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Shake Table Studies on Soil-Abutment-Structure Interaction in Skewed Bridges
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Soil-abutment-structure interaction could affect the seismic response of bridges considerably. Skew angle might significantly influence the mobilized passive resistance of the backfill soil and the behavior of soil-abutment system due to the large induced in-plane rotations and translation of the superstructure, coupled with variations in stiffness and strength of backfill soil in skewed abutments. The current Seismic Design Criteria (SDC, 2013) of the California Department of Transportation (Caltrans) does not consider the skew angle effect on the passive capacity of soil-abutment systems. Effect of skew angle could be important for bridges under seismic forces and skewed integral abutments subjected to thermal expansion. Previous experiments on skewed abutments were undertaken by gradually increasing lateral loads under static conditions, with no dynamic effect simulated. Earthquake forces are dynamic which were not studied before. Another issue in all the previous abutment tests is that it was assumed there is always full contact between the superstructure end diaphragm and the abutment under lateral loading resulting in a uniform load transfer across the width of the abutment. The abutment wall elements in these tests did not rotate about a vertical axis because of the test setup. In reality, however, the superstructure tends to undergo in-plane rotation that results in changing contact point between the superstructure and the abutment and rotation of the abutment wall about vertical axis. The overall objective of the current study was to investigate experimentally and analytically the effect of skew angle on the abutment soil response under realistic dynamic earthquake loading and develop recommendations on modeling of skewed abutments for application in bridge research and design. The experimental study was focused on soil-abutment-structure interaction in skewed bridges under dynamic loading based on large-scale shake table tests at the University of Nevada, Reno. Three 5.5-ft high abutment walls at three skew angles of 0º, 30º, and 45º with a projected width of 10 ft in the direction of motion were impacted by a bridge superstructure and pushed in the longitudinal direction of the bridge into a 25 ft long by 19 ft wide engineered backfill soil in a stationary timber box. The abutment walls were allowed to rotate to further simulate actual bridge abutments realistically. Analytical studies were performed by developing FLAC3D models of the shake table tests in the current study. The analytical models simulated the abutment wall and backfill under the static uniform and non-uniform displacement loading on the wall. Design recommendations were developed by evaluating the most recent available models estimating the passive forcedisplacement relationships of the abutments considering the effect of skew.
Report No. CCEER-17-04