Thin-walled, metallic structures are widely used across many engineering industries and are a popular choice due to their high load bearing capacity to self weight ratios. Interactive buckling is a common and potentially dangerous form of instability in these structures. The current work aims to investigate interactive global-local buckling in an I-section compression strut with rigidly
rotating flange-web joints, using primarily an analytical approach.

The analytical approach uses the Rayleigh-Ritz method, combined with continuous displacement functions to formulate a system of ordinary
differential and integral equations, describing the equilibrium states of the strut. Initially, weak axis global-local buckling interaction
is considered where both the flange and the web components of the cross-section contribute to the local buckling mode, owing to the
rigidly rotating flange-web joint. The solutions are validated using a finite element (FE) model, showing excellent comparisons. The strut
is then considered to be braced in the weak axis, thus susceptible to strong axis global-local buckling interaction. The strong axis global
buckling mode and local buckling of the flange and web components are first considered separately, revealing a neutrally stable and stable
post-buckling response respectively. The buckling modes are then combined in an analytical model, enabling them to act simultaneously;
it is found that the critical, global buckling mode has a neutrally stable post-buckling path, which then becomes highly unstable when the
local buckling mode is triggered and mode interaction is observed. The solution is validated against an FE model and shows excellent
comparisons.

Imperfection sensitivity of the strut is then investigated, revealing that the structure is sensitive to both global and local initial
geometric imperfections. The shape of the local imperfection to which the strut is most sensitive is also identified and it shows a greater
sensitivity when both global and local imperfections are present simultaneously. The solutions for an example strut with imperfections
is compared to an FE model, again showing excellent comparisons.

Parametric studies are conducted to investigate the effect of varying the geometry of the strut. Both the strut length and cross-section
height are varied in independent studies, identifying the geometries that give rise to the most interactive, and therefore most
undesirable, behaviour in the structure. The implications of the identified behaviour on the design of similar structures is
discussed. The post-buckling behaviour of a thin-walled I-section strut, buckling under either weak or strong axis global-local mode
interaction with rigidly rotating flange-web joints has therefore been established at a fundamental level, using an analytical approach.