The Forth Estuary Transport Authority ordered the investigation after nine cracked nuts were spotted during a routine investigation last year.
The nuts secure 192 cable bands to the bridge's main cables. Each cable band is held in place by a number of 35mm-diameter high tensile steel bolts, pre-tensioned to a load of about 80t.
All the cable band nuts and bolts were replaced in the late 1990s as part of a project to replace the hanger ropes. The investigation has identified a number of design and specification decisions and construction methods that may have contributed to the cracking, including replacement of the original imperial bolts with metric versions with a thinner section than the originals. The nuts are small compared with similar ones on other suspension bridges. Another factor was the use of a higher grade of steel, which meant that the nuts are less flexible than the originals. Misalignment of washers may have led to uneven loading in the nuts. In addition, the protective coating was inadequate and allowed moisture to cause damage.
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.
