Improving the performance of conventional base isolation systems by an external variable negative stiffness device under
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EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION
Earthq Eng & Eng Vib (2020) 19: 985-1003
October, 2020
DOI: https://doi.org/10.1007/s11803-020-0609-3
Improving the performance of conventional base isolation systems by an external variable negative stiffness device under near-fault and long-period ground motions Sandhya Nepal† and Masato Saitoh‡ Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama, Japan
Abstract:
Recent studies have shown that base-isolated objects with long fundamental natural periods are highly influenced by long-period earthquakes. These long-period waves result in large displacements for isolators, possibly leading to exceedance of the allowable displacement limits. Conventional isolation systems, in general, fail to resist such large displacements. This has prompted the need to modify conventional base isolation systems. The current work focuses on the development of an external device, comprising a unit of negative and positive springs, for improving the performance of conventional base isolation systems. This unit accelerates the change in the stiffness of the isolation system where the stiffness of the positive spring varies linearly in terms of the displacement response of the isolated objects. The target objects of the present study are small structures such as computer servers, sensitive instruments and machinery. Numerical studies show that the increase in the damping of the system and the slope of the linear function is effective in reducing the displacement response. An optimal range of damping values and slope, satisfying the stability condition and the allowable limits of both displacement and acceleration responses when the system is subjected to near-fault and long-period ground motions simultaneously, is proposed.
Keywords: base isolation; near-fault earthquakes; long-period earthquakes; negative stiffness; variable stiffness
1 Introduction In recent decades, high-tech facilities and equipment, such as computer servers, art monuments, medical facilities and machinery, are often worth more than the building itself (Tsai et al., 2008), and their failure may cause massive loss (such as economic loss and data loss) for the social communities. The failure of such critical structural content may be due to overturning, excessive displacement, or excessive acceleration (Lopez Garcia and Soong, 2003a, 2003b). For example, although modern equipment such as a hard disk drive (HDD) can withstand up to 1.0 g (10 m/s2) (WorkSafe Technologies Corporation), there is old equipment still in use that has low acceleration bearing capacity up to 0.3 g (3.0 m/s2). This equipment needs some sort of protective system which can dampen the high acceleration response of the isolated mass to its bearing limit or even lower. Based on experimental studies conducted for structural contents such as medical facilities, electronic systems Correspondence to: Sandhya Nepal, Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura
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