This project evaluates the Lost River bridge, which is a 68-foot, single-span monolithically poured, reinforced concrete, slab-girder bridge. The bridge is located in southern Klamath County and exhibits significant shear and flexural cracking in the exterior girders. This damage caused the bridge to be closed temporarily in 2003; it was reopened after epoxy injection repairs with a posted load of 10 tons per single axle and 17 tons per tandem axle. Epoxy injection is not typically used as a structural repair and there is some uncertainty as to the accuracy of the load posting. This project seeks to determine the accuracy of this load posting by answering the research question “can natural frequencies determined using low-force, harmonic forced vibration be used to enhance a series of grillage models for the purpose of refining the load rating of a reinforced concrete bridge to confirm or modify a posted load based on shear capacity?” To answer this question a girder-line model and a grillage model were constructed and refined using field investigations of the structure. The structure’s natural frequencies for several modes were evaluated using the RDSETGO system and the compressive strength of the concrete was measured using a non-destructive rebound (Schmidt) hammer testing method. This process was unable to determine a refined load rating for the governing shear condition. The main success of the project was the development of a six-step, generalized approach to dynamic modeling and enhanced load rating. This report details the approach to accurately and efficiently model any bridge with the goal of a refined load rating.
