In early 2010 Freyssinet Makers, the Northern structural repair division of Freyssinet Ltd, was appointed by Balfour Beatty Civil Engineering to carry out bridge bearing replacement and associated works on one of the world’s most significant long span suspension bridges. The Forth Road Bridge is located in Scotland, connecting Fife and the north east of Scotland with capital city Edinburgh and the south. When the bridge was opened in 1964, it was the fourth longest in the world, and the longest outside the United States, with a main span of 1006 meters between the two towers. In total, the structure is over 2.5 km long. Since the structure was designed in the 1950s, the traffic loading on the bridge bearings has increased significantly. This can be attributed to a higher volume of traffic on UK roads, and also to an increase in the weight and number of heavy goods vehicles. Maintenance to the Forth Road Bridge is ongoing and a large capital programme to strengthen the bridge is also continuing. In 2009 the Forth Estuary Transport Authority (FETA) embarked on a competitive tender process for the Viaduct Bridge Bearing Replacement project, estimated to total £15 million. The contract is the biggest single contract awarded since the bridge opened. It involved the replacement of all the mechanical bridge bearings on the Forth Road Bridge’s north and south approach viaducts, which sit at either end of the bridge on each shore, connecting the A90 and M90 with the main body of the suspension bridge. The steel bearings support the bridge deck and enable it to move as required by changes in temperature. The contract also required an extensive overview of the concrete pier condition in conjunction with concrete repair works and the installation of a discreet anode CP system. Balfour Beatty Civil Engineering was selected as the Main Contractor for the project, carrying out the works under the supervision of consulting engineers Atkins. Freyssinet Ltd was chosen to supply and install the bridge bearings, and complete associated concrete repair and cathodic protection works on the structure. The Company was able to demonstrate a successful track record with this type of work, including previous involvement with a similar project in Scotland on the Tay Bridge. Consulting Engineers Atkins were responsible for designing the complex sequence of temporary works, permanent box girder and pier head strengthening to allow the structure to be supported on temporary jacks. In addition they supplied the detail design for the cathodic protection system.
Balfour Beatty coordinated the whole project construction sequence including supplying extensive scaffold access platforms, extension of the reinforced concrete pier head and strengthening of the steel box girder at each temporary jacking support point.
40No. permanent mechanical bearings were specially designed to BS 5400, and manufactured in Freyssinet Ltd’s factory in Telford in quality controlled conditions that exceed the requirements of BS EN ISO 9001. There were two types of bearing to be installed – fixed rocker and sliding guiding, each weighing a massive 2500kg. These replaced the existing mechanical rocker and pin roller bearings which were badly worn and corroded.
Freyssinet also designed and supplied 35No. temporary support bearings and over 100No. hydraulic jacks to meet the requirements of the temporary support and restraint works. In addition Freyssibar was supplied and stressed to assemble and fix the longitudinal restraint frames onto the structure. Freyssinet are responsible for all the hydraulic jacking and monitoring of the bridge deck to allow removal of the existing bridge bearings. The monitoring operation comprised of a complex system of 17No. electronic sensors fixed to each pier location (19 in total). Each sensor was wired to a data-logger which captured all sensor data every 2 minutes; vertical / horizontal displacement, air temperature, pier verticality and pressures in the hydraulic support system. The system monitored the movement of the structure 5 days before the jacking operation, during the bridge bearing replacement, and for 20 days after the new bridge bearings were installed. The bridge bearing replacement work by Freyssinet considered also the careful coordination of hydrodemolition works, steel cutting and drilling, and heavy lifting and positioning techniques to all allow the careful and safe positioning of each bearing into an extremely restricted access gap.
In addition to the bearing installation works, Freyssinet installed an extensive cathodic protection system in to areas as identified and designed by Atkins. The whole system utilisied three anode systems; titanium mesh, titanium ribbon and discreet mixed metal oxide anodes. Over 14,000m of titanium ribbon anode strip was installed into the concrete faces of the pier head cross beams and legs to form part of the CP system. Freyssinet came up with an innovative technique that utilised a chase cut into the concrete using a remote operated wall saw. This cut up to 60m per day without exposing operatives to excessive hand arm vibration or concrete dust. Corrosion Control Services (CCSL), a sister company of Freyssinet, were involved in the procurement of specialist materials and supplied the necessary specialist site supervision / technical commissioning as required by the contract The Forth Road Bridge is a category ‘A’ listed structure and consent had to be granted for the works to be carried out. Selection of all repair materials and methods of placement were carefully considered in technical trials prior to the start of the works in order to meet the requirements of the consent. The project has now been running for 12 months and is currently on schedule to be complete by the end of 2012. Freyssinet Ltd are currently involved with the tendering process for future works on the Forth Road Bridge, and hope to continue their successful collaboration with the FETA and the other contractors working on this landmark structure.