Engineering marvel that has become victim of its own success

IT WAS hit by a huge storm in construction, is pounded by juggernauts scarce imagined by its designers and is now feeling its strength ebbing away as its main supports corrode.

The Forth Road Bridge was an engineering marvel when it opened - the longest suspension bridge in the world outside the United States.

However, 42 years on, the span is a sorry sight, its future uncertain as it struggles to cope with twice as much traffic as it was designed for, and facing the potential need for a main cable transplant that could see it out of action for three years.

The bridge, opened by the Queen on 4 September, 1964, was an instant hit, quadrupling the number of vehicles crossing the Queensferry passage in its first year compared to the old ferries. However, it went on to become a victim of its own success, with traffic numbers climbing inexorably to some 24 million last year.

But the problem has not just been sheer numbers. While light vehicles like cars and vans account for nine in ten of those crossing, disproportionate damage is being caused by heavy lorries.

The bridge's designers faced 24-tonne trucks as their heaviest contenders, but their weight has since nearly doubled to 44 tonnes. In addition, the advent of high-pressure lorry tyres has placed even further strain on the bridge.

As a result, while carriageway resurfacing was once an occasional chore, it is now required every few years. The massive disruption caused during weekend closures of the southbound lanes two years ago is about to be repeated on the other side of the bridge for four months next year.

The crossing is like an increasingly frail and worn-out patient whose every vital organ has had to be repaired or replaced. The towers have been strengthened, joints renewed and many other key parts of the structure overhauled.

Tolls should have ended more than a decade ago, but this was shelved because of the huge cost of the major work to keep the bridge open.

However, the knock-out blow could be an ailment no-one had predicted - premature corrosion of the cables that hold up the bridge.

Alarming evidence of the problem in New York bridges prompted a precautionary check on the Forth, only for engineers to find to their horror that things were far worse than anyone had imagined. Their only consolation was that the Severn Bridge - the Forth's younger sister - had suffered even worse, and has already had to restrict lorries to one lane.

The Forth Road Bridge may well be casting envious glances over to the Forth Bridge, its big red sister, on which huge amounts of money have been lavished to keep it in rude health for well over a century.

Also, while the railway bridge enjoyed a year-long party and sensational fireworks display for its 100th birthday in 1990, a planned laser show to mark the Forth Road Bridge's 25th was a disastrous non-event.

With plans for a 40th celebration passed over, there is now doubt that the younger crossing will still be fully functioning when she hits 50 in 2014 - by which time lorries may already have been banned.

The seeds of the bridge's predicament may have been sown during her six-year construction, which started in 1958. Checks on the corroded main cables have revealed that storm damage and bad workmanship may have had a part to play.

Alastair Andrew, the bridgemaster and general manager of the Forth Estuary Transport Authority, has said corrosion has been detected where some of the thousands of pencil-thin wires in the cables crossed each other, when they should be parallel.

He said this may have been caused by wires being tangled in near hurricane-force winds during construction, adding: "It is clear there was less than perfect workmanship" - although this may just reflect the standards of the time.

Professor Roland Paxton, an engineering historian at Heriot Watt University, said engineers often ignored the lessons of the past. He said Thomas Telford had abandoned parallel wires for chain bars in bridge construction nearly 200 years ago because of the corrosion risk.

He said: "Engineers do not want to know about history - they always reckon techniques are much better now than they were. In the 1960s, they did not think corrosion would be a problem because the wires were galvanised and that would be enough to protect them."

From pilgrims' passage to industrial icon

PEOPLE have lived on the rocky Queensferry peninsula since Neolithic times, yet the first evidence of an established crossing dates back only to 11th century pilgrims en route to St Andrews and Dunfermline.

The name of the passage at the narrowest point of the Firth of Forth derives from Queen Margaret, the wife of King Malcolm III, who encouraged the water link to the ecclesiastical centres of Scotland.

The ferry became the busiest in Scotland, providing essential transport for farmers' cattle and horses. However, the Forth Bridges Visitors Centre Trust said that by 1760 its users found it slow, disagreeable and dangerous.

Major improvements were made in the early 1800s by civil engineer John Rennie, which included a new signal house, ramped piers and pier lights that could accommodate boats at any state of the tide.

These piers, which still exist, continued to be used by ferries until the opening of the Forth Road Bridge in 1964.

Although the Romans may have been the first to consider a bridge, firm proposals were not developed until centuries later.

A planned tunnel between South Queensferry and Rosyth was abandoned in 1807, while an 1818 suspension bridge scheme was also shelved.

Steam then made the railways the fastest land transport and, after a ferry between Granton and Burntisland linked trains on either side of the Forth in 1850, the iron road became the first to bridge the crossing in 1890.

The Forth Bridge, which was this year voted by Scotsman readers as their favourite national icon, was described by Alex Salmond, the SNP leader, who nominated it, as standing "impervious to time and the elements - a reminder of a golden age when all was possible if the country dared to dream".

For nearly 75 years, trains sped across the red cantilever structure, leaving ferry-queuing cars in their wake, before it was joined by its slender silver sister. Many millions of pounds have been spent maintaining the Victorian wonder, but contrary to popular belief, experts said it was over-designed by only 15 per cent following the Tay Bridge disaster 11 years earlier.

FORTH ROAD BRIDGE BY NUMBERS

42: The age of the bridge in years

11.5 million: Its original cost

8 million: The cost of approach roads and link bridges

3,300: The length of the main span in feet

17th: The world ranking of the bridge among longest suspension bridges

1.6 miles: Total length of bridge and viaducts

11,618: The number of 0.2in-thick wires in each of the two main cables

30,800 mile: The total length of wires in main cables

39,000 tons: The weight of steel in bridge construction

208 ft: The height of the bridge deck above the Forth

215 tons: The weight of heaviest vehicle to have crossed bridge

1995: The year tolls were originally due to have been lifted

890,000: The number of vehicles using the ferries the year before the bridge opened

4 million: How many vehicles crossed the bridge in its first year

24 million: The number of vehicles crossing the bridge last year

295 million: The number of vehicles going over bridge since it opened

1740: Date of the first plan for a road crossing at Queensferry

7: The number of workers who died during the bridge's construction

1: The toll charge for cars - northbound only

11.2 million: Total tolls collected last year

210 million: Total tolls collected

100: The number of permanent staff

10 years: How long it takes to paint the underside of the bridge deck