Interactive buckling in thin-walled rectangular hollow section struts
Creators: name: Shen, Jiajia
Publisher: name: Imperial College London
Year: 2018

description: Thin-walled rectangular hollow section (RHS) struts are widely used in current structural engineering practice due to their mass efficiency and relative ease of manufacture. Owing to their optimized geometric properties, they are vulnerable to local--global interactive buckling and exhibit highly unstable post-buckling behaviour with severe imperfection sensitivity when the local buckling load is close to the global buckling load. The current work investigates the underlying mechanism of local–global interactive buckling of RHS struts using both analytical and finite element (FE) approaches. Variational models formulated using analytical techniques, describing the nonlinear local–global mode interaction in thin-walled RHS struts with varying flange–web joint rigidity, different strut lengths and geometric imperfections under pure compression, are presented. A system of nonlinear differential and integral equations subject to boundary conditions is formulated and solved using numerical continuation techniques. For the first time, the equilibrium behaviour of such struts with different cross-section joint rigidities is highlighted with characteristically unstable interactive buckling paths and a progressive change in the local buckling wavelength. Studies on the effects of strut length identify the boundaries for the four distinct length-dependent zones, where different characteristic post-buckling behaviour are exhibited. The most unstable zone is demonstrated to have a considerably narrower range than previously determined owing to the consideration of more realistic corner boundary conditions within the cross-section. Imperfection sensitivity studies identify the high degree of sensitivity of struts exhibiting mode interaction. They also reveal that local and global imperfections are relatively more significant where global and local buckling are critical respectively. Moreover, a unified local geometric imperfection measurement based on equal local bending energy is proposed to determine the most severe local imperfection profile. It reveals that the most severe profile is modulated rather than periodically distributed along the strut length for purely elastic case. For verification and extensive parametric study purposes, a nonlinear FE model, which considers material nonlinearity, geometric imperfections, and residual stresses, is developed within the commercial package Abaqus. The classical solutions and experimental results from two independent studies are used to verify and validate the FE model, both of which show excellent comparisons. The validated FE model is then used to verify the variational model, which also shows excellent comparisons in local buckling wavelengths, cross-section deformation profile, ultimate load and the mechanical behaviour. Finally, parametric studies on geometric properties, material nonlinearity and residual stresses are conducted using the developed FE model to understand the behaviour of RHS struts exhibiting mode interaction in more practically realistic scenarios. Based on the numerical results and existing experimental results from the literature, the current design rules for thin-walled welded RHS struts are assessed by means of reliability analysis in accordance with Annex D of EN1990. A modified Direct Strength Method (DSM) equation has been proposed and it is shown to provide a better ultimate load prediction than it does presently.