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The Varina-Enon Bridge is a cable-stayed, post-tensioned segmental box girder bridge in Richmond, Virginia. A large flexural crack was noted by inspectors in July 2012 which prompted a number of investigations into the current condition of the bridge. Particular focus has been put on prestress losses which have a significant impact on the strength and serviceability of the bridge. Previous work has been conducted to monitor the behavior of the bridge and to back-calculate effective prestress. This was done using field data from a long-term data collection system in the bridge as well as a finite element model which includes a staged-construction analysis. Creep and shrinkage are accounted for using the CEB-FIP '90 model code. Effective prestress in the Varina-Enon Bridge is back-calculated using live load strain data from the long-term data collection system. Previous work has overestimated live load moment since the influence of the crack opening has not been accounted for. This research refines the methods used to determine live load moment from live load strain. Two new methods are developed based on influence lines matching crack gauge data during a live load event. The new methods are compared to the method used in previous studies. Results using two elastic moduli for concrete are compared for each method of live load moment calculation. Finally, back-calculated effective prestress values are compared against effective prestress from the finite element model. Depending on the method used for live load moment calculation, back-calculated effective prestress ranged from 167.4 ksi to 170.8 ksi. Both new methods for live load moment calculation yielded slightly smaller values for effective prestress compared to the method used previously. Increasing the elastic modulus from 6000 ksi to 6200 ksi increased back-calculated effective prestress values from an average of 168.3 ksi to 168.6 ksi. For elastic moduli of 6000 ksi and 6200 ksi, the finite element model returned an effective prestress of 170.3 ksi and 170.8 ksi, respectively. / Master of Science / Prestressing in concrete uses steel tendons to apply a compressive force to a structure. This technique allows for stiffer and lighter structures with longer span lengths to be built. The force in the steel tendons decreases over time, and this is called prestress loss. Prestress losses can have a significant impact on the strength and service life of a structure, so estimating the magnitude of prestress loss is of great importance in prestressed concrete structures. The Varina-Enon Bridge is a cable-stayed, prestressed concrete box-girder bridge in Richmond, Virginia. In July 2012, cracking was observed in the bridge, and this prompted several investigations into its performance. This research calculates effective prestress (prestress force leftover after prestress loss) in several ways. A long-term data collection system collects sensor data which is used to calculate effective prestress experimentally, and a computer model is used to determine effective prestress computationally. Effective prestress results from sensor data are slightly smaller than results from the computer model. However, the differences in results are fairly small, and all values are within expectations, so it is concluded that the Varina-Enon Bridge has not experienced more than expected prestress losses.