Study on the Ductile Behavior of Fixed-head Piles

For fixed-head piles subjected to seismic loading, they usually sustain large flexure curvature demands at the pile heads when the pile cap undergoes a horizontal displacement. For this type of pile, it is cost effective to design the piles as ductile members to absorb the earthquake energy, instead of elastic ones, which are normally adopted in conventional seismic design. Consequently, the displacement ductility capacity of a pile is an important concern when the ductile design is applied to piles under seismic loading. This study performs parametric analysis to investigate the influencing factors of the displacement ductility capacity of a fixed-head pile. In addition to the curvature ductility, the results of the parametric analysis show that the over-strength ratio of the pile section is another important factor that influences the displacement ductility capacity of the pile. Furthermore, this study develops simple formulae to estimate the displacement ductility capacity of a fixed-head pile in cohesive and cohesionless soils. The formulae have three parameters: the sectional over-strength ratio, curvature ductility capacity, and a modification factor considered for soil nonlinearity. For demonstrating the applicability of the proposed formulae, this study further applies them to seismic design of the pile foundation of wall piers.
On the other hand, the softening in flexural strength of a RC pile is commonly observed in either experiments or numerical analysis. This study performs a parametric study on its mechanism. The results show that the softening in flexural strength of pile sections is due to the loss of concrete strength and the softening degree of flexural strength is significantly influenced by the axial force level on the pile section. For this type of pile section, its sectional over-strength ratio is small and the displacement ductility capacity of the pile is accordingly small.