THE FORTH ROAD BRIDGE 04 February 2011

The Forth Road Bridge is one of the world’s most significant long span suspension bridges. With a main span of 1006 metres between the two towers, it was the fourth longest in the world and the longest outside the United States when it opened in 1964. In total, the structure is over 2.5 km long. A staggering 39,000 tonnes of steel and 125,000 cubic metres of concrete was used in its construction.

Freyssinet were appointed the sub contractors in Spring 2010 by Balfour Beatty to develop a hydraulic jacking and monitoring system to lift the bridge deck at each support structure on the northern and southern approach spans. Each support structure on the approaches (15 piers, 2 towers and 2 abutments) required to be lifted by 2mm whilst maintaining live traffic over the structure and the existing articulation of the deck. A complex system of vertical and horizontal jacks was installed in conjunction with a series of temporary bridge bearings, designed and supplied by Freyssinet, to support the structure temporarily whilst the existing bridge bearings were replaced. Each support pier required 8 temporary vertical support bearings and jack stacks weighing approx 600kg each, and 4–8 horizontal jacks to control any existing locked in forces. These support points were continually monitored to record loading information during and after the jacking operation.

A computerised monitoring system was established by Freyssinet in advance, during and post jacking operation, to record, deflections, pressures and temperatures at each support structure. When the structure was lifted, the 47 year old bridge bearings were removed using hydro demolition techniques and a system of temporary support works. The old bearings were replaced with new Freyssinet bridge bearings; fixed rocker and sliding rocker types. Freyssinet designed the new bridge bearings to very constrictive tolerances with each replacement bearing weighing approximately 2.4 tonnes.

In addition to the bearing works, Freyssinet are contracted to install an impressed current cathodic protection (CP) system, and minor concrete repairs, to areas of the supporting structures. The overall CP system utilises: discreet anodes, titanium mesh ribbon anodes and titanium mesh anode overlay. There is over 14,000 lin m of mesh ribbon to be installed between the 19 support structures alone.
Works are due to complete in Summer 2012.
Why not learn more about Freyssinet’s bridge bearing replacement services for both repair and new structure projects?