Seismic behavior & design of shallow fdn

Seismic Behavior and Design of Shallow Foundations : 

Seismic Behavior and Design of Shallow Foundations University of Idaho Engineering Outreach CE-566 Earthquake Engineering By Greg Luttrull

Seismic Behavior and Design of Shallow Foundations : 

Seismic Behavior and Design of Shallow Foundations Topic Discussed: Foundation Design Process Foundation Modeling Assumptions Seismic Building Codes Soil-Structure Interaction (SSI) Nonlinear Soil Behavior PEER Design Recommendations

Seismic Behavior and Design of Shallow Foundations : 

Seismic Behavior and Design of Shallow Foundations Shallow Foundations Design Process Geotechnical investigation Structure Analysis/Load Calculations Foundation Geometry Reinforced Concrete Design

Seismic Behavior and Design of Shallow Foundations : 

Seismic Behavior and Design of Shallow Foundations Foundation Modeling Rigid foundation/soil system assumption Rigid assumption Usually results in a conservative load demand leads to higher cost May not accurately portray structural performance under seismic loading. NEHRP Commentary on the Guidelines for the Seismic Rehabilitation of Buildings provides an illustration of the effects of foundation flexibility.

Seismic Behavior and Design of Shallow Foundations : 

Seismic Behavior and Design of Shallow Foundations Seismic Building Codes United States national seismic code in the early 1990’s based on life safety, or collapse prevention and performance of the structure was not well defined. Damage to structures during the Loma Prieta and 1994 Northridge earthquakes in California brought renewed interest to improve the practice of earthquake engineering. National codes have been revised to include provisions modeling the flexibility of the soil-structure interface.

Seismic Behavior and Design of Shallow Foundations : 

Seismic Behavior and Design of Shallow Foundations Soil-Structure Interaction (SSI) Defined as the process “in which the response of the soil influences the motion of the structure and the response of the structure influences the motion of the soil.” Good description of the SSI provided in the commentary for NEHRP Provisions for Seismic Regulation for New Buildings and Other Structures.

Seismic Behavior and Design of Shallow Foundations : 

Seismic Behavior and Design of Shallow Foundations Nonlinear Behavior of Shallow foundations Structural performance can be significantly affected by nonlinear behavior of the soil beneath shallow foundations. Nonlinear behavior dissipates energy and reduces the ductility demands on the structure. Energy dissipation is accompanied by permanent deformation and rotation of the foundation.

Seismic Behavior and Design of Shallow Foundations : 

Seismic Behavior and Design of Shallow Foundations Shallow foundation behavior under cyclic loading Gajan et al. performed a series of tests on a large centrifuge for the nonlinear load-displacement behavior of shallow foundations subjected to vertical, lateral slow cyclic and dynamic loading. The nonlinear load-displacement behavior discussed in the paper includes moment-rotation relationships, horizontal force-sliding relationships, settlement-rotation relationships, and settlement-sliding relationships.

Seismic Behavior and Design of Shallow Foundations : 

Seismic Behavior and Design of Shallow Foundations Moment-Rotation Relationship During foundation rocking at high amplitude rotations one side of the footing would uplift forming a gap at the same instant the opposite side would yield caused a rounding of the surface of the soil Rounding of the soil surface also led to the degradation of the rotational stiffness

Seismic Behavior and Design of Shallow Foundations : 

Seismic Behavior and Design of Shallow Foundations PEER Recommendations In a paper published by PEER, Gajan et. al. propose recommendations for design of shallow foundations subjected to seismic loading. Propose shallow foundations be designed with well-defined moment capacity and exhibit ductile nonlinear behavior when the moment capacity is exceeded Permanent settlements from foundation rocking should be characterized and incorporated into the design

Seismic Behavior and Design of Shallow Foundations : 

Seismic Behavior and Design of Shallow Foundations PEER Recommendations Practical tools for modeling of the soil foundation interaction must be available. Two models discussed Beam-on-Nonlinear-Winkler-Foundation (BNWF) model Contact Interface Model (CIM) [4] Further validation with full-scale field performance data would be useful to gain additional insights and confidence in the models.

Seismic Behavior and Design of Shallow Foundations : 

Seismic Behavior and Design of Shallow Foundations Conclusions Elastic design (fixed foundations) may lead to conservative demands on structures under seismic loading and may not depict the true soil structure system behavior. Current codes make provision for non-linear foundation behavior which in effect reduces the demand on the structure. As engineers better understand nonlinear soil behavior and modeling tools become available, they will be able to better predict the performance of a structure subjected to seismic loading.