Jonathan D. Bray

Ground motion evaluation procedures for performance-based design

The objective of performance-based earthquake engineering (PBEE) is the analysis of performance objectives with a specified annual probability of exceedance. Increasingly undesirable performance is caused by increasing levels of strong ground motion having decreasing annual probabilities of exceedance. Accordingly, the evaluation of ground motion intensity measures is a vital component of PBEE. This paper provides a brief synthesis of ground motion analysis procedures within a performance-based design framework, and is a summary of a recent report to which the reader is referred for details. The principal topics addressed are probabilistic characterizations of source, path, and site effects, with the discussion of these effects focusing principally on applications in active regions such as California.

Selecting a suitable time step for discrete element simulations that use the central difference time integration scheme

The distinct element method as proposed by Cundall and Strack uses the computationally efficient, explicit, central difference time integration scheme. A limitation of this scheme is that it is only conditionally stable, so small time steps must be used. Some researchers have proposed using an implicit time integration scheme to avoid the stability issues arising from the explicit time integrator typically used in these simulations. However, these schemes are computationally expensive and can require a significant number of iterations to form the stiffness matrix that is compatible with the contact state at the end of each time step. In this paper, a new, simple approach for calculating the critical time increment in explicit discrete element simulations is proposed. Using this approach, it is shown that the critical time increment is a function of the current contact conditions. Considering both two‐ and three‐dimensional scenarios, the proposed refined estimates of the critical time step indicate that the earlier recommendations contained in the literature can be unconservative, in that they often overestimate the actual critical time step. A three‐dimensional simulation of a problem with a known analytical solution illustrates the potential for erroneous results to be obtained from discrete element simulations, if the time‐increment exceeds the critical time step for stable analysis.

Empirical Relationships for Frequency Content Parameters of Earthquake Ground Motions

The frequency content of an earthquake ground motion is important because it affects the dynamic response of earth and structural systems. Four scalar parameters that characterize the frequency content of strong ground motions are (1) the mean period (Tm), (2) the average spectral period (Tavg), (3) the smoothed spectral predominant period (To), and (4) the predominant spectral period (Tp). Tm and Tavg distinguish the low frequency content of ground motions, while To is affected most by the high frequency content. Tp does not adequately describe the frequency content of a strong ground motion and is not recommended. Empirical relationships are developed that predict three parameters (Tm, Tavg, and To) as a function of earthquake magnitude, site-to-source distance, site conditions, and rupture directivity. The relationships are developed from a large strong-motion database that includes recorded motions from the recent earthquakes in Turkey and Taiwan. The new relationships update those previously developed by the authors and others. The results indicate that three site classes, which distinguish between rock, shallow soil, and deep soil, provide a better prediction of the frequency content parameters and smaller standard error terms than conventional “rock” and “soil” site classes. Forward directivity significantly increases the frequency content parameters, particularly Tm and To, at distances less than 20 km. Each of the frequency content parameters can be predicted with reasonable accuracy, but Tm is the preferred because it best distinguishes the frequency content of strong ground motions.

An Empirical Geotechnical Seismic Site Response Procedure

Abstract A simplified empirically based seismic site response evaluation procedure that includes measures of the dynamic stiffness of the surficial materials and the depth to bedrock as primary parameters is introduced. This geotechnical site classification scheme provides an alternative to geologic‐based and shear wave velocity‐based site classification schemes. The proposed scheme is used to analyze the ground motion data from the 1989 Loma Prieta and 1994 Northridge earthquakes. Period‐dependent and intensity‐dependent spectral acceleration amplification factors for different site conditions are presented. The proposed scheme results in a significant reduction in standard error when compared with a simpler “rock vs. soil” classification system. Moreover, results show that sites previously grouped as “rock” should be subdivided as competent rock sites and weathered soft rock/shallow stiff soil sites to reduce uncertainty in defining site‐dependent ground motions. Results also show that soil depth is an ...

Mechanisms of Seismically Induced Settlement of Buildings with Shallow Foundations on Liquefiable Soil

Seismically induced settlement of buildings with shallow foundations on liquefiable soils has resulted in significant damage in recent earthquakes. Engineers still largely estimate seismic building settlement using procedures developed to calculate postliquefaction reconsolidation settlement in the free-field. A series of centrifuge experiments involving buildings situated atop a layered soil deposit have been performed to identify the mechanisms involved in liquefaction-induced building settlement. Previous studies of this problem have identified important factors including shaking intensity, the liquefiable soils relative density and thickness, and the buildings weight and width. Centrifuge test results indicate that building settlement is not proportional to the thickness of the liquefiable layer and that most of this settlement occurs during earthquake strong shaking. Building-induced shear deformations combined with localized volumetric strains during partially drained cyclic loading are the dominant mechanisms. The development of high excess pore pressures, localized drainage in response to the high transient hydraulic gradients, and earthquake-induced ratcheting of the buildings into the softened soil are important effects that should be captured in design procedures that estimate liquefaction-induced building settlement.

Centrifuge Testing to Evaluate and Mitigate Liquefaction-Induced Building Settlement Mechanisms

The effective application of liquefaction mitigation techniques requires an improved understanding of the development and consequences of liquefaction. Centrifuge experiments were performed to study the dominant mechanisms of seismically induced settle- ment of buildings with rigid mat foundations on thin deposits of liquefiable sand. The relative importance of key settlement mechanisms was evaluated by using mitigation techniques to minimize some of their respective contributions. The relative importance of settlement mechanisms was shown to depend on the characteristics of the earthquake motion, liquefiable soil, and building. The initiation, rate, and amount of liquefaction-induced building settlement depended greatly on the rate of ground shaking. Engineering design procedures should incorporate this important feature of earthquake shaking, which may be represented by the time rate of Arias intensity i.e., the shaking intensity rate. In these experiments, installation of an independent, in-ground, perimetrical, stiff structural wall minimized deviatoric soil deformations under the building and reduced total building settlements by approximately 50%. Use of a flexible impermeable barrier that inhibited horizontal water flow without preventing shear deformation also reduced permanent building settlements but less significantly.

Correlation between ground failure and soil conditions in Adapazari, Turkey

Ground failure in Adapazari, Turkey during the 1999 Kocaeli earthquake (Mw ¼ 7.4) was severe. In four central downtown districts, where more than 1200 buildings collapsed or were heavily damaged, hundreds of structures tilted and penetrated into the ground due in part to liquefaction and ground softening. Based on a multi-institutional subsurface investigation program, soil conditions along four lines in which ground failure was surveyed after the earthquake are classified into four generalized subsurface site categories. This classification is primarily based on the presence or absence of shallow and intermediate depth liquefiable soils. Observations of ground failure are found to correlate well with site categories that are susceptible to liquefaction according to current state-of-the-art methods without strict adherence to the Chinese criteria. Soils that liquefied were found to meet the liquid limit and liquidity index conditions of the Chinese criteria. However, soils that liquefied did not typically meet the clay-size condition for liquefiable soils by the Chinese criteria. q 2002 Published by Elsevier Science Ltd.

Shear modulus and material damping of municipal solid waste based on large-scale cyclic triaxial testing

Representative dynamic properties of municipal solid waste (MSW) are required to perform reliable seismic analyses of MSW landfills. A comprehensive large-scale cyclic triaxial laboratory testing program was performed on MSW retrieved from a landfill in the San Francisco Bay area to evaluate the small-strain shear modulus, and strain-dependent normalized shear modulus reduction and material damping ratio relationships of MSW. The effects of waste composi- tion, confining stress, unit weight, time under confinement, and loading frequency on these dynamic properties were evaluated. The small-strain shear modulus depends primarily on waste composition, confining stress, unit weight, and time under confinement. The normalized shear modulus reduction and material damping curves for MSW depend on waste composition and confining stress. Based on the results of this study and a review of literature, strain-dependent shear modulus reduction and material damping relationships are recommended for use in landfill design.

Large-scale direct shear testing of municipal solid waste

Large direct shear testing (300 mm x 300 mm box) of municipal solid waste (MSW) collected from a landfill located in the San Francisco Bay area was performed to gain insight on the shear response of MSW. The study investigated the effects of waste composition, confining stress, unit weight, and loading rate on the stress-displacement response and shear strength of MSW. The amount and orientation of the fibrous waste materials in the MSW were found to play a critical role. The fibrous material had little effect on the MSWs strength when it was oriented parallel to the shear surface, as is typically the case when waste material is compressed vertically and then tested in a direct shear apparatus. Tests in which the fibrous material was oriented perpendicular to the horizontal shear surface produced significantly stronger MSW specimens. The test results indicate that confining stress and loading rate are also important factors. Based on 109 large-scale direct shear tests, the shear strength of MSW at low moisture contents is best characterized by cohesion=15 kPa, friction angle=36 degrees at a normal stress of 1 atmosphere, and a decrease in the friction angle of 5 degrees for every log-cycle increase in normal stress.

Mobile Phones as Seismologic Sensors: Automating Data Extraction for the iShake System

There are a variety of approaches to seismic sensing, which range from collecting sparse measurements with high-fidelity seismic stations to non-quantitative, post-earthquake surveys. The sparse nature of the high-fidelity stations and the inaccuracy of the surveys create the need for a high-density, semi-quantitative approach to seismic sensing. To fill this void, the UC Berkeley iShake project designed a mobile client-backend server architecture that uses sensor-equipped mobile devices to measure earthquake ground shaking. iShake provides the general public with a service to more easily contribute more quantitatively significant data to earthquake research by automating the data collection and reporting mechanisms via the iShake mobile application. The devices act as distributed sensors that enable measurements to be taken and transmitted with a cellular network connection. Shaking table testing was used to assess the quality of the measurements obtained from the iPhones and iPods on a benchmark of 150 ground motions. Once triggered by a shaking event, the devices transmit sensor data to a backend server for further processing. After a seismic event is verified by high-fidelity stations, filtering algorithms are used to detect falling phones, as well as device-specific responses to the event. A method was developed to determine the absolute orientation of a device to estimate the direction of first motion of a seismic event. A “virtual earthquake” pilot test was conducted on the UC Berkeley campus to verify the operation of the iShake system. By designing and fully implementing a system architecture, developing signal processing techniques unique to mobile sensing, and conducting shaking table tests to confirm the validity of the sensing platform, the iShake project serves as foundational work for further studies in seismic sensing on mobile devices.