The hybrid test was composed of an experimentally tested isolated two story moment frame that represented a superstructure and a numerically simulated lumped mass substructure. A unidirectional shake table was constructed for the real time loading. The shake table, shown in Figure 1, consists of a large steel platform isolated on linear guide rails. The friction in the rails was specified as less than 10%. The steel platform is 5.8 m long by 2 m wide. The platform is supported at six points, directly below the locations of the frame columns. The shake table is driven by an MTS actuator with +/- 50 cm of stroke and 667 kN force capacity. The table is controlled using an MTS 493 Real-Time Controller. The digital controller provides closed-loop PID, differential Feedforward, and Delta-P control capabilities.
The basic outline of the hybrid test is shown in Figure 2. Earthquake excitation is input into the base of the numerical substructure, modeled in OpenSees [8]. The absolute displacement at the top of the numerical substructure is the target displacement, which is sent to OpenFresco [9, 10]. OpenFresco serves as the middleware which is used to interface the numerical substructure with the experimental superstructure through the control system. A predictor-corrector algorithm is then used to bridge the difference between the analysis time step size and the smaller control system time step size. The command displacement generated by the predictor-corrector algorithm is sent to the controller which controls the actuator driving the shake table. The resulting displacement of the table and the shear force under the physical specimen – recorded using loads under each of the isolators – are measured. The measured displacement is fed back into the controller and predictor-corrector algorithm while the measured shear force is fed into the numerical OpenSees model for the next analysis time step.