Experimental validation of a simplified model for rolling isolation systems

Abstract

Rolling isolation systems (RISs) protect mission-critical equipment and valuable property from earthquake hazards by decoupling the dynamic responses of vibration-sensitive objects from horizontal floor motions. These responses involve the constrained rolling of steel balls between bowl-shaped surfaces. The light damping of steel balls rolling between steel plates can be augmented by adhering thin rubber sheets to the plates, thereby increasing the rolling resistance and decreasing the displacement demand on the RIS. An assessment of the ability of lightly-and heavily-damped RISs to mitigate the hazard of seismically induced failures requires high-fidelity models that can adequately capture the systems' intrinsic nonlinear behavior. The simplified model presented in this paper is applicable to RISs with any potential energy function, is amenable to both lightly-and heavily-damped RISs, and is validated through the successful prediction of peak responses for a wide range of disturbance frequencies and intensities. The validated model can therefore be used to compute the spectra of peak floor motions for which displacement demands equal capacity. These spectra are compared with representative floor motion spectra provided by the American Society of Civil Engineers 7-10. The damping provided by rolling between thin viscoelastic sheets increases the allowable floor motion intensity by a factor of 2-3, depending on the period of motion. Acceleration responses of isolation systems with damping supplied in this fashion do not grow with increased damping, even for short-period excitations. Figure 1. Exploded view of a rolling equipment isolation system.level or return period is of particular importance in the probabilistic seismic hazard analysis of contents protected by isolation. Estimating the displacement demand for an equipment isolation system corresponding to a specific installation, building site, and hazard level requires, in part, a predictive model of the isolation system behavior.To date, researchers have focused primarily on the single-axis behavior of equipment isolation systems, neglecting the coupling between transverse responses. Experimental tests on equipment isolation systems are sparse [4,[8][9][10], especially for multi-axis disturbances [11]. The prediction of the response of equipment isolation systems and their ability to protect building contents requires models that can capture the observed nonlinear behavior of actual isolation systems subjected to multi-axis shaking. Accordingly, the focus of this paper is on the experimental validation of a multi-axis, nonlinear model of rolling isolation systems (RISs) to further attenuate responses.Rolling isolation systems [12] are widely used to isolate mainframes, LAN racks, electronics enclosures, telecommunications switches, and other mission-critical equipment and valuable property. Museums around the world have adopted isolation systems to protect objects (such as The Statue of Hermes and The Gates of Hell) from earthquake-induced floor motions [13]...

Assessment of a rolling isolation system using reduced order structural models

“…[27], when supplemental damping is included in an RIS, the displacement demands of the RIS are decreased, improving the performance and better protecting the isolated object. In all cases, the distribution functions for the lightly-damped RIS fall to the left (lower peak displacements) than those for the heavily-damped RIS.…”

“…Both approaches act to increase the rolling resistance and thus increase energy dissipation, decrease isolator displacements, and improve performance over lightly-damped RISs. The latter approach, referred to as a heavily-damped RIS, is considered in the present study [27].…”

“…The total acceleration floor responses at three heights -1st story, mid-height, and roof, as indicated in Fig. 2 -are evaluated and then applied to the validated models for lightly-and heavily-damped RISs [19,27]. The peak relative displacement (d max R ) and the peak total acceleration (a max T ) of the RISs are recorded.…”

“…[27] by examining the performances of a lightly-damped RIS and a heavily-damped RIS located within earthquake-excited structures (Fig. 1).…”

“…Finally (Section 5) the performance of RISs, located at varying heights within the buildings, is assessed. The bidirectional floor responses of the seismically excited building are applied to the experimentally-validated models for lightly-and heavily-damped RISs [19,27], and empirical cumulative distribution functions (CDFs) of peak equipment responses (accelerations and displacements) are compared and design guidelines are presented.…”