24TH OCTOBER 2008
The Tay Bridge Disaster
4A6(1)- Structures Case Study
GROUP 7: OWEN COYLE
CHRIS FITSIMONS
NIALL MCLOUGHLIN
JEAN DIERES MONPLAISIR
STEVEN WALSH
Introduction
The Tay Railway Bridge was first opened in 1878 after 6 years of construction. It was
designed by Sir Thomas Bouch, an English Civil Engineer. When built, the bridge was the
longest in the world; stretching for almost 2 miles across the firth of Tay and held together by
some 80,000 rivets. The bridge consisted of 85 spans which carried a single rail track. Of the
85 spans, 72 spanned 44 metres and 13 were navigation spans. For the 44 metre spans, the
spanning girders of the bridge were below the level of the track. The navigation spans had a
much larger span of 74 metres and were spanning girders above the level of the track (i.e. the
train runs through a tunnel of girders). These girders were supported by piers consisting of 6
columns set at a max height of 26.8 m above the water level, in order to provide clearance for
ships. The columns were connected together by bracing members. The most important
stiffening bracing members were the diagonal ties. The piers were supported by caisson
foundations i.e. large concrete cylinders sitting on the bed of the estuary. Between the bases
of the columns of the pier and the top of the caissons were two courses of stone masonry. The
bolts to hold the columns in place were only anchored into the top two courses of masonry.
The construction of the bridge was the largest engineering project in Britain at the time until,
on the 28th of December 1879, there was a large storm in the Tay area. It was recorded that
there was a gale force wind of 10-11 (on the Beaufort scale) blowing through the Tay estuary,
perpendicular to the bridge. The storm caused considerable damage to the bridges’ structure
which ultimately resulted in the central navigation spans of the bridge collapsing into the
Firth of Tay at Dundee. Tragically there was a train on the bridge at the time which
plummeted into the river, killing 75 people.
Theories of Collapse
There are 3 main theories that attempt to explain why the Tay bridge collapsed. However,
none of these are universally accepted. The theories are as follows;
1. Wind Theory
This suggests that the bridge was simply not strong enough to withstand the wind on that
night. At the time of the collapse, there was a very strong, gusty wind blowing - estimated
to be Force 10/11 on the Beaufort Scale, and hit the bridge at right angles from the west.
As the piers were only anchored into the top 2 courses of masonry, a large moment
caused by the high winds and tall piers was induced and it lifted the anchoring bolts. This
induced a larger force into the ties which provided lateral bracing for the bridge. The
force in the critical tie (the tie which would be expected to fail first) was the inner tension
tie in the second level above the base was too large and so this tie failed. In turn,
additional force is transferred through the tie above it and it also fails. This leads to
failure in the other ties and so the pier no longer acts as 6 columns braced together but as
2 sets of 3 columns, reducing the lateral stiffness to 1/3 of the original pier. This leads to
the pier swaying violently and, as the train enters, an additional strong gust of wind blows
the bridge beyond it’s limits. The columns under compression forces fail and the bridge
collapses into the firth. This failure then develops horizontally. All 13 navigation spans
collapse but the other spans do not because they are shorter, lower in elevation and,
hence, have a significantly greater capacity to resist damage.
2. Fatigue Theory
This theory, summarised on the website: http://www.open2.net/forensic_engineering,
claims that dynamic effects caused the cast iron lugs in the bridge fail by means of
fatigue. This proposal is based on the eye witness reports that the high girder piers
oscillated from side to side whenever a train crossed the bridge. Upon inspection of
photographs taken of the failed components, it was noticed that the failure of the cast iron
lugs was due to fatigue rather than overstressing. In this theory, there is an attempt to take
away from the Train Derailment Theory (see below) by stressing that the train crossed the
bridge at six that evening and managed to get across the bridge, despite the high winds. It
was noted, however, that sparks flew from the wheels as if it were being pushed over by
the wind. Eye witnesses have also stated that the bridge was oscillating both up and
down, and from side to side. The theory states that these oscillations, in combination with
extremely high wind pressures, caused some tie bars on the bridge to fail. These tie bars
serve to stabilise the bridge and the argument that fatigue in these members caused the
ultimate failure of the bridge is plausible.
3. Train Derailment Theory
The Train Derailment Theory is based on the belief that the train crossing the bridge at
the time of failure was a major factor in the collapse of the bridge. It proposes that a kink
in the tracks caused the train to derail and the resulting uplift of the train attributed to
additional aerodynamic forces. Fatigue in the lugs and poor maintenance record of the
structure also attributed to the failure. It is claimed that one of the carriages then struck
the bridge and the resultant shock experienced by the pier caused the cast iron lugs
(connecting the wind bracing members to the columns) to fracture which lead to the
subsequent collapse of the pier structure. The bridge lost lateral stability and was then
blown over. This theory is supported by the fact that the girder closest to Wormit, in the
high girder section of the bridge, had been dropped during construction of the bridge.
Although it was straightened and reused it returned to its bent shape over time. Thus as a
result, the rail tracks developed a kink.
Summary
It is likely that the Tay bridge collapsed due to a combination of factors. It was poorly
designed, built and maintained. One vital factor contributing to the failure was that wind
loading was severely under-estimated. The structure was designed to take a wind force of 10
pounds per square foot, when in fact the normal value used at the time was 40 to 50 pounds
per square foot. Bouch made the mistake of using the lowest value for wind loading
recommended to him, which was a strange development because even in past projects he
used a higher factor of safety. In addition, the windward columns were not properly anchored
to the foundation and an inferior design of the piers was used. Had the building of the Tay
Bridge received better financial backing, Sir Thomas Bouch may not have compromised on
the design. Due to the project being behind schedule, over budget and being such a large
project in magnitude, his professional judgment was probably clouded. Bouch underestimated
the loading and overestimated the strength.
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 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. To this day, however, there is still speculation as to the
fundamental cause of the collapse.
Conclusion
One big lesson learnt from this disaster is that professional engineers need to operate at a high
level of integrity. Clients still try to pressure consultants to cut costs and hence reduce
safety. The professional engineer must not buckle to such pressure. After the disaster
precautionary measures were introduced to try and ensure nothing like this happened again.
These measures included:

All Bouch's bridges were examined and reinforced or rebuilt.

Steel was approved by the Board of Trade for use in bridges.


Designs using cast iron columns were banned.
Regular and frequent inspections of bridges were made during and following
construction by Board of Trade personnel.

A maximum wind pressure of 56 per square foot for design of bridges and rules for
applying this specification to bridges of different construction were recommended.
Appendix A:
Wind theory failure mechanism