LAUSR

A modified concrete capacity design method is available to predict the ultimate tensile strength (Nu ) 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 Nu of the combined mode (Ncomb ), this study included 144 unconfined tension tests on screw anchors from three manufacturers in three diameters and two effective embedment depths (hef ) per diameter. Approximately 80% of the tested anchors failed in the combined mode. The differences in Ncomb among the three manufacturers were insignificant despite their varied thread design. Ncomb was found to be linearly correlated to the parameter hef 1.3 but was found independent of the anchor diameter. A regression model was developed to specifically predict Ncomb . The model showed a good fit with the test results.