Experimental investigation of buckling of thin-walled cylindrical shells subjected to combined bending and torsion

The objective of this paper is to provide and evaluate experimental results for buckling of thin cylindrical shells under combined bending and torsion appropriate for applications in steel wind turbine towers. Wind turbine towers are commonly made of steel cylindrical shells with large diameter-to-thickness ratios, making stability an essential concern. Towers are subject to complex loading including compression, bending, shear, and torsion, with the combination of bending and torsion at the top of the tower being a controlling load case. Though there have been a number of studies into the stability and design of cylinders subjected to isolated loading conditions (e.g. compression or bending), investigations into the structural response of thin-walled cylinders under combined bending and torsion remain scarce. To address the knowledge gap, an experimental study was carried out on the stability of steel cylinders under combined bending and torsion. A total of 48 cylinders with diameter-to-thickness ratios ranging between 127 and 320 were tested under varying bending and torsion combinations as commonly experienced in wind turbine towers. To gain further insight into the imperfection sensitivity of the shells, a 3D laser scanner was used to determine geometric imperfections of each test specimen prior to testing. The combined bending and torsion test setup, instrumentation, loading procedures, and structural response, including ultimate resistances, load-deformation characteristics and failure modes are reported. The test results are aimed to provide a firm basis for the validation of numerical models and development of advanced design approaches, such as the Reference Resistance Design, for cylindrical shells under combined bending and torsion. Future work will involve creating laboratory scale and full-scale wind turbine tower finite element models and providing improved guidance on combined bending and torsion demands.