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Contractors World - UK & Ireland Vol 2 No 8
its main cable opened up to check for signs of corrosion.
Although the main cable showed no exterior signs of
deterioration, the concern was that corrosion might be
present inside – as had recently been discovered in the
cables of older American long span suspension bridges.
When the cable was opened up, the bridge authority
was surprised to find that 8-10% of the cable’s strength
had been lost as a result of corrosion – despite the cable
at that time being just 40 years old. Although such a loss
is significant, it hasn’t necessitated traffic restrictions. It is
crucial, however, that the corrosion is halted to prevent
this happening in the future.
Engineers have now protected the cable with waterproof
elastomeric wrap and fitted a dehumidification system in a bid to stem the progress of the deterioration.
Dehumidification has already been used on
the Forth Road Bridge in other areas and in the main
cables in newer bridges in Japan and Sweden, but this is
the first time it has ever been used inside a main cable
that’s more than four decades old.
Three panels on the east cable were opened in
early 2008 to allow an internal inspection of the wires
forming the main cables (two of the panels were
opened previously in 2004 during the first internal
inspection).
This second internal inspection in 2008 showed
that the projected envelope of the predicted strength
of the cables, determined from the first inspection,
was still valid except that the second inspection
extended the timescale for potential loading
intervention. In addition, the second inspection
results indicated that it appeared more likely that the
rate of deterioration was predicted to follow the more
optimistic line.
Therefore, from the 2008 results it was considered
more likely that any restriction in loading would not
have to be considered until between 2017 and 2021.
A tender submitted by C Spencer Ltd., for the total
sum of £2,573,310 to carry out a third inspection was
approved by the FETA Board in October 2011. Flint &
Neill Limited (F&N) were appointed as Engineers by
the Authority to assist with contract procurement and
supervision of the works.
This third internal cable inspection will allow another
point to be plotted on the cable strength loss versus time
graph and be the first indication of the effectiveness of the
dehumidification system. This inspection will also increase confidence in the prediction of the future strength of
the main cables.
However, it will not provide absolute values as there is a recognised degree of error in determining cable
strengths from such a small sample of wires. In addition, it is recognised that the determination of the condition
of the wires which in turn affects predicted strength, is based on the subjective judgement of the engineers and
inspectors carrying out the detailed inspection.
Further calculations of cable strength will need to be obtained in future years to increase confidence levels in the
capacity of the cables. It is almost certainly the case that some degree of uncertainty concerning the magnitude of
future strength loss of the main cables will remain and the cables will require to be continually monitored, and be
subject to a regime of internal inspections and strength evaluations, for the remainder of the service life of the
bridge. However, it is expected that this uncertainty will diminish with continual monitoring and after each future
inspection.
Following preliminary cable inspections, dehumidifying
systems where installed and the cables wrapped in special
elastomer for additional protection.