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Commonly used analysis procedures tend to segregate the “structural” components of the foundation (e.g., footing, grade beam, pile, and pile cap) from the supporting (e.g., soil) components. The “structural” components are typically analyzed using standard strength design load combinations and methodologiesmethods, whereas the adjacent soil components are analyzed using allowable stress design (ASD) practices, in which earthquake forces (that have been reduced by R) are considered using ASD load combinations, to make comparisons of design forces versus allowable capacities. These “allowable” soil capacities are typically based on expected strength divided by a safety factor of safety, for a given level of potential deformations.When design of the superstructure and foundation components is performed using strength-level load combinations, this traditional practice of using allowable stress design to verify soil compliance can become problematic for assessing the behavior of foundation components. The 2009 NEHRP provisions (FEMA 2009a) contain two resource papers (RP 4 and RP 8) that provide guidance on the application of ultimate strength design procedures in the geotechnical design of foundations and the development of foundation load-deformation characterizations for both linear and nonlinear analysis methods. Additional guidance on these topics is contained in ASCE 41 (2014b).
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