MS16.1 - Recent advances in vibration control of structures with isolation and/or energy dissipation devices

The need to control and reduce the dynamic responses of a structure is strongly found. A standard technique to lower structural vibration is to use elements capable of dissipating energy. Here, the proposal to add a hysteretic element makes it possible to obtain a double result: to increase the structure dissipation characteristics and to introduce a nonlinearity which can potentially facilitate the spreading of input energy among the system vibration modes. The viscoelastic tuned mass damper (TMD) proposed by Den Hartog [1] was a great achievement in the vibration mitigation of structures; recently the beneficial effects of nonlinear devices connected to structures has been extensively demonstrated [2]. A hysteretic vibration absorber (HVA), which is described by the Bouc-Wen model, has the advantage that the restoring force combines the elastic and dissipation characteristics without the need of a damper. Moreover, its strong nonlinear characteristics activate phenomena, such as modal interactions which promotes the transfer of energy from the directly excited mode towards other modes [3]. The case of internal resonance 1:1 which resembles the Den Hartog proposal is first dealt with. Other than being a simpler solution, its effectiveness is similar to that of viscoelastic TMD, but only in a definite excitation range, due the dependence of its characteristics on the oscillation amplitude. Moreover, in this case no typical phenomena of nonlinear dynamics are activated. Different is the case of internal resonance conditions n:1, with ???? > 1 which promotes the occurrence of a rich variety of nonlinear phenomena. For the case of internal resonance 2:1, the appearance of a novel mode around the first resonance through a bifurcation mechanism involves the second mode in the response, with a beneficial effect on the vibration amplitude of the directly excited first mode. Two peaks appear in the FRC around the first resonance, for a certain value of excitation. The reduction of the response due to the HVA with respect to the uncontrolled case is evident: the picture seems similar to that of Den Hartog TMD, but instead it is completely different. A novel peak is not due to the addition of a degree of freedom, as in the case of TMD, but to an interaction with the second mode; this advantageous behaviour still remains in a large range of frequency and intensity of excitation. The case illustrated here demonstrates the efficiency of introducing a hysteretic element for the passive control of structural vibrations, but other internal resonance conditions could be advantageously investigated [1] Den Hartog, J.P. (1934) Mechanical Vibrations; McGraw-Hill: New York, NY, USA. [2] Vakakis, A.F. (2017) Intentional utilization of strong nonlinearity in structural dynamics. Proc. Eng., 199, 70–77. [3] Casini, P.; Vestroni, F. (2022) The role of the hysteretic restoring force on modal interactions in nonlinear dynamics. Int. J. Non-Linear Mech., 143, 104029.

An inerter is a mechanical two-terminal device that creates a force proportional to the relative acceleration between its terminals. A unique feature of this device is that it has the potential to produce high dynamic mass while its static mass remains at a fixed value. These devices, used as tuned inerter dampers (TIDs) at lower levels of multi-storeyed structures, have been noted in the existing literature to be superior alternatives to tuned mass dampers (TMDs), since they do not add high static mass on the top. The present study focuses on investigating the inelastic dynamic behavior of frames fitted with inerters. The study takes into account features such as various aspects of inelastic frame behavior (viz., strain hardening, pinching, and stiffness and strength degradation), bending-torsion coupling, irregularities in elevation (leading to weak and (or) strong ground floors), and transient nature of the ground motions. The focus of the study is on investigating the role of TIDs in (a) reducing the response and ductility demand, (b) enhancing load carrying capacity in linear regime by delaying the inception of yielding, (c) reducing the hysteretic energy absorbed and hence the damage to the system, and (d) modifying the magnitude of base shear transferred to the foundations. The question of optimal placement of TIDs, especially when the frames are prone to bending-torsion coupled oscillations, is also considered. We begin by validating the mathematical model for a frame with an inerter by comparing the model predictions with experimental observations made on a three-story shear frame tested on an electro-mechanical shake table. This is followed by two studies: Study-1: Here, we focus on the behavior of a five-storeyed inelastic building frame model fitted with a TID at the ground floor level and subject to transient earthquake support motions. The difference that the TID makes to ductility demand, hysteretic energy absorbed, and global damage indices is investigated. Study-2: Here, we consider a one-story frame asymmetric in plan, subjected to two-component earthquake support motions. The influence of location and number of TIDs on the bending-torsion coupled inelastic behavior of the frame is investigated. The presence of the inerter as TID in these studies is observed to be beneficial in terms of reducing response features such as the ductility demand, total hysteretic energy absorbed, and measures of global damage. On the other hand, the study notes that the magnitude of the base shear transferred to the foundation increases due to the presence of TID. The study points towards the need for investigating the behavior of large-scale realistic building frames fitted with inerter-based devices, developing strategies for choosing the optimal placement and number of inerters, and allowing for random nature of the earthquake ground motions in the models to take into account diversity of frequency content and duration.