Implementation of SFSI and site effect models in PEER methodology - 1342005

Project goals and objectives

The Year 7 project by the PIs consisted of a benchmark study of a code-compliant structure, which was carried out in collaboration with researchers at Stanford (Deierlein) and Caltech (Beck). During the course of that project, several critical "gaps" within the PEER methodology were identified with respect to the implementation of models for site effects and soil-foundation- structure interaction (SFSI) effects. Some of those issues were addressed during our Year 8 project, although not as many as we had originally anticipated because of roll-over of the Year 7 work into Year 8 (accordingly, some of our Year 8 work is rolling into Year 9). Our proposed Year 9 work is continuing to fill the gaps identified during the benchmark project and to package tools developed during previous years' work for application. Specific objectives include the following:

1. Integration of the results of site-specific ground response analyses (i.e., using appropriate equivalent-linear or nonlinear analysis) within probabilistic seismic hazard analyses (PSHA). Packaging of appropriate methods within OpenSHA.
2. Development and implementation of models to predict ground motion variations between the foundation level of buildings and the free-field (i.e., kinematic interaction models). The key technical issues to be addressed here are phase shift (equivalently expressed as coherency) of foundation/free-field motions and the development of simple models for the ratio of foundation/free-field response spectra (RRS).
3. Package of time history selection protocols into a database system already being developed by PEER staff as part of the NGA project. The key issue here is to ensure that critical information (search parameters) needed by engineers performing time history selection is available within the database. 4. Implementation and packaging in OpenSees of spring and dashpot models developed by co PI Taciroglu.
4. Assessment of previously developed foundation models. Key issues associated with this assessment include (a) identification of input parameters (and their uncertainties), (b) quantification of model variability (driven by both uncertainty in input parameters and the differing attributes of available foundation models), and (c) evaluation of model functionality (numerical stability) when models are utilized beneath multiple footings of a building structure. Work on this task will be closely related to a follow-on benchmark study that would likely involve a shear wall building. Accordingly, our work will be contingent on participation of other groups (Deierlein and Beck) and their schedules. Some of this work may occur in Year 10.

Role of this project in supporting PEER's mission (vision)

This work will help enable implementation of the PEER methodology including the effects of site condition on ground motion, the effects of kinematic soil-structure interaction on seismic demand, and the effects of inertial soil-structure interaction on building response. The major impact of our effort is to provide packaged tools for IM characterization, time history selection, and SFSI analysis (typically utilized in IM to EDP analyses).

Methodology employed

Scope items are discussed with reference to the specific goals/objectives described above.

• Task 1: Implementing within OpenSHA routines that allow the results of 1D nonlinear geotechnical ground response analyses to be integrated into PSHA. The available routines that are being implemented were developed by Baturay and Stewart (2003) and Bazzurro and Cornell (2004). This work began in Year 8 and will be completed in Year 9. The programming of Java applets for use in the OpenSHA web-based software will be facilitated by Ned Field and his OpenSHA programming team.
• Task 2: Our work on the phase shift/RRS issue will proceed as follows. We will begin by evaluating phase differences (coherency) between foundation and free-field motions. We will attempt to identify frequency bands with small phase shift (typically low frequencies) and frequency bands with arbitrary phase shift (typically large frequencies), and the boundary frequencies separating those bands. Based on those observations, we will attempt to develop guidelines for evaluating foundation/free-field phase differences. Then, for a specified transfer function amplitude and the phase shift guidelines, we will utilize a large suite of recorded, free- field time histories to calculate compatible time histories for the foundation motion. We will then evaluate ratios of response spectra (RRS) for the foundation/free-field time histories. Based on previous experience documented in the FEMA-440 report, for some time histories we expect RRS to follow the transfer function amplitude up to frequencies as high as about 5 to 10 Hz. For others, RRS will be nearly unity regardless of period. We will attempt to relate the RRS/transfer function ratio to ground motion characteristics such as mean period as well as relevant site/source characteristics. Engineering models will be provided to estimate RRS given transfer function ordinates and the relevant ground motion/site/source characteristics. As noted previously, this was originally to be a Year 8 task. The work was pushed back to Year 9 because of rollover of Year 7 work into Year 8 and because of the supplemental tasks that have been added to Year 8 (see Tasks 1 and 3).
• Task 3: Development of a user-interface within the NGA database to enable time history selection that accounts for parameters of interest (near-source parameters, epsilon, magnitude, distance, etc.). This work is collaborative with the NGA database manager on staff with PEER. Work on this task has actually begun in Year 8. Communications between this project team and the NGA project management have not always been optimal (our emails going unreturned, etc.), which has impeded timely progress on this work. Support and direction from PEER management will be needed.
• Task 4: Implementation of soil-foundation interaction macroelement models into OpenSees. These models have already been developed through funding from Caltrans and PEER and have been validated for pile-foundations. The model will be validated for shallow foundations using centrifuge test data and relevant documentation will be provided.
• Task 5: Foundation models have been developed with PEER support by a number of investigators (Kutter, Hutchinson, Taciroglu). This task involves an assessment of those models with respect to the issues outlined above (input parameters, model variability, model functionality). This model assessment work involves application of existing SFSI elements for wall foundations in a single shear wall/foundation system within a typical code-compliant building and in a centrifuge experiment (discussed in December 2005 and January 2006 meetings). Ongoing work is investigating the sensitivity of model predictions to non-physical model parameters.

Brief Description of previous year's achievements, with emphasis on accomplishments during last year (Year 8)

We have completed the implementation of a nonlinear Vs30-based model for site effects into OpenSHA (now available in the attenuation relationship module under the title "Choi and Stewart" in www.opensha.org). An output example from this module is shown along with outputs from Abrahamson & Silva's 1997 attenuation results:

In the area of SSI simulation, the following work has been completed:

1. Using three-dimensional finite element simulations a coupled vertical-lateral macroelement response model for deep foundations was developed. A journal article in this topic is nearing completion.
2. A prototypical macroelement model was developed for SSI in shallow foundations. Preliminary sensitivity studies have been completed of the foundation response to macroelement parameters, and model calibrations were performed using UC Davis centrifuge test results as a basis. This model currently provides an uncoupled lateral-vertical response of the soil-foundation system and was shown to be numerically robust under quasi-static, cyclic loading. Currently, we are developing calibrated finite element models of UC Davis tests. These models will aid us in developing coupled lateral-vertical versions of the macroelement models for shallow foundations.
3. Preliminary studies were performed for converting impedance (frequency-domain) models of SSI into discrete time-domain filters using weighted least-squares techniques. These studies indicated that there is no universal weighting scheme and that the optimal values of the weighting parameters depend on the input ground motion. Automatic and adaptive procedures to determine optimal weighting parameters for a given ground motion are being considered.

Other similar work being conducted within and outside PEER and how this project differs

• The Task 1 work is synergistic with the OpenSHA model development being undertaken by SCEC and USGS. The proposed work complements that effort by implementing state-of-the-art site models into the platform.
• The Task 2 work is a natural follow-on to PI Stewart's previous participation in the ATC-55 project.
• Task 3 will be completed in collaboration with the NGA group. We have initiated contact with the NGA IT manager, and we will have our first meeting with her at the end of November 2005.
• The Task 4 work is an extension of the project that the PI and the Co-PI are completing for Caltrans. The implementation of the developed models in OpenSees is not only beneficial to the earthquake research community in general, but also for Caltrans engineers who might like to use OpenSees as a validation and analysis tool.
• The Task 5 work is part of a collaborative effort with model developers within PEER (Tara Huitchinson and Bruce Kutter) as well as Gregg Deierlein and Helmut Krawinkler.

Describe any instances where you are aware that your results have been used in industry

Site response models developed in earlier phases of work (Years 6–7) have been used by a number of consultants and also researchers across the U.S.

Expected milestones & deliverables

• - Web-based Java applets for PSHA that include various site effect models
• - Development and dissemination of RRS and phase shift guidelines
• - Web-based time history selection tools integrated with NGA database
• - New OpenSees models for deep/shallow foundation-soil interaction elements
• - Documentation of foundation model assessment results

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