Following the discovery of nine cracked heavy duty nuts on the Forth Road Bridge, an interim report has concluded that all 1,888 similar nuts on the bridge should be replaced.
Bridge authority FETA ordered an investigation after bridge inspectors discovered nine cracked nuts during a routine inspection last year. Consulting engineers FaberMaunsell were appointed to carry out laboratory testing and a desk study of similar details on suspension bridges throughout the world.
The nuts 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’s investigation has identified a number of design and specification decisions and construction methods that may have contributed to the cracking:
- The replacement of the original 1 ½ inch bolts with metric M39 bolts resulted in the nuts having a thinner section than the originals
- The dimensions of the new nuts are particularly small compared with similar nuts on other suspension bridges
- The much higher grade of steel specified means the nuts are harder and less flexible than the originals
- Misalignment of washers may have led to uneven loading in the nuts
- The protective coating on the nuts was inadequate and allowed moisture to cause damage
Four of the failed nuts have already been replaced on the west cable, using access platforms already in place. The five failed nuts on the east cable will be replaced in the course of this year.
“Thanks to the vigilance of our inspection team this problem was identified at an early stage and the bridge remains perfectly safe,” said Bridgemaster & Chief Engineer Barry Colford.
“Cracks have been found in only nine out of 1,888 similar nuts. Nevertheless, our investigation has concluded that any of these nuts could potentially fail in future, so the recommendation is to replace them all as a precaution. We’re planning to carry out further tests in the laboratory before bringing a final report to the FETA Board later this year.”
The interim report states that legal advice has been sought by FETA to establish whether there is any liability on the part of the designer or the contractor who replaced the nuts in the 1990s. The outcome depends on the investigation’s final report, due later this year.