Worlds of David Darling > Children's
Encyclopedia of Science > Spiderwebs to Skyscrapers > Chapter 4
SPIDERWEBS TO SKYSCRAPERS:
The Science of Structures
a book in the eXperiment! series by David Darling
4. Sky High
Tall structures are the best-known and most spectacular human-made landmarks
in the world. From the Great Pyramid of Cheops in Egypt to the Empire State
Building in New York City, they impress anyone who looks at them.
|The John Hancock Center
|Cantilevers connected to a
central core support the great weight of some skyscrapers
But tall buildings also serve an important practical purpose today. By spreading
upward instead of outward, a skyscraper makes the best possible use of limited
space in the center of a city.
For example, the 100-story John Hancock Building in Chicago provides homes
for 1,500 people and work space for a further 3,500. A restaurant one thousand
feet above the ground and a swimming pool and grocery store on the forty-fourth
floor are among its other sky-high facilities.
A Pressing Problem
Weight is the biggest problem in the construction of high buildings. The
load on the lower sections of a very tall building is only slightly less
than that of the foundations. For this reason, the top section of a skyscraper
must be made of lighter materials. Most multistory buildings have frames
consisting of steel girders that become thinner toward the top.
As with all structures, a skyscraper must be designed so that the weight
of the upper parts is properly supported by the parts underneath. One way
to do this is to build a very strong central "core," like the trunk of a
tree. The floors of the building are then held up by powerful arching beams,
known as cantilevers, that stick out from this core like a tree's branches.
High Winds Beyond this Point
After weight, the next greatest force acting on a tall structure is the
pressure of moving air. Winds tend to blow much harder on the upper floors
of a skyscraper than on those lower down. As a result, there is a powerful
bending force on the base.
To resist wind pressure and prevent the top of a building from moving too
much, thick steel beams are used to stiffen the outer walls. These may consist
of heavy beams that run right across and around the skyscraper or diagonal
beams that make an X-shaped pattern. Diagonal beams were originally used
to stiffen just the central core of tall buildings, but now they are also
found on the outside walls.
Another way to reduce sway is to make a skyscraper narrower at the top than
at the bottom. This lessens the area of the walls where the wind speeds
are highest, so that the sideways force on the top parts of the building
Even the strongest skyscrapers, however, sway to some extent. The wind force
acting over one face of a tall building may be as much as two thousand tons.
This can cause the top of a building to sway back and forth by as much as
Hanging in Midair
You will need:
- Six books of the same size
- Several similar coins (pennies or nickels, for example)
- A table
What to do:
Arrange the books as shown in the diagram. The table represents the
core of a skyscraper. The books represent a cantilever, jutting out
from the core, supporting the floors of the building. Place several
coins at the edge of the top book of the cantilever. These represent
the weight of the floors the cantilever has to support. What happens?
If the cantilever is not strong enough, reduce the stagger of the
books until they can support their load. Notice the type of curve
the projecting ends of the books make.
Try arranging the books in a different way so that they stick out
as much as the cantilever and support the same load. Is this possible?
Taking it further:
Try doing the experiment with arches of different heights (keeping
the gap between the books the same). Which shape of arch is the strongest?
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
- Two rulers
- Cutting knife
- 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.
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.
| Tall skyscrapers may sway
as much as 3 feet in a strong wind
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.
A Model City
You will need:
- Containers of various sizes and shapes, including boxes and
- A large wood or cardboard base
- A blow dryer
- A desk lamp or flashlight
- Tape and glue
What to do:
Stick the containers to the base to make a model of the city. Make
the model realistic by drawing out a street plan and arranging the
buildings in well laid out blocks.
Cut small strips of paper and fold them around the pins to make little
flags. The paper should be able to spin around freely if you blow
on it. Stick the flags on various parts of your model – at street
level and on the tops and sides of the buildings.
Set up the fan so that it is level and pointing at the tops of your
model skyscrapers. Turn all the flags so that they are at right angles
to the direction the fan is pointing. Turn on the fan. What happens
to the flags? Which parts of the city have felt the greatest effect
of the wind? Move the fan to a new position, set up the flags as before,
and repeat the experiment. Describe your results.
Darken the room and use the lamp or flashlight to represent the sun.
Move the light over the model in a high arc. Notice how the shadows
of the buildings move and change size. Do any of the shadows of the
taller buildings extend beyond the downtown area? If so, these shadows
might fall on parts of the city where many people have their homes.
Decide if this is a problem and, if so, redesign your model to reduce
the effects of shadowing.
Taking it further:
Invent your own section of a city or use ideas from a city that you
know well. You might also study the effects, for example, of demolishing
one of the skyscrapers in your model and replacing it by a much higher
Instead of an entire section of a city, you could focus on just a
small cluster of skyscrapers. One of the effects of tall buildings
is that they tend to cause winds to spiral down to street level. You
could study this by putting several high structures together, marking
their sides and the model streets with pin flags, and then directing
air from a powerful fan at the tops of the buildings. Can you produce
a spiraling air pattern? If so, can you get rid of the wind at street
level by rearranging the buildings or spacing them farther apart?
Read about this effect and find out how architects attempt to avoid
the problem in practice.