13 Conclusion
It should be noted that this investigation was not prompted by the discovery of any concerns with the Forth Bridge Cables but as a prudent response to the result of findings in the USA. When the original scope for this initial investigation was determined, the severity of the findings was not anticipated.
Notwithstanding the above, it is apparent that the Forth Bridge main cables have deteriorated to a greater extent than that expected or anticipated prior to the initial inspection. An assessment of the rate of future deterioration has been derived, based upon the information that is presently available. However, this information is an amalgam of fact, theory and hypothesis and as such the assessment can only be verified by a subsequent inspection, probably within five years of the first inspection. It is therefore prudent and necessary for FETA to investigate the full range of options available to them to prevent or minimise the restriction or loss of use of the Forth Bridge in the foreseeable future. These options are expected to include measures to seek to retard the rate of deterioration and to reduce the loading in the main cables.
We conclude that the Faber Maunsell / Weidlinger team has performed the initial internal inspection and cable strength calculations in accordance with accepted practice in the United States and in general conformance with the NCHRP Guidelines. Certain assumptions have been made that differ from the NCHRP methods but they are reasonable and appropriate considering the particular characteristics of the Forth Road Bridge versus the older US bridges upon which the NCHRP study relied on as case studies.
We have reviewed the load assumptions and find that the dead load calculations are reasonable and that very close agreement has been reached between the assessor and the checker. The live load calculations are slightly conservative, however this will have only a small impact on the final global factor of safety
We have independently calculated the current cable strength and are comfortable that a reasonable estimate of the current loss of strength is around eight percent. We have also reviewed the projected strength degradation calculations and find them to be potentially very conservative, depending on the degradation assumptions made. A slight change in the degradation model yields a significant increase in cable life. However, the reliability of the calculation process for the prediction of the rate of future deterioration is inherently low. The process has never been calibrated against actual bridges in the United States nor elsewhere and relies on assumptions that can't currently be validated.
The degradation models become moot if prompt intervention is implemented to stop or slow the corrosion process. It has been the practice in the United States to take action when serious deterioration has been found and such action is recommended.
It is our considered opinion that while the corrosion is of serious concern if left unchecked, there is no urgency to limit the loading on the bridge at this time. However, it is strongly recommended that action be taken to protect the cables from further deterioration as soon as practical.