Masonry arch bridges - towards a hierarchical assessment framework

Masonry arch bridges are estimated to account for more than 40% of European bridges. Often more than 100 years old these bridges have performed well in service and are arguably the most durable and sustainable bridge type. However, the gradual deterioration of materials with time, coupled with the increase in loading from modern road and rail vehicles, make re-assessment, maintenance, repair and strengthening inevitable in order to ensure that safety, performance and serviceability are sustained at an acceptable level. While numbers of masonry arch bridges in the US are lower these issues are equally relevant. Bridge managers and owners use a variety of assessment algorithms to identify safe load capacities. The underlying theory behind different assessment approaches varies. The required bridge data also varies from method to method as do the dimensional models used. This diversity of assessment methods explains, in part, the resulting scatter of assessment ratings achieved for a single bridge. What is often more difficult to rationalize is the anomaly of an apparently higher level, or more rigorous, assessment algorithm resulting in a lower assessment rating. UCD is working with the National Roads Authority in Ireland to rationalize a hierarchical framework of assessment algorithms whereby increasing assessment effort is rewarded by demonstrable convergence toward the ultimate capacity of the bridge. The aim is to develop an algorithm hierarchy for masonry arch bridges. As part of this work 12 masonry arch bridges, characteristic of the range of bridges on the Irish road network, have been assessed using three assessment methods. This paper presents a discussion of the assessment approaches, the assessment ratings achieved and the trends in assessment ratings of the different assessment approaches vis-a-vis each other and also measured responses, to passing weighed vehicles, for two of the bridges.