Serviceability-Based Dynamic Load Rating of a LT20 Bridge

County LT20 bridges are bridges with span lengths less than 20 feet. Considered minor structures, these bridges are not included in the National Bridge Inventory System (NBIS) and do not receive the benefits of federally-mandated bridge evaluations. These bridges are rated using analytical procedures based on observations made during visual inspections and are almost never load tested. This article reports on a serviceability-based dynamic load rating of an LT20 bridge. Ambient excitation has been suggested to nondestructively estimate the remaining load capacity of these bridges for rating purposes. In this study, undertaken to determine the accuracy of the load capacity prediction, a two-lane concrete deck steel girder bridge was studied using measured modal characteristics and static load testing. The authors hoped to confirm the dynamic load test results through static load testing. The results showed the potential of the proposed testing methodology. For all practical purposes, the stimated deflections from the three tests (static load test, impact test, and ambient traffic test) all fall in the same orders of magnitude with a statistical variation within 30%. The authors conclude that the method is a viable technique as a supplement for existing evaluation of those LT20 bridges by suggesting a service load capacity based on the measurement of global stiffness and allowable service deflection limits.