Chin-Hsiung Loh

Application of the Empirical Mode Decomposition-Hilbert Spectrum Method to Identify Near-Fault Ground-Motion Characteristics and Structural Responses

In this article, the empirical mode decomposition method combined with the Hilbert spectrum method (EMD + HHT) is used to analyze the free-field ground motion and to estimate the global structural property of building and bridge structure through the measurement of seismic response data. The EMD + HHT method provides a powerful tool for signal processing to identify nonlinear and nonstationary data. Based on the decomposed ground-motion signal, the absolute input energy of each decomposed wave was studied (the fling step [pulselike wave] can be separated from the recorded near-fault ground motion). Through application of the EMD + HHT method to building and bridge seismic response data, the time-varying system natural frequency and damping ratio can also be estimated. Damage identification from seismic response data of buildings and bridges, particularly from the Chi-Chi earthquake data, is also described. Manuscript received 31 July 2000.

APPLICATION OF OFF-LINE AND ON-LINE IDENTIFICATION TECHNIQUES TO BUILDING SEISMIC RESPONSE DATA

The objectives of this paper are to present a comparison of the dynamic characteristics of a seven-storey reinforced concrete building (Van Nuys-Holiday Inn) identified from four recorded strong-motion response data (Whittier earthquake, Landers earthquake, Big Bear earthquake and Northridge earthquake). In the analysis, time-domain methods for estimating the system parameters and the modal properties of the building are studied. Both off-line and on-line identification algorithms are applied to these seismic response data. Under the assumption of a linear time-invariant system the ARX model and ARMAX model are used. Comparison of the identification results using different models are made. In addition, recursive procedures are adapted as on-line identification and the time-varying modal parameters are estimated. For structural systems under strong earthquake excitation, a recursive identification method, adaptive forgetting through multiple models (AFMM), is introduced to identify systems with rapidly changing parameters. Through the analysis of the seismic response data of the building subjected to four earthquakes the identification algorithm and the identification results are discussed.

Identification of Fei-Tsui arch dam from both ambient and seismic response data

This purpose of this paper is to study the dynamic characteristics of the Fei-Tsui arch dam using the seismic response data and the ambient vibration data. For the identification of dam properties from seismic response data, the multiple inputs from the abutment of the dam to represent the nonuniform excitations of seismic input motion are considered, and the ARX model is applied using the discrete-time linear filtering approach with least-squares approximation to identify the dynamic characteristics of the dam. The system modal dampings, natural frequencies and frequency response functions are identified. A comparison of the identified modal parameters is made among different seismic events. Post-earthquake safety evaluation of the dam can be made based on the identified model. Finally, the ambient vibration test of the dam is performed to identify the mode shapes along the dam crest.

Ground motion characteristics of the Chi-Chi earthquake of 21 September 1999

The purpose of this paper is to investigate the ground motion characteristics of the Chi-Chi earthquake (21 September 1999) as well as the interpretation of structural damage due to this earthquake. Over 300 strong motion records were collected from the strong motion network of Taiwan for this earthquake. A lot of near-field ground motion data were collected. They provide valuable information on the study of ground motion characteristics of pulse-like near-field ground motions as well as fault displacement. This study includes: attenuation of ground motion both in PGA and spectral amplitude, principal direction, elastic and inelastic response analysis of a SDOF system subjected to near-field ground motion collected from this event. The distribution of spectral acceleration and spectral velocity along the Chelungpu fault is discussed. Based on the mode decomposition method the intrinsic mode function of ground acceleration of this earthquake is examined. A long-period wave with large amplitude was observed in most of the near-source ground acceleration. The seismic demand from the recorded near-field ground motion is also investigated with an evaluation of seismic design criteria of Taiwan Building Code. Copyright

GA-based optimum design of a shape memory alloy device for seismic response mitigation

Damping systems discussed in this work are optimized so that a three-story steel frame structure and its shape memory alloy (SMA) bracing system minimize response metrics due to a custom-tailored earthquake excitation. Multiple-objective numerical optimization that simultaneously minimizes displacements and accelerations of the structure is carried out with a genetic algorithm (GA) in order to optimize SMA bracing elements within the structure. After design of an optimal SMA damping system is complete, full-scale experimental shake table tests are conducted on a large-scale steel frame that is equipped with the optimal SMA devices. A fuzzy inference system is developed from data collected during the testing to simulate the dynamic material response of the SMA bracing subcomponents. Finally, nonlinear analyses of a three-story braced frame are carried out to evaluate the performance of comparable SMA and commonly used steel braces under dynamic loading conditions and to assess the effectiveness of GA-optimized SMA bracing design as compared to alternative designs of SMA braces. It is shown that peak displacement of a structure can be reduced without causing significant acceleration response amplification through a judicious selection of physical characteristics of the SMA devices. Also, SMA devices provide a recentering mechanism for the structure to return to its original position after a seismic event.

Nonlinear identification of dynamic systems using neural networks

A neural-network-based method is proposed for the modeling and identification of a discrete-time nonlinear hysteretic system during strong earthquake motion. The learning or modeling capability of multilayer neural networks is explained from the mathematical point of view. The main idea of the proposed neural approach is explained, and it is shown that a multilayer neural network is a general type of NARMAX model and is suitable for the extreme nonlinear input-output mapping problems. Numerical simulation of a three-story building and a real structure (a bridge in Taiwan) subjected to several recorded earthquakes are used here to demonstrate the proposed method. The results illustrate that the neural network approach is a reliable and feasible method.

Directionality and simulation in spatial variation of seismic waves

Abstract The purpose of this paper is to study the directionality in spatial variation of seismic waves. Characteristics of mutual correlations of seismic ground motion of different points for different components of frequencies were analysed through the definition of normalized covariance. Two variables were involved in the study of directionality, the incident angle of wave propagation projected into the horizontal ground plane and the relative angle. A mathematical model is presented to include the directionality in spatial variation of seismic waves. From the analysis of frequency-wave number spectra, the simulation of spatially two-dimensional stochastic waves can be modelled using a deterministic model for the low frequency range of seismograms. Both directionality and simulation in spatial variation of seismic waves were studied based on the data recorded by the SMART-1 array.

Time domain estimation of structural parameters

Abstract The extended Kalman filter is applied to a system identification problem of seismic structural systems. This paper presents an identification method for an equivalent linear system, a bilinear hysteric restoring system and a bilinear hysteretic restoring system with stiffness degradation effect. For the accuracy of this present proposal, the justification of the method is investigated on numerically simulated data on response of a known system as well as a known degrading system. The proposed method is formulated in detail. It is then applied to identify the hysteresis behaviour of two buildings which were subjected to earthquake loads.

Empirical Study of Sediment‐Filled Basin Response: The Case of Taipei City

We analyze the site response of the Taipei basin using the records obtained by the Taiwan Strong Ground Motion Instrumentation Program (TSMIP) network. Records of 66 earthquakes of M=2.6-6.5 with a hypocentral depth varying from 1 km to 118 km and hypocentral distances of up to 150 km are studied for 35 stations located within this triangle-shaped alluvium structure. The site response is obtained in terms of spectral ratios calculated by dividing of the site spectrum by the reference spectrum estimated for a hypothetical “very hard rock” site. The recently developed empirical source scaling and attenuation models for the Taiwan region are used for the reference spectra calculation. This approach allows us to evaluate the variability of spectral ratios due to uncertainties introduced by source and propagation path effects and variability in the site response itself. The characteristics of site response in the Taipei basin depend on the properties of soil deposits and, in general, may be described by 1-D models. However, there are some peculiarities of spectral ratios that show the influence of subsurface topography.

An efficient analysis of structural response for multiple-support seismic excitations

Abstract A response spectrum method is proposed for the seismic analysis of a multisupport structure subjected to spatially varying ground motion. Based on theoretical results by Der Kiureghian and Hofer, an approximate and efficient analysis of the frequency integral of the cross-correlation function of multisupport excitation is presented. A two-span beam-like structure is studied to demonstrate the application of this proposed method and to characterize the influence of the differential support motion. Treatment of the various combinations of phase relationships between supports is also discussed.