Pull-out behaviour of pre-installed anchor bolts in concrete transition segments for wind turbines

The transition segment plays a critical role in ensuring the structural integrity of wind turbine hybrid towers. Although the pull-out behaviour of conventional anchor bolts embedded in concrete has been extensively investigated, existing studies have mainly focused on fully embedded or shallowly embedded anchors in ordinary concrete members. Limited studies have been reported into the semi-embedded anchor bolts used in concrete transition segments. To address this research gap, a total of six specimens were prepared and tested under pull-out loading. The failure modes, load-displacement responses, and strain distributions of the specimens were recorded and analysed in detail, together with the key factors influencing their load-carrying capacities. Subsequently, a finite element (FE) model was developed and validated against the test results in terms of the load-displacement curves, ultimate pull-out capacities, and failure modes. Based on the validated model, an extensive parametric study including 124 FE models was conducted. The results revealed that providing supplementary reinforcement and increasing the reinforcement ratio could significantly enhance the pull-out capacity. Furthermore, the ultimate capacity increased with greater embedment depths and edge distances. The applicability of existing design methods for predicting the pull-out capacity of anchor bolts was evaluated using the obtained test and FE results. These existing methods were demonstrated to provide unsafe predictions, primarily because they typically neglect the contribution of steel reinforcement and are predominantly established based on shallowly embedded anchors. A new design method was therefore proposed based on theoretical analysis and numerical simulations, providing accurate predictions with a mean predicted-to-reference capacity ratio of 1.01 and a coefficient of variation of 0.036.