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1 S u m m a r y This report presents a comprehensive research program dedicated to bridges carrying light rail transit loading, including those subjected to both light rail and highway traffic loadings. The research is composed of two phases: a state of the art review and technical investigations. Of interest are the behavior of light rail bridges, live loads and associated forces, railâtrainâstructure interaction, and load factor calibration to implement the Load and Resistance Factor Design (LRFD) method. Technical findings are used to develop guide specifications, which complement the contents of the AASHTO LRFD Bridge Design Specifications (BDS). Light rail bridges are designed to accommodate the AW4 design category, consisting of train weight and fully occupied passengers, in conjunction with embedded, ballasted, and direct fixation tracks. A number of research projects have been conducted to examine the behavior of rail bridges, whereas most of them are concerned with conventional heavy-haul and high-speed trains. Limited information is available on the response of bridges subjected to light rail transit. Existing design guidelines for light rail bridges are developed with agency- specific live loads or based on the provisions of AASHTO LRFD BDS and the American Railway Engineering and Maintenance of Way Association (AREMA). Technical discrepancy exists, because AASHTO LRFD BDS and AREMA are not related to light rail transit loadings. Although the existing guidelines are useful and provide valuable sources for the design of light rail bridges, there is no consensus in many occasions [e.g., dynamic load allowance (DLA) or impact varies from 10% to 40% of a static load]. Two design philosophies are currently implemented in the light rail bridge community, namely, the LRFD and Allow- able Stress Design (ASD) methods. In the case of agencies adopting the LRFD approach, load factors are directly taken or modified from AASHTO LRFD BDS. Refined calibration is required for light rail bridges, since the characteristics of light rail transit are different from those of highway traffic. The behavior of five constructed bridges is monitored when subjected to light rail trains, with an emphasis on in situ train loadings, girder strains, displacements, dynamic charac- teristics, live load distributions, and multiple presence events. Finite element models are formulated to study various aspects associated with light rail bridges, such as flexural and shear responses, DLA, live load distributions, deflections along with user comfort, centrifu- gal and longitudinal forces, thermal loading, rail break, and the effect of combined light rail and highway traffic loadings. A standard live load model is developed to generate uniform design outcomes, regardless of transit agency. Load factors are calibrated, particularly for the strength and fatigue limit states of light rail bridges. Proposed AASHTO LRFD Bridge Design Specifications for Light Rail Transit Loads