ABSTRACT: Masonry arch bridges comprise a significant proportion of the bridge stock both nationally and internationally. They have proven to be enduring structures, far surpassing their design lives and with relatively low life-cycle costs. New masonry arch bridges are not generally constructed. However, their ageing materials paired with ever increasing traffic loads have led to the necessity for their periodic re-assessment. Masonry arch bridge design was predominantly by rule of thumb and their re-assessment has been subject to a number of simplified assumptions. One such simplification, the bridge “effective width” is studied in this paper with the use of LUSAS finite element analysis software. A preliminary study was carried out on a masonry arch, assessing six point load locations. Arch displacements, axial stresses and axial force envelopes were determined. This informed a more complex study, where parameters such as: arch stiffness, fill stiffness, fill depth, arch shape, arch span/rise, abutment stiffness, ring thickness and bridge skew were studied. This model was verified using displacement test data obtained from the literature. The effective widths of a series of bridges with varying parameters were studied. Statistical analysis was used to determine the most influential factors on the effective widths of masonry arch bridges. Once these were determined, a formula for the calculation of effective width was derived and compared with the effective width prescribed in the current design code. KEY WORDS: Masonry arch, Transverse load distribution, Finite element analysis, Effective width. 1 INTRODUCTION  Although new masonry arch bridges are not generally constructed, they have been used extensively for the last 2000 years. Many masonry arch bridges were constructed by ‘rule of thumb’, and with ever increasing traffic loading there is a need for their reassessment in order to determine their true load capacity. Masonry arch bridges account for a significant proportion of global bridge stock. In Ireland, 36% of the bridge stock on national roads comprises masonry arches [1]. By international standards for strategic transport routes, this is high. Currently, the modified MEXE method of analysis [2] is the most popular method of assessment of masonry arch bridges. This method has, however, proven to be inaccurate in many instances [3]. Analysis through computer modelling programs such as LimitState Ring and Archie M is becoming increasingly popular and these packages offer essentially two dimensional analysis with simplified transverse distribution rules. Masonry arch bridges comprise many components that complicate their analysis. The principal element is the arch barrel, which reacts against large abutments and piers. The arch barrel supports spandrel walls, which retain backfill over the arch. The fill material comprises soils and gravels that are placed and compacted over the barrel and between the spandrel walls to provide a formation layer for the road pavement. The masonry bridge is completed with parapet walls to retain errant vehicles and the like. There is a number of specific terms used in the description of masonry arch bridges, which are illustrated in Figure 1. Figure 1. Masonry arch bridge features Although numerous studies have been undertaken to analyse arch behaviour along the span, few explore how the arch distributes applied forces transversely. In practice, loads are not applied uniformly across the arch width and thus the distribution of loads needs consideration. Skewed bridges have significant three dimensional characteristics. The current prescribed analysis for masonry arch bridges maintains that a skew arch transfers loads across the shortest span available. This induces large shear stresses on the obtuse corners, with the abutments providing rotational resistance to torsional moments. The aim of this paper is to assess transverse load distribution in masonry arch bridges subject to point loads at different locations and to compare the findings with the current prescribed analysis method set out in BD21/01 [4]. In addition the load distribution within skewed arch bridges is assessed. A study of transverse load distribution in masonry arch bridges Helen Dunne 1 , John J. Murphy 1 , Kieran Ruane 1,2 1 Department of Civil, Structural and Environmental Engineering, Cork Institute of Technology, Bishopstown, Cork, Ireland 2 RPS Consulting Engineers, Innishmore, Ballincollig, Co. Cork email: helen.dunne@mycit.ie, john.justinmurphy@cit.ie, kieran.ruane@rpsgroup.com