The UC San Diego Large High-Performance Outdoor Shake Table (LHPOST), which was commissioned on October 1, 2004 as a shared-use experimental facility of the National Science Foundation (NSF) Network for Earthquake Engineering Simulation (NEES) program, was upgraded from its original one degree-of-freedom (LHPOST) to a six degree-of-freedom configuration (LHPOST6) between October 2019 and April 2022. The LHPOST6 is a shared-use experimental facility of the NSF Natural Hazard Engineering Research Infrastructure (NHERI) program. A mechanics-based numerical model of the LHPOST6 able to capture the dynamics of the upgraded 6-DOF shake table system under bare table condition is presented in this report. The model includes: (i) a rigid body kinematic model that relates the platen motion to the motions of components attached to the platen, (ii) a hydraulic dynamic model that calculates the hydraulic actuator forces based on all fourth-stage servovalve spool positions, (iii) a hold-down strut model that determines the pull-down forces produced by the three hold-down struts, (iv) a 2-D and various 1-D Bouc-Wen models utilized to represent the dissipative forces in the shake table system, and (v) a 6-DOF rigid body dynamic model governing the translational and rotational motions of the platen subjected to the forces from the various components attached to the platen. In this report, the rigid body dynamics is studied utilizing the platen twist (combination of platen translational and rotational velocities) and wrench (combination of force and moment resultants acting on the platen) following principles from the robotic analysis literature. The numerical model of the LHPOST6 is validated extensively using experimental data from the acceptance tests performed following the shake table upgrade, and the model predictions of the shake table system response are found to be consistently in very good agreement with the experimental results for tri-axial and six-axial earthquake shake table tests. The validated mechanics-based numerical model of the LHPOST6 presented in this study can be coupled with finite element models of shake table test specimens installed on the rigid platen to study the dynamic interaction between the shake table system and the specimens. Another important potential use of the model is to improve the motion tracking performance of the LHPOST6 through either off-line tuning of the shake table controller and/or development of more advanced shake table controllers.
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