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Dr
Peter Lewis gives his opinion on why the Tay Bridge collapsed.
He
is a senior lecturer in the Department of Materials Engineering
at the Open University. He also acts as a consultant forensic
engineer, mainly in accidents that result in litigation.
How did you come to investigate
the Tay Bridge disaster?
"The
Materials Engineering Department at the Open University were
developing a course in forensic engineering and we thought
that it would be useful, and educational, to look at one of
the great disasters from the past; and the Tay Bridge is probably
the most important structural failure within the last two
centuries in Britain.
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Theory Summary
From the study of the pictures taken immediately after the
accident, tied in with the witness testimony from the Board
of Trade inquiry, we think that the disaster is more complicated
than we’ve been led to believe by others. It wasn’t a static
failure by wind pressure but rather there were almost certainly
dynamic effects on the bridge itself well before the disaster.
These led to gradual deterioration of the ironwork supporting
the high girders. And come the night of the storm, the bridge
piers were no longer capable of supporting the applied load.
Within that we think that fatigue, crack propagation, probably
played an important role.
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What
do you think happened on the night of the disaster?
The most important event is the passage of the earlier six
o’clock train. This train crossed the bridge despite the very
strong winds and it was noted that sparks flew from the wheels
as if it were being pushed over by the wind. The bridge itself
might have been oscillating from side to side to cause the
same effect or it might have been a combination of the two.
Either way, it got to safety on the other side and we think
that the combination of wind pressure plus oscillations on
the bridge probably caused the failure of a large number of
tie bars (which stabilise the bridge).
When the second train, the mail train from Edinburgh, came
to the signal box at the start of the bridge the wind was
equally strong as far as we know. It proceeded along the low
girder section of the bridge satisfactorily, although again
sparks were seen by one of the signalmen as he watched it
disappear over the bridge. Then the train entered the high
girder section and between piers four and five the bridge
started to collapse.
We believe that the passage of the previous train had caused
a lot of structural damage - in other words the bridge was
in a critical state. The locomotive they used was very much
heavier than normal. The six o’clock train was only pulled
by a small tank engine whereas the mail train was pulled by
a very heavy 35 tonne loco.
The extra load and the state of the bridge led to collapse
of one of the first five piers. We don’t know which one exactly.
So there followed a succession of pier failures, not just
one pier. If the piers had been in a good structural state
then it could have withstood the failure of one pier alone
but in fact what happened was that all twelve of the high
piers failed. And they failed spectacularly; most of the piers,
ten of the twelve piers were actually left flush with their
platforms. In other words, almost all traces of the cast iron
piers had vanished. The girders were found by the side and
they didn’t move very far; they’re actually quite close. At
pier one they’re only 16 feet away, which means that the high
girders just slipped off.
It’s the total state of collapse of all these piers which
we think must point towards deterioration of the ironwork
of all the piers.
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How
does this theory match the actual evidence on the ground?
We base this picture of steady deterioration on eye witness
evidence from the painters and fitters. I think there are
about ten of them in all who gave evidence to the inquiry.
They were working on all of those piers in the high girder
section and they reported that when a train passed over, the
piers oscillated from side to side as well as up and down
and forward and back. So they were all oscillating in three
different directions.
This oscillation hadn’t been seen at all a year previously
when it was tested by the Board of Trade. So that leads us
to the view that there was steady deterioration.
There’s a second piece of entirely independent evidence which
supports this. The inspector of the bridge, Mr Noble, discovered
that the joints were rattling when he stood on the platforms.
This is a symptom of deterioration because the joints in the
diagonal tie bar are meant to be very tight. But if all those
joints were loose, then the structural integrity of the piers
was in question. If a pier is not held and braced then it
loses a lot of its integrity. And unfortunately Mr Noble didn’t
tell the engineer, Bouch, of the problem. What he did do was
he tried to fix it himself by knocking in shims (small pieces
of metal). But these actually made the problem worse because
he didn’t re-tighten the tie bars. He effectively left them
loose but stopped them rattling. And we’re led to believe
from the evidence that he did between a hundred and a hundred
and fifty of these joints, which is a large number of loose
and untrustworthy joints in a bridge of that size.
If you look at the photographs from the Board of Trade inquiry
set of photographs then you can see which component parts
were weakest. It’s immediately obvious that the lower lugs
which held the tie bars in place have all fractured. And they
must have fractured before the final collapse because they
are lying on the platforms. You could think, ‘well this might
have been a consequence of the accident’, but if you look
there’s very little trace of any piers left on most of the
pier platforms. Now, if those lugs had been breaking while
the piers were still upright then they would naturally fall
on to the platform. But of course if they’d broken as the
piers were falling then you would see very little trace of
any lugs on the platforms. And the number of lugs is such
that they must have arisen from the failure of many tiers,
there are just so many of them.
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So, I’m sure that they must have occurred before the final
collapse, and as a result the tie bars were being weakened
by the passage of previous trains.
There is also some supporting evidence from lugs in situ on
the few columns that remained on some of the platforms. You
get partial cracks, so-called sub-critical cracks, where a
component breaks part way and they are characteristic of fatigue
crack propagation in a large structure. You find fatigue cracks
at stress concentrations at the holes and lugs which is where
we see the cracks on the columns that remain. On several fractured
surfaces from lugs we can see traces of crack arrest paths
from hidden defects like blowholes - they’re very clear evidence
of fatigue crack growth.
The bridge fell in a very regular way which is a bit puzzling.
It fell in a wave form, as three arcs in fact, one for each
of the big girders and it’s so uniform, there’s very little
difference between the ends and the middle. That it’s just
that the piers are in a very similar state of very poor structural
integrity.
To have fallen in such a regular way, I think, shows that
the piers must have been in a state of very poor structural
integrity and that this state was very similar across all
the piers in the high girder section.
If you read the proceedings of the inquiry the three commissioners
and the barristers keep referring to the racking of the piers.
The structure of these piers was such that they weren’t tied
together at the top, there were two L-shaped girders which
each connected three columns - essentially creating two sets
of columns which were only held together by the tie bars.
It would have been much better, much safer, to have connected
all of the columns with a single girder, which would have
been hexagonal in shape. But they weren’t connected and it’s
clear when the inquiry talks about a racking action that they
mean the sets of columns swaying in parallel. So, all the
stress is being taken by the tie bars which, I think, supports
the idea that they were the first to fail, and caused the
disaster.
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Does
the speed of the wind play a factor in your theory?
The problem with the wind is that people couldn’t measure
it exactly in those days. In fact, one of the outcomes of
the Tay Bridge inquiry was that instruments like anemometers
(used to measure wind speed) were developed.
Clearly the wind cannot have been that high if the train reached
the high girders themselves, it had to cross a third of the
bridge to actually get to the high girders across the open
estuary. And although sparks were flying from the wheels,
which indicates there was wind pressure against it, it was
by no means critical or high, at that stage anyway.
But, of course, the wind played a role because you know it
applied a moment, a levering effect, against the piers. But
it wasn’t the only effect, the state of the pier remains leads
us to believe they’re the major cause of the disaster and
not the wind alone.
In fact, because we believe that the bridge was progressively
getting weaker, it was going to collapse before long anyway.
The storm on that night probably made it happen sooner, rather
than later; cracks only grow in one direction - towards a
critical failure.
Because all the evidence on the wind strength depends on indirect
evidence it is difficult to state with any certainty what
it actually was. Benjamin Baker used the state of walls and
the state of the signal boxes in the area to estimate the
wind strength. For example, there were three signal boxes
very close to the start of the bridge. Although one had its
chimney-pot knocked off there were no broken windows which
you’d expect with the hurricane force winds that were suggested.
And remember that it wasn’t a closed structure with a very
large surface area. This was an open lattice structure and
presenting very little resistance to the wind.
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Do
you have an ethos that guided your investigation?
In any accident you must look at the direct evidence and the
fresher the evidence the better. It’s essentially the philosophy
we used. You must tackle the direct evidence without looking
to secondary or tertiary sources for opinion and come to your
own view of what that evidence shows. That’s how I work as
a forensic engineer.
How do you rate the quality of the
BOT inquiry, both against today’s standards and also those
of the 1880s?
The inquiry sat within a few days of the disaster up in Dundee,
which was I think an achievement in itself and it started
interviewing eyewitnesses which was good because you’re getting
direct testimony when it is fresh in the minds of the witnesses.
Then they moved to Westminster and received the expert evidence.
Some of which was a bit mixed and looking in hindsight, they
could have been more systematic in their approach and recorded
more evidence. But they did take a lot of photographs and
they did test some of the surviving materials so they were
able to tie together quite a reasonable picture for the time.
However, although fatigue was beginning to be known about
then, the expert engineers seem to have neglected to look
at the fracture surfaces in any detail.
But they reached their conclusions remarkably swiftly compared
to today, six months from start to finish. It was essential
in that day and age because the public wanted a result quickly,
especially as Bouch was planning to build another bridge over
the Forth. And people wanted some reassurance that another
whole train could not be lost in another such disaster. He’d
also constructed quite a few other bridges to not dissimilar
designs, so all those bridges had to be tested for their integrity.
So yes, they did the right thing I think.
Overall, I think they did a pretty good job, although obviously
today things would be examined in much greater detail, which
would take much longer."
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