Steam trains, bridges and potential catastrophe

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. 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. 1,421. 1,422. 1,423.

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