Summary Active vibration control techniques using inertial‐mass actuators have gained some level of acceptance in civil structures. Recent research indicates the effectiveness of this technique in mitigation of human‐induced excitation in pedestrian structures such as floors and footbridges. However, there are some drawbacks associated with the use of inertial‐mass actuators which needs to be dealt with. One of the main disadvantages of using inertial actuators is their stroke saturation non‐linearity. When the stroke saturation phenomenon happens, excessive movement of inertial mass along the stroke hits the ends of actuator and can destabilize the system or even damage the actuator. This paper presents a novel velocity feedback control strategy to robustly prevent stroke saturation of inertial actuator. Two inner loops are added into a direct velocity feedback (DVF) control loop. First inner loop is a proportional‐derivative (PD) controller based on the measured displacement of inertial mass. The second inner loop is implemented as a DVF gain modifier based on the actuator mass displacement over‐range. This adaptively reduces the DVF gain, by an amount proportional to the over‐range by displacement ratio, when inertial mass displacement is predicted to exceed the certain limit. Theoretical and experimental study of the control strategy is examined on a laboratory scale floor structure using an inertial‐mass actuator. Both the results demonstrate the effectiveness of the proposed control strategy. The displacement of actuator's mass is kept within the stroke limits while satisfactory control performance is maintained.

Using Vibro-Impact Nonlinear Energy Sinks (VI NESs) is one of the novel strategies to control structural vibrations and mitigate their seismic response. In this system, a mass is tuned on the structure floor, so that it has a specific distance from an inelastic constraint connected to the floor mass. In case of structure stimulation, the displaced VI NES mass collides with the inelastic constraint and upon impacts, energy is dissipated. In the present work, VI NES is studied when its parameters, including clearance and stiffness ratio, are simultaneously optimized. Harmony search as a recent meta-heuristic algorithm is efficiently specialized and utilized for the aforementioned continuous optimization problem. The optimized attached VI NES is thus shown to be capable of interacting with the primary structure over a wide range of frequencies. The resulting controlled response is then investigated, in a variety of low and medium rise steel moment frames, via nonlinear dynamic time history analyses. Capability of the VI NES to dissipate siesmic input energy of earthquakes and their capabilitiy in reducing response of srtructures effectively, through vibro-impacts between the energy sink's mass and the floor mass, is discussed by extracting several performance indices and the corresponding Fourier spectra. Results of the numerical simulations done on some structural model examples reveal that the optimized VI NES has caused successive redistribution of energy from low-frequency high-amplitude vibration modes to highfrequency low-amplitude modes, bringing about the desired attenuation of the structural responses.

In the field of structural dynamics, system identification usually refers to building mathematical models from an experimentally-obtained data set. To build reliable models using the measurement data, the mathematical model must be representative of the structure. In this work, attention is given to robust identification of nonlinear structures. We draw inspiration from reduced order modelling to determine a suitable model for the system identification. There are large similarities between reduced order modelling and system identification fields, i.e. both are used to replicate the dynamics of a system using a mathematical model with low complexity. Reduced Order Models (ROMs) can accurately capture the physics of a system with a low number of degrees of freedom; thus, in system identification, a model based on the form of a ROM is potentially more robust. Nonlinear system identification of a structure is presented, where inspiration is taken from a novel ROM to form the model. A finite-element model of the structure is built to simulate an experiment and the identification is performed. It is shown how the ROM-inspired model in the system identification improves the accuracy of the predicted response, in comparison to a standard nonlinear model. As the data is gathered from simulations, system identification is first demonstrated on the high fidelity data, then the fidelity of data is reduced to represent a more realistic experiment. A good response agreement is achieved when using the ROM-inspired model, which accounts for the kinetic energy of unmodelled modes. The estimated parameters of this model are also demonstrated to be more robust and rely on the underlying physics of the system.

System identification of engineering structures is an established area in the structural dynamics research community. It is often used to characterise certain physical properties of a structure using the data measured from it. For structures exhibiting nonlinear behaviour, physics-based approaches are used where a form of nonlinearity is synthesised and parameters are estimated using the data, or probabilistic approaches are investigated to tackle the model uncertainty of structures. However, to build reliable models, the estimated parameters from the measurement data must reflect the true underlying physics of the structure. Therefore, Reduced-Order Models (ROMs) can be used as the surrogate models, where the nonlinear parameters of the ROMs are having a meaningful relation with the physical parameters of the system. In this work, we propose nonlinear system identification in the context of using some recently developed ROMs which account for the kinetic energy of unmodelled modes. It is shown how ROMs may be used to represent low-order, accurate models for system identification. Identification of a nonlinear system with strong modal coupling is demonstrated, using simulated data, while the estimated ROM response shows good convergence with that of full order system. Similarly, the estimated parameters match with those of directly computed ROM.