The Tay Bridge Disaster

 Railway bridge built in 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 28 th December 1879, during a storm with gale force winds between  75 people Killed.

ndex/the-collapse-mechanism ndex/the-collapse-mechanism

Theory 1: Wind Load GGGGale force wind along the Tay estuary- strikes perpendicular to the bridge. MMMMoment induced by the wind lifts the anchoring bolts. TTTTies that provide majority of the lateral bracing fail. AAAAs result- Piers act as 2 sets of 3 columns rather than 6 braced columns. LLLLateral stiffness is reduced by 2/3.  P P P Piers sway and collapse under the wind load.

Theory 2: Fatigue Load CCCCast iron lugs that support tie bars fail by fatigue rather than by overstressing. SSSSupported by eye-witness accounts that the girder piers oscillated whenever a train crossed the bridge. PPPPhotographs after the failure show that the lugs did fail by fatigue. TTTThe oscillations, in combination with extremely high wind pressures, caused tie bars on the bridge to fail. SSSSince the tie bars serve to stabilise the bridge, failure of the bridge due to lug fatigue is plausible.

Theory 3: Train Derailment CCCCast iron lug fatigue and poor rail maintenance contribute to a kink forming on track. TTTTrain derails as it travels over the kink- aided by strong cross winds. OOOOne of the carriages strikes the pier. SSSShock induced by collision causes the lateral bracing to fracture. LLL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: 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).