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Bill Dow gives his opinion on why the Tay Bridge collapsed.
He
is a retired principal lecturer in physics and head of science
at Dundee College of Education (now part of Dundee University).
He holds a BSc Honours in Physics, an ME in applied maths
and is a fellow of the Royal Astronomical Society.
How did you come to investigate
the Tay Bridge disaster?
"I was brought up on the story of the Tay Bridge, with
a particular family connection. My great grandfather had his
own foundry and he, together with the owners of another big
foundry in Dundee, seriously thought about tendering for this
bridge.
My mother always declared that he had said that the wrong
bit of the bridge fell down. Now, I don’t know what he meant,
but I have a suspicion that what he probably meant was that
the most difficult part of the bridge to construct was the
bit between pier 15 and pier 22 which was founded on piles
driven into mud flats.
Of course, it was also within the living memory of my grandparents,
who used to tell me all sorts of lurid stories about what
happened on that night - half of which weren’t true!
When I was a student at St Andrews University I used to travel
on the present Tay Bridge at least once a week. When you look
down from the present bridge you can see the remains of the
old bridge, and this really prompted me to take an active
interest. There were always umpteen theories as to why the
bridge fell down and I thought to myself, well why not investigate
this. Since then it’s really been a life long interest!
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Theory
Summary
There was always a question, was the train involved in the
disaster? Gradually, as I teased out a lot of the material
which had been put together by the people who salvaged the
remnants of the bridge, it became pretty obvious to me that
at least one vehicle had been off the rails for quite some
distance, at least the length of one girder.
Although there were probably many things involved, I believe
that this derailment played a key part in the fall of the
bridge. In short, I think that the rear carriages of the train
derailed and ran into one of the coverplates. The force of
this impact would shatter the cast iron lugs leaving the bridge
in a high wind without its proper structural support.
I also think that the girder closest to Wormit, in the high
girder section of the bridge, had bent during the lifetime
of the bridge. This girder had been dropped during construction
and was bent. It was straightened out and reused, but I believe
over time it started to return to the bent shape. As a result
the rail tracks which ran over it developed a kink. I think
that this kink, combined with the high wind, may have been
responsible for the derailment.
The presence of the 'kink' was well
documented in accounts of the permanent way, but they never
came to light during the inquiry.
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What
do you think happened on the night of the disaster?
I went through the papers in the old North British Railway
Company's archive and I came across two absolutely crucial
letters that showed that two girders had actually fallen into
the river in February of 1877. And this fall is quite well
documented, it concerned the two girders spanning piers 28,
29 and 30.
The inquiry was told that two girders had actually fallen
into the river in February of 1877, during the bridge’s construction.
It was fully published in the press, it wasn’t hushed up at
all. But what isn’t so well known is that the construction
company realised that if they were going to get the bridge
finished by September of 1877 they just didn’t have enough
time to make two extra girders over and above what they already
had to make anyway. Each girder took about four weeks and
required a spring tide for raising. So
they left one at the bottom and made a completely new one
to replace it, but they repaired the other girder that had
fallen in. It was literally, according to the manager, fished
out, straightened and put back up. And, in my view, that was
the girder, between piers 28 and 29, which had a major part
to play in the disaster.
The girder needed to be straightened out because when it fell
into the river bed it bent due to the fact that the river
bed was not flat, but curved as a result of scour. Now when
you straighten anything out it’s never quite as strong as
it ever was before. And when that girder was put back up they’d
obviously managed to straighten it sufficiently well that
during all Major Hutchinson’s tests early in 1878 everything
appeared to be OK.
Now that girder was riveted to the next 4 girders, so the
end of the girder must have been held straight by means of
the other girders behind it. But because of the hammering
from trains going over it that girder would have reverted
to the shape, or as close as it could, that it had on the
bottom of the river.
As the girder bent, it would have forced the rails to bend
with it, and there were certainly reports that as the trains
came along the low girders from Wormit and entered the high
girders the engines nodded into the girder. In other words
it went slightly downwards and then went slightly eastwards.
After that the rails gradually crept back into the normal
line of the rest of the girders.
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On
the night of the disaster the wind was coming from the west
(so it’s described as a westerly wind), which was virtually
at right angles to the bridge and it was definitely about
force ten or force eleven. The training ship Mars was in a
good position to give this information. Its true speed in
miles per hour as far as I know was never measured, nor was
the speed of the gusts. As the train entered the high girder
section it would have gone slightly, but sharply, towards
the right, or the east. This would have put an impact on the
girder which would have tended to drive the girder initially
to the west and then to the east as it travelled among the
bent girder. That is exactly what happened; the engineer declared
that this girder, had moved 20 inches to the east before it
was pulled off its rollers. Apparently a tracing of that exist
to this day, but I’ve never seen it. But it certainly was
referred to in the inquiry.
Whether anything came off the line there or not I just don’t
know. But the bridge was such that if a carriage, but not
an engine or a tender, came off the rails it could be towed
for quite some distance. It might have torn up some of the
floorboards, but it wouldn’t have done any major structural
damage to the bridge.
In fact, the wooden floorboards came loose as the girders
landed in the river, most of which were simply swept out into
the North Sea. However, some of the floorboards were washed
up by the outgoing tide at, what was then the village of,
Broughty Ferry. Some of the floorboards which were recovered
had marks from the treads of the carriage ‘tyres’.
As the train was somewhere about the third or the fourth high
girder out of Wormit there was an incredibly hard gust of
wind. As a result, either the second class carriage was blown
off then, or it was already off the line. Now, the mathematics
which was done for the inquiry about the force required to
upset the second class carriage was completely wrong. It doesn’t
allow for the Bernoulli effect, where you get a lifting force
when there is a high speed wind across the top of the carriage
and a low speed wind below it. They actually reckoned that
it would take a force of about 33 pounds per square foot to
displace that carriage. British Rail during my time would
not trust their mark two steel carriages on the present bridge
if the wind could exert a force of more than 30 pounds per
square foot. There’s obviously something wrong here if the
inquiry gave greater stability to an old lightweight wooden
carriage than to a modern steel carriage.
That severe gust pushed it sufficiently far east that it actually
hit one of what we call the cover plates at the end of the
fourth girder. At that point the 135 tonnes of train is going
at about 25 miles per hour, which is quite enough to demolish
essential parts of the bridge - particularly the brittle cast
iron lugs. The train was effectively a 135 tonne battering
ram travelling at 25 miles per hour..
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When
the second class carriage hit this plate the big, heavy guard’s
van ran into it from behind and totally demolished the second
class carriage, and in the process demolished itself as well.
What was found were the undercarriages of the second class
carriage and the guard’s van telescoped so hard together that
in fact they initially thought they were recovering one vehicle
when in fact there were two vehicles there. And it was quite
difficult to disentangle the two.
It required very serious forces to drive two vehicles together.
That just does not happen by chance. That does not happen
by the two vehicles landing in the water. This happens when
in fact the vehicles are travelling at speed and one is being
stopped and the other one is running into it. So that was
where I began to reckon that the train played more than just
a cursory part in it.
So, effectively, you’ve got a hammer blow from the train’s
impact with the bridge which would break all the cast iron
lugs. And incidentally Bouch did not design the lugs, that
was one of the things that Bouch left to Gilkes (the contractor
who built the bridge). He said to Gilkes, look you can put
the ties on either by wrought iron collars or wrought iron
bands, or you cast them on. And Gilkes realised it was a lot
easier to cast them than to use wrought iron bands, so he
cast them. Bouch shouldn’t have passed that responsibility
on to the contractor.
As a result of the impact the bridge had lost all the cross
bracing of the wind ties which were there to resist wind pressure
- they’d all broken off at the lugs. With a wind blowing there
was not enough stability in the vertical columns by themselves,
so the bridge just went down to the east.
Now the other question you need to answer is why did all the
high girders fall and not any of the low girders. All the
girders were riveted in sets, the high girders were riveted
into three sets. At the Wormit end there were five 245 foot
girders riveted together. The middle four girders were riveted
together as another set and at the Dundee end there were another
set of four girders riveted together.
It
is essential to realise that the rails were laid from one
end of the bridge to the other in 24 feet lengths with half
inch expansion joints - there were no special expansion joints
at the point where one girder met the next.
Now, before any bit of the high girders started to fall you
not only had to pull the girders off their rollers (which
sat between the piers and the girders) and off various joints,
but you also had to sever four steel running rails. These
were of 75 pounds per yard and made of rolled steel, so they
were tough stuff.
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So the rails had to be broken somehow; from what we can see
in the photographs the rails broke at the fishplates (the
joints between two lengths of rail), which would be the easiest
place to break. But the photographs of the surviving piers
seem to show one of the rails actually severed, and that would
really take some serious force.
So
what I think happened is: the set of high girders nearest
Wormit went first due to the impact of the carriages with
the coverplate. In falling they pulled the next set of girders
over because although the sets of girders were separated by
expansion joints, the rail tracks did not have an expansion
joint at the same place - so there were some very strong ties
between the sets of girders. As the second set of girders
fell it would have pulled the third set of girders over, in
the same way.
But this didn’t bring down the low girders at either the Wormit
end or the Dundee end, although the photographs show that
the surviving piers had all their wind ties broken and the
rails are bent towards the east showing that as the high girders
went over they had a jolly good pull on these surviving piers
towards the east as well as downwards.
Not only did those piers survive the pull, they also survived
the break and they survived the rest of the gale. And because
these two surviving piers were constructed in the same way
as the piers that fell, this is proof to me that something
more than just the wind was needed to bring the bridge down.
Because despite having lost all the wind ties they still survived
the rest of the gale. The sole reason that they did was that
the girders each of them was supporting were riveted into
sets of four.
So, you’ve got four low girders, riveted together and supported
by five piers. But at one end the pier is effectively out
of action because it's lost all its wind ties, and at the
other end the pier is effectively only half a pier as it is
supporting the end of another set of girders. But the three
in the middle are still in business and what it means is that
four girders are managing to survive in a wind when there’s
only three, or three and a half, piers able to withstand that
wind. Now that shows a pretty high factor of safety.
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How
does this theory match the actual evidence on the ground?
The high girders were riveted together in three sets; one
set of four closest to Dundee, one set of five nearest Wormit,
and one set of four in the middle. As the set nearest to Dundee
and the middle set fell into the river on the east side of
the bridge, each set of girders remained riveted together.
The reason we know this is that they were found in the river
still riveted together, and they were in lovely little arcs.
But the five girders that made up the set closest to Wormit
were found with only four of them still riveted together,
the girder nearest Wormit had broken away and was found at
quite a different angle from the rest. It was obviously suffering
from forces which none of the other girders were suffering
from, in order to sever the rivets which were holding it to
the other girders and land in such a jaunty angle.
That’s why I think that this kink in the rails had a lot to
do with the actual disaster. You’ve got to explain why this
one particular girder broke away and why it lies at a jaunty
angle. I think that over time the girder started to return
to the shape that it had bent into when it had fallen into
the river during the bridge’s construction.
You can also see in the photographs that all of the cast iron
lugs have been broken, but there’s no failure of wrought iron.
There have been those who said that the wrought iron wind
ties should have been much thicker - but they’d burst their
lugs. It was the cast iron lugs that failed in all cases,
which ties in with my theory of the train hitting a cover
plate.
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Does
the speed of the wind play a factor in your theory?
You can’t get away with the train and the train alone bringing
the bridge down, that would be pretty speculative. You had
to get a series of things; not only did you have to get the
vehicle derailed, you had to get it pushed far enough towards
the east that it would have hit the cover plate - which would
have meant a deviation of at least two feet to the east.
But I also think bits of the bridge were damaged and the ‘repairs’
carried out by Noble were really putting the bridge out of
alignment. He was stuffing wedges down in-between the sling
plates and this would have destroyed the line of some of the
wind ties and they really distorted the positions of the lugs
as well.
The wind speed, whatever it was, was critical to derailing
the train, and probably combined with some existing damage
to the bridge, it helped to bring down the high girders. But
it didn’t bring down the rest and that is the big mystery,
especially thinking about the piers at either end of where
the high girders had been. The gale went on for at least an
hour after the high girders fell and these piers, despite
the quite serious damage they endured as the high girders
parted company from them, still survived the rest of the gale.
So, despite the damage there was enough factor of safety left
in them to survive the winds. And this is why I’m quite convinced
that the wind, and the wind alone, just would not do it. This
is where the critical speed comes in. The wind has got to
be strong enough to bring down the high girders, but not strong
enough to bring down the low girders. So I think if you’ve
got a wind and a wind alone theory you’ve got to explain why
anything survived at all really.
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Do
you have an ethos that guided your investigation?
During my time with the Air Force I was trained that if ever
a disaster occurred, ‘For God’s sake get the information.’
What was rammed into us was, ‘Get where everything was. Find
out the position of everything , it’s very vital to know where
things are relative to the others.’ Now that kind of information
was just never taken for the Tay Bridge disaster.
This training has definitely shaped how I approached this
investigation. I look for the damage. What was actually done?
That was the Air Force rule, ‘We don’t want just what you
can explain, we want to know all the damage whether you can
explain it or not. Don’t start making up theories before you
know all the evidence, particularly without knowing where
everything has landed.’
So, essentially, my ethos is to make sure that I shape the
theory to answer all the evidence; where was the train found,
where were the bodies found, where were the carriage roof
lights found, where were the wheels?
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How
do you rate the quality of the BOT inquiry, both against today’s
standards and also those of the 1880s?
I think it was a top class inquiry of its day, but it had
its faults.
After the disaster, the Board of Trade appointed three assessors.
It was a wreck so they got the Wreck Commissioner, Rothery.
It was a railway wreck so they got the Chief Inspector of
Railways, Yolland. It was a civil engineering disaster so
they got the President of the Institution of Civil Engineers.
Who better than these three? It shows that the Board of Trade
got, in their opinion, the top three men.
But they acted like church mice while the person who conducted
the inquiry was the Sheriff of Forforshire, a man called Trayner,
who later became one of the top judges in Scotland. Now what
happened there was that Trayner conducted the inquiry but
the assessors wrote the report.
Recently, when Lord Cullen conducted the "Piper Alpha"
inquiry, he asked the questions. He had assessors sitting
alongside him, giving advice on what to ask and giving an
opinion as to the quality of the answers. In other words,
the assessors could "steer" the questions towards
areas which they thought important and suggest further questions
depending on the answers. But in the final analysis, it was
Lord Cullen who came to his own conclusions and wrote the
report.
In
the case of the Tay Bridge Inquiry, the two engineers could
not agree with Rothery, the inquiry chariman, so two separate
repoerts were written, one by the engineers and one by Rothery.
The engineers did not apportion blame but Rothery blamed Bouch
and Bouch alone. There were other essential differences between
the two reports. The Board of Trade should have insisted on
the three assesors presenting an agreed report, but it never
did."
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