Limit state analysis and performance assessment of fixed-head concrete piles under lateral loading

Deep foundations of buildings and bridges often involve the use of concrete piles that are restrained from rotation at the pile-head. Under lateral seismic loads, however, the fixity at the pile/pile-cap connection induces a large curvature demand at the pile-head, with potential for failure in the pile. Severe damage of pile foundations had been observed in recent earthquakes. In foundations where yielding of piles cannot be avoided, the level of damage needs to be carefully controlled, particularly if a certain performance level is to be guaranteed for the structure. In order to limit the level of damage in the foundation, design of piles requires a careful assessment of the curvature ductility demand under an imposed lateral displacement.; For a fixed-head pile subjected to a large lateral load, sequential yielding occurs with the largest curvature demand occurring at the pile head, followed by a reduced stiffness due to the formation of the first plastic hinge, and then by a fully plastic mechanism after the development of the second plastic hinge. Identification of these yield-limit states facilitates the development of a kinematic relation between the curvature and displacement ductility of the pile. In this dissertation, a simple analytical model relating the global displacement to the local inelastic deformation is proposed for fixed-head pile foundations. The analytical model enables the pile displacement, curvature demand and subsequent severity of local damage to be estimated for a wide range of pile and soil properties. The pile response predicted by the proposed model is compared with the results of finite element analyses with soil modeled by a series of p-y elements. The performance of fixed-head piles under various levels of displacement ductility factors are also assessed for a wide range of soil properties. In particular, the local curvature ductility demand is compared with the curvature ductility capacity provided by the level of confining steel specified in current seismic provisions. Considerable curvature ductility demand implying significant damage may be expected in the pile for even a moderate level of foundation yielding. Results highlighting the influence of soil and structural parameters, and the design implications are also specified.