Cable Band Bolts
Last updated on 3 Jul 2013
Current works on site:
In March 2012 an inspection revealed new damage to cable band bolt assemblies on the Forth road Bridge, including two cable bands with more than one compromised bolt assembly. Emergency repairs were scheduled and the replacement of all the cable band bolts and nuts was brought forward.
A contract was subsequently awarded to C Spencer Ltd for the sum of £4,058,725.00 to replace all 944 bolt assemblies as a matter of urgency, in order to reinstate the structural integrity of the hanger system and minimise the risks to cross Forth traffic.
Design of the permanent and temporary works has progressed well and the procurement and testing of the new bolt assemblies and the access systems has commenced. The first two cable access platforms were lifted onto the bridge using overnight carriageway closures during the week commencing 13 August 2012.
Working on the main cables is very weather dependent and an allowance has been made in the programme for possible delays mainly due to wind.
Progress made on the project up to June 2013 means that the risk of uncertainty on the final outturn cost has reduced significantly since last winter and will continue to reduce during the remainder of the project until final completion toward the end of 2013.
The bolt assemblies that are being replaced are used to secure 192 “cable bands” to the bridge’s main cables – metal castings over which the steel rope hangers holding up the roadway are looped. Each cable band is held in place by a number of 35 mm diameter high tensile steel bolts, pre-tensioned to a load of around 80 tonnes
All the cable band nuts and bolts were replaced in the late 1990s as part of a larger project to replace the hanger ropes.
The bridge authority originally ordered an investigation after bridge inspectors discovered nine cracked nuts during a routine inspection in 2007. Consulting engineers FaberMaunsell (now Aecom) were appointed to carry out laboratory testing and a desk study of similar details on suspension bridges throughout the world.
In 2009 an interim report into the failure of nine heavy-duty nuts on the Forth Road Bridge concluded that all 1,888 similar nuts on the bridge should be replaced as and when resources and access permitted.
The investigation's final report was subsequently delayed after one of the replacement nuts was found to have cracked again in December 2009. A new access platform had to be designed and built, and the nut was replaced again in October 2010. Laboratory tests were then completed and a scheme to replace all of the nuts developed.
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