Working on the Forth Road Bridge in Scotland is an infamously ongoing project and replacing the bearings on the bridge was no mean feat for Freyssinet, as Teresa Fitzherbert finds out.
- Strengthening work becomes biggest job to date
- Jacking and monitoring the bridge
- The importance of early contractor involvement
When the Forth Road Bridge opened in 1964 it was the fourth longest in the world and the longest outside of the United States. At 1.5 miles long and comprising 55,000 tonnes of steel held together with more than 8 million rivets, maintaining it was going to be a big job.
So big in fact, that the phrase “like painting the Forth Bridge” has entered popular lexicon to describe a neverending endeavour.
Yet this was not going to put off Freyssinet Makers when it was appointed by Balfour Beatty Civil Engineering to complete the refurbishment works on the bridge in 2010.
Strengthening work becomes biggest job to date
Since it was designed in the 1950s, the volume and weight of traffic over the bridge, which connects Fife and the north-east of Scotland with Edinburgh and the south, has hugely increased.
Work to strengthen and maintain the bridge is an ongoing process and a 15-year capital investment programme, worth more than £100m, was put forward by the Forth Estuary Transport Authority in 2009.
Approximately £15m of that went on the Viaduct Bridge Bearing Replacement project, for which Freyssinet Makers was responsible.
The contract was the biggest single award since the bridge opened. It required Freyssinet to replace all the mechanical bridge bearings on the bridge’s north and south approach viaducts to support the bridge deck and let it move with the changing temperature.
“When you are jacking and monitoring a structure of this sort you can really see it as a living thing”
Graham Stanford, Freyssinet Structural Repairs
Freyssinet was also contracted to give a widespread overview of the condition of the concrete pier, along with concrete repair works and the fitting of an impressed current cathodic protection system. The works were carried out under the supervision of consulting engineer Atkins.
According to Freyssinet Structural Repairs business manager Graham Stanford, one of the crucial aspects in the project’s success was that Feryssinet got involved early. “The job was very successful for us and proved to be so because we were neatly embedded within the project team, which included Balfour, Atkins and the client, Network Rail,” he says.
“This enabled us to be fully involved with the methodology for the project and to be there giving advice when it was needed.”
Jacking and monitoring the bridge
The existing bearings on the bridge were worn and corroded from decades of Scottish weather. To carry out the works, Freyssinet designed and manufactured 40 permanent mechanical bearings to BS 5400. The two types of bearings were fixed rocker and sliding guided, with each weighing 2.4 tonnes.
Freyssinet also designed and supplied 35 temporary support bearings and used more than 100 hydraulic jacks for the temporary support and restraint works.
The company also carried out monitoring of the bridge deck to allow the removal of the existing bridge bearings. The system monitored the movement of the bridge for five days before the jacking operation, during the replacement, and for 20 days after the new bridge bearings were installed.
“This job was very demanding but the most interesting part for us was the bridge jacking and the need for remote monitoring,” says Mr Stanford. “When you are jacking and monitoring a structure of this sort, you can really see it as a living thing that expands, contracts, bends and vibrates.”
The importance of early contractor involvement
The company’s biggest challenge came when installing the carthodic protection system. “The need to fit about 14 km of anode ribbon on the concrete faces for the cathodic protection was our greatest challenge. We did this by using remove sawing techniques,” Mr Stanford says.
The work was finished on time in August 2012. “Being involved at an early stage and taken seriously as a key member of the technical team ensured a good result for everyone,” he says.