Flexural Strength and Ductility of Extended Pile-Shafts. I: Analytical Model

An analytical model, based on the commonly used equivalent cantilever concept, is developed for assessing the local ductility demand of a yielding pile-shaft when subjected to lateral loading. For elastic response of the pile-shaft, an equivalent depth-to-fixity is assumed, which can be derived by equating the lateral stiffness of the cantilever to that of the elastic soil-pile system. In adapting the equivalent cantilever model to yielding pile-shafts, however, the depth-to-maximum-moment is assumed to occur at a depth above the depth-to-fixity. The lateral strength, which depends on the depth-to-maximum-moment, is determined using the flexural strength of the pile and the ultimate pressure distribution of the soil. By assuming a concentrated plastic hinge rotation at the depth-of-maximum-moment, a kinematic model relating the local curvature ductility demand to global displacement ductility demand is developed. The kinematic relation is shown to depend on the aboveground height, depth-to-maximum-moment, depth-to-fixity, and equivalent plastic hinge length. The model is illustrated using a pile-shaft embedded in cohesive and cohesionless soils.