Decoupled model-based real-time hybrid simulation with multi-axial load and boundary condition boxes

Real-time hybrid simulation (RTHS) is a cost and space efficient alternative to shake table testing for seismic assessment of structural systems. In this method, complete structural systems are partitioned into numerical and physical components and tested at real earthquake velocities. Well-understood components of the structure are modeled in finite-element numerical models. Meanwhile, the physical substructure, which often contains the highly nonlinear and numerically burdensome components is fabricated and tested in a laboratory facility. Testing at real earthquake velocities is useful to obtain nonlinear rate-dependent material behaviors. Realistic reproduction of seismic conditions for structural assessment has required researchers to develop multi-axial RTHS capabilities. In such developments, multiple actuators are arranged at the boundary condition with the physical specimen to impose realistic displacements and rotations. But, varying degrees of dynamic coupling exist between the actuators in multi-axial boundary conditions. Controllers and kinematic transformations are developed for the tracking action of the actuators to compensate for the amplitude and phase discrepancies between target and measured displacement signals, otherwise stability issues are likely to result. In this paper, a multi-axial framework is introduced for RTHS testing, using a Load and Boundary Condition Box (LBCB) at the University of Illinois at Urbana-Champaign. The previously developed multi-axial RTHS framework for the LBCBs compensates for actuator dynamics in Cartesian coordinates; this approach lacked stability robustness when testing stiff specimens. The distinguishing feature of the proposed framework is that tracking compensation is executed in the actuator coordinates. The differences between the previous and proposed multi-axial RTHS frameworks are explored in detail herein. This paper presents the components of the framework and the describes a six-degree-of-freedom moment frame RTHS experiment. Finally, experimental results are discussed and directions for future research efforts are considered.