The Pantheon Dome, completed in 128 A.D., was one of the most impressive engineering accomplishments of its time -- indeed, for all time. Although Romans had been building small domed bathhouses and temples for centuries, no other dome would match the Pantheon Dome's 143-foot diameter for another 15 centuries. The dome was constructed primarily of concrete, a material rarely used at that time for anything besides foundations. This allowed

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The Pantheon Dome, completed in 128 A.D., was one of the most
impressive engineering accomplishments of its time
--

indeed, for all
time. Although Romans had been building small domed bathhouses
and temples for centuries, no other dome would match the Pan
theon
Dome's 143
-
foot diameter for another 15 centuries.


The dome was constructed primarily of concrete, a material rarely
used at that time for anything besides foundations. This allowed
workers to more easily taper the thickness of the dome walls, whic
h
ranges from 5 feet near the top to more than 20 feet where the dome
meets the building's vertical walls. Another impressive feature of the
Pantheon Dome is the oculus, or "eye", which provides a 27
-
foot
opening to the sky. The oculus is located 143 feet
above the building's
floor, making the dome as high as it is wide.


The Pantheon's completion was certainly a crowning achievement for
its designer, Hadrian, the 13th emperor of Rome. It did, however,
present its share of engineering problems. Most import
ant of these
was that the dome required significant lateral support in the form of
enormous concrete "step rings" near where the elegantly arching dome
meets the vertical walls of the lower part of the building.


Throughout most of a dome, inward and upwa
rd forces counteract the
force of gravity
--

the weight of the dome itself. These opposing
forces cause the individual sections of the dome to push hard against,
or compress, one another. Where compression is high, the dome is
stable. At the lower sections
, where the dome meets the building's
vertical walls, however, the necessary compressive force is missing.
To save the dome from collapse, Pantheon engineers placed the
massive concrete step rings around the base of the dome. These rings
are heavy enough t
o resist the outward force of the dome and have
kept the dome stable and relatively free from cracks for nearly 2,000
years.


Can you think of a dome in a public building, religious building, sports
stadium or other structure near where you live? Describe
the parts of
the dome that are in tension and compression. How did the people
who built this dome solve the problems of tension and compression?

The narrator states that, "...compression makes the ring strong and
stable." Why is this true? Explain the forc
es involved?

The narrator explains that, "...about halfway down, the ring pushes
out. This is called tension." Why does this happen? Can you explain the
forces involved?

The step rings solved a problem that the Pantheon Dome had. Can you
find structures ar
ound you that required some additional feature to
take care of a problem they had? Are there outward forces on the
walls of our building, of the swings sets in the playground, and in
other structures?