Minimum Spiral Reinforcement Requirements and Lateral Displacement Limits for Prestressed Concrete Piles in High Seismic Regions
Several code-based equations exist today for design of the minimum transverse reinforcement required in the potential plastic hinge region of prestressed concrete piles. However, the reinforcement requirements of these equations differ drastically in some cases by as much as a factor of three. Furthermore, there is no expectation for the ductility capacity of prestressed pile sections in seismic regions, nor do the code-based equations allow the designer to account for a desired ductility when determining the minimum confinement reinforcement in prestressed concrete piles. For this reason, a rational study presented in this report was undertaken to develop an equation that determines the minimum quantity of Grade 60 spiral reinforcement necessary to achieve a target ductility over a given range of axial loads in prestressed concrete piles. Based on a parametric study, it is suggested that the prestressed piles should be designed to have a ductility capacity of 18 unless shown otherwise that a lower target value could be used. In this case, the developed equation facilitates reduced amounts of spiral reinforcement to be quantified. In addition, an axial load limit for the prestressed piles in seismic regions is presented together with a definition for idealized moment-curvature response of these piles.
Using the soil types defined by ASCE-7, the study established displacement limits for the piles designed with the recommended amounts of spiral confinement. These limits, which increase with reduced stiffness and strength of the soils, indicate that confinement reinforcement in piles supported by weak soils can be significantly reduced as large lateral displacements of piles should be prevented to ensure satisfactory seismic response of the superstructure.