Benchmarking study of software for one-dimensional, nonlinear seismic site response analysis with pore water pressure generation
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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
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.
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).
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
· 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.
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.
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.
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 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
· 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
the five tasks is outlined in more detail below:
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
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.
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
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.
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
The deliverable for this task will be a technical report
summarizing the supplemental data collection.
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.
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
|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: firstname.lastname@example.org 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|
|Index Terms:||Seismicity, Earthquake resistant design, Highway bridges, Pore water pressures, Benchmarks, Software, State departments of transportation, Nonlinear programming, |
Bridges and other structures