Load carrying capacities of exciting bridges need to be properly assessed for the safe operation and efficient maintenance of bridges. Conventionally, the load carrying capacities of bridges have been evaluated from proof loading tests with test trucks by measuring static and dynamic responses, such as deflections or strains under known loading conditions. However, it is very hard to carry out loading tests on the expressway bridges with a large amount of traffic, since full or partial block of traffic during the tests may cause not only inconvenience to drivers but also increase of the logistic cost and time. To overcome these difficulties, an alternative method is proposed using ambient acceleration data without traffic control. The proposed method consists of ambient vibration measurement, experimental modal analysis, correction of the initial finite element (FE) model based on the identified modal properties, and estimation of the load carrying capacity using the updated FE model. The proposed method has been verified through field tests on a decommissioned prestressed concrete girder bridge by comparing with the results of the conventional truck loading tests.