Content uploaded by Tadahiro Kishida
Author content
All content in this area was uploaded by Tadahiro Kishida
Content may be subject to copyright.
A preview of the PDF is not available
Probabilistic Seismic Hazard Analysis for Maximum Seismic Shear Stresses in Soils Using Improved Ground-Motion Parameters
Abstract and Figures
Maximum seismic shear stresses (t max) have been recognized as one of the important parameters in design practice. This study develops ground-motion parameters for t max and implements these in probabilistic seismic hazard analysis to provide the t max distribution of deep soil layers for design purposes. The application of improved ground-motion parameters for t max is demonstrated at the Oakland International Airport, where a thick Young Bay Mud deposit exists under the artificial fill. Model biases in the predictive equations of seismic shear-stress reduction coefficients (r d) are evaluated by comparison with the site response analyses performed with a wide range of input ground motions. Based on these results, we introduce improved ground-motion parameters for t max (I tau) as a linear combination of spectral accelerations, implemented in probabilistic seismic hazard analysis to calculate seismic hazard curves. Conditional mean spectra are calculated, given I tau at 10% in 50 years to illustrate the variations in frequency contents with depth compared with the uniform hazard spectra. Finally, t max is calculated with depth by using hazard values of I tau and compared with the peak-ground-acceleration-based and uniform-hazard-spectra-based calculations. Analysis results show that t max will be underestimated for deep soil layers by peak-ground-acceleration-based calculation if the median value of r d is used in design practice.
Figures - uploaded by Tadahiro Kishida
Author content
All figure content in this area was uploaded by Tadahiro Kishida
Content may be subject to copyright.
... Kramer and Mayfield (2007) considered the combination of PGA and M w for liquefaction analysis because the seismic demand of the liquefaction potential depends on both parameters. Kishida and Tsai (2013) developed the ground-motion parameter for maximum seismic shear stress (t max ) by combining spectral acceleration (Sa) values (PGA, 0.2 and 1.0 s, respectively) and incorporated these values into PSHA. These studies demonstrate the importance of considering additional dependent parameters when estimating seismic demand for design purposes. ...
... K max distributions are calculated inFig. 17 by multiplying PGA with r d by Kishida et al. (2009a), where r d values are conditioned on UHS [a UHS-based approach as defined by Kishida and Tsai (2013)] and CMS given PGA [a PGA-based approach as defined by Kishida and Tsai (2013)] inFig. 15. ...
... K max distributions are calculated inFig. 17 by multiplying PGA with r d by Kishida et al. (2009a), where r d values are conditioned on UHS [a UHS-based approach as defined by Kishida and Tsai (2013)] and CMS given PGA [a PGA-based approach as defined by Kishida and Tsai (2013)] inFig. 15. ...
The conventional liquefaction potential assessment methods (also known as simplified methods) profoundly rely on empirical correlations based on observations from case histories. A probabilistic framework is developed to incorporate uncertainties in the earthquake ground motion prediction, the cyclic resistance prediction, and the cyclic demand prediction. The results of a probabilistic seismic hazard assessment, site response analyses, and liquefaction potential analyses are convolved to derive a relationship for the annual probability and return period of liquefaction. The random field spatial model is employed to quantify the spatial uncertainty associated with the in-situ measurements of geotechnical material.
Equivalent number of cycles (Ncyc) is a significant factor in assessing liquefaction potential during earthquakes. However, the effects of Ncyc are considered a deterministic value by using a magnitude scaling factor, and the uncertainties of these effects have not been included in the current design practice. This paper calculates Ncyc within the soil mass by deconvolution analysis as a function of the period of soil layer from the ground surface (Ts) and the soil property (b) using a wide range of acceleration time histories. The predictive model of Ncyc is developed as variable dependent on earthquake magnitude, peak ground acceleration (PGA), and ratio of spectral acceleration, Ts and b. Then, the Ncyc model is combined with the ground-motion prediction equation of PGA and the prediction equation of seismic shear-stress reduction coefficient (rd) to obtain the prediction equation of the seismic demand of the liquefaction potential (K1cyc). The standard deviation of K1cyc is also modeled on the basis of the aleatory variability of PGA, rd, and Ncyc with correlations between these residuals. This predictive model of K1cyc is implemented in probabilistic seismic hazard analysis (PSHA) to show the impact of these uncertainties on design practice.
The paper describes a performance-based approach to the evaluation of liquefaction potential, and shows how it can be used to account for the entire range of potential ground shaking. The result is a direct estimate of the return period of liquefaction, rather than a factor of safety or probability of liquefaction conditional upon ground shaking with some specified return period. As such, the performance-based approach can be considered to produce a more complete and consistent indication of the actual likelihood of liquefaction at a given location than conventional procedures. In this paper, the performance-based procedure is introduced and used to compare likelihoods of the initiation of liquefaction at identical sites located in areas of different seismicity. The results indicate that the likelihood of liquefaction depends on the position and slope of the peak acceleration hazard curve, and on the distribution of earthquake magnitudes contributing to the ground motion hazard. The results also show that the consistent use of conventional procedures for the evaluation of liquefaction potential produces inconsistent actual likelihoods of liquefaction.
The vertical ground motion component is disregarded in the design of ordinary highway bridges in California, except for the bridges located in high seismic zones (sites with design horizontal peak ground acceleration greater than 0.6 g). The influence of vertical ground motion on the seismic response of single-bent, two-span highway bridges designed according to Caltrans Seismic Design Code (SDC-2006) is evaluated. A probabilistic seismic hazard framework is used to address the probability of exceeding the elastic capacity for various structural parameters when the vertical component is included. Negative mid-span moment demand is found to be the structural parameter that is most sensitive to vertical accelerations. A series of hazard curves for negative mid-span moment are developed for a suite of sites in Northern California. The annual probability of exceeding the elastic capacity of the negative mid-span moment is as large as 0.01 for the sites close to active faults when the vertical component is included. Simplified approaches based on the distance to major faults or the median design peak acceleration show that there is a large chance (0.4 to 0.65) of exceeding the elastic limit if the current 0.6 g threshold is used for the consideration of vertical ground motions for ordinary highway bridges. The results of this study provide the technical basis for consideration of a revision of the 0.6 g threshold.
The seismic response of levees in the Sacramento-San Joaquin Delta, where the subsurface soils include thick deposits of highly organic soils, is evaluated. One-dimensional (1-D) and two-dimensional (2-D) equivalent-linear analyses were performed that accounted for variability in ground motions, dynamic properties, and soil profiles. Regression models were developed for: (1) the ratio of spectral accelerations at levee crests computed by 2-D versus 1-D response analyses, (2) stress reduction factors from 1-D site response analyses and seismic coefficient reduction factors for various failure surface depths from the 2-D response analyses, and (3) Newmark sliding block displacements computed for the input NEHRP site D ground motions and the computed seismic coefficient time series. The results of these regression models are compared to those obtained in previous studies involving different soil conditions, geometries, and motions. Newmark sliding block displacement hazard curves were calculated for a representative site in the Sacramento-San Joaquin Delta, and the contributions of various uncertainties to the displacement hazard curves are described.
Seismic site response and site effects models are presented for levees in the Sacramento-San Joaquin Delta where the subsurface soils include thick deposits of highly organic soils. Sources of uncertainty that contribute to the variation of seismic wave amplification are investigated, including variations in the input ground motions, soil profiles, and dynamic soil properties through Monte Carlo simulations of equivalent-linear site response analyses. Regression models for seismic wave amplification for levees in the Delta are presented that range from a function of peak outcrop acceleration alone to a vector of response spectra ordinates and soil profile parameters. The site effects models were incorporated into a probabilistic seismic hazard analysis for a representative location, and the relative impacts of the various models on the computed hazard are evaluated.
Probabilistic seismic hazard analysis (PSHA) is conducted because there is a perceived earthquake threat: active seismic sources in the region may produce a moderate-to-large earthquake. The analysis considers a multitude of earthquake occurrences and ground motions, and produces an integrated description of seismic hazard representing all events. For design, analysis, retrofit, or other seismic risk decisions a single “design earthquake” is often desired wherein the earthquake threat is characterized by a single magnitude, distance, and perhaps other parameters. This allows additional characteristics of the ground shaking to be modeled, such as duration, nonstationarity of motion, and critical pulses. This study describes a method wherein a design earthquake can be obtained that accurately represents the uniform hazard spectrum from a PSHA. There are two key steps in the derivation. First, the contribution to hazard by magnitude M, distance R, and ɛ must be maintained separately for each attenuation equation used in the analysis. Here, ɛ is the number of standard deviations that the target ground motion is above or below the median predicted motion for that equation. Second, the hazard for two natural frequencies (herein taken to be 10 and 1 Hz) must be examined by seismic source to see if one source dominates the hazard at both frequencies. This allows us to determine whether it is reasonable to represent the hazard with a single design earthquake, and if so to select the most-likely combination of M, R, and ɛ (herein called the “beta earthquake”) to accurately replicate the uniform hazard spectrum. This closes the loop between the original perception of the earthquake threat, the consideration of all possible seismic events that might contribute to that threat, and the representation of the threat with a single (or few) set of parameters for design or analysis.
Significant factors affecting the liquefaction (or cyclic mobility) potential of sands during earthquakes are identified, and a simplified procedure for evaluating liquefaction potential which will take these factors into account is presented. Available field data concerning the liquefaction or nonliquefaction behavior of sands during earthquakes is assembled and compared with evaluations of performance using the simplified procedure. It is suggested that even the limited available field data can provide a useful guide to the probable performance of other sand deposits, that the proposed method of presenting the data provides a useful framework for evaluating past experiences of sand liquefaction during earthquakes and that the simplified evaluation procedure provides a reasonably good means for extending previous field observations to new situations. When greater accuracy is justified, the simplified liquefaction evaluation procedure can readily be supplemented by test data on particular soils or by ground response analyses to provide more definitive evaluations.
Effects of irregular series of earthquake-induced cyclic stresses and the value of initial stress conditions at the time of consolidation on the liquefaction resistance of sand are discussed by examining the results of several series of cyclic triaxial tests that have been performed at the University of Tokyo. Relationships Tokyo, Relationships established in a graphical form between the applied shear stress and the residual pore water pressure, that is, the pore water pressure which remains after thorough application of irregular time histories of shear stress. A practical method is proposed to evaluate pore water pressures as well as factor of safety against liquefaction that can develop in a horizontal deposit during an earthquake. The method is applied to analyze the liquefaction of a sand deposit and a detailed investigation was carried out for cyclic strength of undisturbed samples.
The perimeter dike is located in seismically active region, the San Francisco Bay Area. The Hayward Fault is the closest active fault to the site, located 5.5 miles east of the site. Based on the findings from recent seismicity studies conducted by the Working Group on Bay Area earthquakes, it is likely that the perimeter dike will be subjected to large ground motions generated by future large seismic events on the active faults in the area. This paper summarizes the results of the vulnerability assessment of the perimeter dike for both the static and seismic loading conditions. The vulnerability assessment is based on seepage, slope stability, overtopping, and seismic analyses. The seismic deformation analysis was performed using a nonlinear finite difference computer program. The seismic analyses indicated that the dike model accurately predicts the deformations observed from the 1989 Loma Prieta earthquake. The same model was then used to predict the seismic deformations of the perimeter dikes for design ground motions.