LEGO tower in the center of the shake table's top surface. To test each structure, create a lateral shaking movement using the same distance of displacement you previously measured. Did all, none or some of the towers fall? If some fell and others did not, what were the differences in height between these towers? In general, did the taller towers fall more frequently than the shorter ones?
• Tip: If none of the towers fell, try testing this activity with taller towers and/or a smaller base size. If all of the towers fell, try testing this activity with shorter towers, a larger base that takes up more space and/or a smaller distance of shake-table displacement.
• Extra: In this activity, you kept the footprint of each tower the same and only changed the height. Try testing LEGO towers with different-size bases. Do you think that by changing the footprint you could make taller buildings more stable? You could also calculate the area of the base (by multiplying its length times its width in centimeters) and divide this by the height for each tower to get the ratio of base to height. How do the towers' base-to-height ratios compare with how they perform on your shake table?
• Extra: Try building towers out of a different material that allows you to test different structural designs. Good materials are straws, popsicle sticks or toothpicks and marshmallows. Try comparing square designs to triangular or hexagonal designs. Try adding extra structural elements to your designs. Can you design a stabler tower? How tall can you build it before it loses stability?