Project Title/ID Number 3D Simulation of Seismic Ground Deformation in OpenSees—2202003
Start/End Dates 10/1/03—9/30/04
Project Leader Ahmed Elgamal (UCSD/F)
Team Members Jinchi Lu (UCSD/GS), Zhaohui Yang (UCSD/PD), Linjun Yan (UCSD/GS)

F=faculty; GS=graduate student; US=undergraduate student; PD=post-doc; I=industrial collaborator; O=other

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1. Project Goals/Objectives:

The Y7 project is mainly focused on developing a number of computational simulation capabilities in OpenSees and applications on 2D/3D bridge-foundation-ground systems. This project includes the following specific tasks:

  1. Analyze the seismic behavior of a large-scale 3D bridge-foundation-ground system. Investigate the dynamic interaction between abutments and bridge structure.
  2. Refine solid-fluid fully coupled formulation and implement new u-p elements for more robust soil liquefaction simulations (in collaboration with Dr. Frank McKenna).
  3. Assist in 2D modeling of Humboldt Bay Bridge and synthesizing PEER probabilistic seismic analysis framework (in collaboration with Prof. Joel Conte).
  4. Develop and implement numerical algorithms for parameter sensitivity analysis of seismic ground response (in collaboration with Prof. Joel Conte).
  5. Improve application of spatial definition of ground motion (Domain Reduction Method) on a fully nonlinear 2D bridge-foundation-ground system (in collaboration with Prof. Jacobo Bielak).

2. Role of this project in supporting PEER’s mission (vision):

The simulation capabilities developed are important components in the PEER computational framework OpenSees for Performance-Based Earthquake Engineering (PBEE) applications. These capabilities are being verified and utilized by PEER researchers in conducting computational simulations such as the Humboldt Bay Bridge testbed.

3. Methodology Employed:

The PEER computational program, OpenSees, is employed as the main research platform.

4. Brief Description of past year’s accomplishments (Year 6) & more detail on expected Year 7 accomplishments:

In Year 6, a large-scale 3D model of the Humboldt Bay Bridge was developed using OpenSees. This model includes many advanced nonlinear modeling capabilities available in OpenSees, such as nonlinear fiber beam-column elements (for bridge piers and foundation piles) and pressure-dependent soil material models. The soil models were partially calibrated based on experimental data from other PEER research projects (e.g., soil sample test data from UC Berkeley, centrifuge test results from UC Davis). Appropriate transmitting boundaries were also included to allow radiation of seismic waves. Preliminary simulation results indicated that the presence of bridge abutments may have a paramount influence on the overall response of the bridge-ground system.

figure 1
Fig. 1. Deformed shape of OpenSees 3D bridge-foundation-ground model subjected to base excitation

In addition, in Year 6, the Domain Reduction Method (developed by Prof. Jacobo Bielak) was employed to study the seismic response of a 2D bridge-foundation-ground system subject to spatially varying earthquake excitation. In this model, the interior domain, including the bridge, pile foundations, and surrounding soil, was defined to be fully nonlinear; whereas the exterior domain was linear elastic with uniform properties. Simulation results using this model showed that the seismic behavior of a bridge can be very sensitive to the angle of incidence of the incoming seismic wave. The commonly used assumption that the structure is only subjected to vertically propagating seismic waves is often non-conservative.

This Y7 project is a continuation of the Y6 project. The specific tasks in Year 7 are:

  1. Investigate the dynamic interaction between abutments and bridge structure. For this study, another 3D bridge-ground model was built (Fig. 3), which is identical to the previous one (Fig. 1) except that the bridge abutments were replaced by piers. The difference in bridge response between these two models highlights the influence of abutments on the system behavior.
  2. Refine solid-fluid fully coupled formulation and implement new u-p elements. In collaboration with Dr. Frank McKenna, modifications are being made in OpenSees to improve the robustness of the existing implementation of the u-p formulation. Moreover, a 9-4-node element (9 nodes for solid and 4 nodes for fluid) will be implemented. This element is best known for its reliable performance in simulating dynamic response of saturated soil including liquefaction. Currently, only a 4-4-node element (u-p formulation) exists in OpenSees.
  3. Extensive collaboration is ongoing with Prof. Joel Conte to exercise the PEER PBEE methodology framework on the Humboldt Bay Bridge testbed. This 2D model of the Humboldt Bay Bridge includes many developed soil nonlinearity algorithms and transmitting boundary effects. It is anticipated that all major tasks of the testbed will be completed in Year 7.
  4. Finally, in close collaboration with Prof. Conte, numerical algorithms have been developed for direct differentiation of a multi-surface cyclic plasticity model to obtain consistent stiffness operator as well as the sensitivity of individual model parameters (e.g., shear stiffness, shear strength, etc.) to global response quantities (e.g., acceleration, displacement, etc.). These algorithms have been implemented and verified in OpenSees.
  5. Preliminary results from the application of the Domain Reduction Method (Fig. 2) show that a significant impedance contrast between the nonlinear interior domain and the linear exterior domain can introduce spurious reflected waves at the interior/exterior boundary. Methods are being sought to reduce such undesirable effects.

figure 2
Fig. 2. OpenSees 2D bridge-foundation-ground model subjected to vertical or oblique plane shear wave

figure 3
Fig. 3. Deformed shape of OpenSees 3D bridge-foundation-ground model (without abutments) subjected to base excitation.

5. Other Similar Work Being Conducted Within and Outside PEER and How This Project Differs:

Large-scale 3D computer simulation of seismic wave propagation and nonlinear site response effects are only at the very early stage of development. Related work is being conducted at Carnegie Mellon Univ. for linear and mildly nonlinear scenarios. A main goal of this project is to more accurately model soil-structural interaction response ranging from linear to highly nonlinear scenarios (e.g., liquefaction). Furthermore, application of the Domain Reduction Method in highly nonlinear site response simulations has not been done before. Advanced PEER research in this area is being conducted by Prof. Jeremic at UC Davis.

6. Plans for Year 8 if project is expected to be continued:

For Year 8, it is proposed to conduct research on:

  1. Refining the 3D bridge model to include solid-fluid fully coupled elements for simulating soil liquefaction and related effects on bridge-foundation systems.
  2. Using the 3D model to carry out probabilistic seismic hazard and engineering demand analyses, following the PEER PBEE methodologies.
  3. Using the 3D model to study the sensitivity of global response quantities on soil material parameters.
  4. Helping test and debug many existing and new components of OpenSees through modeling of the Humboldt Bay Bridge.

7. Describe any actual instances where you are aware your results have been used in industry:


8. Expected Milestones & Deliverables:

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