Effect of Shallow Foundation Rocking on Dynamic Response of Bridges
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Rocking as an acceptable mode of seismic response has been extensively studied and has been shown to potentially limit local displacement demands. Rocking can act as a form of isolation, reducing displacement and force demands on a bridge, thereby allowing for design of smaller footings and members. As part of a collaborative effort to develop guidelines for the design of bridges supported on piers that rock on their foundations a series of shake table tests of a simple ¼ scale inverted pendulum reinforced concrete bridge column was conducted. These tests are among the first to consider three components of excitation. Testing levels included design and maximum credible earthquake scenarios that created an inelastic response in the test specimen. Concurrent centrifuge testing to determine the inelastic response of soil when similar to-scale systems were allowed to rock developed data which could be used with the shake table tests to create and validate analytic models of the complete rocking response. Analysis shows that full-scale systems allowed to rock on their foundations reduce displacement and force demands without creating a global instability. Analytic models show that short period fixed bases systems tend to have amplified displacements when allowed to rock for a variety of soil profiles.