Time-Dependent Monitoring and Modeling of I-35W St. Anthony Falls Bridge. I: Analysis of Monitoring Data

For the successful implementation of long-term monitoring strategies of prestressed concrete structures, the expected behavior of the structure must be accurately quantified before anomalous or damage-related readings can be properly identified. In situ structures may be subject to large variations in temperature, which can have a significant impact on measured deformations, and continued creep and shrinkage of the concrete further complicate long-term predictions. The goals of this and a companion paper are to present the methodology for extracting the time-dependent behavior of a posttensioned concrete box girder bridge from structural monitoring data in the presence of changing temperatures (this paper) and to compare predictions of long-term time-dependent deformations computed using finite-element analysis with the extracted time-dependent monitoring data (the companion paper). To investigate the interactions between temperature and time-dependent behavior for in situ monitoring data, strains and expansion joint deflections from the St. Anthony Falls Bridge, a posttensioned concrete box girder bridge on I-35W in Minneapolis, Minnesota, were collected over a period of 5 years. A methodology based on linear regression was used to separate the time-dependent deformations from the temperature-related deformations given a variable coefficient of thermal expansion (CTE). The total temperature-related deformations were captured by functions based on the average bridge temperature, the thermal gradient through the depth of the superstructure, and the average squared temperature of the bridge, which was proposed because the CTE was observed to vary with temperature. On examination of the extracted time-dependent readings, the deformation rates were found to decelerate during the winter and accelerate during the summer. To enable direct comparison between the measured results and the creep and shrinkage predictions from finite-element models assuming constant temperature, an Arrhenius-adjusted time formulation was used, which normalized the measured deformations under varying temperatures to those expected from a constant reference temperature. This procedure for processing the time-dependent measured data enables a comparison with time-dependent finite-element model results conducted at constant temperature.