Experimental Results of Stability of Cylindrical Shells under Combined Bending and Torsion

Modern wind turbines are often supported by tubular steel towers made from globally conical, locally cylindrical shells with relatively large diameter-to-thickness ratios-approximately between 100 and 300-which enables the tower material to be used as efficiently as possible. Wind turbine towers face complex loading resulting from both environmental and operational load cases and are sensitive to geometrical imperfections that inevitably arise during the fabrication process. Whereas bending often controls at the base of turbine towers, the upper sections are controlled by combined bending and torsion. Although extensive studies have been conducted on the stability and design of cylinders subjected to isolated actions, investigations into the structural response of thin-walled cylinders under combined actions, such as bending and torsion, remain limited. To address this knowledge gap, an experimental program was carried out to study the structural behavior of thin-walled steel cylinders under combined bending and torsion. A total of 48 cylinders were tested with varying diameter-to-thickness ratios and torsion-to-moment ratios found in wind turbine towers. To gain insights into the imperfection sensitivity of these tests, a laser scanner was used to measure the geometric imperfections of each specimen before testing. The test setup, instrumentation, loading procedures and structural response of the cylinders, including ultimate resistances, load-deformation characteristics, and failure modes, are reported. The primary objective of this study is to provide benchmark test data for the validation of numerical models and the development of advanced design methodologies, such as reference resistance design (RRD), for cylindrical shells under combined bending and torsion. Future work will involve formulating guidelines for using laser-scanned data to evaluate geometric imperfections, developing laboratory- and full-scale wind turbine tower finite element models, and ultimately providing improved design guidance on combined bending and torsion.