Some of you may have seen
this film before --
this is the very famous
Tacoma Narrows Bridge collapse
in Tacoma, Washington state,
1940.
The bridge became known
as "Galloping Gertie"
because she -- she?
Is a bridge female? I don't know.
She was wobbling like this
for quite a long time,
and notice this twisting motion.
The bridge was far too flexible.
It was designed by a chap
called Leon Moisseiff,
no stranger to suspension bridge design,
but in this case he pushed the limits
just that little bit too far
and paid the price.
Thankfully, nobody was killed.
But this bridge collapse stopped
suspension bridge development
dead in its tracks.
For 10 years nobody thought
about doing another suspension bridge.
There were none.
And when they did emerge in the 1950s,
they were an understandable overreaction,
this sort of oversafe response
to what had happened.
But when it did occur in the mid-60s,
there was indeed a step change --
an innovation,
a technological step change.
This is the Severn Bridge in the UK.
Notice the aerodynamically
streamlined cross section
in the center there.
It's also a box which makes it
very torsionally stiff --
that twisting motion which we saw
at Tacoma would not happen here.
And it's also really lightweight,
and as we'll see in a moment,
lightweight is really
important for long spans,
and everybody seems
to want us to build longer spans.
