Department: Structural Engineering
Name: Patrick Richard Wilson
Email: prwilson @ ucsd.edu
Grad Year: 2008
In order to improve the seismic design of highway bridges, a collaborative effort is underway to study the behavior of typical highway bridges during earthquakes. Facilities of the George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES) are being used to conduct experiments at the University of Nevada, Reno (UNR) and the University of California, San Diego (UCSD). The focus of this discussion is a series of tests concerning the bridge abutment soil-structure interaction, conducted at the Large High Performance Outdoor Shake Table (LHPOST) at UCSD.
In highway bridge design, a seat abutment with a sacrificial back-wall is proposed. The purpose of this system is to protect the pile foundation of the abutment by allowing the back-wall to become sheared off upon impact from the bridge superstructure. This system allows for the force-displacement-capacity of the abutment to be developed primarily from the passive resistance provided by the backfill. The UCSD test model assumes the back-wall has been sheared off of the abutment stem. Passive Resistance to displacement of the back-wall into the backfill was measured in static tests. Inertial forces from the backfill on the back-wall were measured in dynamic tests.
Exciting results from these tests will be the included in this discussion. Static test results show that the passive resistance provided by a CALTRANS approved fill can be lower than specified in the seismic design criteria. This result can have a significant impact on the displacement demand for bridge columns. Dynamic test results suggest that the resistance provided by the abutment backfill can vary with changes in ground acceleration during an earthquake. During dynamic tests, pressures measured on the back-wall increased and decreased by as much as 40% of the peak passive resistance provided by the fill. Current design specifications do not consider these effects. The results from the UCSD abutment tests will be used to calibrate soil models which can then be used in conjunction with varying bridge models to determine the influence of the abutment soil-structure interaction on overall bridge performance.
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