This study describes a novel method to determine the natural frequencies and mode shapes of a single-span prestressed voided slab bridge to perform finite element (FE) model updating using common design software to determine a field-calibrated load rating. Other researchers have used expensive accelerometers and specialized software to determine the modal properties of a bridge to support model updating. These methods can be expensive to implement and may require lane or bridge closures to complete. The research in this report answers the following questions: Can a refined bridge load rating for short single-span bridges without skew be conducted using an efficient iPod-based acceleration sensing system, harmonic forced vibration, and manual FE model updating based on field-measured modal parameters? Can this approach be refined to be feasible for DOT implementation? To answer these questions iPods were used to record the modal properties of the study bridge in Klamath Falls, Oregon. Four FE models were created to show the effects of modifying certain properties (modifying the Modulus of Elasticity of the voided slabs, the end conditions, and the stiffness of the lateral elements) on the AASHTO load rating of a structure. The methods described in this report are adequate for determining the natural frequencies and mode shapes of a bridge within an absolute uncertainty of ±0.118 Hz and ±0.107 g. Moreover, they show that the field-calibrated rating model decreases the load rating of the study bridge by 25 percent, with agreement between the field calibrated model and field-tested frequencies within one to four percent.
