Axial response and material efficiency of tapered helical piles

Different techniques have been proposed to increase the bearing capacity of open-ended piles.Welding helices to the shaft and tapering the pile shaft could be used simultaneously to enhance the static and dynamic behaviors of these piles.This paper subjects the bearing capacity,stiffness,frictional behavior,and material efficiency of the tapered helical piles to scrutiny.Tapered helical piles are introduced herein as an alternative option to improve the material efficiency of hollow piles.Based on the Taguchi method,a series of experiments was designed and conducted.The axial responses of tapered helical piles are also investigated using finite element analyses.The results derived from loadedisplacement curves and strain gages are used to characterize the axial compression responses of tapered helical piles.The effects of tapered angle,helices diameter and helices distance are examined using dimensionless parameters,and the degree of contribution of these factors is calculated on each of the enumerated variables individually.Experimental results show that the shaft friction resistance of tapered helical piles increases continuously with the pile head settlement.Furthermore,the effect of tapered wall on the shaft friction resistance is more tangible at low stress levels.The results showed that the relative material efficiency factor of the optimum pile could be 2.5 times that of unoptimized pile with a similar quantity of material.

Effect of foundation flexibility on ductility reduction factors for R/C stack-like structures

The most important parameter used to determine force reduction factors in force-based design procedures adopted in the current seismic codes is the structural ductility. For a structure supported on a flexible foundation, the ductility factor could be affected by foundation compliances. The ductility factors given in the current codes are mostly assigned ignoring the effect of SSI and therefore the objective of this research is to assess the significance of SSI phenomenon on ductility factors of stack-like structures. The deformed configuration of stack-fike structures is idealized as an assemblage of beam elements considering nonlinear moment-curvature relations, while a linear sway-rocking model was implemented to model the supporting soil. Using a set of artificial records, repeated linear and nonlinear analyses were performed by gradually increasing the intensity of acceleration to a level where the first yielding of steel in linear and nonlinear analyses is observed and a level corresponding to the stack collapse in the nonlinear analysis. The difference between inelastic and elastic resistance in terms of displacement ductility factors has been quantified. The results indicate that foundation flexibility can decrease the ductility of the system and neglecting this phenomenon may lead to erroneous conclusions in the prediction of the seismic performance of flexibly-supported R/C stack-like structures.