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Benchmarking study of software for one-dimensional, nonlinear seismic site response analysis with pore water pressure generation

Description:

A survey of State Departments of Transportation (DOTs) and consultants performed for NCHRP Synthesis 428 – Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions - revealed that one-dimensional (1-D) equivalent–linear analysis is the "de facto"standard for site response analysis of State DOT highway facilities at those locations where site-specific ground response analyses are conducted in accordance with provisions in the 2014 AASHTO "Load and Resistance Factor Design (LRFD) Bridge Design Specifications" and the 2011 American Association of State Highway and Transportation Officials (AASHTO) "Design Guidelines for Seismic Bridge Design". However, users have concerns about the applicability of equivalent-linear analyses for the cases for which site-specific response analyses are most useful; i.e., soft soil sites, liquefiable sites, and sites subjected to very strong shaking. While nonlinear 1-D site response analyses are beginning to be used in practice to address these concerns, considerable uncertainty exists with respect to the appropriate manner in which to employ and interpret such analyses.

The primary concern over the applicability of equivalent linear site response analyses is due to the pore water pressure generation and dissipation that accompanies cyclic loading of saturated soils. If the generated pore water pressures are sufficiently large, soil stiffness and strength are significantly reduced. Ultimately, in some soils, liquefaction can occur due to this pore pressure generation. These phenomena are not captured by equivalent linear analysis. In a nonlinear site response analysis with pore water pressure generation and dissipation, response of soil to cyclic loading accounts for generation of excess pore water pressure during cyclic shearing of the soil as well as dissipation of these excess pore water pressures during and after the cyclic loading. The generation, dissipation, and redistribution of pore water pressure influences the soil stiffness (modulus) and strength (shear stress) during shaking, resulting in a more realistic simulation of site response if these effects are captured properly.

Consequently, the use of the potentially more realistic nonlinear models for 1-D seismic site response analysis that incorporate pore pressure generation and dissipation is becoming more common in DOT practice, especially among their consultants. However, DOT adoption of these nonlinear analyses is restrained by uncertainty as to how to develop the input parameters required for the nonlinear models and by the lack of well-documented validation studies for these models. A number of benchmarking exercises have been conducted to evaluate the accuracy of equivalent linear site response analysis and design guidance is available on their use. Furthermore, Kwok et al. (2007) describe results of a benchmarking study on nonlinear site response analyses (i.e., nonlinear analyses that do not account for pore pressure generation). However, no comprehensive benchmarking study has been performed for nonlinear stress analyses with pore pressure generation, and no design guidance on the use of these types of analysis is available.

Noting the absence of such a study, National Cooperative Highway Research Program (NCHRP) Synthesis 428: "Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions" concluded that “Given the value and increasing use of nonlinear effective-stress analysis for site class E (soft soils) and Site Class F (liquefiable soils and very soft clays in the profile), a rigorous benchmarking study of 1-D nonlinear software with pore water pressure generation should be conducted.” The results of this work will be useful for the effective and economical seismic design of all types of highway structures by bridge and foundation designers throughout the country.

Objective:

The primary objective of this research project is to (1) develop a better understanding of the input parameters required for 1-D nonlinear site response analysis, with pore water pressure generation; (2) perform rigorous benchmarking of available software that validates those models; and (3) provide guidance to designers on the use of these models. This will facilitate the design of safer and more economical bridges because State DOTs would now be able to (1) effectively address the mandate in AASHTO specifications for site-specific evaluation of earthquake design ground motion (i.e., the acceleration response spectrum) for ground conditions termed Site Class F; (2) evaluate more accurately the response of other sites where nonlinear soil response is important (i.e., site Class E sites and sites with very high design ground motions); and (3) take advantage of a reduction in mapped design ground motions of as much as 33% in cases where pore pressure generation could lead to a reduction in design ground motions. The final product will be a comprehensive report that summarizes the validation studies and provides practical guidance on the mechanics and steps required for developing design ground motions, characterizing the site, evaluating the necessary soil properties, and performing an appropriate nonlinear effective stress site response analysis that includes pore pressure generation and dissipation.

This research will focus on 1-D nonlinear software with pore water pressure generation and dissipation for the following reasons:

· The current practice of using one dimensional equivalent-linear modeling does not effectively account for pore pressure generation or its effect on site response, and this deficiency leads to unreliable ground response predictions. Furthermore, while the one dimensional equivalent linear method is not recommended when the levels of shaking-induced shear strains are “high,” there is no consensus on the limiting (“high”) shear strain level for use of this approach.

· The alternative of using 1-D stress nonlinear site response analysis also neglects the explicit interaction of pore fluid with the soil matrix. While total stress analysis is an acceptable simplification under many conditions and is numerically efficient, it is not realistic for conditions where the soil is saturated and pore pressure development is of concern.

· While multi-dimensional (2-D and 3-D) nonlinear total and effective-stress analyses also lack practical guidance, their use is relatively limited. Therefore, the need for developing guidance for these types of analysis is not as urgent. Furthermore, the proper use and limitations of 1-D nonlinear effective stress analyses need to be understood before moving on to multi-dimensional analyses.

· Owners who wish to take advantage of the benefits of a 1-D nonlinear effective stress site response analysis need guidance to be able to specify a scope that can be effectively performed to produce a useful result.

Benefits:

Seismic design is an important consideration with respect to the resiliency and sustainability of transportation infrastructure in many areas of the US. Besides the potential loss of life and structural damage that can directly result from inadequate seismic design, the indirect costs associated with damage due to inadequate seismic design can be a significant source of earthquake-induced losses. In fact, these indirect costs, including loss of access during response and recovery activities, disruption of business activities, environmental impacts due to increased travel time, and sociological and psychological impacts, can outweigh costs associated with direct impacts in a major earthquake. Conversely, overly conservative seismic design may not only waste limited construction funds but may have adverse effects by increasing seismic loads to levels exceeding the design levels and shifting load paths to unanticipated elements within the structure.

AASHTO specifications for seismic design, including both the AASHTO LRFD Bridge Design Specifications and the Guide Specifications for LRFD Seismic Bridge Design, mandate site-specific evaluation of earthquake design ground motions (i.e., the acceleration response spectrum) for ground conditions termed Site Class F. In the AASHTO specifications, Site Class F soils are soft clay sites. These AASHTO specifications also allow discretionary use of site-specific analyses for other ground conditions and a reduction in design ground motions from those in the AASHTO seismic hazard maps by as much as 33% if justified by a site-specific ground motion analysis. Some State DOTs are taking advantage of this site response reduction provision, particularly in cases where pore pressure generation effects are significant, including cases where this could lead to liquefaction. Furthermore, there is some evidence that the AASHTO site factors used to adjust mapped values of design ground motions for local ground conditions may be inappropriate under some conditions. For example, they may not be appropriate for short period structures (fundamental period of the structure, To _< 0.5 sec) at shallow bedrock sites (i.e., depth to bedrock less than 150 feet), and for structures with a relatively long predominant period (To_ > 1.0 sec) at deep soil basin sites [e.g., depth to bedrock greater than 500 feet]. Site-specific analyses are also being used in these circumstances as an alternative to the use of AASHTO site factors.

For years, the equivalent-linear stress approach, as programmed in 1-D site response analysis codes, has been the primary method used to evaluate the influence of local ground conditions on earthquake design ground motions on a site-specific basis. However, this type of analysis has limitations: (1) at sites where strong shaking results in large shear strain response, leading to nonlinear site response effects; (2) at sites where there is a potential for significant seismically induced pore water pressure buildup, including soil liquefaction, because equivalent-linear analysis cannot consider the effects of pore pressure generation; and (3) at soft clay sites subject to moderate intensity/long-duration motions, as equivalent linear analysis cannot consider the effects of cyclic degradation or he non-linearity of soft soil response under even moderate intensity loading.

A number of nonlinear site response analysis methods have become available over the past decade and are now being used in practice, including methods that can account for shallow bedrock site response, deep soil basin effects, soft soil site response, and pore water pressure generation. Significant expertise is required to conduct and interpret the results from these newer methods, often leading to questions about the validity of results. For instance, experience with the newer nonlinear analysis methods show that strains (and hence stiffness reduction) may become more localized than in an equivalent-linear total stress analysis. As a result, details of the soil profile, particularly soft layers and impedance contrasts, can have a larger effect on the results of a nonlinear analysis than they do on the results of an equivalent-linear analysis.

Furthermore, available methods for nonlinear site response analysis (with and without pore pressure generation) require significant expertise and numerous discretionary decisions. For example, the analysis requires selection of an appropriate suite of time histories and a determination as to whether the small strain modulus and other soil properties should be measured in the field and/or laboratory or obtained using correlations. These analyses also require decisions on the extent of sensitivity analyses that should be employed. Much more expertise and discretionary decision making is required with nonlinear site response methods than with conventional equivalent-linear analysis and the need for expertise and guidance is greatest with analyses that consider pore pressure generation and dissipation.

Commentary within the AASHTO specifications cautions the reader of potential issues when conducting site-specific ground motion studies, but the commentary does not provide guidance on the nature of these issues or on how or when to consider these potential issues. This lack of guidance raises concerns as to whether estimates of site-specific ground motion analysis results in excessive project construction costs when ground motion response is overestimated or unacceptable risk to the public when ground motion response is underestimated.

Seismic design presents a rather unique challenge to the engineer in that sometimes conventional “conservative overdesign” in the face of uncertainty can lead to unanticipated damage due to shifting load paths and changes in structure response characteristics. Therefore, the designer cannot arbitrarily increase forces or stiffen structural elements to accommodate uncertainty. Instead, safe and economical seismic design requires accurate estimates of both the seismic loads and of the anticipated seismic response of structural elements to these loads.

Given the increasing use of the more realistic nonlinear models for 1-D seismic site response analyses by State DOTs, especially among their consultants; and the importance of the use of nonlinear effective-stress analysis for site class E (soft soils) and Site Class F (liquefiable soils and very soft clays in the profile), and a rigorous benchmarking study of 1-D nonlinear software with pore water pressure generation and dissipation is warranted and should be conducted.

The objective of this research is consistent with the AASHTO Highway Subcommittee’s Strategic Plan on Bridges and Structures, which calls for addressing the grand challenge of advancing the AASHTO specifications (Grand Challenge 4). The goals of Challenge 4 are to (1) provide clear, concise technical guidance to the practicing bridge engineer, (2) to understand the limit states required for safe, serviceable and economical bridges and highway structures, and (3) to develop enhanced reliability-based design and evaluation provisions addressing these limit states in a clear and concise manner relatively consistent with traditional highway-bridge practice and effort while incorporating new or enhanced construction materials and processes. More specifically, the need for further work, clarification and incorporation of contemporary seismic design provisions into LRFD was identified as an area of technical importance; and completion and adoption of state-of-the-art seismic design provisions was identified as an important activity/ research need under Grand Challenge 4.

Furthermore, the NCHRP 428 synthesis literature review and survey (consisting primarily of State DOTs (included the AASHTO Highway Subcommittee on Bridges and Structures, Technical Committee T-3 States, their consultants, and select academic researchers) identified the need for benchmarking and guidance on 1-D nonlinear software with pore water pressure generation as the top research need to improve practices and procedures for site-specific evaluations of earthquake ground motions. Therefore, this research project may be considered a continuation of the NCHRP 428 as it is designed to address significant unresolved issues concerning site specific ground motion analysis; and the need for guidance identified by the FHWA, based on technical assistance requests from some State DOTs.

The end results of this work will be useful for the effective and economical seismic design of all types of highway structures by bridge and foundation designers throughout the country. This result would be accomplished through appropriate modifications to the AASHTO_ LRFD Bridge Design Specifications_ and the AASHTO_ Guide Specifications for LRFD Seismic Bridge Design_. These modifications will require approval by the appropriate AASHTO Committees (e.g. T-3, T-5 and T-15).

Related Research:

Some limited information on nonlinear site response analysis with pore water pressure generation and dissipation can be found in the following publications:

· AASHTO Guide Specifications for LRFD Seismic Bridge Design, 2nd Ed.

· AASHTO LRFD Bridge Design Specifications, 7th Ed.

· FHWA-NHI-11-032: LRFD Seismic Analysis and Design of Transportation Geotechnical Features and Structural Foundations

· NCHRP Synthesis 428: Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions

· NASCE 7-10, ASCE 7-15 (pending), ASCE 4, ASCE 43-05

· NRC RG 1.208: A Performance-Based Approach to Define the Site-Specific Earthquake Ground Motion

· S.L. Kramer (1996). Geotechnical Earthquake Engineering. Prentice Hall.

· Anderson, D.G., Shin, S., and Kramer, S.L. (2011) “Observations from Nonlinear, Effective-Stress Ground Motion Response Analyses following the AASHTO Guide Specifications for LRFD Seismic Bridge Design,” 90th Annual Meeting of the Transportation Research Board.

Additional information may be found in design guidance prepared by several state departments of transportation bridge sections. Most of the State DOTs documents follow, in some way, the general guidelines for conducting a site response analysis outlined in AASHTO documents. Some documents discuss the use of equivalent-linear analysis while others discuss the use of nonlinear site response analyses with and without pore water pressure generation. However, with the exception of NRC RG 1.208: A Performance–Based Approach to Define the Site-Specific Earthquake Ground Motion, these documents do not provide sufficient guidance on the mechanics and steps required for developing design ground motions, characterizing the site, evaluating the parameters that describe the nonlinear stress-strain-pore pressure response of the site soils, or on performing and interpreting the site response analysis. Furthermore, little to no guidance is provided on the limitations of these types of analyses.

The above documents summarize the state of practice for nonlinear site response analysis with pore water pressure generation in the United States. However, additional information may be available from international sources and from research reports and papers published in refereed journals. In particular, papers and reports on well-documented physical model tests and case histories of sites subject to seismic loading may yield additional useful information. Therefore, a comprehensive search and synthesis of the available information on this topic will be conducted as a first step in the proposed research.

Tasks:

The tasks necessary to accomplish this research objective include:

· Task 1: Identify, Collect, and Synthesize Available Data. This task will include identifying existing computer programs to be considered in benchmark study; collecting existing data and guidance for those programs; collecting site response and pore pressure generation that can be used for benchmarking purposes, and synthesizing the information to identify gaps in data required to perform the benchmarking study. The data collected will include input data for the computer programs (e.g., input motions, soil profile, and soil properties); field data on pore pressure generation in earthquakes and under simulated earthquake loading conditions (e.g., blast loading, vibroseis loading); centrifuge test results; and previous numerical analyses from the US and abroad.

· Task 2: Develop Work Plan. A work plan will be developed based on the results from Task 1. Work plan elements will include field and model testing to supplement the data collected in Task 1 for the benchmark analyses (Task 3), rigorous benchmark studies (Task 4), and a report that will provide guidance for the validated computer software programs (Task 5).

· Task 3: Develop Supplemental Data for Benchmark Analyses. This task will include performing field, shaking table, and/or centrifuge tests to supplement existing benchmarking data.

· Task 4: Conduct Benchmark Analyses. In this task, analyses will be conducted using the computer software identified in Task 1 to perform 1-D nonlinear site response analysis, with pore water generation. The results of the analyses will be compared to the data collected in Task 1 and Task 3 to identify best practices for and limitations of 1-D nonlinear effective stress site response analysis.

· Task 5: Prepare Final Report. The final product will be a comprehensive report that summarizes the benchmarking studies and provides guidance on the use of computer software for 1-D nonlinear site response analysis with pore water generation; and draft revised LRFD guide specifications with commentary that will serve as a basis for ballot item development.

Each of the five tasks is outlined in more detail below:

  1. will provide the baseline for work plan development.

    • Available information on software for 1-D nonlinear seismic site response with pore water pressure generation, the reliability and limitations of these models, and recommendations for developing the input for this type of analysis from around the world will be collected and synthesized to establish the current state of knowledge and practice.

    • Information on program input will include recommendations for establishing input ground motions (including the number of time histories to be employed in an analysis), for developing the site profile (e.g., the number of borings or soundings), and for establishing the required soil properties (e.g., availability of empirical correlations, types and numbers of laboratory tests, interpretation of lab and field data).

    • Available data on pore pressure generation under real and simulated earthquake loads, including ground motion data (from the surface and below ground) and pore pressure records captured in the field during earthquakes, from blast loading, and from tests employing large mechanical vibrators) and from shaking table and centrifuge model tests will also be collected as part of this task,

    • The deliverable from Task 1 will be a report of findings summarizing the available information on 1-D nonlinear software, with pore water pressure generation, recommendations for and limitations on the use of the applicable computer software, available data for benchmarking these analyses, and gaps in knowledge about software use and in the data available for benchmarking the software.. Software for nonlinear site response with pore water generation that will be considered in this task include DMOD2000, CyberQuake, DEEPSOIL, 1-D FLAC, and 1-D Plaxis. The report should also address the limitations of equivalent linear analyses and non-linear total stress analyses at soft soil sites subject to strong ground motions.

  2. Task will provide a detailed plan for the supplemental data acquisition, benchmarking the available codes for 1-D nonlinear site response analyses with pore pressure generation, and development of draft specifications and commentary.

    • Supplemental data acquisition (Task 3) may include centrifuge testing, large scale shaking table tests, and field tests involving blast loading and large mechanical vibrators. The supplemental data should complement available data such that that the combined data sets encompass the range of representative soft soil liquefiable sites likely to be encountered in US design practice.

    • The complete data set should then be used to validate/benchmark the computer software programs identified in Task 1 for further consideration (Task 4). The results of the benchmark analyses will be used to develop guidance on their proper use and limitations for these types of analyses and to develop draft specifications and commentary (Task 5).

    • This work plan will be submitted as a draft for review and comment by the Review Panel and then for final approval following response to Review Panel comments.

  3. T will collect data to fill in the data gaps identified in Task 1 such that data sets representative of soft ground conditions (Site Class E and F soils) in areas expected to be subject to high levels of strong shaking and site susceptible to significant pore pressure generation are available for the benchmark studies.

    • Supplemental data development may include performing centrifuge tests or shaking table tests or field testing using blast loading or using the large mechanical vibrators developed for the National Science Foundation Network for Earthquake Engineering Simulation (NEES) program.

    • The testing will focus on simple but representative configurations with the expectation that more complex configurations will be addressed using the software programs calibrated on the centrifugal test results.

    • The deliverable for this task will be a technical report summarizing the supplemental data collection.

  4. Ta will include performing 1-D nonlinear site response analyses, with pore water generation, using the identified computer software packages to analyze the site response and pore pressure generation data collected in Task 1 and Task 3.

    • The analyses will include parametric studies to evaluate sensitivity of results to the soil profile and input motions and soil properties and to identify limitations on the use of these programs, such as in situations where the soil is expected to exhibit dilatancy.

    • The deliverable for this task will be a technical report summarizing the numerical analyses and findings from this task.

  5. will provide guidance on when an effective-stress nonlinear site response analysis is warranted, on the strengths and limitations of available software for such analyses, on selection of appropriate input ground motions, on development of the site profile (e.g., soil properties, minimum depth), analysis procedures, and interpretation of results (e.g., criterion for liquefaction).

    • The report will include draft LRFD guide specifications, with commentary, that will serve as a basis for ballot item development and consideration by AASHTO Committees T-3 and T-15.

    • This task includes submission of both draft and final reports.

Sponsoring Committee:AKB50, Seismic Design and Performance of Bridges
RNS Developer:Edward Kavazanjian, Jr., PE, PhD, D.GE, NAE. Associate Professor of Civil Engineering, Arizona State University, Tempe, AZ, 85287-3005; Tel: 480-727-8566; Email: edkavy@asu.edu Donald Anderson, PE, PhD, D,GE, Principal Geotechnical Engineer, CH2M Hill, 1100 112th Avenue NE, Suite 400 Bellevue, WA 98004-4504; Email: Donald.Anderson@CH2M.com
Date Posted:03/16/2016
Date Modified:03/25/2016
Index Terms:Seismicity, Earthquake resistant design, Highway bridges, Pore water pressures, Benchmarks, Software, State departments of transportation, Nonlinear programming,
Cosponsoring Committees: 
Subjects    
Highways
Design
Geotechnology
Bridges and other structures

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