An important objective of axially statically load testing deep foundations is to determine load-transfer behavior. Integral to this objective is determining internal forces at various locations within the deep-foundation element. These internal forces are usually obtained from strain measurements. Strain measurements do not determine internal forces directly, and to convert strain to internal force, it is conventionally assumed that both the foundation’s composite-section elastic modulus, E, and total cross-sectional area, A, must be known. However, only the product EA, the axial rigidityof the foundation, must be known. Using applied test loads and measured strains, the Incremental Rigidity (“IR”) method determines the relationship between axial rigidity and strain at a strain gage level. From this relationship, measured strains can be converted to internal forces without having to know either a deep-foundation element’s elastic modulus or cross-sectional area.