Today, we face the ghost of yesterday's bridge
failure. 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.
In his fine book on design
failures, Henry Petroski hits us with this remark:
The paradox of engineering design is that
successful ... concepts devolve into failures,
while the colossal failures contribute to the
evolution of ... inspiring structures.
He uses the collapse of the Tacoma
Narrows Bridge to make his point. The Tacoma Narrows
Bridge was finished in 1940. That was 57 years after
the Brooklyn Bridge capped off John Roebling's
brilliant career as the creator of modern suspension
bridges. For two generations, one suspension bridge
triumph after another fed growing overconfidence.
The Tacoma Narrows Bridge was then the third
longest suspension bridge in the world. But its
designers had done something they hadn't done
before. The bridge was only two lanes wide -- not
meant for heavy traffic. So they used light
construction. The finished bridge was spectacular
-- really spectacular -- a delicate buoyant
structure that seemed to hang in the air like a
giant spider web. It seemed plenty strong enough.
The structural design of bridges was well worked
out by then. But it was also more flexible than the
ones before it. After all, wasn't it good to be
strong like a willow tree?
And so the bridge immediately picked up its
nickname, Galloping Gertie. Driving across it in
any kind of wind turned into a roller-coaster ride.
Four months after it opened, a higher wind than
usual blew down through the Narrows. This day, as
the Bridge danced in the wind, it turned into a
kind of mad dervish. The road was closed, and movie
cameras were brought up to record what was to be
the most famous bridge collapse of all time.
After that, bridge designers quickly learned how to
cope with the aerodynamic forces they'd been
ignoring for years. And we haven't seen another
failure like that one since.
But aerodynamic failures weren't new when the
Tacoma Bridge went down. Engineers had simply let
themselves forget the past. 86 years earlier, in
1854, the longest suspension bridge in the world
spanned the Ohio River at Wheeling, West Virginia.
It was five years old when it failed. The
Wheeling Intelligence reported the
failure:
... we saw ... the whole structure heaving and
dashing [in the wind] with tremendous force. For a
few moments we watched it with breathless anxiety,
lunging like a ship in a storm; at one time it rose
to nearly the height of the tower, then fell and
twisted and writhed. [Then] down went the immense
structure from its dizzy height to the stream below
with an appalling crash and roar.
And 28 years before that, in 1826, one
of the earliest suspension bridges had crashed just
the same way in England.
We engineers make a terrible mistake when we forget
where we've already been, because history will come
back to haunt us.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
(Theme music)
