Holdfast Training Services Trial Blog

Hello all,

You may or may not have heard about the problems associated with the Forth Road Bridge near Edinburgh.  I have been passed some info that I though may interest some of you out there….

The images below are from a load test done a couple of weeks ago.  Achieved using lots of full gritter lorries with drivers on danger money wearing armbands!  Some interesting facts:

• The bridge spans just over a kilometre between the piers.  It was the 4^thlongest suspension bridge in the world when opened in 1964.
• The piers are 156m tall (about half the height of the Eiffel tower, and about the same as Lomond or NEV from seabed to flare tip)
• The bridge contains 40,000Te of steel
• The maximum tension in the main cables is about 25,000 tonnes at the mid span
• The main cable tensile stress is incredibly high at 1050MPa (N/mm²), which is about the same as a 17 stone man suspended from a 1mm²guitar string (nice image there – sorry)
• Each tower supports 8000Te imparted from the main cables.

The part that failed was an inverted steel ‘goalpost’ that is attached to the bottom of one of the deck hanger cables.  It almost certainly suffered fatigue damage, ie too much stress cycling caused by heavy lorries.  When it was opened in 1964 it was designed for 25,000 vehicles/day, and the maximum lorry weight was 24Te.  Today it sees 70,000 vehicles/day and the maximum lorry weight is 44Te.  If the steel component had completely parted, the consequences are debatable.  In the worst case it could have caused a section of the road deck between the hangers to tip, possibly causing the combination of 4 or 5 heavy lorries (that would cause such a failure) to crash.