Structural behaviour of horizontal joints in steel fibre reinforced concrete wind turbine towers

The horizontal joints in steel fibre reinforced concrete (SFRC) wind turbine towers are critical regions governing overall structural safety. However, existing design methods oversimplify the complex stress state under combined bending–torsion, resulting in significant deviations in predicted torsional resistance. To improve prediction accuracy, this study experimentally and numerically investigates the mechanical behaviour of SFRC tower horizontal joints under pure torsion and combined bending–torsion. Five 1:10-scale SFRC tower specimens were designed and tested, and a finite element (FE) model was developed, validated, and used for parametric analyses of concrete strength, geometric dimensions, prestress level, friction coefficient, and initial bending level. Both the experimental and FE results show that bending-induced joint decompression and opening reduce the effective contact area and substantially decrease torsional resistance. Relative to the pure-torsion baseline, the ultimate torsional resistance decreases by 44.9% when the initial bending level reaches the joint-decompression threshold. The observed failure modes are governed primarily by interfacial frictional slip at the horizontal joint and material cracking of the SFRC tower wall. Comparisons with FE reference values show that the DIN-based method exhibits substantial scatter and underestimates several post-opening cases, whereas T/CEC 5008 generally overestimates the simulated resistance. A modified calculation method is proposed based on the distinct failure modes and joint-opening behaviour. The proposed method achieves a mean predicted-to-simulated ratio of 1.08 with a coefficient of variation of 0.06, providing a more accurate basis for the torsional design of large-scale concrete wind turbine towers.