Aspects of Structural Design of Drilled Shafts for Flexure

Drilled shafts are often designed to carry lateral loads, as in cantilevered or tied-back retaining walls, bridge abutments, building and bridge piers subjected to asymmetric vertical loads or lateral loads from wind, vessel impact or seismic events. The nature of construction of drilled shafts sometimes involves the introduction of structural flaws. Flaws large enough to be detected by non-destructive evaluation methods (NDE) are almost always repaired or the drilled shaft is replaced. However small flaws could remain undetectable and may adversely affect the moment capacity of drilled shafts, especially when they occur near the positions of maximum moments. It is therefore important to investigate the effect of such undetectable minor structural flaws on the flexural capacity of drilled shafts. A numerical model was selected, verified and calibrated against data obtained from an experimental study that addressed the effect of various types of various minor structural flaws on the flexural strength of drilled shafts. The adopted numerical model, after calibration, reproduced moment capacities for the tested shaft specimens with sufficient accuracy (avg. error of 2.8 %). The calibrated numerical model was then used to study the effect of a wide range of flaws on the flexural capacity of drilled shafts. Reduction factors were calculated for worst-case scenarios of single flaws as well as flaw combinations assumed to occur simultaneously at positions of maximum moments. The numerical study revealed that the most severe reduction in flexural capacity for a single flaw was about 27 percent and was associated with a void flaw that occupied 15 percent of the cross section and penetrated inside the concrete shaft core. Reductions in flexural capacities were also calculated for cases were more than one flaw occurred simultaneously.