“The Tay Rail Bridge Disaster”
Structures Group Research Report
Submitted by ;
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Colin McFadden
Paul O Donoghue
Ruaidhri Farrell
Julie Ann Cushen
INTRODUCTION
The first Tay rail bridge was completed in February 1878. Thomas Bouch was
responsible for the design, construction and maintenance of the bridge. Most of
his bridges were lattice girders supported on slender cast iron columns braced
with wrought iron struts and ties. When the Tay Bridge was complete Thomas
Bouch received a knighthood. The bridge was officially opened on 26th
September 1877 when a party of directors crossed over in a train pulled by the
engine Lochee.
The Tay Bridge was nearly two miles long, consisting of 85 spans and at the time
was the longest bridge in the world. The spans carried a single rail track; 72 of
these spans were supported on deck spans, the remaining 13 navigation spans
were through girders. These "high girders", as they were known, were 27 ft high
with an 88 ft clearance above the high water mark. It was these spans, which fell.
At the time of the collapse Bouch was working on the design of the proposed
Forth Bridge. In consequence, the design of the bridge was transferred to
Benjamin Baker, Sir John Fowler and William Arrol.
COLLAPSE THEORIES
-Train Theory
This theory attributed to Bill Dow, a retired lecturer in Physics, Dundee College of
Education, says that the train came off the track due to a kink in the rails and
uplift of the train attributed to aerodynamic forces. In consequence, one of the
carriages hits the bridge and the shock experienced by the pier causes the cast
iron lugs connecting the wind bracing members to the columns to fracture leading
to the subsequent collapse of the pier structure.
-Dynamic Theory
This theory, attributed to Dr. Peter Lewis, senior lecturer in the Department of
Materials Engineering at the Open University, claims that dynamic effects are
much more important than had been previously realised. It is claimed that
dynamic effects caused the fatigue failure of the cast iron lugs. The evidence for
the dynamic effects is based on the eyewitness reports from painters and fitters
that the high girders piers oscillated from side to side whenever a train crossed
the bridge. On the basis of close inspection of high quality photographs (which
show some limited evidence of crack arrest lines) of the failed parts it is claimed
that the failure of the cast iron lugs was due to fatigue rather than overstressing.
The dynamic theory is not supported by any dynamic analysis. Whilst brittle grey
cast iron has fatigue properties it is rare for it to fail by fatigue especially lowstrength, high Phosphorous cast iron, which was used at the time of the Tay
Bridge disaster. Weak cast irons usually fail catastrophically due to the result of
overstressing. Research carried out by Castings Technology International
(previously called the British Cast Iron Research Association) found the following
results:
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The fatigue limit of weak cast irons can be increased to a remarkable
degree by previously under stressing at or just below the fatigue limit.
Weak cast irons show a remarkable ability to absorb overstress, both in
the notched and unnotched condition.
Cast irons in general have low notch sensitivities in fatigue.
Meteorological Office records show that during the 22 month life of the bridge it
suffered gale force winds on a total of only 9 days. The only time on which
stronger than gale force was recorded was on the evening of the collapse.
The following are facts:
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The wind force at the time of the collapse was exceptionally high. The
structure was not designed to take wind force
The measured (post collapse) strength of the tie/lug combination was less
than the already inadequate design value.
The windward columns were not properly anchored to the caissons and
the uplift of their bases caused an increase in the tie forces.
AFTERMATH OF THE TAY BRIDGE DISASTER
Although the Tay Bridge was a remarkable engineering feat at its time, there
were significant faults in the design due to a lack of knowledge in the field of wind
pressure and a lack of sufficient funds for the project, which forced the designer
Sir Thomas Bouch to reduce the number of cast iron columns from six to four.
Problems arose in the type and maintenance of the materials used in
construction of the Tay Bridge and the very large wind loading on the structure,
especially on the night of the failure on the 26th of December 1879. The bridge
was constructed from cast iron, which is notoriously brittle and weak compared to
steel, it was noted that the iron was cracking due to the very cold weather. It was
also determined that the lugs holding the bracing were snapping off from being
“too tight”. The iron, produced in a foundry near the site due to the financial
constraints, was also of very poor quality and there was a record of girders being
used despite being twisted and in one case a high girder was lost to a storm for a
night.
As a result of the inquiry into the Tay Bridge disaster, several measures were put
into place. Firstly the use of cast iron columns was banned and the steel used to
construct the bridges had to be approved of by the Board of Trade. Secondly all
of Bouch’s bridges were examined and where necessary reinforced. The board
of Trade also appointed new designers to a project they had already contracted
Bouch to design. Another result of the disaster was that every bridge had regular
visits from Board of Trade personnel to insure that the structure was being built
and maintained correctly. The most important result of this disaster was the
founding of a Royal Commission on "Wind Pressure on Railway Structures". The
commission carried out an extensive series of tests to determine the pressures
due to winds at varying speeds, which would be vital for bridges since they can
be very exposed, especially in valleys where the wind can be funnelled and
magnified. They also examined cases of overturned carriages due to wind and
eventually came up with guidelines so as to prevent the reoccurrence of the
event.
CONCLUSION
A Court of Inquiry was set up to try and ascertain the reason for the collapse of
the bridge. The Court of Inquiry report concluded that, "The fall of the bridge was
occasioned by the insufficiency of the cross bracing and its fastenings to sustain
the force of the gale." The Court of Inquiry indicated that if the piers, and in
particular the wind bracing, had been properly constructed and maintained, the
bridge could have withstood the storm that night, albeit with a low factor of safety
- 4 to 5 was the norm at the time. Sir Thomas Bouch was held chiefly to blame
for the collapse in not making adequate allowance for wind loading.
As a result of the Tay Bridge disaster and subsequent inquiry several measures
were introduced:
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All Bouch's bridges were examined and reinforced or rebuilt. The contract
for the new Forth Bridge (near Edinburgh) with Bouch as designer was reassigned to Fowler, Benjamin Baker and Arrol in 1882.
Cast iron was prohibited for bridge construction. Steel was approved by
the Board of Trade as a replacement.
Board of Trade personnel were instructed to regularly inspect all new
bridges during and after construction.
A Royal Commission on "Wind Pressure on Railway Structures" was set
up in 1881. Its work included extensive survey of wind speeds and
pressures using anemometers at numerous locations and the subject of
wind loading started to be investigated intensively.
The Tay Bridge was victim of an unfortunate combination of human and natural
factors so Sir Thomas Bouch may not be entirely to blame. Tom Martin, co
author of “The Tay rail bridge disaster - A reappraisal based on modern analysis
methods” may be correct in suggesting that if the building of the Tay Bridge had
been a better financed, Bouch may have not compromised the strength and
stability of the piers. Due to the project being behind schedule and over budget,
his professional judgement was probably clouded, for example he had intended
to use sets of 8 columns instead of 6 for each. Also Bouch did not design the
entire bridge lugs that were one of the things that Bouch left to Gilkes (the
contractor who built the bridge) designed the lugs and this was not considered at
the Court of Inquiry. The materials used for the bridge were of inadequate
quality. At the time of construction relatively little was know about wind loading
compared to now. This information would have been important when designing
the bridge. Overall, the Tay Bridge disaster emphasises the fact that engineers
must design and oversee projects to the highest integrity to eliminate the same
scenario from occurring again.
REFERENCES
http://www.tts1.demon.co.uk/tay.html
http://www.open2.net/forensic_engineering/riddle/riddle_01.htm
http://www.dundeecity.gov.uk/centlib/taybridge/taybridge.htm