Kenzo Toki

Active Control of Seismic Response of Structures

A new closed-open-loop optimal control algorithm is proposed that has been derived by minimizing the sum of the quadratic time-dependent performance index and the seismic energy input to the structural system. This new control law provides feasi ble control algorithms that can easily be implemented for applications to seismic-excited structures. We developed optimal control algorithms, taking into account the nonlinearity of the structural system for applying a control force to a structural system subjected to gen eral dynamic loads. The formulation of a predictive control law has been developed in which emphasis is placed on compensation for the time delay due to measurement process and the control action. These optimal algorithms are simple and reliable for on-line con trol operations and effective for a structural system with a base isolation mechanism. The control efficiency affected by two weighting matrices included in the performance index is investigated in detail. Numerical examples are worked out to demonstrate the control effi ciency of the proposed algorithms.

Simulation of stochastic waves in a non-homogeneous random field

Abstract Stochastic waves are simulated in a non-homogeneous field; layered media with irregular interfaces. Observed waves are specified at one or more points, and the proposed procedure simulates waves at arbitrary points for which no motion has been proposed, using only information from observed records. Stochastic waves are assumed to be composed of a deterministic component (trend wave) and a stochastic component (random wave). We propose a simple trend model that uses the Fourier spectrum of the observed wave. The kriging method is used for the optimum interpolation of random waves. According to the conditional simulation, random stochastic waves were generated on a non-homogeneous random field. The simulated waves are coincident with known time histories at specific points. To check the validity of the procedure developed, we calculated the waves in layered media with irregular interfaces using the discrete wave-number method and compared them to the waves simulated by our procedure. This procedure that includes the kriging technique provides an efficient means by which to simulate the stochastic waves of a non-homogeneous random field.

Simulation of fault rupture process by the stochastic finite element method

It has been shown that the finite element method which utilizes the joint element for representing the fault plane is a promising tool to analyse the fault rupture process and to predict the near field ground motion. In such analyses, however, the spatial distribution of stress and strength along the fault must be known and has been assumed to be deterministic. Then, as a next step of the analysis, the spatial distribution of stress and strength on the fault are considered to be random variables and the effects of spatial variation are discussed by making use of the Monte Carlo simulation and the first-order approximation method.

Analytical representation of phase characteristics for source time function modeled by stochastic impulse train

Abstract In order to discuss the relationship between the lower and higher frequency components of earthquake source spectra, we deal with impulse train model as source time function of earthquake, because spectral characteristics of source time function depend on occurrence times of impulse function which corresponds to small extent on the fault. Then, the spectral characteristics of source time function are obtained analytically and numerically from the stochastic viewpoints: namely, on one hand, the trend of impulse train dominates the frequency characteristics in low frequency range, and on the other hand, the fluctuation from the trend settles high frequency range. Furthermore, it is shown that the spectral properties of source time function can be determined using only two parameters which are number of impulses n and the probability density function of occurrence time of impulses f T ( t ).

A work softening joint element used in dynamic analysis of soil-structure

This paper introduces an elasto-plastic joint element characterised by strain hardening and softening in the analysis of dynamic soil-structure interaction. The phenomena of separation and sliding on the contact surface between soil and structure can be better simulated and the process can also be described. The interaction problems in a typical soil-structure system are analyzed in terms of elasto-plastic joint element as well as elastic ones. The results show that the elasto-plastic joint element is much better than the elastic one in modelling, especially in that the relative displacements accross the joint element can be much greater than that of the elastic case. Separation and sliding are not only related to the coefficient of friction and cohesion but also to their changes with plastic volumetric strain.

Dynamic behaviour and identification of non-uniform ground by the discrete wave number method

Abstract Powells method for minimizing a function of several variables without calculating derivatives is applied to recorded earthquake motion on the ground surface to identify ground characteristics that have irregular profiles. The identifications are made by designating the shear wave velocity depth and width of the irregularity of surface ground as unknown parameters and are based on the least square fit between the amplitude of the transfer function determined from accelerograms recorded at two observation sites and the corresponding transfer function calculated from the response analysis of a ground model. The discrete wave number method is used to analyze the response of ground with a non-uniform profile for the incidence of SH waves. The effect of the initial assumed values on the convergence is studied by evaluating the square error between the theoretical transfer function and that calculated from the parameters identified. The dispersive trend found for the accelerograms is explained by the calculated response of a ground model with a non-uniform profile.

Assessment of the vertical distribution on seismic ground motion

It is very important for the facilities such as nuclear power plants to infer seismic force loading on the earthquake stability assessment of the building foundation and the surrounding slope. The purpose of this paper was to propose a method to evaluate underground seismic coefficients, taking into account dynamic response along the depth in horizontally multi-layered ground. The dynamic property of the seismic coefficient was analyzed on the basis of earthquake records observed at hard and soft rock sites mostly found in Tertiary deposits and sedimentary ground sites of the Pleistocene and Holocene epoch. The evaluation methods of a vertical distribution on underground seismic coefficients were proposed for a few calculation methods on the classified layered ground. Extended evaluation for underground seismic coefficients was confirmed with respect to some multi-layered ground during strong motion.