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Seismic Response of a Full-scale 5-story Steel Frame Building Isolated by Triple Pendulum Bearings under 3D Excitations
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A full-scale shake table test of a 5-story steel moment frame building was carried out as part of a collaborative NEES/E-Defense research program. The building was tested in three configurations: isolated with triple pendulum bearings (TPB), isolated with lead rubber bearings combined with cross linear sliders (which is not discussed in this report), and fixed base. The test provided full-scale response data of both the isolation system and the isolated structure and demonstrated the efficiency of the isolation system in reducing the demands in the isolated structure. A 3-dimensional TPB element with a general friction model that accounts for the variation of friction coefficients on both velocity and vertical force was developed to predict the response of individual TPB and the overall isolation system. The element accounts for both vertical-horizontal coupling behavior and bidirectional coupling of TPB. The horizontal behavior of the element is based on a series combination of bidirectional elastic-plastic springs and the circular gap elements. The new TPB element was verified by the full scale test data and has been implemented in OpenSees so that it is available for general use. The analytical model of the specimen building was also developed and validated by the test data from both the isolated base and fixed base tests. The following modeling assumptions were shown to best present the response characteristics of the tested structure: (1) beams were modeled as nonlinear elements with resultant composite sections, (2) moment connections were modeled using a Krawinkler panel zone model, and (3) energy dissipation was represented by Rayleigh damping calibrated to include higher mode effects observed in the test data, along with additional interstory dampers. The vertical component of the excitation was shown to amplify the horizontal response of both the fixed base and isolated base structures. This coupling effect was small in the tested fixed base configuration relative to the isolated base configuration. The calibrated analytical model was used to identify 3 main sources of this amplification: (1) vertically and horizontally coupled modes of the structure, (2) rocking of the structure on the isolation system due to vertical flexibility of the isolation and support system along with uplift, and (3) the vertical-horizontal coupled response of friction bearings. Only the first source of coupling was applicable to the fixed base building.
Report No. CCEER-15-01