Today, a strange bridge. 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.
By design or by chance,
you've all seen the Steelers playing in
Pittsburgh's Three Rivers Stadium. It's
called that because it lies just north of the point
where the Allegheny and Monongahela Rivers meet to
form the Ohio. The rivers form a letter Y centered on Pittsburgh. The
stadium is just north of the juncture.
Today, bridges and tunnels criss-cross the three
rivers. But when Pittsburgh was mushrooming into one of our
major industrial cities, two centuries ago, bridges
were a crying concern.
The young Prussian engineer John Roebling came to America in
1831 and tried to form a Utopian community north of
Pittsburgh. Ten years later he gave up farming and
went to work developing wound-steel cables for
pulling barges. Then he realized he could
revolutionize bridges if he used his cables to
suspend them. Roebling began in Pittsburgh: an
aqueduct suspended across the Allegheny, a
1500-foot bridge over the Monongahela. He went on
to become America's greatest bridge builder. He
bridged Niagara Falls. He eventually gave us his
masterpiece, the Brooklyn Bridge. But the wildest
Roebling bridge never got built. Joseph Tarr and
Stephen Fenves tell about Roebling's Tripartite
Bridge.
The Tripartite Bridge was to lie right where the
three rivers meet. It was laid out like the center
of a Peace Symbol - three branches radiating from a
central point to link the three parts of the city.
Each branch was a single span about a quarter-mile
long.
The Pennsylvania governor okayed it in 1846, and
stock went on sale. But competing forces rose up
against the bridge. It would put ferryboat
operators out of business. It would deflate real
estate values in the city center. People were told
it would block river traffic and take cash away
from other important projects. In the end, no stock
was sold, and the project was put on ice.
If the bridge offered any real problem, it was in
the design. Suspension-bridge cables carry huge
loads. They're usually anchored solidly into the
earth. But here one end of each leg had to anchor
at a central pier out in the water. Cable forces
from three directions would have to balance one
another at that pier.
Could Roebling have done it? Maybe he could've, but
he died before the project was resurrected in 1871.
This time, developers turned to Roebling's son
Washington, who was finishing the Brooklyn Bridge.
This time they gave up on John Roebling's
cable-balancing act. This time one of the links
would be a solid truss bridge that could anchor
cables from the other two suspension branches.
But people still wouldn't invest. The project died
a second time, and we're left with all the what
ifs. Look at the tip of Manhattan, so much like
Pittsburgh with a river on either side. Instead of
the Brooklyn Bridge we could be looking at a
three-way bridge connecting Manhattan, Brooklyn,
and New Jersey. Technology is filled with what
ifs and almosts like that. Maybe we
should spend more time with the wonderful not
quites of invention.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
(Theme music)
