Worlds of David Darling > Children's
Encyclopedia of Science > Spiderwebs to Skyscrapers > Chapter 1
SPIDERWEBS TO SKYSCRAPERS:
The Science of Structures
a book in the eXperiment! series by David Darling
1. Preparing the Ground
Structures, large and small, are all around us and serve many different
purposes. The ones we tend to think of first are those that people have
made, such as houses, bridges, dams, and skyscrapers. Certainly, these can
be very spectacular. The Sears Tower in Chicago, for instance, soars 110
stories and 1,454 feet above street level. Twin television masts on the
roof takes its total height to eighteen hundred feet, or over a third of
The Sears Tower (center left) soars above the city of Chicago
However, there are also many amazing natural structures. Spiders' webs,
birds’ nests, trees, caves, and mountains are just some of the "buildings"
that are put up without any human help. In fact, we could not be builders
ourselves if it were not for our skeleton – that vital structure inside
each one of us.
Basics of Building
A structure is simply a thing or a framework that provides support. It might
be a bridge that helps hold up a road across a river. Or it might be a house
that gives support to the roof and ceiling over our heads.
A Roman aqueduct built over 2,000 years ago
If the structure is a good one it will be strong enough to stand up to all
the forces that are likely to act on it over a long period of time. This
requires careful planning and design. Which shapes and arrangements of parts
are best suited to the task? What materials should be used?
We also need to think about what a structure will rest upon. How can a building
be prevented from simply sinking under its own weight?
The Need for Support
You will need:
- A large plastic bowl
- Large flat stones
- A square open-topped container
- A ruler
- A pen
- Several heavy books of equal weight
- Four pieces of wood ½" thick, 1" long, and ½" less
wide than the sides of the container (see diagram)
- Four pieces of wood as above but 2" long
- Four pieces of wood as above but 4" long
- A piece of stiff cardboard slightly bigger than the top of the
What to do:
Pack the flat stones tightly in the bottom of the bowl to a depth
of 2". On top of this put a 2" layer of gravel and, finally, a 2"
layer of sand. Make sure that each layer is smooth and even.
On the side of the container mark a scale in tenths of an inch, starting
from zero at the open end. Place the open end of the container lightly
on the surface of the sand. Its sides represent the outer walls of
a building. Gently place one of the books on top of the container.
The weight of the book represents the downward load acting on the
walls. Look at the scale on the side of the container and write down
how much the walls have sunk into the sand. Add several more books,
one at a time, and repeat the measurement each time.
Remove the container. Take the four smallest pieces of wood and push
them into the sand along the lines made by the sides of the container.
The top of the wood should be exactly level with the surface of the
sand. Place the container so that its walls sit on the wooden "foundations."
Again, gradually load the container with books and after adding each
book, measure the amount by which the walls have sunk.
Remove the container and the 1" pieces of wood. Repeat the experiment
with the 2" pieces and the 4" pieces. Try to explain what you find.
Plot your results in the form of a graph.
Place the sheet of cardboard 1" under the surface of the sand. Make
sure it is level. Push in the 1" pieces of wood so that they rest
on the cardboard. Place the container on the wood and load it with
books, one at a time, measuring how much the walls sink as the load
is increased. Compare the results with those you obtained earlier
for the 1” pieces without the cardboard underneath. What can you deduce
The bigger and heavier a building is, the more it pushes on the ground beneath
it. Because of this, all but the lightest structures must sit in specially
prepared supports known as FOUNDATIONS.
The foundations of the Georgia Dome in Atlanta
The soft upper layers of soil are dug away from below where the structure
will stand. In the case of an ordinary house, trenches several feet deep
are dug along the lines of the supporting walls. Then these trenches are
filled with concrete that quickly sets hard.
Larger, heavier structures, such as bridges and skyscrapers, need much deeper
foundations to prevent them from sinking. Just how deep the foundations
have to be depends on the makeup of the ground beneath the site. Where the
ground is soft, the foundations must go down a long way. If there is solid
rock near the surface, on the other hand, the foundation can sit directly
Huge skyscrapers such as those in New York City would have been impossible
to build in cities such as Houston, Texas, or London, England, until quite
recently. The reason is that the island of Manhattan consists almost entirely
of bare granite – a very hard and strong type of rock. Houston and
London, on the other hand, rest on thick beds of soft clay.
|These drawings show a foundation made with
a thick raft of concrete (top), and another constructed with piles
driven into rock
To carry the weight of a building such as a skyscraper, special foundations
known as "piles" are used. These are concrete or steel columns hammered
into the ground until they penetrate solid rock or hard-packed soil. Alternatively,
piles can be made by boring shafts into the ground and failing them with
If the underlying rock or firm soil lies too deep to reach with ordinary
foundations, engineers may put down a thick raft of concrete right across
the site. This spreads the load as widely as possible over the weak soil.