Earthquakes and Structures, Vol. 14, No. 1 (2018) 33-44 DOI: https://doi.org/10.12989/eas.2018.14.1.033 33 Copyright © 2018 Techno-Press, Ltd. http://www.techno-press.com/journals/eas&subpage=7 ISSN: 2092-7614 (Print), 2092-7622 (Online) 1. Introduction Differences in the amplitudes and phase angles of the seismic ground motions (GMs), recorded over extended areas has been recognized for many years. In the early days, lack of an adequately large set of recorded data prevented quantification of the phenomenon and the development of approaches for the incorporation of the effect into the dynamic analysis of nuclear safety related structures (forthcoming ASCE-4). Abrahamson (2007), in a separate study referenced herein, presents a state-of-the-art representation of the coherency function based on a large number of densely spaced ground motion recordings. Coherency functions define the relationships between ground motion at separate locations as a function of two parameters (1) the separation distance between the locations and (2) the frequency of the ground motion. For coherent motion and vertically propagating seismic waves, the amplitude at all locations is the same such that the coherency function is unity. For incoherent motion, the amplitude at separated distances is different and the coherency function is below unity (Abrahamson 2005, 2007). For fixed base NPP Horizontal and vertical ground motions are subject to incoherency and have been included (Abrahamson 2005, EPRI 2006). Corresponding author, Professor E-mail: kim2kie@kunsan.ac.kr a Masters Graduate E-mail: kaiser_ce_07@kunsan.ac.kr b Senior Researcher E-mail: shljrl@kepco-enc.com Novak and Hindy (1979), Hindy and Novak (1980) were the pioneers to introduced the ground motion’s coherency losses as a mathematical description in the earthquake engineering field. Early in the nineteen ninety eighties (Loh et al. 1982, Der Kiureghian et al. 1992, Abrahamson et al. 1991, 1993) addressed the phenomenon of incoherency based on the investigation on the earthquake acceleration recorded over limited regions. Later, comprehensive researches has been carried out by Luco et al. (1986) on rigid foundation, Hao et al. (1996) on asymmetric structures, Monti et al. (1996), Harichandran et al. (1996), Saxena et al. (2000), Kim et al. (2003), Chakraborty and Basu (2008), Mwafy et al. (2011) on nonlinear responses of long span bridges, highway bridges subjected to incoherent motions. Recently, Hossein et al. (2013) proposed method to generate spatially varying accelerograms using neural networks. According to Ghiocel et al. (2009) the effects of motion incoherency on the computed In-structure-response- spectra (ISRS) are significant for both the rock and the soil sites for NPP structures. Sayed et al. (2015) investigated spatial variation of El-Centro (1940) record for BI-NPP, Adanur et al. (2016), recommended to consider incoherent motion for suspension bridges isolated with LRBs. Seismic (base) isolation is a mature technology in the civil engineering realm for protecting structures from the effects of moderate and severe earthquake shaking. Base- isolation being limitedly used in nuclear power plants, is a practical strategy that ensure the structural safety and flexibility in moderate to strong earthquake. Ali Hadidi et al. (2016) provided design of base-isolated buildings subjected to near fault motions, Mavronicola et al. (2014) investigated responses of bilinear LRB isolator under pulse- like motions. In addition, Murase et al. (2013) proposed a hybrid base-isolation system to improve seismic performances Effect of the incoherent earthquake motion on responses of seismically isolated nuclear power plant structure Kaiser Ahmed 1a , Dookie Kim 1 and Sang H. Lee 2b 1 Department of Civil Engineering, Kunsan National University, Gunsan-si, 54150, Republic of Korea 2 Power Engineering Research Institute, KEPCO E&C, Gimcheon, Republic of Korea (Received November 22, 2016, Revised January 15, 2018, Accepted January 20, 2018) Abstract. Base-isolated nuclear power plant (BI-NPP) structures are founded on expanded basemat as a flexible floating nuclear island, are still lacking the recommendation of the consideration of incoherent motion effect. The effect of incoherent earthquake motion on the seismic response of BI-NPP structure has been investigated herein. The incoherency of the ground motions is applied by using an isotropic frequency-dependent spatial correlation function to perform the conditional simulation of the reference design spectrum compatible ground motion in time domain. Time history analysis of two structural models with 486 and 5 equivalent lead plug rubber bearing (LRB) base-isolators have been done under uniform excitation and multiple point excitation. two different cases have been considered: 1) Incoherent motion generated for soft soil and 2) Incoherent motion generated for hard rock soil. The results show that the incoherent motions reduce acceleration and the lateral displacement responses and the reduction is noticeable at soft soil site and higher frequencies. Keywords: incoherent earthquake motion; base-isolated nuclear power plant; seismic response