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1. Evaluation of Rolling-Type Isolation Systems for Seismic Hazard Mitigation

   https://doi.org/11244/300332  

   Calhoun, Skylar Josef ( August 2018 , The University of Oklahoma Libraries)

   null (Ed.)

   Nonstructural components within mission-critical facilities such as hospitals and telecommunication facilities are vital to a community's resilience when subjected to a seismic event. Building contents like medical and computer equipment are critical for the response and recovery process following an earthquake. A solution to protecting these systems from seismic hazards is base isolation. Base isolation systems are designed to decouple an entire building structure from destructive ground motions. For other buildings not fitted with base isolation, a practical and economical solution to protect vital building contents from earthquake-induced floor motion is to isolate individual equipment using, for example, rolling-type isolation systems (RISs). RISs are a relatively new innovation for protecting equipment. These systems function as a pendulum-like mechanism to convert horizontal motion into vertical motion. An accompanying change in potential energy creates a restoring force related to the slope of the rolling surface. This study seeks to evaluate the seismic hazard mitigation performance of RISs, as well as propose and test a novel double RIS. A physics-based mathematical model was developed for a single RIS via Lagrange's equation adhering to the kinetic constraint of rolling without slipping. The mathematical model for the single RIS was used to predict the response and characteristics of these systems. A physical model was fabricated with additive manufacturing and tested against multiple earthquakes on a shake table. The system featured a single-degree-of-freedom (SDOF) structure to represent a piece of equipment. The results showed that the RIS effectively reduced accelerations felt by the SDOF compared to a fixed-base SDOF system. The single RIS experienced the most substantial accelerations from the Mendocino record, which contains low-frequency content in the range of the RIS's natural period (1-2 seconds). Earthquakes with these long-period components have the potential to cause impacts within the isolation bearing that would degrade its performance. To accommodate large displacements, a double RIS is proposed. The double RIS has twice the displacement capacity of a single RIS without increasing the size of the bearing components. The mathematical model for the single RIS was extended to the double RIS following a similar procedure. Two approaches were used to evaluate the double RIS's performance: stochastic and deterministic. The stochastic response of the double RIS under stationary white noise excitation was evaluated for relevant system parameters, namely mass ratio and tuning frequency. Both broadband and filtered (Kanai-Tajimi) white noise excitation were considered. The response variances of the double RIS were normalized by a baseline single RIS for a comparative study, from which design parameter maps were drawn. A deterministic analysis was conducted to further evaluate the double RIS in the case of nonstationary excitation. The telecommunication equipment qualification waveform, VERTEQ-II, was used for these numerical simulations. Peak transient responses were compared to the single RIS responses, and optimal design regions were determined. General design guidelines based on the stochastic and deterministic analyses are given. The results aim to provide a framework usable in the preliminary design stage of a double RIS to mitigate seismic responses. 

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2. Fundamental Dynamics of 3-Dimensional Seismic Isolation

   Eltahawy, W. ( January 2017 , 16th World Conference on Earthquake Engineering)

   Seismic isolation systems for buildings are generally selected to achieve higher seismic performance objectives, such as continued operation or immediate occupancy following a design earthquake event. However, recent large scale tests have suggested that these objectives may be compromised if the shaking includes large vertical acceleration components that are damaging to the nonstructural components and contents. Some research has been conducted to develop three dimensional isolation systems that can isolate the structure from both the horizontal and vertical components of ground motion. In several cases, systems have been proposed without much justification of the target design parameters. Rocking has been noted as a potential concern for structures with 3D isolation systems, and complex systems have been proposed to control the rocking. In this study, the fundamental dynamic response of structures with 3D isolation systems is explored. Target horizontal and vertical spectra for a representative strong motion site were developed based on NEHRP recommendations, and horizontal and vertical ground motions were selected that best fit the target spectra when the same amplitude scale factor was applied to all three motion components. Using a simple model of a rigid block resting on linear isolation bearings, the following aspects are evaluated for a wide range of horizontal and vertical isolation periods: response modes and severity of rocking, horizontal and vertical displacement demands in the isolation bearings, and attenuation of both horizontal and vertical accelerations in the structure relative to the ground acceleration. Preliminary results point to a number of useful observations. For example, rocking appears to be an issue only if the horizontal and vertical isolation periods are closely spaced. Helical spring isolation systems that have been applied to a few structures have this characteristic. However, if the horizontal isolation period is large relative to the vertical isolation period, troublesome rocking can be avoided. In addition, other researchers have proposed systems with vertical isolation periods on the order of 2 seconds, which require large displacement and damping capacity. However, preliminary results suggest that vertical isolation periods as low as 0.5 seconds will be effective in attenuating the vertical acceleration. Limiting the vertical isolation period will make design of a 3D isolation system more feasible with respect to vertical displacement capacity and avoiding rocking. 

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3. Moghimi, G. and Makris, N., 2022. Seismic response of yielding multistory steel buildings with pressurized sand dampers. Journal of Structural Engineering ASCE, published on-line: 10.1061/ (ASCE)ST.1943-541X.0003364

   https://doi.org/https://doi.org/10.1061/(ASCE)ST.1943-541X.0003364  

   Gholamreza Moghimi, S.M.ASCE ( April 2022 , Journal of structural engineering)

   Abstract: This paper investigates the seismic response analysis of the 9-story SAC building equipped with pressurized sand dampers, a new type of low-cost energy dissipation device where the material enclosed within the damper housing is pressurized sand. The strength of the pressurized sand damper is proportional to the externally exerted pressure on the sand via prestressed steel rods; therefore, the energy dissipation characteristics of a given pressurized sand damper can be adjusted according to a specific application. The strong pinching behavior of pressurized sand dampers was characterized with a previously developed three-parameter Bouc-Wen hysteretic model that for this study was implemented in the open source code OpenSees with a C++ algorithm, and it was used to analyze the seismic response of the 9-story SAC building subjected to six strong ground motions that exceed the design response spectrum for all soil categories. The paper shows that for the family of strong ground motions used in this study, pressurized sand dampers with strength of the order of 5%–10% of the weights of their corresponding floors were able to keep the interstory drifts of the 9-story SAC building at or below 1%, while base shears and peak plastic hinge rotations were reduced in the damped configuration. Supplemental damping produced mixed results on floor accelerations; nevertheless, in most floors, peak accelerations were reduced. DOI: 10.1061/(ASCE)ST.1943-541X.0003364. 

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4. 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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5. Energy Exchange between Two Sub-systems Coupled with a Nonlinear Elastic Path

   Sen, O.T. ; Singh, R. ( May 2018 , NOVEM 2018 Conference)

   The chief objective of this paper is to explore energy transfer mechanism between the sub-systems that are coupled by a nonlinear elastic path. In the proposed model (via a minimal order, two degree of freedom system), both sub-systems are defined as damped harmonic oscillators with linear springs and dampers. The first sub-system is attached to the ground on one side but connected to the second sub-system on the other side. In addition, linear elastic and dissipative characteristics of both oscillators are assumed to be identical, and a harmonic force excitation is applied only on the mass element of second oscillator. The nonlinear spring (placed in between the two sub-systems) is assumed to exhibit cubic, hardening type nonlinearity. First, the governing equations of the two degree of freedom system with a nonlinear elastic path are obtained. Second, the nonlinear differential equations are solved with a semi-analytical (multi-term harmonic balance) method, and nonlinear frequency responses of the system are calculated for different path coupling cases. As such, the nonlinear path stiffness is gradually increased so that the stiffness ratio of nonlinear element to the linear element is 0.01, 0.05, 0.1, 0.5 and 1.0 while the absolute value of linear spring stiffness is kept intact. In all solutions, it is observed that the frequency response curves at the vicinity of resonant frequencies bend towards higher frequencies as expected due to the hardening effect. However, at moderate or higher levels of path coupling (say 0.1, 0.5 and 1.0), additional branches emerge in the frequency response curves but only at the first resonant frequency. This is due to higher displacement amplitudes at the first resonant frequency as compared to the second one. Even though the oscillators move in-phase around the first natural frequency, high amplitudes increase the contribution of the stored potential energy in the nonlinear spring to the total mechanical energy. The out-of-phase motion around the second natural frequency cannot significantly contribute due to very low motion amplitudes. Finally, the governing equations are numerically solved for the same levels of nonlinearity, and the motion responses of both sub-systems are calculated. Both in-phase and out-of-phase motion responses are successfully shown in numerical solutions, and phase portraits of the system are generated in order to illustrate its nonlinear dynamics. In conclusion, a better understanding of the effect of nonlinear elastic path on two damped harmonic oscillators is gained. 

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