Performance-Based Evaluation of Bridge on Liquefiable Soils Using OpenSees - 2412005

Project Title—ID Number Performance-Based Evaluation of Bridge on Liquefiable Soils Using OpenSees - 2412005
Start/End Dates 10/1/05 – 9/30/06
Funding Source PEER-NSF
Project Leader (boldface) and Other Team Members Steve Kramer (UW/F), Pedro Arduino (UW/F), Hyung-Suk Shin (UW/GS)
F=faculty; GS=graduate student; US=undergraduate student; PD=post-doc; I=industrial collaborator; O=other

Project goals and objectives

The goal of this project is to show that the PEER PBEE methodology, as expressed in the PEER framing equation, can be used with the OpenSees analytical tool to allow owners to make better decisions about seismic design and risk mitigation. The project will show how the OpenSees program can be used to develop EDP|IM fragility relationships for a bridge founded on liquefiable soils, and how those fragility curves can be used in the PEER PBEE framework to evaluate hazards for a complex soil-pile-structure system. The project will demonstrate the capabilities of OpenSees to (a) predict soil movements due to lateral spreading, (b) evaluate soil- pile-structure interaction in liquefiable soils, and (c) estimate the reliability of the performance predictions. The project will also demonstrate the capabilities of several important components of OpenSees.

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

The project will show that the PEER PBEE methodology, as expressed in the PEER framing equation, can be used with the OpenSees analytical tool to allow owners to evaluate performance of an existing structure located on poor soils. It will illustrate the interaction between structural and geotechnical damage mechanisms. In combination with the project of Bray and Martin, this project will demonstrate the benefits associated with detailed response analyses in terms of identifying damage mechanisms, soil-structure interaction effects on performance, and reduction of model uncertainty.

Methodology employed

The performance of a bridge resting on favorable soil conditions is being evaluated by Stojadinovich and others. In this project, the same bridge will be assumed to lie in an area underlain by liquefiable soil conditions. The performance of the bridge will be evaluated in the PEER framework using the OpenSees analytical tool; in a parallel project, Bray and Martin will evaluate the performance of the bridge using simpler, empirical procedures.

Evaluation of the performance of the bridge requires development and testing of an OpenSees model of the bridge, its foundations, and the soils that support it. The model must be capable of computing the free-field response of the site, including pore pressure generation, redistribution, and dissipation in liquefiable soils along with the effects of those phenomena on the stiffness and strength of the soils, the interaction of the pile foundations with the liquefiable soils, including the effects of pore pressure changes on that interaction, and the response of the above-ground portions of the structure.

The validated OpenSees model is being used to investigate the dependence of appropriate EDPs on various IMs. This dependence is being studied first deterministically to develop a good understanding of the mechanisms by which cyclic and permanent deformations develop in the soil, foundations, and structure, and the nature of the demands imposed on the structure and foundations by the response of the system to different input motions. Uncertainty in the EDP|IM relationship will be investigated using relatively simple and computationally efficient methods such as the sensitivity analysis procedures implemented in OpenSees by Conte and the reliability module of Haukaas. EDP|IM fragility curves will also be investigated using Monte Carlo methods.

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

Much of the work to date has centered upon building and testing of an OpenSees model of the bridge, foundations, and soil deposit. Several iterations of soil geometry and material properties have been required to identify a soil profile that is amenable to analysis using the OpenSees approach and the empirical approach of Bray and Martin. The structural model was implemented with the assistance of Kevin Mackie. An OpenSees model was developed and tested using the equivalent pier concept of foundation modeling (i.e. modeling pile groups as a single shaft with appropriate material properties). The model was subjected to a series of ground motions representing four return periods (figure below shows relative efficiencies of different IMs). Further discussion with the other investigators and with Caltrans engineers led to the decision to model the pile foundations individually (at least individual rows in the two- dimensional model). A new mesh adopting this approach has been developed and is being tested at this time. This model will be previewed at the May 4 project meeting, and then used for multiple analyses.

Other similar work being conducted within and outside PEER and how this project differs The project has numerous interdependencies with other Year 9 projects. For conciseness, the interdependencies are summarized in tabular form below.

Principal Investigator(s) Needed Inputs Provided Outputs
Bray Martin Estimates of appropriate uncertainties involved in estimation of performance using simplified procedures. OpenSees pile-soil interaction models for testing against simplified procedures.
Stojadinovic Input on appropriate EDPs for structural damage due to liquefaction; insight into appropriate DMs and DVs for those EDPs. OpenSees model for aboveground portions of bridge system. EDP|IM fragility curves for requested and appropriate EDPs.
Elgamal Input on proper use of Elgamal constitutive model. Results of OpenSees verification testing.

The project is most closely related to the project of Bray and Martin who are evaluating the same bridge/foundation/soil system but using simpler, empirical procedures for soil and soil- foundation response. The results of these two projects will allow evaluation of the marginal benefits of performing detailed soil-foundation-structure interaction analyses using an advanced modeling tool like OpenSees.

We are unaware of similar work being conducted outside of PEER.

Expected milestones & deliverables

  • OpenSees complete model with individual piles: May 4, 2006
  • OpenSees analyses with suite of ground motions: June 30, 2006
  • OpenSees sensitivity analyses: July 31, 2006
  • Identification of most significant geotechnical/foundation parameters: August 30, 2006
  • Investigation of effects of geotechnical uncertainties on EDPs: September 30, 2006

Member company benefits


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