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Vibration based structural health monitoring has become more common in recent years as the required data acquisition and analysis systems become more affordable to deploy. It has been proposed that by monitoring changes in the dynamic signature of a structure; primarily the natural frequency; one can detect damage. This approach to damage detection is made difficult by the fact that environmental factors; such as temperature; have been shown to cause variation in the dynamic signature in a structure; effectively masking those changes due to damage. For future vibration based structural health monitoring systems to be effective; the relationship between environmental factors and natural frequency must be understood such that variation in the dynamic signature due to environmental noise can be removed. A monitoring system on the I-35W St. Anthony Falls Bridge; which crosses the Mississippi River in Minneapolis; MN; has been collecting vibration and temperature data since the structures opening in 2008. This provides a uniquely large data set; in a climate that sees extreme variation in temperature; to test the relationship between the dynamic signature of a concrete structure and temperature. A system identification routine utilizing NExT-ERA/DC is proposed to effectively analyze this large data set; and the relationship between structural temperature and natural frequency is investigated.

Since the opening of the I-35W Saint Anthony Falls Bridge in 2008; over 500 sensors have been collecting data to better understand the behavior of post-tensioned concrete box girder structures. Recent research in the accelerometers installed on the bridge indicates they can be effectively used in a vibration-based structural health monitoring system; but previous studies have shown that natural frequency alone may not be sufficient to determine the performance of the structure. Vertical displacements were believed to be a simpler performance measure as direct comparisons can be made with design calculations and maintenance guidelines. To avoid the shortcomings of conventional displacement measurement options; this study focuses on using the currently installed accelerometers to estimate the vertical displacements of the southbound bridge. The proposed technique utilizes up-to-date modal parameters within a dual Kalman filter to estimate the vertical displacements of the structure from noisy acceleration measurements. When applied to the I-35W Saint Anthony Falls Bridge; it was found that the dual Kalman filter approach captures only dynamic displacements due to relatively slow loading (e.g.; traffic loading and thermal loading) and the corresponding low-frequency static displacements are likely too small for GPS measurements due to the high stiffness of the structure.