Juin-Fu Chai

Dynamic responses of bridges subjected to near-fault ground motions

Abstract The motivation of this paper is to study the dynamic behavior of bridge structural systems subjected to both near‐fault and far‐field ground motions. A highway bridge containing a hinge supported continuous girder with six concrete piers was selected for this study. The Chi‐Chi (Taiwan) earthquake was selected as a first set in this study to provide the near‐fault earthquake characteristics. On the other hand, another earthquake record was selected to illustrate far‐field earthquake characteristics for comparison. The results show that near‐fault earthquake ground motions cause more ductility demands and base shear than far‐field earthquake ground motions.

Consideration of the near-fault effect on seismic design code for sites near the Chelungpu Fault

Abstract In this paper, based on the concept of UBC97, the modification of the design response spectrum in the current Taiwan seismic design code for sites near the Chelungpu Fault is proposed by defining the so‐called near‐source factors to increase the seismic capacity of buildings against near‐fault ground motions. The near‐source factors can be defined for different structural period ranges by the near‐fault attenuation functions for PGA and spectral acceleration demands at 0.3 and 1. 0 second periods, respectively. These attenuation functions are determined from the data recorded at sites near the Chelungpu Fault during the Chi‐Chi (Taiwan) Earthquake on September 21, 1999. Furthermore, the normalized inelastic near‐fault design base shear coefficients can be defined by the normalized near‐fault design response spectrum as well as the reduction factor for a specified ductility ratio and structural period as defined by the seismic design code. Finally, based on the near‐fault ground motion data recorded during the Chi‐Chi (Taiwan) Earthquake, the demand of the normalized inelastic base shear coefficients can be determined numerically by the EPP model. It can be used to verify the capacity of buildings defined by the modified seismic design code to resist the threat of near‐fault ground motions.

Modeling of phase spectrum to simulate design ground motions

Abstract In this paper, the concept of group delay time is used to model the phase spectrum on each separated frequency range according to the compact support of Meyer wavelet. The regression equations to predict the mean value and standard deviation of group delay times can be developed on the basis of the recorded earthquake set. Based on the predicted mean value and standard deviation for a target site, the sample of group delay time at a certain discrete frequency can be either generated randomly by a specified probability density function or simulated conditionally by applying the Kalman filtering technique to consist of the earthquake data observed at nearby stations. Then, the phase spectrum can be modeled by integrating the simulated group delay times. On the basis of the modeled phase spectrum, the design ground motion can be simulated by the iteration process of modifying the Fourier amplitude, such that the associated spectral response acceleration will be compatible with the design response spectrum as specified by the seismic design code.

The transition matrix for the scattering of elastic waves in a half‐space

Abstract In this paper, the transition matrix for elastic waves scattering from an alluvium on an elastic half‐space is developed. Bettis third identity is employed to establish orthogonality conditions among basis functions that are Lambs singular wave functions. The total displacements and associated tractions for both the surrounding half‐space and alluvium are expanded in a Rayleigh series. After the boundary conditions are applied, the T‐matrix can be obtained. Explicit forms of the basis functions are derived for the two‐dimensional anti‐plane and in‐plane problems. The linear transformation is utilized to construct a set of orthogonal basis functions. The transformed T‐matrix is related to the scattering matrix and it is shown that the scattering matrix is symmetric and unitary and that the T‐matrix is also symmetric. Some typical scattering cases induced by incident plane waves are illustrated for verification.

Experimental study for seismic demands on in-cabinet equipment in NPPs

This study aims to establish ordinary seismic demands imposed on safety-related electrical instruments mounted in electrical cabinets (referred to as in-cabinet equipment) in nuclear power plants and to understand the basic characteristics of the dynamic responses for electrical cabinets. An existing cabinet used in the Maanshan nuclear power plant was adopted as a specimen for a shaking table test. The vibration responses and spectral amplification factors of the tested cabinet under different shaking intensity levels, as well as the variation of total mass and anchorage locations of additional weights attached to the cabinet, are studied and summarized in this study. The results of this preliminary study can contribute to developing a complete test plan for motor control centers in the Lungmen nuclear power plant in future studies.

Resonance analysis of a 2D alluvial valley subjected to seismic waves

The T-matrix formalism and an ultrasonic experiment are developed to study the scattering of in-plane waves for an alluvial valley embedded in a two-dimensional half-space. The solution of the in-plane scattering problem can be determined by the T-matrix method, where the basis functions are defined by the singular solutions of Lambs problems with surface loading in both horizontal and vertical directions. In the experiment, a thin steel plate with a semicircular aluminum plate attached on the edge is used to simulate the two-dimensional alluvial valley in the state of plane stress. Based on the spectra of displacement signals measured at the free edge of the scatterer, the resonance frequencies where the peaks appear can be identified. It can be shown that the nondimensional resonance frequency is one of the characteristic properties of the scattering system. Furthermore, it is noted that the nondimensional resonance frequencies measured experimentally are in good agreement with those calculated theoretically.

A simplified method for the evaluation of seismic demands on in-cabinet equipment in motor control center type cabinets in nuclear power plants

Abstract For numerical analysis or shaking table tests of in-cabinet equipment, in-cabinet response spectrum (ICRS) is adopted as required seismic demand. Several kinds of simplified concepts are commonly used to obtain ICRS. One of the concepts is using floor response spectrum multiplied by the in-cabinet amplification factor (AF). Other concepts generate ICRSs from FRSs (floor response spectra) by frequency or time domain analysis. However, the obtained ICRS through these methods are usually too conservative or complicated. This paper aims to provide a convenient but more precise method to evaluate ICRS for in-cabinet equipment in motor control center (MCC) type cabinets. Shaking table tests were conducted to investigate the dynamic characteristics of the MCC type cabinet. Based on the observation of the test results, global modes of the cabinet were simulated by a lumped-mass model, and local modes were simulated by a detailed finite element model of the plate at which in-cabinet equipment is anchored. The accuracy of the proposed numerical method was verified by comparing results with those from tests and other analysis methods.

Numerical Analysis of a RHR Piping System Subjected to Seismic Loading

The objective of this study is to build a credible numerical model using SAP2000 for fragility analysis of RHR piping system, and to establish the load pattern of a cyclic loading test to identify the seismic vulnerability of the system. The RHR piping system selected for test and numerical analysis is duplicated from a part of the RHR system in a nuclear power plant (NPP) in Taiwan, and this part is distributed between the floor of the reactor building (RB) to the wall of the reinforced concrete containment vessel (RCCV) of the sample NPP.The numerical model for the sample RHR piping system was developed, and the nonlinear response-history analysis was conducted using input motions compatible with the floor response spectrum at the anchor points of the sample piping system subjected to Safe Shutdown Earthquake (SSE). Based on the distribution of resultant inertial forces and the responses at critical locations of the piping system under SSE, the magnitudes and locations of the equivalent concentrated static loads were determined and used in the pushover analysis to estimate the capacity of the RHR piping system. The numerical results will be verified by the aforementioned cyclic loading test. More studies are on the way including shaking table test and fragility analysis for the sample piping system to further identify the seismic performance and risk of the system.Copyright

Experimental Study on Seismic Behavior of a Typical Sprinkler Piping System in Hospitals

Based on the immediate needs of emergency medical services provided by hospitals after strong earthquakes, this paper is to introduce a research program on assessment and improvement strategies for typical configuration of sprinkler piping systems in hospitals. The study involved component tests and subsystem tests. Cyclic loading tests were conducted to investigate inelastic behavior of components including concrete anchorage, screwed fittings of small bore piping and mechanical couplings. Parts of horizontal piping systems of the aforementioned seismic damaged sprinkler piping system were tested using shaking table tests. Furthermore, the horizontal piping subsystems with seismic resistant devices such as braces, flexible pipes and mechanical couplings were tested.The test results show that the main cause of the damaged case is the poor shear capacity of the screwed fitting of the small-bore tee branch. The optimum improvement strategy to achieve higher nonstructural performance level for the horizontal piping subsystem is to strengthen the main pipe with braces and to decrease shear demands on the tee branch by flexible pipes. The hysteresis loops and failure modes of components were further discussed and will be used to conduct numerical analysis of sprinkler piping systems in future studies.Copyright

Cyclic Loading Test and Numerical Analysis of Flange Joint and Reducer of RHR Piping Systems

According to the seismic risk assessment results presented in the Final Safety Analysis Report (FSAR) for a nuclear power plant in Taiwan, the failure of Residual Heat Removal (RHR) piping system occurs in both of the two accident sequences with the highest contributions for core damage. The seismic performance of RHR piping system depends on the capacity of its components, such as supports, flanged joints and reducers. For the need of seismic response-history analysis of RHR piping systems, we developed detailed numerical models of flanged joint and reducer using finite element analysis software (ABAQUS and SAP2000). The proposed finite element models were verified by the experimental results. The pure bending tests with four-point cyclic loading were conducted for sample flanged joint and reducer to investigate their mechanical behaviors. The displacement and rotation responses identified from the tests are in good agreement with the results of numerical analysis. A preliminary simplified model of flanged joints was also proposed in this study to improve the efficiency of numerical analysis for RHR piping system.Copyright