The Forth road bridge is 46 years old and is developing a few aches and pains.
Its cables are losing their strength, prompting a decision to build a new bridge by 2016 to take much of the traffic travelling over the Firth and leaving the middle-aged behemoth to carry lighter public transport loads.
But even so the current road bridge will need its joints replaced within the next few years and until then they will need extra care and attention in the form of regular inspections and maintenance.
Cue a £1.75M project to put in a new underdeck access platform to replace a rather less robust arrangement. The new platform will allow easy access to the joints for inspection and maintenance and ultimately their replacement, all without causing severe traffic disruption above.
Client for the scheme is the Forth Estuary Transport Authority; the contractor is Raynesway Construction and design is by Atkins.
The work involves fabricating and installing four remote access gantries 45m from each expansion joint; stairways to replace existing ladders; upgrading the existing network of walkways to the appropriate loading for working platforms; and fabricating and installing a multi-tier walkway system in and around the main Demag expansion joints.
“The fabricated steel sections are manoeuvered into place and put togther like a Meccano set”
Graham MacAlpine, project manager
“We had to design the platform to accommodate the 1.7m movement of the roller shutter joints, so the walkways are fixed on one side and slide on the other to mirror the joint movement,” says Atkins site engineer David Bishop.
The bridge can sway up to 7m when the winds pick up and it bounces up and down 2-3m with the traffic so the construction team, including scaffolders from Harsco and the steel fixers from Miller Callaghan, have learned to roll with what feels like a living bridge.
It’s amazing how quickly you get used to it,” says Raynesway project manager Graham MacAlpine.
“The fabricated steel sections are manoeuvred into place by a combination of hoists and manhandling,” MacAlpine says. “They range in weight from 10kg to 130kg and were designed so they can be carried in through the access points on the deck and put together like a large Meccano set.
Raynesway did the work on the south side of the bridge first, and lessons from that experience have been carried over to the north.
“We did have some problems with fitting tools in for the bolting so we have changed some of the angles on the sections,” he says.
The job is currently 10 weeks ahead of programme with a predicted finish in November.
Up the road at the Tay road bridge, Raynesway has completed a project to replace the bearings at the southern Fife abutment.
Five old bearings were replaced with four new, larger bearings, two under each box girder.
The abutment wall had to be extended and new bearing shelves constructed.
Meanwhile, strengthening walls were built inside the box girders.
These walls take the loads from temporary jacks and the new bearings. The bridge deck was jacked up during night closures to allow the new bearings to be installed.