Sights and sounds of a decaying Forth Road Bridge

These wires have now snapped at the rate of more than one a month for the past 17 months – a stark illustration of how the life expectancy of the 44-year-old bridge is shortening.

The wire breaks – detected by microphones embedded inside the cables – come on top of 86 found during a previous inspection four years ago.

But they tell only part of the story. The real worry facing bridge officials is how quickly corrosion of the remaining wires is sapping the strength of the crossing.

Now, The Scotsman has learned that the clearest picture of when traffic bans may have to be imposed on the crossing will be provided by a new inspection over the next month.

The results, which are expected by April, could determine whether a threatened lorry ban in just five years' time has to be brought forward – or can be pushed back.

Hauliers said they were praying planned repairs would be successful, to prevent the Fife economy being hammered by such severe restrictions.

Engineers will check the section of the cables with the worst-known corrosion for the first time in four years to see whether their previous predictions are still correct.

Corrosion and wire breaks among the wires that make up the 2ft thick cables have already cut the bridge's strength by 8-10 per cent.

A further 10 per cent loss of strength of the cables, which hold the bridge up, would first force a lorry ban, and then complete closure of the crossing in 11 years' time.

The calculations are based on the corrosion continuing unchecked at the fastest possible rate, and the bridge's strength falling below a minimum safety threshold.

When the bridge opened in 1964, the cables had a safety factor of 2.59 – or 2.59 times the strength required to hold up its own weight and that of traffic.

Vehicles account for 15 per cent of the total weight, half of which comes from lorries.

However, cable inspections in 2004 showed the bridge's safety factor had been cut to 2.23 times the strength required.

The Forth Estuary Transport Authority (Feta) said the minimum safety factor was two – a margin taking account of possible extreme weather, the heaviest traffic loads and any unknown deficiencies in the bridge structure.

It is currently calculated that this level could be reached in 2013 if the corrosion is not halted.

Officials will also use next month's scheduled inspection to ensure microphones detecting the wires snapping are working properly.

A total of 22 wire breaks have been recorded by the 866,000 acoustic monitoring system since it was installed in August 2006.

These are in addition to 86 wire breaks found during inspection of ten sections of the cables at various points on the bridge in 2004.

However, Feta, which runs the bridge, hopes it can halt the corrosion by blowing dry air into the cables to dry them out. But installation of the dehumidification equipment required will not be completed until October next year because of the traffic disruption it has caused.

Feta officials said it will not be known until at least 2011 whether the process has worked – when construction of a replacement bridge is due to start.

However, the new bridge, to be built alongside at a cost of up to 4.2 billion, will not be finished until at least 2016.

Feta has ordered next month's inspection so its results can be compared to those to be taken once dehumidification has started.

The cables will be opened and several 10ft-long wire samples removed for analysis. Replacement wire will be compressed on to the strands to maintain their strength.

The work, which is expected to take three weeks, will involve a 180ft long section of the east cable at its lowest point, half way across the bridge.

This section centres on a 60ft stretch which is thought to have been the worst corroded among those checked four years ago because it was where water had collected after seeping down the inside the cable.

However, Feta officials said they remained puzzled as to why this section was not the worst one for wire breaks among those checked four years ago.

They were also surprised to find the same section of the west cable was found to be less corroded, despite the prevailing westerly wind.

Feta said there had also been no pattern in frequency or location to the wire breaks detected by the microphones, which are positioned every few hundred yards along the cables.

The system is designed to give early warning of new problems.

Feta has said it would open up an area of the cable for inspection if several wire breaks were detected close to each other.

Barry Colford, Feta's depute general manager, said he could not predict the results of next month's inspection – or what implications they would have for the potential 2013 lorry ban.

He told The Scotsman: "It may alter that date, it may not. We hope it will be in the same 'envelope' as the previous results, but we do not know whether it will be slightly up or down.

"It should give us a better idea of how the bridge's safety factor is likely to change over the coming years.

"It will allow us to re-evaluate concerns that the safety factor could fall below two.

"The current rate of wire breaks is not causing undue concern."

Gavin Scott, the head of policy in Scotland for the Freight Transport Association, said: "We are praying the dehumidification does work so any lorry ban can be pushed back until a new crossing is in place.

"If not, you might as well switch off the lights in Fife."

Alan Russell, the chief executive of Fife Chamber of Commerce, said: "We must do everything possible to have a new bridge completed before any restrictions have to be applied on the Forth Road Bridge.

"That is vital to give businesses confidence to invest in Fife in the future. Planning decisions must be fast-tracked if at all possible."

Neil Greig, the Scotland director for the Institute of Advanced Motorists' Motoring Trust, said: "It is important to know exactly when the bridge is likely to face problems.

"I hope the inspection results will reassure drivers that everything possible is being done."

Phil Flanders, the Scotland director of the Road Haulage Association, said major roadworks on the Forth Road Bridge would provide a sign of things to come.

He said that next year's round-the-clock lane closures, due to be imposed while joints on the deck which carries the carriageways are replaced, should concentrate minds.

He said: "There will be even greater disruption, with all lorries having to travel via the Kincardine Bridge, if there is a ban on heavy goods vehicles on the Forth Road Bridge imposed four years later."

Red for danger: Colour-coded warning system graphically illustrates damage

WHEN Forth Road Bridge officials started a precautionary inspection of its main cables in 2004, they were shocked by the extent of corrosion found.

The move followed problems found in several suspension bridges in the United States, and the Forth was the first to be checked in Britain. Analysis of samples taken from ten points on the bridge's cables, which stretch the length of the crossing on either side of its deck, made sobering reading.

A colour-coded, cross-section illustration was created of the 11,618 wires inside the cables at the worst section found (pictured above). It shows the east cable at its lowest point, halfway across the bridge, on which next month's new inspection will focus.

The diagram is based on a system devised by US government highways experts – a four-stage guide in which red, yellow, green and black represent degrees of corrosion, with red the most severe and black the least. The central black areas are comparatively tiny compared to the other colours.

The Forth Road Bridge "represented" the UK on the US-led working group which drew up the code of practice for such cable inspections.

The cross-section shows most of the worst affected wires, shaded red, are towards the outside of the 2ft thick cable, although there are also areas of the highest-level corrosion closer to the centre.

The arrows show where wooden wedges were driven into the cluster of wires inside the cable's protective wrapping to inspect the interior.

This section was the most corroded part of the cable inspected in 2004, although it did not have the greatest number of wire breaks. The Forth Estuary Transport Authority, which runs the bridge, said looking at the diagram alone does not show the remaining strength of that part of the cable.

It said complicated calculations were required because the extent of corrosion in each wire provided only an indication of its residual strength.

The corrosion is thought to have been caused by water seeping into the cables – with the worst section suffering from water collecting at the cable's low point.

However, it remains unclear whether the way the cable was made when the bridge was built in the early 1960s played a part. There remains the possibility that bad workmanship and storm damage while the cables' wires were spun from one end of the bridge to the other could have had a role.

However, corrosion of cables on the Severn Bridge, which is two years younger than the Forth, has been found to be even worse.