Extended permissible limit state assessment method for masonry arch bridges

Masonry arch bridges form a significant proportion of the transport infrastructure of many countries, including the UK. However, ensuring their continued use can be challenging in some cases, due to increasing traffic loadings potentially undermining the structural performance of some bridges and a lack of widely recognised methods for the serviceability assessment of these types of structures. Traditionally, bridge assessment has been carried out on the basis of Ultimate Limit State (ULS) criteria with a factor of safety applied to obtain the magnitude of service load that can be routinely applied. However, this approach is unable to determine whether a bridge is able to carry increased traffic loading or is more prone to rapid deterioration.

Findings from recent research on the in-service behaviour of masonry arch bridges have led to the development of a new simplified assessment method that introduces separate calculations for Ultimate Limit State and Permissible Limit State (PLS), where the latter corresponds to the level of load that can be indefinitely applied without causing any load-induced damage to the bridge. The new simplified PLS assessment method has been described in the recently published CIRIA C800 guidance.

This paper presents the theoretical basis of the CIRIA C800 PLS simplified approach from the perspective of shakedown theory and proposes an extended methodology that reduces the conservatism of the current approach in the case of long span and low span/rise ratio bridges. Supporting parametric studies and comparisons with experimental data are presented.

It is highlighted that the simplified PLS approach requires knowledge of fewer material properties than a ULS assessment. For Level 1 and Level 2 assessments, it is therefore more straightforward for assessing engineers to apply to masonry structures where uncertainty in material properties is an inherent issue.