r - The Hong Kong Polytechnic University

The 5th Cross-strait Conference on Structural and Geotechnical Engineering (SGE-5)
Hong Kong, China, 13-15 July 2011
UNCERTAINTY IN NONLINEAR SEISMIC GROUND RESPONSE ANAYLYSES
O. L. A. Kwok
Department of Civil Engineering,
National Taiwan University, Taipei, Taiwan. Email: annieonleikwok@ntu.edu.tw
ABSTRACT
One-dimensional seismic ground response analyses are seldom performed using nonlinear procedures because
parameter selection and code usage protocols are poorly documented and understood, the effect of parametric
variability on the analysis results is generally unknown, and the benefits of nonlinear analysis relative to the
widely-used equivalent-linear analysis are un-quantified and unclear. In this paper, effect of parametric
variability on predictions using nonlinear ground response analysis will be discussed. Sources of variability
considered include material properties such as dynamic material curves and velocity profiles. Various vertical
array sites are used in the study. It is found that the standard deviation of predictions is dominated by material
curve variability at small periods and velocity variability at medium to large periods respectively.
KEYWORDS
nonlinear, ground response analysis, one-dimensional, uncertainty.
INTRODUCTION
Theoretical modeling of one-dimensional site response can generally be accomplished using equivalent linear or
nonlinear analysis. Equivalent-linear ground response modeling is commonly performed in practice as it
requires the specification of well-understood and physically meaningful input parameters (shear-wave velocity,
unit weight, modulus reduction and damping). Nonlinear ground response analyses provide a more accurate
characterization of the true nonlinear soil behavior, but their implementation in practice has been limited, which
is principally a result of poorly documented and unclear parameter selection and code usage protocols.
Moreover, previous studies have thoroughly investigated the sensitivity of site response results to the input
parameters in equivalent-linear analyses (e.g. Roblee et al., 1996), but this level of understanding is not
available for the input parameters used in nonlinear analyses. The objective of current study is to evaluate the
uncertainties in predictions by nonlinear ground response modeling due to various sources of variability,
including material properties and modeling schemes. Vertical array sites are used in this study as the uncertainty
due to input ground motion can be eliminated and predictions from nonlinear analyses can be compared directly
to the data (recordings).
SOURCES OF VARIABILITY
Modeling Schemes
The ground response analyses are performed by using various nonlinear codes including DEEPSOIL (Hashash
and Park 2001, 2002; Park and Hashash 2004), D-MOD_2 (Matasovic 2006), a ground response module in the
OpenSees simulation platform (Ragheb 1994; Parra 1996; Yang 2000; McKenna and Fenves 2001;
opensees.berkeley.edu), SUMDES (Li et al. 1992), and TESS (Pyke 2000). All of these codes are time-domain
analysis methods which allow soil properties within a given layer to change with time as the strains in that layer
change. Like a structure, the layered soil column is idealized either as a multiple degree of freedom lumped
mass system or a continuum discretized into elements with distributed mass with appropriate boundary
conditions. Soil material models employed range from relatively simple cyclic stress-strain relationships
(DEEPSOIL, D-MOD_2 and TESS) to advanced plasticity models (OpenSees and SUMDES).
Material Properties
The four vertical array sites considered in this study include KGWH02 (from the Japanese Kiknet network of
strong motion stations), Lotung (Taiwan), Turkey Flat and La Cienega (both of them are in California of the
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