The Tay Bridge Disaster

Railway bridge built in 1878- after 6
years of construction.

Designed by Sir Thomas Bouch.

Longest bridge in the world at time of
construction- stretching 2 miles.

Girders spanning 44 metres.

Girders supported by piers consisting of
6 columns- 26.8m height to allow for
ship clearance.

Rail line on top of girders.

Rail line enclosed in a ‘cage’ of steel
members.

Failed on 28th December 1879, during a
storm with gale force winds between 1011.

75 people Killed.
http://taybridgedisaster.co.uk/i
ndex/the-collapse-mechanism
Theory 1: Wind Load

Gale force wind along the Tay estuary- strikes perpendicular to the
bridge.

Moment induced by the wind lifts the anchoring bolts.

Ties that provide majority of the lateral bracing fail.

As result- Piers act as 2 sets of 3 columns rather than 6 braced
columns.

Lateral stiffness is reduced by 2/3.

Piers sway and collapse under the wind load.
Theory 2: Fatigue Load

Cast iron lugs that support tie bars fail by fatigue rather than by
overstressing.

Supported by eye-witness accounts that the girder piers oscillated
whenever a train crossed the bridge.

Photographs after the failure show that the lugs did fail by fatigue.

The oscillations, in combination with extremely high wind pressures,
caused tie bars on the bridge to fail.

Since the tie bars serve to stabilise the bridge, failure of the bridge
due to lug fatigue is plausible.
Theory 3: Train Derailment

Cast iron lug fatigue and poor rail maintenance contribute to a kink
forming on track.

Train derails as it travels over the kink- aided by strong cross winds.

One of the carriages strikes the pier.

Shock induced by collision causes the lateral bracing to fracture.

Lack of lateral stability allows the bridge to be blown over by strong
winds.
Summary

Likely that a combination of the factors outlined caused the failure.

Major flaw was the wind load considered- Bouch used 10 pounds
per square foot (the lowest value recommended to him).

Typical wind load values adopted at the time: 40-50 pounds per
square foot.

Windward columns weren’t properly anchored.

Could have been avoided with better financial backing- Higher
design loads could be used and better quality steel (no cast iron).