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Forty years after it first came into service, the internal
condition of the main cables on the Forth Road Bridge is currently being
evaluated -- the first time such a project has been carried out in the UK.
Opened by The Queen in September 1964, the Forth RB is the oldest
major suspension bridge in this country and now a Category A listed structure.
Since 1964 the number of vehicles using it has grown from four million a year to
over 24 million, and there have been a number of large-scale improvement and
strengthening projects to allow for the increased loads.
The suspended structure weighs around 16,000 tonnes and provides a
main span of 1006 metres. Each of the main cables from which the deck is
suspended consists of 11,600 individual high-tensile wires arranged in 37
strands, which are compacted into a circular shape, 600mm in diameter. This
structure is painted with a paste made from red lead powder and linseed oil and
wrapped circumferentially with galvanised wire, which is then coated with
protective paint.
One of the main purposes of the test programme is to investigate
the condition of the individual wires that form the cables. While non-invasive
inspections are carried out regularly on suspension bridges in the UK,
inspecting wires within the cable involves an invasive technique used in the USA
but never used before in the UK. It requires the outer wrapping wire to be
uncoiled, wooden wedges to be driven into the cables to open them and then
sample wires to be removed for inspection and analysis. In the case of the Forth
bridge, this work is being carried out at five positions on each cable, some of
them 80 metres above the road deck.
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