It's a magnificent bridge.
But it's quite easy to design a bridge if the span's quite
short and the load it's carrying is almost negligible.
But it's a lot harder if the bridge is longer and the load
it's carrying is a lot larger.
And that was a challenge faced by the engineers in the early
19th century who were building the railways across Britain.
So this is a Great Western tank engine
from about the 1930s.
The sort of thing you'd see on a branch line train.
And even though it's a small locomotive, you can see how
massive it is, how heavy it is.
But it's not just its weight that damages bridges.
And to understand that, I'll just explain a little bit
about how it works.
So these are the cylinders.
Inside the cylinder there's a piston, it's very much like a
car engine.
But when steam's admitted to the cylinder, it pushes the
piston back and that forces this connecting rod here back.
But can you see the crank pin here which is
attached to the wheel?
The crank pins in line with the centre of the wheel.
No matter how much steam you admit to the cylinder, it's
not going to be able to move the engine at all.
So this is the left-hand side of the engine.
Here we are on the right-hand side of the engine.
Now, you'd expect that the wheels would be 180 degrees
out of phase, the crank pin will be up here, but it's not.
The wheels are actually 90 degrees out of phase.
The crank pin's at the bottom.
And that means when it's in this position, when steam
flows into the cylinder it can push against the wheel and
rotate the wheel.
And that means that the engine can always start from a
stationary position whatever attitude the wheels are in.
Let's try that with the camera somewhere else.
I've moved my crank's around so they're 90 degrees to each
other, which is just like the steam engine.
I can't find the pedal.
There, that's got it.
I cycle every day and it's just really very obvious.
It feels like cycling on a kangaroo.
But it's very uneven, and I imagine you can
see that, feels it.
And this sort of uneven loading causes
real damage to bridges.
That's about a ramming speed.
This is very odd.
So here's my model bridge.
Just ignore this bit, it's just a guide rail.
This is the bridge itself.
It's a steel bar.
And when I force down on it, it deflects downwards and the
bottom of the bar goes into tension.
So I've got a strain gauge that's fixed to the bottom of
the bar, and that measures the tensions.
That measures how much load I'm applying to the bridge.
So here's my steam engine.
It doesn't look much like the steam engine.
But it's just a trolley that I can move across the bridge, so
I'll do that now.
And here's the trace from the strain gauge.
It starts from zero here, builds to a maximum when the
train's in the middle of the bridge, and then drops back
down to zero.
And we know that steam engines don't run smoothly like that.
Because of the cranks, they apply a
cyclic load to the bridge.
So we can see here how we've replicated that.
We've got this disc with an out of balance mass on it, and
we can spin the disc around as we're
driving across the bridge.
So let's see the effect of that now.
So we've got the new trace here with all the spikes on
it, and the old trace, this smooth one behind.
And you can see two things.
The maximum load on the bridge is higher.
But also, you've got lots and lots more load cycles applied
to the bridge.
In the 19th century, railway engineers called this cyclic
loading hammer blow.
The material that they could use, cast iron,
was a hopeless material.
For one thing, it's very brittle.
But also, in the size of casting you'd need to make a
railway bridge, it will be just covered in defects.
So we've got a small specimen of cast iron, and we've put
our deliberate defect in it.
This is to replicate the size of defect that you get in a
large casting.
And we're going to use this test machine to apply cyclic
load to this specimen using this hydraulic cylinder here.
So here we've put a specimen in the machine.
We're supporting it at both ends just like a railway
bridge would be supported on pillars.
And we're using the test machine to apply cyclic
loading to the specimen in the centre.
And failures like this happened in practise.
A famous example's in 1847, when a bridge carrying a
passenger train over the River Dee failed.
A bridge designed by Robert Stephenson, causing
significant loss of life.
And it isn't just history.
Today we have to design structures that are subject to
cyclic loading, whether they're bridges or aircraft.
And it's only through advances in material technologies that
we are able to design these structures with more
confidence.
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