Text from an email covering note to H&F Council dated Friday 8 January 2021
Following our discussion on the Mott MacDonald report (pdf 446KB), I can now summarise the position in respect of the current CCSO(s). As the author of the various CCSOs I believe it is important that the context of the various reports is clear to H&F, and so I provide some more detail to the overall position in the attached briefing paper.
It is clear that the Mott MacDonald report is another step on the way to a fuller understanding of the bridge structure and behaviour. We can say that it makes the case well about the efficacy of the temperature control system which is key to maintaining a stable position for the structure. That said, we must remind ourselves that there are a number of other matters not yet addressed before any consideration could be given to reviewing the current CCSO(s). These include (as major items):
- We must still inspect as fully as possible the west side pedestals; and
- We must still demonstrate by practical experience that the temperature controls are effective over the full range of ambient temperatures likely to be experienced; and
- The MM report must be subjected to an independent Cat 3 check; and
- We must be aware that a failure to set a date for remedial works must mean closure of the bridge; indefinite use of temporary measures is not acceptable.
The principal finding of the report is that provided the temperature controls remain effective, and anchor chain loading remains in a close range towards the centre of the full range of likely loads that would otherwise be imposed with seized rollers, the NE pedestal structure is likely to remain stable. It does not remove the 'unknowability' of the absolute chain loading, but the analysis allows that to recede in importance.
It is worth noting that the report stops short of recommending any practical change to the current CCSO(s) – that is to be expected as it necessarily represents a focus on a single pedestal in a system of pedestals and other structures in the bridge.
That leaves a position in which the current CCSOs remain unchanged.
David Crawley
Director - Xanta Limited