Total replacement of the problem truss end links on the Forth Road Bridge was recommended by consultant Fairhurst five years ago, it has been confirmed. But the work was never approved, mainly due to the need for lengthy closures of the crossing before the new Queensferry Crossing was open.
Instead, work began earlier this year to strengthen the brackets that support the top end of the links and transfer loads into the tower structure. This involves welding on plates inside the tower itself, and work was in progress on the north west tower when the problem at the lower end of the link at the north east tower was discovered last week.
As there are no hanger cables next to the towers, the truss end links carry the loads from the extreme ends of the deck trusses. This prevents differential vertical movement between adjacent trusses and ensures that there is a smooth running surface transition and good alignment of the main expansion joints at the towers. They also help resist lateral wind loads.
An analysis by consultant Aecom around six years ago indicated that the end of the deck would drop by 150mm under dead weight alone if a pair of the links should fail. This would almost certainly lead to massive buckling of the adjacent expansion joint. A major repair operation would be needed, involving long term closure of the bridge.
Fairhurst’s preferred long term solution was to insert new links above the deck truss. This would have been expensive and would inevitably cause serious traffic disruption. A fix that would keep the bridge safe until the new crossing opens without the need for major closures was preferred.
It was the upper brackets that caused most concern; hence bridge manager Amey’s inspection team’s surprise when the problems with the lower end of the link were discovered.
An Amey spokesman said that the movement joint between the link and the bottom chord of the deck truss had been checked as road gritting lorries were driven overhead, and that it appeared to be working. Strengthening operations on all eight pairs of links are heavily dependent on weather conditions.
Originally designed more than 50 years ago for 4M crossings a year at a time when maximum HGV weight was 24t, the bridge is now subject to 24M crossings annually, many of them by HGVs weighing up to 44t. The towers had to be strengthened in the 1990s and the deck trusses have been upgraded several times.