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1. Experimental Evaluation of the Performance of a Nonlinear Dual-Mode Vibration Isolator/Absorber System

   Bin, P.; Harvey, Jr. (February 2021, Proceedings of the 2021 International Modal Analysis Conference XXXIX)

   null (Ed.)

   Floor isolation systems (FISs) are used to mitigate earthquake-induced damage to sensitive building contents and equipment. Traditionally, the isolated floor and the primary building structure (PS) are analyzed independently, assuming the PS response is uncoupled from the FIS response. Dynamic coupling may be non-negligible when nonlinearities are present under large deflections at strong disturbance levels. This study investigates a multi-functional FIS that functions primarily as an isolator (i.e., attenuating total acceleration sustained by the isolated equipment) at low-to-moderate disturbance levels, and then passively adapt under strong disturbances to function as a nonlinear (vibro-impact) dynamic vibration absorbers to protect the PS (i.e., reducing inter-story drifts). The FIS, therefore, functions as a dual-model vibration isolator/absorber system, with displacement dependent response adaptation. A scale experimental model—consisting of a three-story frame and an isolated mass—is used to demonstrate and evaluate the design methodology via shake table tests. The properties of the 3D-printed rolling pendulum (RP) bearing, the seismic gap, and the impact mechanism are optimized to achieve the desired dual-mode performance. A suite of four ground motions with varying spectral qualities are used, and their amplitudes are scaled to represent various hazards—from service level earthquake (SLE), to design basis earthquake (DBE), and even maximum considered earthquake (MCE). The performance of the multi-functional FIS is established and is described in this paper. 

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2. MULTI-DIRECTIONAL SHAKE TABLE REAL-TIME HYBRID SIMULATIONS OF FLOOR ISOLATION SYSTEMS IN BUILDINGS

   Vega, E; Harvey, S; Ricles; J; Cao, L; Malik, F; Marullo, T (July 2024, World Conference on Earthquake Engineering proceedings)

   Seismic resiliency includes the ability to protect the contents of mission-critical buildings from becoming damaged. The contents include telecommunication and other types of electronic equipment in mission-critical data centres. One technique to protect sensitive equipment in buildings is the use of floor isolation systems (FIS). Multi-directional shake table real-time hybrid simulation (RTHS) is utilized in this paper to validate the performance of full-scale rolling pendulum (RP) bearings, incorporating multi-scale (building– FIS–equipment) interactions. The analytical substructure for the RTHS included 3D nonlinear models of the building and isolated equipment, while the experimental substructure was comprised of the FIS. The RTHS test setup consisted of the FIS positioned on a shake table, where it is coupled to the analytical substructure and subjected to multi-directional deformations caused by the building’s floor accelerations and equipment motion from an earthquake. Parametric studies were performed to assess the influence of different building lateral load systems on the performance of the FISs. The lateral load resisting systems included buildings with steel moment resisting frame (SMRF) systems and with buckling restrained braced frame (BRBF) systems. Each building type was subjected to multi-directional ground motions of different sources and hazard levels. Details of the experimental test setup, RTHS test protocol and main preliminary results on the multi-directional testing of an RP-based FIS are described. Challenges in conducting the multi-axial RTHS, including the nonlinear kinematics transformation, adaptive compensation for the actuator-table dynamics, along with the approaches used to overcome them are presented. The acceleration and deformation response of the isolated equipment is assessed to demonstrate the effectiveness of the FIS in mitigating the effects of multi-directional seismic loading on isolated equipment in mission-critical buildings. 

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3. Friction-induced instability and vibration in a precision motion stage with a friction isolator

   https://doi.org/10.1177/1077546321999510  

   Wang, Jiamin; Dong, Xin; Barry, Oumar R; Okwudire, Chinedum (March 2021, Journal of Vibration and Control)

   null (Ed.)

   Motion stages are widely used for precision positioning in manufacturing and metrology applications. However, they suffer from nonlinear premotion (i.e. “static”) friction, which adversely affects their speed and motion precision. In this article, a friction isolator is used as a simple and robust solution to mitigate the undesirable effects of premotion friction in precision motion stages. For the first time, a theoretical study is carried out to understand the dynamic phenomena associated with using a friction isolator on a motion stage. Theoretical analysis and numerical simulation are conducted to examine the dynamical effects of friction isolator on a proportional–integral–derivative-controlled motion stage under LuGre friction dynamics. The influence of friction isolator on the response and stability of the system is examined through theoretical and numerical analyses. Parametric analysis is also carried out to study the effects of friction isolator and friction parameters on the eigenvalue and stability characteristics. The numerical results validate the theoretical findings and demonstrate several other interesting nonlinear phenomena associated with the introduction of friction isolator. This motivates deeper nonlinear dynamical analyses of friction isolator for precision motion control. 

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4. Shake Table Tests of a Coupled Primary Structure-Floor Isolation System

   https://doi.org/10.17603/ds2-r06w-fy29  

   Harvey, Philip; Bin, Puthynan (January 2021, Designsafe-CI)

   A small-scale rolling-pendulum (RP) isolation system with gap restrainers was tested under swept-sine surveys (chirp) of varying amplitude. These tests were used to characterize the properties of the experimental small-scale floor isolation system (FIS) that was subsequently installed in a small-scale three-story primary structure (PS). The PS-FIS system was tested under four historic ground motions through shake table testing. These tests were used to assess the coupling between the FIS and the PS. Additionally, cyclic load-displacement tests were run to characterize the response of the shock absorbers used in the FIS. All the data from these experiments have been processed to quantify the performance and characteristics of the PS-FIS and its components, as documented in the data report and in Bin (2021). 

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5. Design and Assessment of Innovative Dual-Mode Rolling Isolation Systems

   https://doi.org/11244/321054  

   Hadikhan Tehrani, Mohammad (August 2019, The University of Oklahoma Libraries)

   null (Ed.)

   Loss of operation or devastating damage to buildings and industrial structures, as well as equipment housed in them, has been observed due to earthquake-induced vibrations. A common source of operational downtime is due to the performance reduction of vital equipment, which are sensitive to the total transmitted acceleration. A well-known method of protecting such equipment is seismic isolation of the equipment itself (or a group of equipment), as opposed to the entire structure due to the lower cost of implementation. The first objective of this dissertation is assessing a rolling isolation system (RIS) based on existing design guidelines for telecommunications equipment. A discrepancy is observed between the required response spectrum (RRS) and the one and only accelerogram recommended in the guideline. Several filters are developed to generate synthetic accelerograms that are compatible with the RRS. The generated accelerograms are used for probabilistic assessment of a RIS that is acceptable per the guideline. This assessment reveals large failure probability due to displacement demands in excess of the displacement capacity of the RIS. When the displacement demands on an isolation system are in excess of its capacity, impacts result in spikes in transmitted acceleration. Therefore, the second objective of this dissertation is to design impact prevention/mitigation mechanisms. A dual-mode system is proposed where the behavior changes when the displacement exceeds a predefined threshold. A new piecewise optimal control approach is developed and applied to find the best possible mechanism for the region beyond the threshold. By utilizing the designed curves obtained from the proposed optimal control procedure, a Kelvin-Voigt device is tuned for illustrative purposes. On the other hand, the preference for protecting equipment decreases as the earthquake intensity increases. In extreme seismic loading, the response mitigation of the primary structure (i.e., life safety and collapse prevention) is of greater concern than protecting isolated equipment. Therefore, the third objective of this dissertation is to develop an innovative dual-mode system that can behave as equipment isolation under low to moderate seismic loading and passively transition to behave as a vibration absorber for the primary structure under extreme seismic loading. To reduce the computational cost of simulating a large linear elastic structure with nonlinear attachments (i.e., equipment isolation with cubic hardening nonlinearity), a reduced order modeling method is introduced that can capture the behavior of such nonlinear coupled systems. The method is applied to study the feasibility of dual-mode vibration isolation/absorber. To this end, nonlinear transmissibility curves for the roof displacement and isolated mass total acceleration are developed from the steady-state responses of dual-mode systems using the harmonic balanced method. The final objective of this dissertation is to extend the reduced order modeling method developed for linear elastic structure with nonlinear attachment to inelastic structures (without attachments). The new inelastic model condensation (IMC) method uses the modal properties of the full structural model (in the elastic range) to construct a linear reduced order model in conjunction with a hysteresis model to capture the hysteretic inter-story restoring forces. The parameters of these hysteretic forces are easily tuned, in order to fit the inelastic behavior of the condensed structure to that of the full model under a variety of simple loading scenarios. The fidelity of structural models condensed in this way is demonstrated via simulation for different ground motion intensities on three different building structures with various heights. The simplicity, accuracy, and efficiency of this approach could significantly alleviate the computational burden of performance-based earthquake engineering. 

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