Postearthquake Vertical Load-Carrying Capacity of Extended Pile Shafts in Cohesionless Soils: Quasi-Static Test and Parametric Studies

The belowground portion of extended pile-shaft-supported bridges is usually prone to earthquake-induced damage during earthquake shakings. The unobservable pile damage below the ground surface makes it difficult and time-consuming to estimate the functionality loss of these damaged bridges and decide whether to reopen them for emergency traffic after an earthquake. Therefore, the reliable and rapid evaluation of the postearthquake vertical load-carrying capacity of extended pile shafts is essential for postearthquake recovery. To this end, one extended pile-shaft specimen in homogeneous sand was first subjected to the lateral cyclic loads applied to the column head in the laboratory to simulate earthquake loads. A pushdown test was then performed on this laterally damaged specimen to determine its residual vertical load-carrying capacity. After that, a finite element model based on the beam on a nonlinear Winkler foundation (BNWF) for the quasi-static test was built and validated using the experimental data. Finally, a comprehensive parametric study was carried out to investigate the postearthquake vertical load-carrying capacity of extended pile shafts, exhibiting a lateral residual displacement, considering the variation of structure and soil parameters. After that, a continuous loss model for the vertical load-carrying capacity of the extended pile shafts in cohesionless soils was developed. Results indicate that the ratio between the height of the aboveground column and the column diameter had a considerable impact on the residual vertical load-carrying capacity due to the P-Delta effect. The greater the ratio, the faster the normalized vertical strength degradation rate. The relative and mass densities of sandy soil and concrete strength had an insignificant impact on the normalized vertical strength. The relationship between the normalized vertical strength and the pile curvature ductility and the residual column drift ratio was described using piecewise linear regression models in logarithmic space, respectively. Based on the loss model, multilevel limit values of the peak pile curvature ductility and the residual column drift ratio were recommended for different traffic capacity levels of extended pile-shaft-supported bridges. This research represents a first step toward developing a rapid postearthquake assessment approach for the extended pile-shaft-supported bridges.