Slender Marine Pile Design for Gravity Loads

The design of slender concrete piling in a marine environment has traditionally been addressed by the American Concrete Institute (ACI 318) Building Code Requirements for Structural Concrete. These piles, typically either solid or hollow prestressed concrete elements, are often limited by the precast concrete facilities available locally. On the U.S. west coast, the maximum dimension is generally limited to 24-inch (61-cm) octagonal piles. Long lengths, resulting in very slender compression elements, often result from multiple factors, including water depth, marginal marine solis potentially increasing depth-to-pile fixity, and marine structure configurations that allow the piles to be subjected to sidesway under lateral loads. Heavy equipment, such as cranes wheel loads or designated heavy uniform load areas often result in large gravity loads in addition to self-weight of the structure. The combination of long pile lengths, potential plumb-pile sway behavior, and high gravity loads results in significant demands for these highly slender elements. Slender concrete piles are often designed by the ACI 318 Slender Effects in Compression Members provisions. Since its introduction, these provisions have embodied the slender column design concept of the American Institute of Steel Construction specifications for steel buildings with a few exceptions. The most notable exception is that, unlike the steel specifications, ACI 318 does not reduce the concrete column capacity directly by the Euler buckling. Rather, ACI 318 relies on a factored axial load and magnified moment concept to account for slenderness. Recognizing this may not address all slender concrete column design conditions, ACI 318 also included requirements to consider sidesway buckling under gravity loads. Pile design of many deep-water marine structures might have been controlled by these provisions. However, recent changes to these provisions have made design of a structure supported by plumb piles even more restrictive.