TAY BRIDGE DISASTER
Group 6
Aoife Byrne
Brian Walsh
Introduction
The British railway industry was greatly expanding during the mid-1800s. Two large estuaries, the Firth
of Forth and just north of it the Firth of Tay, formed a major barrier between Edinburgh (located on the
south bank of the Firth of Forth) and the north of Scotland. Travelling around these estuaries was very
impractical and inefficient. In 1870 it was decided a railway bridge would be constructed across the Firth
of Tay.
Thomas Bouch (1822-1880) was appointed the head engineer for the project, which was considered the
largest engineering project in Britain at the time. He was responsible for the design, construction and
maintenance of the bridge.
The bridge was approximately 2miles in length, making it the longest bridge in the world at the time.
Construction was completed in 1878. The bridge was tested and approved by the Board of Trade
.Thomas Bouch received knighthood for his work.
On the 28th of December 1879, at approximately 19.15, the bridge collapsed during a gale-force storm.
A train travelling across the bridge fell into the sea along with the 75 passengers and staff as the bridge
collapsed. There were no survivors, apart from the train itself which was recovered from the river and
reinstated for service. The collapse of the Tay Bridge was and still is considered the greatest disaster in
British Structural Engineering.
This report will detail the design of the Tay bridge, the disaster itself and the reasons for its collapse.
Design of Bridge
The bridge design consisted of wrought iron lattice girders supported by cast iron columns. It was
approximately 2 miles in length and was composed of 85 spans supporting a single railway track.
The midpoint of the bridge consisted of 13 navigation spans, each 74m in length. The girders located in
the navigation span were constructed above the track level, creating a tunnel which the train travelled
through. These girders were referred to as the ‘high girders’ and they provided the clearance for ships to
travel under the bridge and up the Firth of Tay.
Figure 1: Elevation of High Girder Span http://openlearn.open.ac.uk
The remaining 72 spans were each 44m in length. Support for the train was below the track level.
The piers that supported the girders consisted of 6 columns, arranged in a hexagonal shape, connected
to each other with a series of ties and struts. These provided stability to columns. Large concrete
cylinders called caissons, embedded in the estuary floor, supported the piers. Stone masonry was placed
between the base of the columns and the caissons. The columns were bolted into the stone masonry,
but the connection did not go as far as into the caisson foundation.
Figure 2: Pier and Column Design http://openlearn.open.ac.uk
Disaster
On the 28th of December 1879, the Tay Bridge collapsed. A gale wind of 10-11 force on the Beaufort
scale was blowing perpendicular to the bridge. The 13 navigation spans came down as a train was
travelling across the same section. The disaster resulted in 75 deaths, there were no survivors.
The Court of Inquiry set up an investigation to determine the reason of the collapse. It was decided 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." Blame was placed on Thomas Bouch for under designing the bridge to
resist wind loading.
Investigation – What happened?!
Underestimated Wind Load: The storm had a wind force of 10-11 on the Beaufort scale which is
equivalent to 89-117km/h, giving a wind loading of up to 680N/mm^2. The bridge was designed to
withstand 10lb/sq.ft of wind loading, or 210N/mm^2. This was the lowest wind loading recommended
to Bouch by advisers. By comparison, French and American designers at the time were using 4 to 5 times
that value. In Bouch’s other design at the time, The Forth Bridge, he used a value of 30lb/sq.ft.
Tie failure: Bouch calculated that the ties would hold 32.5 tons (25.9 tonnes), but when surviving ones
were tested, they only held 24.1 tons (21.9 tonnes). Part of the reason for this is that the holes for
connections to the columns were cast using a tapered insert (for easy removal) and in similar projects
would normally have been drilled so that the sides were parallel. Bouch admitted in the court of enquiry
that he did not specify for this to be done, even though he knew it was wrong not to. When the main
ties failed (the longest ones in the elongated hexagon), they failed in a zip-like manner, starting at the
bottom and working upwards.
Masonry Fixing: The columns
were only anchored to the top two
layers of masonry, these were
lifted during the storm. This put
extra pressure on the bracing, and
contributed to their failure.
When the ties failed, the elongated
hexagonal arrangement of the 6 columns no longer acted in that arrangement, but rather as two sets of
three braced columns. This is a much weaker arrangement, and more liable to collapse.
For safety designers normally underestimate the strength and overestimate the load. This was not the
case with the design of this bridge.
It is noted in the court of enquiry’s report (1880) that the appointee of the board of trade, responsible
for inspecting the bridge prior to opening, did much testing and calculations for severe vertical loading,
but did not make any calculations or tests for wind loading.
Alternative Theories
Train Derailment: This theory was used by Bouch as a defence during the court of enquiry. This theory
says that the train derailed and hit one of the supporting columns, causing the bridge to collapse. This
theory was discredited by the court of enquiry, who said that a derailment could account for one section
of the bridge, but was not a plausible explanation as to why so much of the bridge had collapsed.
The more recent theory about fatigue is concerned with the dynamic effects of the loading and
unloading, but later studies found the evidence to support this theory weak.
Conclusion
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 bridge was under-designed for wind load, using forces that were 75-80% lower than the norm at the
time. The strength of the bracing ties was also over-estimated, and important detailing was not specified
by the designer.
References
http://taybridgedisaster.co.uk
http://openlearn.open.ac.uk/mod/oucontent/view.php?id=397893&section=3.1
http://www.open2.net/forensic_engineering/index.html
The Fall of The Tay Bridge, Swinfen.
http://www.railwaysarchive.co.uk/docsummary.php?docID=107