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Abstract: Assessment of the remaining load carrying capacity of masonry arch bridges has been studied by many researchers in the recent years. Most of the masonry bridges have been built several decades ago, and have been designed for live loads and service conditions that have been changed over time. The residual strength of those structures cannot be predicted, in a reliable manner, because of time dependent effects such as fatigue and creep, environmental defects such as cracking and corrosion, participation of non-structural elements such as fill material and pavement layers, variation in restrain conditions, and differences between as-built and design specifications. To overcome such restrictions, field tests provide a dependable solution. On the basis of construction material, masonry arch bridges may be divided into three groups, namely, brickwork arch, stone arch, and plain concrete arch. Numerical and experimental studies of brickwork and stone arches have been reported frequently. Despite frequent study of brick masonry arches, load test of plain concrete arches have been published only in a few documents. There are a large number of old arch bridges in Iran that have been serving as railway bridges for more than seventy years. Field load testing of an old railway bridge in km 24 of Tehran-Qom railway has revealed important characteristics of the bridge and has proven that there is still large capacity under service load. This paper reports results of a study to predict the maximum and yield load capacity of a plain concrete arch bridge. The bridge has already been tested under vertical load static up to 5000 KN. For figure out the behavior of these types of structures, model updating is necessary. Appropriate modeling of arch bridges is the subject that is highly regarded by researchers in recently year. The complexity in the behavior of these structures will seek more accurate studies and researches, so the field tests are an integral part. In this article, uses numerical modeling in order to determine yield strength and ultimate strength of Akbar Abad Bridge. This bridge is a plain concrete arch and consists of five identical 6m and formerly has tested, due to operational and field limitation, possibility of loading until limit ultimate and Set maximum load capacity and yield strength has not been possible. In this paper with ANSYS software, an accurate modeling has done. In this model has used strain analysis. Considering the existence of initial cracks in bridge, they have been modeled as empty space. Behaviors of the material are assumed nonlinear and have used Dracker-Prager criterion. Finally, according to pattern of cracks appeared in test and result of field testing of a similar bridge (km-23 of Tehran-Qom railway), the ultimate load and yield strength is estimated and The pattern of formation of plastic hinges conforms to the patterns reported in the literature for arch masonry bridges.

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