Refined Evaluation of Effective Prestress in the Varina-Enon Bridge

Abstract

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.