PhD Candidate, University of Maryland at college park
This research proposes the use of rate-independent linear damping for the protection of inter-story isolated structures. Inter-story isolated structures have an isolation layer installed at an intermediate story, a unique contrast to more traditional base-isolated structures. The major benefit of inter-story isolation is the creation of nominally independent structural systems above and below the isolation layer. The accelerations of the floors above the isolation layer are reduced when compared to a conventional structural system at the expense of large isolation layer displacements. In retrofit applications where extra stories added using inter-story isolation, the installation is simple and disruption-free (assuming there is adequate gravity load capacity in the existing columns). Additionally, the base shear demand on the total structure is not significantly increased. As with traditional base isolation, large displacements can lead to undesired behavior and damage in the isolation layer. Supplemental control methods are needed to restrict large displacements without increasing the accelerations above the isolation layer. Rate-independent linear damping provides direct control over displacement, a desirable feature for low-frequency structures. When low-frequency structures are subjected to high-frequency ground motions, rate-independent linear damping produces similar response displacements and velocities in comparison to other damping types; however, the damping forces and resulting floor accelerations are substantially smaller. These features are desirable for the isolation layer; however, in rate-independent linear damping, the restoring force is proportional to displacement and in phase with velocity, a non-causality that has limited its practical applications. A filter-based approach is proposed to determine a causal approximation of rate-independent linear damping which can then be tracked by a semi-active damper in the isolation layer. The approach is demonstrated through the shake table real-time hybrid simulation of a 14-story inter-story isolated structure. The isolation layer, supplemental MR damper control device, and upper stories are experimentally represented while the lower stories are numerically modeled. The results compare well to noncausal numerical simulations in both damping forces and structural responses, achieving the desired displacement suppression without amplifying accelerations.
Abstract: Interstory isolation systems have recently gained popularity as an alternative for seismic protection, especially in densely populated areas. In inter-story isolation, the isolation system is incorporated between stories instead of the base of the structure. Installing inter-story isolation is simple, inexpensive, and disruption free in retrofit applications. Benefits include nominally independent structural systems where the accelerations of the added floors are reduced when compared to a conventional structural system. Furthermore, the base shear demand on the total structure is not significantly increased. Practical applications of inter-story isolation have appeared in the United States, Japan, and China, and likewise new design validation techniques are needed to parallel growing interest. Real-time hybrid simulation (RTHS) offers an alternative to investigate the performance of buildings with inter-story isolation. Shake tables, standard equipment in many laboratories, are capable of providing the interface boundary conditions necessary for this application of RTHS. The substructure below the isolation layer can be simulated numerically while the superstructure above the isolation layer can be tested experimentally. This configuration provides a high-fidelity representation of the nonlinearities in the isolation layer, including any supplemental damping devices. This research investigates the seismic performance of a 14-story building with inter-story isolation. A model-based acceleration-tracking approach is adopted to control the shake table, exhibiting good offline and online acceleration tracking performance. The proposed methods demonstrate that RTHS is an accurate and reliable means to investigate buildings with inter-story isolation, including new configurations and supplemental control approaches.
Pub.: 21 Dec '16, Pinned: 30 Jun '17
Abstract: The simple constant hysteretic damping model is known to be non-causal although it is used often in diverse branches of engineering. In this paper the response of a single degree of freedom oscillator having linear hysteretic damping under arbitrary force excitation has been studied after deriving the impulse response function of the system. Some shortcomings of the results available in literature have been pointed out. It has been shown that the damping model can be practically used for calculating the response of a physical system when the damping is small and the force has small duration.
Pub.: 31 May '16, Pinned: 03 Jul '17
Abstract: It is proven that linear oscillatory systems with hysteretic damping in the form of complex stiffness and/or complex elastic moduli satisfy the causality principle: the response of such a system to an arbitrary external force cannot appear earlier than the onset of the force. The proof, based on a rigorous solution to the problem of forced oscillations, is presented in detail for an oscillator with a complex stiffness, as well as in a brief explanation for a system with N mass. It is also shown that these systems are Lyapunov-unstable. A comparison is made to other linear hysteretic damping models.
Pub.: 24 May '13, Pinned: 03 Jul '17
Abstract: The linear rate-independent damping model is non-causal. But its mathematical form is so attractive that this model and its enhancements are still considered as contemporary research. The linear band-limited hysteretic vibrator (LBLHV) is one such suggested modification for suppressing the non-causality effect. However, its response properties have not been delineated. One of the properties, the unitary impulse response function, will be investigated systematically by the residue theorem here. It was shown that, first, the LBLHV is still non-causal, and the asymptotic rate of the impulse response precursor (IRP) is O(1/t) as time approaches negative infinity. Second, the IRP is no longer monotonic, but composed of two oscillating components. Third, the response at t = 0 can be set to zero by appropriately combining the lower and upper limits of the pass band.
Pub.: 14 Feb '08, Pinned: 03 Jul '17
Abstract: In the present paper, a linear model for multi-degree-of-freedom systems with rate-independent damping is proposed to the purposes of dynamic response prediction and identification. A viscoelastic model with memory, equivalent to the ideal hysteretic model as for the energy dissipation properties, but causal and physically consistent in both the time and the frequency domain, is developed by adopting the Maxwell–Wiechert kernel function and by requiring the loss modulus to be substantially independent of frequency in a specified range of interest. The finite element model of the equivalent viscoelastic system is constructed and its equations of motion are shown to be uncoupled, in terms of modal coordinates, by the real-valued eigenvectors of the conservative system. An augmented state-space formulation, which encompasses, besides the customary displacements and velocites, a number of internal variables devoted to represent the viscoelastic memory, is then provided for the sake of system identification. Mechanical and modal properties of the equivalent viscoelastic model are finally illustrated by means of numerical examples.
Pub.: 25 Sep '14, Pinned: 03 Jul '17
Abstract: Semi-active dampers are used in base-isolation to reduce the seismic response of civil engineering structures. In the present study, a new semi-active damping system using variable amplification will be investigated for adaptive base-isolation. It uses a novel variable amplification device (VAD) connected in series with a passive damper. The VAD is capable of producing multiple amplification factors, each corresponding to a different amplification state. Forces from the damper are amplified to the structure according to the current amplification state, which is selected via a semi-active control algorithm specifically tailored to the system’s unique damping characteristics. To demonstrate the effectiveness of the VAD-damper system for adaptive base-isolation, numerical simulations are conducted for three and seven-story base-isolated buildings subject to both far and near-field ground motions. The results indicate that the system can achieve significant reductions in response compared to the base-isolated buildings with no damper. The proposed system is also found to perform well compared to a typical semi-active damper.
Pub.: 01 Dec '06, Pinned: 03 Jul '17
Abstract: Authors: Roberto Villaverde Article URL: http://www.tandfonline.com/doi/full/10.1080/15732479.2016.1187634?ai=z4&mi=3fqos0&af=R Citation: Structure and Infrastructure Engineering Publication Date: 2016-05-27T06:23:53Z Journal: Structure and Infrastructure Engineering: Maintenance, Management, Life-Cycle Design and Performance
Pub.: 27 May '16, Pinned: 03 Jul '17
Abstract: In this paper, the performance of a nonlinear base-isolation system, comprised of a nonlinearly sprung subfoundation tuned in a 1∶1 internal resonance to a flexible mode of the linear primary structure to be isolated, is examined. The application of nonlinear localization to seismic isolation distinguishes this study from other base-isolation studies in the literature. Under the condition of third-order smooth stiffness nonlinearity, it is shown that a localized nonlinear normal mode (NNM) is induced in the system, which confines energy to the subfoundation and away from the primary or main structure. This is followed by a numerical analysis wherein the smooth nonlinearity is replaced by clearance nonlinearity, and the system is excited by ground motions representing near-field seismic events. The performance of the nonlinear system is compared with that of the corresponding linear system through simulation, and the sensitivity of the isolation system to several design parameters is analyzed. These simulations confirm the existence of the localized NNM, and show that the introduction of simple clearance nonlinearity significantly reduces the seismic energy transmitted to the main structure, resulting in significant attenuation in the response.
Pub.: 01 Mar '05, Pinned: 03 Jul '17
Abstract: This paper presents a control algorithm for seismic mixed base isolation, combining passive isolators and semi-active viscous dampers. The objective is to limit base displacement while avoiding undesirable amplification of the response of the non-isolated modes. To this end, the proposed algorithm takes into account the constraints on the damping coefficient of the semi-active damper and information on the excitation. It is based on the approximate iterative solution, at each control time step, of a nonlinear inhomogeneous constrained optimal control problem. An autoregressive model is used to obtain, at each control time step, a prediction of the upcoming excitation in a short time interval ahead. Numerical simulation results demonstrate the efficacy of the above method, especially in improving floor response spectra, and its superiority with respect to clipped-optimal algorithms.
Pub.: 30 May '17, Pinned: 03 Jul '17