Abstract

There are various buildings in a city. Each building has its own natural period and its original structural properties. When an earthquake occurs, a variety of ground motions are induced in the city. The combination of the building natural period with the predominant period of the induced ground motion may lead to disastrous phenomena in the city. Many past earthquake observations demonstrated such phenomena repeatedly. Once a big earthquake occurs, some building codes are upgraded. However, it is true that this repetition never resolves all the issues and new serious problems occur even recently. In order to overcome this problem, a new paradigm has to be posed. To the author’s knowledge, the concept of “critical excitation” and the structural design based upon this concept can become one of such new paradigms. Earthquake inputs are uncertain even with the present knowledge and it does not appear easy to predict forthcoming events precisely both in time-history and frequency contents. For example, recent near-field ground motions (Northridge 1994, Kobe 1995, Turkey 1999 and Chi-Chi, Taiwan 1999) and the Mexico Michoacan motion 1985 have some peculiar characteristics unpredictable before their occurrence. It is also true that the civil, mechanical and aerospace engineering structures are often required to be designed for disturbances including inherent uncertainties due mainly to their “low rate of occurrence”. Worst-case analysis combined with proper information based on reliable physical data is expected to play an important role in avoiding difficulties induced by such uncertainties. Approaches based on the concept of “critical excitation” seem to be promising. The most critical issue in the seismic resistant design is the resonance. One of the promising approaches to this phenomenon is to shift the natural period of the building through seismic control and to add damping in the building. However it is also true that the seismic control is under development and more sufficient time is necessary to respond to uncertain ground motions. In this talk, the role of critical excitation methods in the design of important structures is discussed extensively. The problem of critical excitation for time-dependent performance indices is treated first. Then the problem of critical excitation for earthquake input energy is discussed. The application of various critical excitation methods to high-rise buildings, base-isolated buildings, nuclear reactor facilities, etc. will be presented.