Today, we design for tension. 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.

I'd like to try an idea on you. The
Washington Monument is 555 feet high, and it weighs 81
thousand tons. The Eiffel Tower,
nearly twice as tall, weighs only ten thousand tons. Now,
scale each down to a height of one foot, reducing every
part in precise proportion. The Washington Monument model
will weigh one pound, but the Eiffel Tower model will
weigh an incredible *1/3th of an ounce*.

What on earth can be so different about the two
structures? The steel in the Eiffel Tower is three times
as dense as the granite in the Washington Monument.
Shouldn't it be far heavier?

The Washington Monument is the tallest masonry structure
ever built. It pushes an upper limit to masonry. Stone
will resist as much loading as steel will in compression,
but it's weak in tension. The stone in the Washington
Monument carries immense compressive loads near the
bottom, but almost no tension. It's
only squeezed; it's not pulled.

The steel Eiffel Tower, on the
other hand, distributes forces throughout a gossamer
network of beams. A single beam might carry tension on
top and be compressed on the bottom. It's contrived so
that every element in a complex structure serves the
whole.

The Washington Monument was finished in 1885. That was
the same year the first tall steel-frame building went up
-- the nine-story Chicago Home Insurance Building. But
those two structures marked an architectural passing of
the torch. After that, real height would be achieved with
the use of light steel frames. The Eiffel Tower was
finished only four years later.

Tall steel-framed buildings weren't the only such idea to
come out of Chicago. Fifty years earlier, the
construction used in your house also originated there --
the familiar idea of making houses from light two-by-four
studs crisscrossed with even lighter pieces of wood.
That's come to be called the Chicago
Balloon Frame.

Before the 1830s, houses were built with heavy
interlocking beams. The first balloon frames looked as
light and gossamer as the Eiffel Tower. They were its
predecessor, for they distributed forces with the same
efficiency as Eiffel's girders. Since then, all kinds of
things have been built that way. Bridges were first, then
houses, skyscrapers, and finally *airplanes*.

A Boeing 727 has to withstand far nastier stress than the
Eiffel Tower. It bears the huge forces of aerodynamics
and of its own engines. But those forces flow through its
members, right down to the stressed aluminum skin around
it. Reduce a 727 to the size of that Eiffel Tower model,
and it will weigh only twice as much -- two thirds of an
ounce instead of just one third.

So a new idea changed everything in the nineteenth
century. It seems paradoxical that the very density of
new and available metals is what led to a whole new
lightness of being.

I'm John Lienhard, at the University of Houston, where
we're interested in the way inventive minds work.

(Theme music)