Development of a Probabilistic Simplified Design Procedure for Assessing the Effects of Liquefaction and Lateral Spreading on Bridges - 2422005

Project goals and objectives

The primary objective of this research project is to develop a simplified design procedure for evaluating the effects of liquefaction and lateral spreading on bridges. This simplified design procedure must be developed to work within the PEER probabilistic performance-based engineering framework in that sources of uncertainty within each step should be incorporated properly. This project will translate pertinent PEER research findings into forms that can be adopted in practice as a probabilistic-based alternative to existing deterministic approaches. The project will demonstrate how the PEER methodology can be effectively used with simpler design-level analysis methods to make informed decisions.

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

This project supports the PEER development and implementation of the Probabilistic-Based Earthquake Engineering methodology through the application and demonstration of the PEER methodology to the problem of evaluating the effects of liquefaction and lateral spreading on bridges.

A number of co-dependencies with other PEER research projects are noted the sections below. Hence, these will not be repeated here except our primary co-dependency with Professor Martin in his closely related research project. Professor Geoff Martin is involved as a senior researcher on this project through a related PEER project (i.e., 2052004). He helped develop the simplified methodology proposed by in the MCEER/ATC Joint Venture Document 49 (2003) “Recommended LRFD Guidelines for the Seismic Design of Highway Bridges” that this project is applying in a probabilistic format. Due to his expertise in this area and his previous extensive work in the area as evidenced by his numerous publications on this topic, Professor Martin's insights and guidance is proving invaluable.

Methodology employed

This Year 9 project is the second year of a three-year project to complete a development and demonstration project of the PEER probabilistic framework within the context of assessing the effects of liquefaction and lateral spreading on bridge foundations.

Mr. Christian Ledezma, a Ph.D. Candidate Graduate Student Researcher, is performing the primary work on this project. He will work under the supervision of Professor Bray. Professor Norm Abrahamson serves as a faculty associate to provide guidance and review on the implementation of aspects of the PEER probabilistic methodology. He has participated on many of the projects supervised by Professor Bray over the last decade and recently initiated a study of probabilistic seismic slope displacement problems.

Professor Geoff Martin, who is funded separately on a related PEER project (2052004) works as a senior researcher with Professor Bray and Mr. Ledezma, as he has extensive experience with the MCEER/ATC simplified deterministic design method that will serve as the initial basis for the new probabilistic procedure. Prof. Martin works with us to ensure we understand and apply the MCEER/ATC design method with the necessary revisions for implementation within the PEER probabilistic framework. It will then be executed to see if it can be done and to identify key unresolved issues.

Revision of the MCEER/ATC simplified method requires that the Ph.D. student researcher, Mr. Ledezma, perform some simplified and advanced analysis to gain insight so that we can revise the procedure in a reasonable fashion. A number of simplifying assumptions were made in developing this practical design procedure and several of these assumptions warrant re- evaluation. These analyses will be performed using the simplified procedure delineated in MCEER/ATC-49 (2003) and with the OpenSees model developed by Professors Kramer and Arduino in a related project (2032004). Without many detailed case histories, the UCD centrifuge studies (Boulanger 2012004) will be vitally important in evaluating the results of the analyses and in judging the appropriateness of the many assumptions involved in the simplified procedure.

However, the primary objective of this research project is to convert an appropriately revised MCEER/ATC methodology into one that can be exercised transparently through the PEER framework. Analytical will be performed in a limited and focused manner to support this objective, without duplicating efforts by other PEER researchers.

This research project capitalizes on the Year 7 PEER work performed by Dr. Thalia Travasarou who worked under the direction of Professor Bray at Berkeley. Travasarou et al. (2004) developed a probabilistic methodology for assessing seismic slope displacements. This methodology is unique in that it includes the case in which combinations of parameters may lead to no seismic displacement, as well as cases where it will likely lead to significant displacement. The relationships developed as part of this previous research are necessary to convert the MCEER/ATC methodology into a probabilistic framework. Instead of calculating displacement by difficult to characterize probabilistically parameters, such as the maximum seismic coefficient (kmax), which is a function of many factors, the Travasarou et al. (2004) method uses a commonly available ground motion intensity parameter, i.e. the spectral acceleration at a degraded slope period. Use of Sa(1.5Ts) as opposed to kmax is important if one wants to incorporate this procedure within the PEER framework (i.e. Sa is a common IM; attenuation relationships and hazard curves are available for Sa, but not for kmax). The method also requires the slope parameters ky and Ts.

This research also takes advantage of the PEER-Lifelines program research results of Professor Seed on probabilistic liquefaction triggering procedures (principally with O. Cetin and R. Moss) and probabilistic lateral spreading (with A. Faris and R. Kayen). It utilizes the probabilistic performance-based earthquake engineering methodology developed by Dr. K. Mackie and Prof. B. Stojadinovic of U.C. Berkeley.

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

This project builds upon the Years 3-5 and 7 PEER projects on identification of efficient intensity measures for earthquake ground motions and development of a probabilistic method for estimating seismic slope displacements. It builds as well upon the first year of this three year project, which is Year 8

This research project capitalized on the Year 7 PEER work performed by Dr. Thalia Travasarou who worked under the direction of Professor Bray at Berkeley. Travasarou et al. (2004) developed a probabilistic methodology for assessing seismic slope displacements. This methodology is unique in that it includes the case in which combinations of parameters may lead to no seismic displacement, as well as cases where it will likely lead to significant displacement. The relationships developed as part of this previous research are necessary to convert the MCEER/ATC methodology into a probabilistic framework. Instead of calculating displacement by difficult to characterize probabilistically parameters, such as the maximum seismic coefficient (kmax), which is a function of many factors, the Travasarou et al. (2004) method uses a commonly available ground motion intensity parameter, i.e. the spectral acceleration at a degraded slope period. Use of Sa(1.5Ts) as opposed to kmax is important if one wants to incorporate this procedure within the PEER framework (i.e. Sa is a common IM; attenuation relationships and hazard curves are available for Sa, but not for kmax). The method also requires the slope parameters ky and Ts.

In Year 8 (the first year of this project), accomplishments include:

1. Developing the foundation design for the bridge design example that will be analyzed by a group of PEER researchers.
2. Worked with the Univ. of Washington researchers to refine the soil profile to be used in the bridge design example.
3. Completed preliminary evaluation of the MCEER/ATC simplified methodology for evaluating the effects of piles in restraining liquefaction-induced lateral spreading.
4. Integrated the Seed et al. (2003) probabilistic liquefaction-triggering procedure and the Travasarou et al. (2004) probabilistic seismic displacement procedure into the deterministic MCEER/ATC simplified methodology and included a probabilistic assessment of the ground motion Intensity Measure. Example results are shown in Figure 1 below.
5. Hosted a one-day workshop on key aspects of the soil-foundation-pile-structure interaction parts of the liquefaction-induced lateral spreading effects on bridge pile foundation problem.

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

Similar work by the PI is not being conducted within PEER or outside of PEER.

Expected milestones & deliverables

The primary deliverable for this project will take the form of a technical report that will summarize the research methods used and present the probabilistic simplified design procedure for evaluating the effects of liquefaction and lateral spreading on bridges. The goal is to demonstrate the use of the PEER methodology through its application to this problem.

The following tasks will be completed as part of this three-year research project:

1. Review of previous and ongoing PEER research related to this problem.
2. Review and apply the current MCEER/ATC-49 simplified design procedure to the proposed standard bridge designs and ground conditions developed by the PEER Thrust Area 2 team.
3. Perform advanced analyses to gain insight so that the simplifying assumptions made in developing the MCEER/ATC-49 practical design procedure can be re-evaluated.
4. Implement the slightly revised MCEER/ATC-49 (and hopefully somewhat validated) simplified design methodology in the PEER probabilistic framework by systematically addresses the uncertainties involved in each step.
5. Perform additional analyses as required to refine and validate the proposed simplified probabilistic design procedure.
6. Prepare a report that documents the work completed and importantly demonstrates how the design method can be implemented in practice.

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