Multi-dimensional Mixed-mode Hybrid Simulation Control and Applications

Hybrid simulation is an effective method for the assessment of the seismic response of structures, combining laboratory testing, computational simulation, and numerical time-step integration of the equations of motion. While this approach has been used for evaluation of the seismic performance of a variety of structures, applications to date have been limited to planar loading and to relatively simple structural systems. In contrast, actions during strong earthquakes are three-dimensional and continuously varying and modern structures can be extremely complex. Further development is required to evaluate the seismic performance of structures, in particular complex structural systems, under realistic loading. The objectives of this study are to develop a multi-dimensional hybrid simulation framework using a six-actuator, self-reaction, loading system, referred to as the Load and Boundary Condition Box (LBCB), for evaluation of the seismic performance of large and complex structural systems and to demonstrate the framework through three-dimensional hybrid simulation of a skew reinforced concrete (RC) bridge. This report contains results for four major tasks that are intended to provide enhanced seismic performance evaluation using advance experimental techniques. The first task is the calibration of the LBCB in global Cartesian coordinates. Due to imperfections in system geometry (e.g., the actuator configuration), errors in the Cartesian measurements are generated from errors in the transformation from actuator to Cartesian space. A sensitivity-based external calibration method is developed to improve the precision by which the LBCB can be controlled in Cartesian space. The second task is to develop, implement, and experimentally verify a mixed load and displacement (mixed-mode) control strategy. A mixed-mode control capability is required, for example, to simulate gravity loads in the axial direction and displacements in the other directions on structural members such as RC piers in hybrid simulation. However, because of the nonlinear nature of the coordinate transformation, mixed-mode control for a multi-axial loading system is still a major theoretical and practical challenge. The mixed-mode control strategy developed in this study accounts for the spatial interaction of actuators both in displacement and load, and the stiffness variation of the structure specimen. The third task is to integrate the control system and its capabilities into a hybrid simulation framework. The framework needs to also incorporate robust network communication for hybrid simulation. The fourth and final task is to validate the hybrid simulation framework through the study of the three-dimensional behavior of a skew RC bridge. First, extensive analyses of skew bridges are conducted to prepare for the hybrid simulation. Subsequently, a smallscale RC pier is experimentally tested as a physical substructure, while the rest of the piers and the bridge deck are analyzed using a finite element model. The mixed-mode control capability is employed to impose on the RC pier simultaneous gravity loads in the axial direction and earthquake-induced displacements in the other directions. The experimental results show that the multi-dimensional hybrid simulation with versatile six degrees-of-freedom loading capability is a promising approach that provides a reliable means for evaluation of the seismic performance of large and complex structural systems.