Howard Hwang

Probabilistic descriptions of resistance of safety-related structures in nuclear plants

Abstract Calculations of reliability of safety-related nuclear plant structures require a knowledge of the probability distributions that describe their resistance. This study considers the applicability of existing statistical data for describing the resistance of steel and reinforced concrete nuclear plant structures. Probability distributions are recommended which can be used in assessing the reliability of containments and other Category I structures, developing fragilities, and selecting appropriate resistance criteria for probability-based structural design.

Microtremor observations of deep sediment resonance in metropolitan Memphis, Tennessee

We measured site resonance periods throughout metropolitan Memphis, Tennessee. The site-resonance was derived from recordings of ambient ‘microtremor’ and is a direct measurement of seismic ground shaking. In the Memphis metropolitan area, the period, T0 (in s) is predicted by the empirical relation T0=−0.39899+7.8922×10−3H−3.0634×10−6H2, [350<H<1100 m], where H is the thickness (in m) of the unconsolidated sediments that overly hard Paleozoic rocks. Assuming that a quarter-wavelength relationship governs the relationship between resonant period, sediment thickness, and average shear wave speed, we estimate the average shear wave speed of unconsolidated sediments as a function of sediment thicknesses: Vs=521.15+0.37459H [350<H<1100 m]. The scatter in the observations, and discordance with earlier work suggests that caution is warranted in applying these values of wave speed indiscriminately to sites in the absence of further detailed work.

Attenuation relations of ground motion for rock and soil sites in eastern United States

This paper presents the attenuation relations of peak ground acceleration and spectral accelerations for rock and soil sites in the central and eastern United States (CEUS). For the bedrock site condition, 56 pairs of moment magnitude M and epicentral distance R are used to simulate ground motion, and for each pair of M and R, 550 samples of ground motion parameters are generated using a seismological model together with random vibration theory and distribution of extreme values. From the regression analyses of these data, the attenuation relations of ground motion parameters for the bedrock site are established. With the aid of appropriate site coefficients, these attenuation relations are modified for the site categories specified in the 1994 NEHRP Provisions. These attenuation relations are appropriate for the assessment of seismic hazards at far-field rock and soil sites in the CEUS.

Seismic response of natural gas and water pipelines in the Ji-Ji earthquake

In order to study the damage patterns of natural gas and water pipelines in the Ji-Ji earthquake, a GIS database and analysis procedures were established. Repair statistics was obtained from major natural gas companies and the Taiwan Water Supply Corporation (TWSC), and entered into the system. Then, repair rates (RR) were calculated. Previously, damage was analyzed without considering the corresponding pipeline material and diameters. In this study, new attempts were made to collect more data including those related to the composition of pipelines to provide a more detailed analysis of the relationship between earthquake forces and the resulting damage. Statistical analysis was also conducted to understand the correlation between RR and seismic parameters such as the peak ground acceleration, peak ground velocity, and spectrum intensity.

Generation of hazard-consistent fragility curves

Abstract This paper presents an analytical method for generating fragility curves of structures. In the proposed method, seismic sources, path attenuation, local soil conditions, and nonlinear building behavior are systematically considered. The uncertainties in the earthquake-site-structure system are quantified by considering the uncertainties in the seismic, site, and structural parameters that define the system. For an illustration, the proposed method is used to generate fragility curves and a damge probability matrix for Smith Hall on the main campus of The University of Memphis, Memphis, Tennessee.

Advances in Earthquake Loss Estimation and Application to Memphis, Tennessee

In recent years, a number of research efforts conducted through the National Center for Earthquake Engineering Research (NCEER) have focused on assessing seismic hazard and vulnerability in the Central United States. These multi-year, coordinated multi-investigator research efforts culminated in two loss estimation demonstration projects for Memphis (Shelby County), Tennessee, that evaluate losses associated with buildings and lifelines, respectively. While conducted independently, these two loss estimation studies share similar approaches, such as the emphasis on using detailed local data. Furthermore, the significance of the projects derives not only from the advances made by individual investigators, but also from the innovations developed in synthesizing the various studies into a coordinated loss estimation effort. This paper discusses the NCEER buildings and lifelines loss estimation projects with emphasis on methodological advances and insights from the loss estimation results.

Reliability assessment of reinforced concrete containment structures

Abstract A reliability analysis method for seismic category I structures subjected to various load combinations is developed and numerical examples are worked out under various assumptions and idealizations. The method falls generally within the so-called level III category within the framework of reliability analysis and design.

Probabilistic seismic analysis of a steel frame structure

Abstract In this study we present a probabilistic seismic analysis of a steel frame structure to generate fragility curves and to investigate the correlation between the response modification factor and the system ductility factor. As an example, we use a five-story hypothetical steel frame structure located in New York City and designed according to the provisions specified in the ANSI standard A58.1-1982 and the AISC ASD specification. We quantify uncertainties in seismic and structure parameters that define the earthquake-structure system by choosing several representative values for each parameter. Then, by using the Latin hypercube sampling technique, we combine these representative values to establish 18 samples of the earthquake-structure system for probabilistic seismic response analysis. The nonlinear seismic analysis is performed by using the DRAIN-2D computer program. For fragility analysis, we define five limit states representing nonstructural damage to collapse of structure and then establish the probabilistic structural capacity corresponding to each limit state. From the probabilistic structural response and capacity, the limit state probabilities at various levels of peak ground acceleration can be computed and used to construct the fragility curves. For the response modification factor R μ , we perform nonlinear and corresponding linear analyses of the 18 samples subject to earthquakes with various levels of peak ground acceleration. The correlation between the R μ , factor and the system ductility ratio is presented. Note that this correlation is based on one steel frame structure. Additional studies are needed to establish appropriate R μ factors for the design of steel structures.

Parametric study of site response analysis

Abstract Analytical methods for site response analysis include many parameters that could affect earthquake ground motions and corresponding response spectra. It is important to investigate the effect of these parameters on site response analysis in order to make confident evaluations of earthquake ground motons at site. This paper presents a parametric study to investigate the effects of site parameters on earthquake ground motions. The parametric study is performed on two hypothetical soil profiles that consist of sand and clay, respectively. The site parameters investigated are the secant shear modulus, low-strain damping ratio, types of sand and clay, location of water table, and depth of bedrock. From the results of the parametric studies, it appears that the secant shear modulus, depth of bedrock, and types of sand and clay have a significant effect on the results of site response analysis. Conversely, the low-strain damping ratio and variations of water tables have only a minor influence on site response analysis.

Seismic fragility analysis of electric substation equipment and structures

This paper presents a seismic fragility analysis of equipment and structures in an electric substation in the eastern United States, using Substation 21 in Memphis as an example. The fragility data of substation equipment and structures can be generated using actual earthquake damage data, experimental data, or analytical approaches. Seismic damage to electric facilities in the eastern United States is rare and information on dynamic testing of electric equipment similar to those installed in the substation is not available; thus, an analytical approach is used to perform the fragility analysis in this study. The fragility analysis results can reveal the expected performance of equipment and structures in the substation in the event of an earthquake. The fragility analysis results also provide the necessary data for the seismic performance evaluation of an entire electric substation and for a system reliability analysis of an electric system.