New schematics have revealed the design of the additional strengthening works on the Forth Road Bridge.
Now a new steel bracket will be attached to the south face of the north east main tower leg, from which the top of the deck truss will be supported via steel strand jacks. It means there will be an additional cable hanger adjacent to the main tower.
Amey engineers are to begin the work within the next few days and it should be completed by mid-February.
Structural monitoring systems are also being installed on all of the eight truss end links and in the future the truss end links will be completely replaced, with the truss supported from the new tower bracket.
Structural Monitoring System
Data from an array of sophisticated sensors fitted to the suspect elements in the deck truss of the troubled Forth Road Bridge will be crucial in the next phase of the strengthening works, it was confirmed today.
Engineers are due to start installing 72mm diameter steel cable hangers from a bracket on the north east tower that will take most of the end loads from one corner of the main deck truss. (See diagram). This is directly above the truss end link that was found to be cracked last November, and which led to the closure of the crossing
Forth Bridge Repairs
Sensors recently installed on the suspect truss end links will ultimately determine how much load will be picked up by the cables as strandjacking takes place.
These include more than 200 strain gauges, along with temperature and tilt sensors. Specialist monitoring company Strainstall diverted a team of its engineers from the nearby Queensferry Crossing, where they were installing Strainstall’s BridgeWatch structural health monitoring system inside the main deck box.
Working in sub-zero temperatures and high winds within the Road Bridge’s open lattice deck trusses, the Strainstall team first installed sensors that informed the decision to re-open the crossing to light traffic only. Then they completed the instrumentation of the truss end links in time for the medium term strengthening works to begin within the next few days.
The sensors will provide data on any distortion of the truss end links as the cables take up the load and should reveal how freely the pin joints at the lower end of the pair of links are moving. As much load as engineers feel is safe will be transferred. No daytime closures will be needed.
This fix should allow the crossing to re-open to HGVs, probably in the middle of next month. Design work on the final strengthening works is on-going, but is likely to involve the total replacement of the existing truss end links.
Again, no bridge closures are envisaged, even though the Queensferry Crossing should be open by this time. It is probable that the new end links will carry the full deck loading, and the additional cables will be removed.