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10 Applicability of NCHRP Guidelines to Forth Bridge
The NCHRP Guidelines offer the only written guidance on the inspection and assessment of parallel wire, aerially spun, main cables for suspension bridges. They were developed in the United States from experiences on American suspension bridges.
However, there are key differences between US and British suspension bridges which may cast doubt on the strict applicability of the Guidelines to the Forth Road Bridge:
- US bridges that have been investigated are generally much older (in the approximate range 65 to 120 years old).
- Environment and climate could be significantly different, e.g. New York summers tend to be hot and humid and the average rainfall is twice that at Forth.
- Standards of inspection and maintenance are considered to be better at Forth Bridge than for some of the US bridges, especially those that are not operated by toll facilities.
- Forth wire quality would be expected to be better and more consistent than wire from older US suspension bridges.
The Guidelines appear to be conservative in several aspects for use in the UK possibly because of some of the differences highlighted above. Weidlinger, as the main authors of the Guidelines, should be aware of these differences and are best placed to assess them, but it is not clear from any information provided within the audit if any allowances have been made other than the revised assumptions made regarding the population of wires that may be cracked. This recognised that the Forth Bridge is younger than those used as case studies when the guidelines were developed.
The Guidelines are relatively new as they were published about a year ago. They have therefore not been used on any bridges other than the two case studies used by Weidlinger. No US bridges have actually gone through a series of repeat inspections that could be used to calibrate the NCHRP methods. However, the calculation methods for current cable strength have been used in principal dating back to the late 1980's when the Williamsburg Bridge cables were evaluated in depth. The concepts presented in the NCHRP Guidelines have been generally accepted in American practice. 6
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