Swaying in the Wind

You will need: Four sheets of stiff paper, each 2 feet by 6 inches Cardboard strips, as shown in the diagram Tape Glue Scissors Two rulers Cutting knife Pen A toothpick Blow dryer A table or kitchen counter next to a tiled wall What to do: Make a model skyscraper as shown in the diagram. Cut fur strips of cardboard each 25" long and 2' wide. Draw a line down the middle of each strip, then score and bend the cardboard along this line. Make a cut 1" long from one end of the line and fold out the two flaps. Glue the sheets of paper to the strips of cardboard to make the structure as shown. Tape the cardboard flaps securely to the table so that one side of the model is almost up against the wall. Tape the toothpick into the right-hand corner of the cardboard frame that is against the wall so that it sticks up at least 1" from the top of the model. Tape one of the rulers to the wall so that the toothpick is pointing at the zero end of the scale. Hold the blow dryer one foot from the top of the left-hand wall and turn it on to the coolest, highest-speed setting. Notice what happens to the model skyscraper. Record how far the toothpick has moved from its zero setting. This gives a measure of the sway of your structure. Now try adding the X-shaped cardboard supports. Glue these securely to each side of the model. Position the blow dryer as before, turn it the same setting, and measure the amount of sway. What do you conclude? Taking it further: Experiment with your own methods to reduce the sway in your model skyscraper. For example, try adding extra strips of cardboard to stiffen the structure or devise ways to make the lower sections of the model heavier than the top. You might also look at how the shape of a structure affects the amount it sways. For example, try testing a structure that tapers toward the top. It is important in this experiment that the attachment of the model skyscraper to the table, or other surface, be very secure. Otherwise, the measurement will include some "give" of the base as well as bending of the whole structure. If the tape does not prevent the bottom of the structure from lifting away in the airstream then the arrangement is not a fair one. This is because it would be allowing an uncontrolled factor to interfere with the quantity under investigation – namely, the bending of the model skyscraper in the artificial wind. As with all the experiments described on these pages, the method given is only a suggestion. You should try out your own ideas and modifications. Perhaps you can devise a batter way of building the skyscraper or measuring the amount of swaying. Try making models of famous skyscraper such as the Sears Tower or John Hancock Center in Chicago, or the Empire State Building in New York. Remember, however, that this will not give a fair comparison of the amount of sway in these buildings since you will not have taken into account differences in internal structure, weight distribution, and so on.

Safety Checks

Tall skyscrapers may sway as much as 3 feet in a strong wind

The design of a new skyscraper has to be thoroughly tested to make sure that it can stand up to all the forces that will act on it. To determine the effect of winds, for example, a plastic scale model may be built that has small tubes with openings at various heights. This model is then put in front of a set of powerful fans. As the moving air from the fans strikes the model, the amount by which the air pressure in the tubes is increased can be measured. If the bending force of the wind seems too high then the design may have to be altered, perhaps by adding more steel beams. Sometimes a model will be made of the entire downtown area of a city. This is because the winds acting on a new building will be determined partly by the position and size of other skyscrapers nearby.

Much use is also made of computers in the early stages of skyscraper design. Programs running on a computer help to show how important parts of the design are affected by the loads they have to bear. The computer can produce color pictures highlighting regions where the force is most intense.

In some parts of the world, earthquakes are a threat – especially to tall buildings that might easily topple over. Skyscrapers in places such as San Francisco need to have special foundations that can absorb the shock of earthquakes. They also need to have supporting frameworks that can bend and vibrate without giving way. Special equipment known as a "shaking table" allows engineers to test whether a new design would be safe in an earthquake.