Tall cylindrical shell structures such as tubular steel wind turbine support towers are frequently subject to combined bending and torsional loading, and as hub heights increase these structures become increasingly driven by the ultimate limit state of buckling. However, the complete theoretical linear and nonlinear buckling interaction between these two reference loading conditions has never been fully quantified. This paper presents a detailed computational parametric investigation into this interaction behaviour using computational buckling analyses undertaken within the capacity curve framework of the evolved Eurocode EN 1993-1-6 (2025) on metal shells, focusing specifically on the documentation of the elastic capacity parameters. A novel approach of helical meshing helps to minimise computational cost while maximising finite element solution quality, particularly for longer torsion-dominated shells. It is documented that relatively small levels of torsion have a near-negligible influence on cylindrical shells dominated by bending loads, a particularly fortuitous finding from the point of view of design, but that the entire elastic interaction relationship is highly nonlinear and length-dependent owing to the eventual development of the ovalisation under bending. The catalogue of behaviours described here will be extended to a complete elastic-plastic characterisation in due course.