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This webinar was co-sponsored by ASCE's Geo-Institute (G-I) and ASCE Continuing Education
Instructors:
Benjamin J. Turner, Ph.D., P.E., M.ASCE
Scott J. Brandenberg, Ph.D., P.E., M.ASCE
Course Length: 1 Hour
Purpose and Background
Kinematic loads from lateral spreading often control the design of bridge deep foundations in seismic regions. This webinar presents a case study that provides a unique opportunity to validate commonly used equivalent-static analysis (ESA) procedures against the documented performance of a pair of bridges in northern Baja California, Mexico, during the 4 April 2010 M 7.2 El Mayor Cucapah Earthquake. This case is particularly valuable because the two bridges performed significantly different in response to lateral spreading - one collapsed, while the other suffered only moderate damage - despite being separated by only a few meters and subjected to essentially the same lateral spreading demand.
In the conventional design of deep foundations for inertial loading effects, superstructure loads are resolved to a top-of-foundation demand and a decoupled, foundation(s)-only analysis is performed. Using the beam-on-nonlinear-Winkler-foundation (BNWF) method in programs such as LPILE is one example. This design methodology is appropriate for inertial loading because the only load path being considered involves inertia demands imposed by the structure onto the foundation and into the soil.
Combining inertia loads with kinematic demands arising from lateral spreading is a more complicated issue. While this method has gained acceptance, it fails to consider that kinematic loads may be distributed to substructure and superstructure components other than the deep foundation (or group) being analyzed. Failing to accurately represent the transfer of kinematic loads to above-ground bridge components can lead to an unsafe design and misinform designers about the true performance of both the foundations and above-ground components.
Back-analysis of the case histories showed that in order to accurately capture the bridge performance, it was necessary to simulate the restraint and load-transfer behavior at the top-of-column to superstructure connection. This load path also plays an important role in the transfer of superstructure inertial forces that may occur simultaneously with kinematic demands. The back-analysis showed that commonly used methods for quantifying and imposing inertial loads on a foundation(s)-only model can result in a drastic overestimate of foundation demands and a simultaneous underestimate of column demands. This webinar presents that captures the contribution of all relevant components to the resistance of inertial and kinematic demands while allowing for simplified analysis of a single bent.
Finally, the performance of a bent that was not subjected to lateral spreading but settled by approximately 50 cm is investigated. In addition to drag loads due to post-liquefaction reconsolidation settlement, the importance of considering reduced side and/or base resistance in soil layers in which there is an increase in excess porewater pressure but not full liquefaction is highlighted.
Primary Discussion Topics
- Basics of lateral spread defined relative to other slope failure mechanisms
- Summarize documented behavior of bridges at the case study site and related cases
- Load path for conventional (e.g., gravity) loads versus kinematic soil loads
- Mechanism by which deep foundations resist lateral spreading
- Summarize current ESA methodologies (e.g., ATC-49, Caltrans, PEER guidelines)
- Modifications to conventional static BNWF analysis necessary for lateral spreading ESA
- Recommendations for combined inertial and kinematic loading
- Define pile pinning
- Axial geotechnical resistance during and after excess porewater pressure generation
Learning Outcomes
Upon completion of this course, you will be able to:
- Perform an ESA for deep foundations subjected to lateral spreading
- Understand how bridge components other than foundations play an important role during lateral spreading, and how to include these in a simplified model
- Identify cases in which a decoupled ESA will not accurately capture behavior
- Communicate effectively between structural and foundation designers regarding
- Explain the structural information needed to perform accurate foundation analyses and the demands placed on above-ground components
- Perform axial pile analysis for liquefaction/increases porewater pressure case
Webinar Benefits
- Gain a better understanding of the mechanisms controlling interaction of laterally spreading soil
- Discuss deep foundations and above-ground structural components
- Achieve more reliable and economical designs that accurately reflect system performance
Assessment of Learning Outcomes
Students' achievement of the learning outcomes will be assessed via a short post-assessment (true-false, multiple choice and fill in the blank questions).
Intended Audience
- Practicing engineers that are involved in foundation design, whether from a structural or geotechnical perspective
- Researchers, including graduate students, that focus on liquefaction or lateral spread issues
- Instructors of graduate-level foundation design courses
Webinar Outline
- Define lateral spreading
- Describe case histories
- Formulation of geotechnical and structural parameters for analysis
- Comparison of results to documented performance
- Comparison between results of decoupled foundation model and model including above-ground components
- Axial pile analysis for liquefaction case
- Summary of recommendations
How to Earn your CEUs/PDHs and Receive Your Certificate of Completion
To receive your certificate of completion, you will need to complete a short on-line post-test and receive a passing score of 70% or higher within 1 year of purchasing the course.
How do I convert CEUs to PDHs?
1.0 CEU = 10 PDHs [Example: 0.1 CEU = 1 PDH]