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Abstract 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. 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 paper. The model includes: (i) a rigid body kinematic model that relates the platen motion to the motions of the 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) Bouc‐Wen models utilized to represent the dissipative forces in the shake table system, and (v) a rigid body dynamic model borrowed from robotic analysis governing the translational and rotational motions of the platen subjected to the forces from the various components attached to the platen. Extensive validation against experimental data shows excellent agreement for tri‐axial and six‐axial earthquake shake table tests. This validated model can be coupled with finite element models of test specimens to study the interaction between the shake table system and the specimens, and it offers potential for enhancing motion tracking performance through off‐line controller tuning or advanced control algorithm development.