Christine A. Goulet

Pitfalls of Deterministic Application of Nonlinear Site Factors in Probabilistic Assessment of Ground Motions

It is common for ground motions to be estimated using a combination of probabilistic and deterministic procedures. Probabilistic seismic hazard analyses (PSHA) are performed to estimate intensity measures (IMs) for reference site conditions (usually rock). This is followed by a deterministic modification of the rock IMs to account for site effects, typically using site factors from the literature or seismic codes. We demonstrate for two California sites and three site conditions that the deterministic application of nonlinear site factors underestimates ground motions evaluated probabilistically for return periods of engineering interest. Reasons for this misfit include different standard deviation terms for rock and soil sites, different controlling earthquakes, and overestimation of the nonlinear component of the site response in the deterministic procedure. This problem is solved using site-specific PSHA with appropriate consideration of nonlinear site response, within the hazard integral.

Assessment of Ground Motion Selection and Modification (GMSM) methods for non -linear dynamic analyses of structures

As non -linear response history analyses are becoming more prevalent in practice, there is a need to better understand how the selection and modific ation (e.g., amplitude scaling or spectrum matching ) of records will influence the resulting structural response predictions . There are currently many methods of ground motion selection and modification available , but little guidance is available to engine ers on which methods are appropriate for the ir specific application. The Ground Motion Selection and Modification Program was formed within the Pacific Earthquake Engineering Research (PEER) Center to address th is issue. This paper presents the curren t met hodology developed by the P rogram as well as sample results from the first pilot study completed in 2006. Preliminary results show that for a first -mode dominated structure, one can improve the prediction of its response by taking into account record prope rties that are important to the non -linear response of the building when selecting and scaling ground motion records.

Comment on “Nonlinear Soil-Site Effects in Probabilistic Seismic-Hazard Analysis” by Paolo Bazzurro and C. Allin Cornell

The authors are to be commended for presenting clear and practical solutions to a problem that has long been vexing to engineers and seismologists—how to link probabilistic assessments of ground-motion hazard with the results of geotechnical ground-response analysis. The term “ground- response analysis” is used here to refer to one-dimensional modeling of shear-wave propagation through a soil column, the response of which will generally be nonlinear for engineering design applications. The purpose of this discussion is to point out some of the key assumptions made in the development of the authors’ solution and to offer a preliminary comparison with an alternative ground-response implementation procedure. The authors present two procedures for implementation of ground-response analysis results into probabilistic seismic- hazard analysis (psha). One procedure involves convolution of amplification factors (af) calculated from ground- response analysis with a site-hazard curve developed for reference-site conditions (usually rock). This solution is given by equations (2)–(4) for the case in which the amplification at frequency f [af( f )] is conditioned only on rock hazard for frequency f , and in equations (5)–(7) for the case in which af( f ) is conditioned on the rock-hazard curve at both frequency f and the fundamental frequency of the soil column ( f sc). The second procedure involves modifying the median and standard deviation of a ground-motion intensity measure estimated from an attenuation relationship for rock- site conditions. The median is modified to account for ground-response effects, including sediment nonlinearity, using equation (14). Standard deviation is similarly modified using equation (15). The modified median and standard deviation are then directly used within the hazard integral in lieu of those from the attenuation relation for rock. A critical assumption implicit to both of these procedures is that site effects are correctly simulated by one- dimensional ground-response analysis. More specifically, this …

Implementation of 1D Ground Response Analysis in Probabilistic Assessments of Ground Shaking Potential

Results of 1D ground response analyses are typically not incorporated into probabilistic seismic hazard analyses (PSHA) in a statistically robust way. Often ground response is incorporated into PSHA using deterministic amplification factors. This simplistic method generates results that are intrinsically arbitrary and often unconservative. The main problem in probabilistically linking PSHA and ground response lies in quantifying the dispersion that is appropriate for use with ground response analysis results. We review two alternative procedures for quantifying this uncertainty and illustrate their differences with respect to dispersion values of spectral acceleration at the surface for various site conditions.