Investigation of a combined failure mode for screw anchors under tension

A modified concrete capacity design method is available to predict the ultimate tensile strength (N_u ) of screw anchors. Screw anchors commonly fail in a combination of concrete breakout and pullout modes. This combined mode is not distinguished from the breakout mode in the modified concrete capacity design method, which may cause confusions to designers. To investigate the N_u of the combined mode (N_comb ), this study included 144 unconfined tension tests on screw anchors from three manufacturers in three diameters and two effective embedment depths (h_ef ) per diameter. Approximately 80% of the tested anchors failed in the combined mode. The differences in N_comb among the three manufacturers were insignificant despite their varied thread design. N_comb was found to be linearly correlated to the parameter h_ef ^1.3 but was found independent of the anchor diameter. A regression model was developed to specifically predict N_comb . The model showed a good fit with the test results.