EFFECTS OF AFTERSHOCKS ON THE BEHAVIOR AND STRUCTURAL INTEGRITY OF WATER TANKS Foteini D. Konstandakopoulou 1 , George D. Hatzigeorgiou 2 ABSTRACT This paper examines the effects of aftershocks on the behavior and structural integrity of water tanks. To be sure, there are many research works that investigated the influence of multiple earthquakes on the response of single- degree-of-freedom systems or 2-D and 3-D multi-degree-of-freedom framed structures. On the other hand, the investigation of seismic r esponse of tanks is limited to single ground motions, e.g., using the ‘design’ earthquake. This study investigates the inelastic response of concrete tanks subjected to seismic sequences, a phenomenon which has not been investigated in the past. Real seismic sequences are examined, where the former have been recorded during a short period of time (up to three days), by the same station, in the same direction, and almost at the same fault distance. It is concluded that due to aftershocks effect, it seems to be defective to take into account only single earthquake records in tanks design process. Keywords: Aftershocks; concrete water tanks; inelastic behavior; repeated earthquakes; structural integrity 1. INTRODUCTION Heavy damage or even collapse of concrete tanks during intense seismic ground motions have been mentioned and acknowledged in the past, e.g., during Long Beach earthquake (1933), Niigata earthquake (1964), Alaska earthquake (1964), San Fernando earthquake (1971), Maule earthquake (2010), amongst others. These failures have lead to direct loss of tanks’ structural integrity as well as to indirect catastrophe such as fires, environmental pollution, etc. It is evident that the reliable seismic design of tanks subjected to intense earthquakes is a vital topic for structural engineering. For this reason, numerous modern seismic codes examined the seismic design of tanks such as AWWA (2005), API 650 (2005), Eurocode 8 (2005) and FEMA-750 (2009). Furthermore, many research studies have been published that examined the behavior of tanks under strong earthquakes, such as Jacobsen (1949), Graham and Rodriguez (1951), Housner (1957, 1963), Veletsos [1974, 1987, 1990], Haroun and Housner (1979, 1981, 1982), Malhotra (1997a,b, 1998) and Minoglou et al. (2013), amongst others. All these studies or standards ignoring the response of tanks subjected to repeated earthquakes, i.e. ignoring the impact of aftershocks on tanks’ behavior and capacity. It should be mentioned that influence of aftershocks on structural response of SDOF systems and 2-D or 3-D building structures has been considered in detail, e.g., in works of Hatzigeorgiou (2010a,b), Hatzigeorgiou and Beskos (2009), Hatzigeorgiou and Liolios (2010), Loulelis et al. (2012), Faisal et al. (2013), Hatzivassileiou and Hatzigeorgiou (2015). The only one work that examined the influence of repeated earthquakes on tanks is that of Konstandakopoulou and Hatzigeorgiou (2017a) but it has exclusively to do with steel tanks and not with concrete ones. Thus, the need to propose an effective method for the inelastic analysis of concrete tanks, taking also into account the influence of aftershocks is obvious. This paper examines the behavior of concrete tanks under the action of repeated earthquakes to cover the abovementioned gap, giving useful conclusions and findings. 1 Adjunct Professor, School of Science and Technology, Hellenic Open University, Patras, Greece, konstantakopoulou.foteini@ac.eap.gr 2 Associate Professor, School of Science and Technology, Hellenic Open University, Patras, Greece, hatzigeorgiou@eap.gr