Question
Is it possible, if the Permissible Limit State (PLS) check is OK, but the Ultimate Limit State (ULS) is not, to simulate the number of cycles of a given traffic load and frequency, that would lead to PLS failure?
Answer
The PLS calculation that is described in CIRIA C800 and used in RING is anticipated to be conservative in that it aims to avoid loadings that would mobilise secondary sources of resistance which require movement of the arch. In principle this would avoid movement induced deterioration, but it is of course not possible to eliminate all movements under load. It is therefore challenging to estimate the number of cycles that could lead to damage to the arch. This would require a much more sophisticated modelling approach.
If an analysis passes the PLS and fails the ULS, this may be indicative of a different issue if you wish the bridge to comply with both limit states. The partial factors adopted for each analysis will reflect the acceptable risk of exceeding the specific limit state.
If the analysis is very close to the limit of the PLS, then it is probably indicative that more regular monitoring is required than where a bridge is significantly far from the PLS.