Tay Bridge Disaster
Group 6
Aoife Byrne
Brian Walsh
Introduction:
• Designed by railway engineer Thomas Bouch to cross the
Firth of Tay, near Dundee, Scotland.
• 6 years to construct, completed in Febuary 1878, after
several delays and design changes.
• 2 miles long, 85 spans: 72 with support from below the
bridge deck and 13 "high girders" where the support was
above the bridge deck, allowing ships to pass under
them.
Disaster!
• All 13 navigation spans or "High Girders" collapsed during
a storm on the 28th of December 1879.
• Force 10-11 winds (89-117km/h) acting at a right angle to
these spans.
• The train crossing at the time fell into the Firth of Tay,
killing all 75 people on board.
• Debris and bodies were still washing up along the river as
long as 4 months later.
What Happened?
• Three theories: Wind, derailment, fatigue.
• Wind is the most widely accepted theory, and is the one
given by the court of enquiry set up to investigate.
• Derailment was used as a defense by Bouch. He told the
court that the train must have hit a kink in the track,
derailed, and hit a supporting column. The court did not
accept this theory, as it does not explain why all 13
navigation spans collapsed.
• Fatigue is a newer theory for the collapse, stating that
dynamic effects, rather than over-stressing, caused the
Lugs connecting the ties to the columns to fail. Recent
studies have shown evidence of this is weak.
Wind:
• Bouch used a wind loading force
of approximately 210kN/m^2 in his calculations, the
norm at the time was 4 to 5 times that.
• Given the strength of the wind that night, acting
at approximately 90 degrees to the surfaces, it had a
loading of up to 680kN/m^2 on the bridge.
• The ties were insufficient for this loading, and in fact
were even weaker in practice than Bouch had calculated.
• The wind caused the windward bolts to lift, putting
extra pressure on the ties, causing them to fail.
• The windward columns failed in tension, and the
downwind columns failed in compression.
Court of Enquiry Findings:
• The court of Enquiry concluded: "The fall of the bridge
was occasioned by the insufficiency of the cross
bracing and it's fastenings to sustain the force of the
gale."
• Thomas Bouch was given full blame for all the faults in
his design, and was given partial blame for poor
workmanship, as he should have given better
supervision, he was also "mostly, if not entirely to
blame" for neglecting to carry out
proper maintenance inspections.
• "What caused the overthrow of the bridge was the
pressure of the wind acting on a structure badly built,
and badly maintained."
Conclusion:
• For safety it is the norm to underestimate strength
and overestimate load when there is any uncertainty.
Bouch did exactly the opposite in the design of this
bridge.