Beyond Uncertainties in Earthquake Structural Engineering

Ground Motion Definition Earthquake events and realized earthquake ground motions are extremely uncertain even with the present knowledge, and it is not easy to predict forthcoming events precisely both in time and frequency (Anderson and Bertero, 1987; Takewaki et al., 1991; 2013; 2011a; Conte et al., 1992; Ariga et al., 2006; Minami et al., 2013; Çelebi et al., 2014). For example, recently reported nearfield ground motions (Northridge 1994, Kobe 1995, Turkey 1999, and Chi–Chi, Taiwan 1999), the Mexico Michoacan motion 1985, and the Tohoku motion 2011, had some peculiar characteristics that could not have been predicted. It is also true that civil, mechanical, and aerospace engineering structures are often subjected to disturbances with inherent uncertainties due mainly to their “low rate of occurrence.” Worst-case analysis (Drenick, 1970; Shinozuka, 1970; Takewaki, 2006/2013; Elishakoff and Ohsaki, 2010), combined with proper information based on reliable physical data, is expected to play an important role in avoiding difficulties caused by such uncertainties. Approaches based on the concept of “critical excitation” seem promising. There are various buildings in a city (Figure 1A). Each building has its own natural period and its idiosyncratic structural properties. Earthquakes trigger various kinds of ground motions in the city. The relation of the building’s natural period with the predominant period of the induced ground motion may lead to disastrous phenomena, as many observations from past historical earthquakes have demonstrated. Once a large earthquake occurs, some building codes are typically upgraded, but such makeshift efforts never resolve all issues and new damage problems have occurred even recently. In order to overcome this problem, a new paradigm has to be posed. In my view, the concept of “critical excitation,” and structural design based upon it, could become a powerful new paradigm. Critical excitation methods were pioneered by Drenick (1970) and Shinozuka (1970). Just as the investigation of limit states of structures plays an important role in the specification of allowable response and performance levels of structures during disturbances, the clarification of critical excitations for a given (group of) structure(s) can provide structural designers with useful information for determining excitation parameters. After Drenick and Shinozuka’s pioneering work (1970), versatile researches have been developed (Iyengar and Manohar, 1985; 1987; Pirasteh et al., 1988; Srinivasan et al., 1992; Manohar and Sarkar, 1995; Pantelides and Tzan, 1996; Tzan and Pantelides, 1996; Takewaki, 2000; 2001a;b; 2008a; Abbas andManohar, 2002; Fujita et al., 2010a;Moustafa andTakewaki, 2010a;b;Moustafa et al., 2010; Moustafa, 2011; Takewaki et al., 2012). Details of critical excitation methods are given in Takewaki (2006/2013). In the case where influential active faults are known during the design stage of a structure (especially an important structure), the effects of these active faults should be taken into account through the concept of critical excitation.While influential active faults are not necessarily known in advance, virtual or scenario faults and their energy can be predefined, especially for the design of important