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Inventory Rating, as defined by the current AASHTO Manual for Bridge Evaluation,3 is that load, including loads in multiple lanes that can safely utilize the bridge for an indefinite period of time.
Operating Rating, defined by the same manual, is the maximum permissible live load that can be placed on the bridge. This load rating also includes the same load in multiple lanes. Allowing unlimited usage at the Operating Rating level will reduce the life of the bridge.
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The inventory rating is an indicator of a structure’s capacity for safely carrying a given live load, including loads in multiple lanes, for an indefinite period of time under normal service conditions.