Experimental Investigation of the Pull-Out Behavior of Semi-Embedded Anchor Bolts in Concrete Transition Sections

To investigate the pull-out behavior of semi-embedded anchor bolts in the concrete transition sections of steel–concrete hybrid wind turbine towers, five 1:2-scale local specimens were tested under monotonic pull-out loading. The effects of reinforcement, end-plate size, number of anchor bolts, and sleeve installation on the failure mode, load–cumulative surface crack-opening displacement response, reinforcement strain, and ultimate pull-out capacity were analyzed. The results show that reinforcement significantly improved both the pull-out resistance and the apparent deformation capacity characterized by cumulative surface crack opening, changing the failure mode from brittle cracking of plain concrete to a ductile combined mechanism governed by end-plate bearing, concrete splitting, reinforcement restraint, and boundary effects. Increasing the end-plate size, changing the anchor arrangement, and installing a sleeve produced only indicative trends in this limited test series because only one specimen was tested for each configuration. Existing methods were also evaluated as benchmark approaches. The characteristic resistance predicted by EN 1992-4 was conservative, partly because of its fractile basis and partly because the tested configuration was outside the assumptions of an unconstrained concrete cone breakout model. The 45° cone method underestimated the capacity of the reinforced specimens but slightly overestimated that of the plain-concrete specimen, whereas the Nilforoush and LEFM methods overestimated the capacity. These findings provide initial experimental evidence for understanding the load-transfer and failure mechanisms of semi-embedded anchor bolts and clarify the applicability and limitations of the existing calculation methods. The development of a reliable predictive model requires a broader experimental database and systematic numerical investigations.