Evaluation Methods for Localized Differential Foundation Movement in Post-Tensioned Concrete Foundations

Shallow-bearing concrete foundations on expansive soils are subject to various loading conditions, most invariably differential movement of the underlying soils. If a concrete foundation does not properly account for the effects of differential soil loading, excessive distortions can occur. Post-tensioning of concrete foundations is a popular engineering solution due to the reduction in quantity of structural members and materials required when compared to a conventionally reinforced foundation. Guidelines for the evaluation of post-tensioned concrete foundations are provided by the Texas Chapter of the American Society of Civil Engineers (ASCE) and the Post-Tensioning Institute (PTI). These guidelines include analyses techniques and limitations for foundation distortion that account for global performance of the foundation system but overlook localized conditions. The lack of localized analyses effectively ignores common performance failures observed in the field, particularly edge lift movement. Through a recent case study, this paper will discuss how industry evaluation methods neglect common localized failures. This paper will also present a proposed methodology on how to properly evaluate a post-tensioned concrete foundation, including global and localized distortion conditions.