PSI - Issue 18

Bilal L. Khan et al. / Procedia Structural Integrity 18 (2019) 108–118

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Bilal L. Khan et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

Earthquakes can cause severe damage to building structures which so often result into loss of property and life. The building structures with substandard detailing or having vertical irregularity, specifically weak storey, are more hazardous to failure against earthquakes (Sim et al., 2012; Pantelides, Hansen, Ameli, & Reaveley, 2017). A ‘weak storey’ is a storey having 80% less stiffness and strength in comparison to the sto rey above it in a building structure (Uniform Building Code, 1997). This can commonly be observed in the existing buildings when no walls are being built on a certain storey due to prescribed usage but other storeys do have walls for residential usage, thus other storeys with walls have more stiffness than the storey with no walls. Due to less resistance of weak storey, it is subjected to larger earthquake forces causing larger lateral deformations than other storeys. This phenomenon results into a weak storey failure. Buildings structures having weak storeys collapse in different earthquake over the world and expensive retrofitting strategies are required to increase their service life (Griffin, 2017). To prevent weak storey failures and for better performance of buildings during earthquakes the effect of seismic forces on buildings is needed to be reduced. The concept of base isolation is used to serve the purpose. Base isolation prevents the transfer of seismic forces from ground to the structure (Tran, Nguyen, & Kim, 2018). It is done by insertion of base isolation devices called base isolators that provide flexibility and energy dissipation properties to the structures (Ismail, Rodellar, & Ikhouane, 2010). The concept of base isolation is increasingly being adopted to reduce the inter storey drifts and floor accelerations. There are different types of base isolation techniques, among that passive control system is the one in which base isolation system is prominent control in reducing the structural responses of non-isolated building (Jangid & Datta, 1995). The passive base isolators are activated by motion of the structure due to earthquake, therefore no external power supply is needed to develop the counter forces. The main concept of base isolation is to increase the fundamental time period of the structure which reduces the accelerations and earthquake forces experienced by the structure. Passive base isolators have limitation as it cannot adapt to varying loading conditions. Passive base isolation system performs well on the designed loading conditions but may not perform well in other situations (Jadhao, Gadi, & Dumne, 2013). It is because the base isolator deformation is limited to its material properties (Jang et al., 2008, Koo et al., 2009). The suitability criteria for base isolation of structures are when the subsoil doesn’t produce occurrence of long period of ground motion. At the site of the structure, minimum of 200 mm of horizontal displacement must be permitted at the base of structure. The lateral loads other than earthquake must be less than 10% of the total weight of the base isolated structure (Khurshid, 2016). The base isolation system is categorized into elastomeric bearing base isolation and sliding/friction bearing base isolation. This study is focused on elastomeric bearing base isolation system which is further classified into High Damping Rubber bearing, Low Damping Rubber Bearing and Lead Core Rubber Bearing. High Damping Rubber Bearing (HDRB) consists of alternate thin layers of high damping rubber and steel plates. The horizontal stiffness of the bearing depends on low shear modulus of rubber. Steel plates serve the purpose of providing high vertical stiffness and also prevent the rubber from bulging out. High damping rubber bearing provides damping in the range of 10% to 20% (Desai & John, 2015). Low damping rubber bearing (LDRB) has two steel plates at each end and many thin steel shim layers with alternate rubber layers. The steel shims serve the purpose of improving the vertical stiffness of the bearing providing no effect on the horizontal stiffness, which is predominated by the shear modulus of the rubber. Damping value is in the range of 2-3% (Desai & John, 2015, Usman & Jung, 2015). A lead core rubber bearing (LCRB) consists of steel shim layers with alternate rubber layers and an energy dissipation lead core. The lead core provides stiffness under service loads and energy dissipation under strong seismic loads. When subjected to minor earthquake the lead-rubber bearing provides stiffness both in lateral and vertical directions. The high elastic stiffness of the lead plug is responsible for this lateral stiffness. The most advantageous property of lead-rubber bearing is to combine the effects of damping, flexibility at lateral load and stiffness at service load into a single compact unit (Desai & John, 2015). Applied or imposed lateral force is one the crucial factors on which performance and efficiency of lead rubber bearing depends on. If the imposed lateral force is small, the lead core would restrain the movement of the steel shims and the bearing would display higher lateral stiffness. As the lateral force increases, the steel shims cause the deformation or yielding of the lead core. Thus, with energy absorbed by the lead core, it develops the hysteretic damping. Consequently, the lateral stiffness of the lead rubber bearing is reduced while its equivalent damping varies from 15% to 35% (Cheng, Jiang, & Lou, 2008).