Worlds of David Darling > Children's
Encyclopedia of Science > Spiderwebs to Skyscrapers > Chapter 2
SPIDERWEBS TO SKYSCRAPERS:
The Science of Structures
a book in the eXperiment! series by David Darling
2. Shaped for Strength
It is much harder to break
an egg squeezing it from end to end than by squeezing it across the sides.
The reason is not that the eggshell is thicker at the ends, but that it
is more highly curved – a fact that is important in the design of
every structure, from a sports stadium to a subway.
Bridge that Gap
You will need:
- Four thick, heavy books of equal size
- A piece of stiff cardboard, 10" long and 3" wide
- Several similar coins (pennies or nickels, for example)
- A ruler
- A pencil
- Several other pieces of cardboard, 3" wide and of various lengths
What to do:
Place two of the books on a table 5" apart. Mark the midpoint of the
cardboard. Lay the cardboard across the gap so that the pencil mark
is exactly at the middle of the gap. Put down two more books on top
of the first pair. Measure the height of the center of the cardboard
above the table.
Place a coin on the midpoint of the cardboard. What happens? Measure
the height of the midpoint above the table. Put another coin on top
of the first and again measure the height of the center of the cardboard.
Repeat this until the cardboard bridge collapses.
Draw lines on the cardboard 2" from either end, and make folds along
these lines. Tuck the 2" flaps under the top pair of books so that
the section in between makes a smoothly curving arch. Repeat the experiment
above, measuring the height of the top of the arch as you add each
coin. What do you notice? Is the cardboard able to support a heavier
load when arched or not? Why do you this should be? Draw a graph for
each of your sets of measurements.
Try doing the experiment with arches of different heights (keeping
the gap between the books the same). Which shape of arch is the strongest?
Beams and Arches
Some of the earliest bridges made by people were probably just logs laid
between opposite banks of a ditch or stream. The log, used in this way,
is an example of a BEAM – a flat structure supported at each end.
The forces acting on a beam and an arch
Beams may be made of wood, stone, concrete, iron, or steel. They are very
common in all types of buildings, old and new, because they are so simple
to make. But they do have a big drawback. Beams will bend or break if too
much weight is put on them. One way to get around this problem is to make
the beam quite thick or build it using a very strong material. Alternatively,
a different type of structure, such as an ARCH, may be used in the beam's
The shape of an arch gives it great strength. The downward force of any
weight is carried away along the arch's smoothly curving sides to its supports.
This spreads out the effect of any load. In a beam, by comparison, the load
is usually greater at certain points than others, so that the structure
has weak spots where it will tend to give way.
Holding Back the Water
DAMS are huge walls of stone or concrete built in the path of a river to
form an artificial lake, or reservoir. The reservoir may be used as a water
supply for towns and cities, or to produce electricity in a power station,
at the foot of the dam.
Types of dam
A dam may take one of several basic forms. The simplest is the embankment
or gravity dam. This slopes up on each side from a broad base (where it
has to withstand the greatest water pressure) to a narrow crest. The Grand
Coulee Dam in Washington is of this type. It uses its enormous weight of
over 20 million tons to hold back the reservoir behind it.
Less bulky dams, which are cheaper and faster to build, must rely on a stronger
shape to resist the force of water acting on them. A dam may be arched,
for instance, so that the water pressure is carried outward to the sides.
In fact, an arched dam is like a very strong arched bridge lying on its
If a dam is arched both up and down and from side to side, then
it has even greater strength. Such a structure is known as a cupola dam.
|The Grand Coulee Dam in Washington is an
embankment, or gravity, dam
All Trussed Up
A single beam, we have seen, can easily bend. However, a framework of beams
joined together is much stiffer and stronger. The strongest of all frameworks
are those in which the beams meet to form triangles. Such an arrangement
is called a truss and is based on the principle that a triangle is the hardest
shape to twist or bend.
Look in the attic of a house and you will find that the roof is held up
by a series of trusses made from thick wooden beams. Over the years, engineers
have experimented with all sorts of truss designs. For example, wooden trusses
were used to support many railway bridges in North America in the last century.
Such structures were found to be especially strong if neighboring triangles
shared the same side. The common side is known as a king post.
Today, trusses are still used in a wide variety of buildings. They stiffen
the supporting structure of tall skyscrapers, form the framework of radio
and television masts, and hold up most of the roofs in our houses, stores,
offices, and schools.
You will need:
- Stapler and staples
- Strips of cardboard, 14" long and 1" wide
- A plastic cup
- Several similar coins (pennies or nickels, for example
- Needle and thread
- Two thick books of the same height
What to do:
Cut cardboard strips and use staples to make a square and a triangle
as shown. Try pulling the sides of these shapes. What do you notice?
Build a box-shaped framework as shown, using cardboard strips, staples,
and tape. Hang the cup by a thread from the center of the framework.
Place the two books 12" apart and put the framework evenly between
them. Measure the height of the of the center of the framework. Put
a coin in the small box and measure the height again. Do this a number
of times and write down your measurements. Can you make the framework
Now make a triangular framework as shown. Repeat the experiment and
compare the results. Which framework is stronger? Can you suggest
Taking it further:
Explore ways of making the box-shaped and triangular frameworks stronger.
Try loading them in various ways to find if they have any weak spots.