Today, we make buildings by stacking stone. The
University of Houston's College of Engineering
presents this series about the machines that make
our civilization run, and the people whose
ingenuity created them.
Jacques Heyman looks at
ancient buildings in his book, The Stone
Skeleton [1]. Then he
tells how the nature of structural design changed
400 years ago. The change took place right after
Galileo wrote a crude theory for calculating
stresses in a cantilever beam. Ever since then,
structural engineers have focused on stress
analysis. They ask what loads cause beams to crack
or arches to collapse. Ever since Galileo, new
kinds of math have steadily made it easier to
answer those questions.
Ancient design was another matter entirely.
Medieval masons didn't study the theorems and
proofs of Euclid. But they did use their squares
and compasses to form marvelous geometric shapes in
stone. The old masons found natural shapes that
stay in balance -- even when they're disturbed. As
long as you don't subject stones to tension, or to
forces that make them slide on one another, masonry
stands up. The old cathedrals have survived
earthquakes and bombing raids that've leveled
cities around them.
In 1675, a generaton after Galileo, the English
scientist Robert Hooke
made a point that helps show how we build with
masonry. First, he said, hang a chain from two
points. It naturally hangs in a state of pure
tension. Now turn the shape of the hanging chain
upside down. You get the shape of an arch that's
loaded in pure compression -- no tension, no stones
sliding on one another.
Heyman inscribes that upside-down hanging-chain
shape on pictures of old arches. He finds it fits
within all of them. Once the arch accomodates that
shape, you don't need any stress analysis.
Unless you load stone so heavily that it breaks
under compression, it won't break at all. And you
don't squash stone. Hagia Sophia, Notre Dame,
Westminster Abbey -- they were all created by the
geometry of stacked stone.
In the century after Galileo we started building
with iron, then steel. Steel is as strong when you
pull it as when you squeeze it. Now we made bridge
trusses with members in tension as well as
compression, and cantilever structures that could
never be rendered in stone. All the while we wove
mathematics about our designs.
But we also lost features with the new structures.
Many old masonry buildings are cracked, but the
cracks hardly matter. It's an old saying that if
masonry stands five minutes after you remove the
forms, it'll stand 500 years. Hagia Sophia
Cathedral in Istanbul, with its dome over 100 feet
across, has stood almost 1500 years.
To build the old buildings of stacked stone, you
began by seeing them, not by calculating them. They
came from an almost incomprehensibly different
corner of the human psyche than steel buildings do.
And that is surely why they touch a different part
of the human psyche as well.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
(Theme music)
