In this paper, one develops a Generalised Beam Theory (GBT) formulation to analyse the local and global buckling behaviour of composite thin-walled members with branched cross-sections made of laminate plate FRP (fibre-reinforced plastic), which takes into account the shear deformability effects. After briefly reviewing the most noticeable differences between the GBT formulations applicable to members with branched and unbranched open thin-walled cross-sections, the paper presents in some detail the steps and procedures involved in performing a GBT cross-section analysis of an arbitrarily branched composite (laminate plate) thin-walled member, which include (i) the identification and characterisation of conventional and shear deformation modes (the latter are not relevant in isotropic members) and (ii) the determination of the corresponding modal mechanical properties. Then, one addresses the numerical implementation of the proposed GBT formulation, which is carried out by means of the finite element method (GBT-based beam element) particular attention is devoted to derivation of the elementary generalised stiffness and geometric matrices, which incorporate all the material coupling effects. Finally, in order to illustrate the application and capabilities of the proposed formulation/implementation, one presents and discusses numerical results concerning the local and global buckling behaviour of shear deformable FRP composite I-section columns with different ply orientations and stacking sequences (see Fig. 1) in particular, one takes advantage of the modal features of GBT to acquire a deeper insight on complex buckling mode interaction phenomena, such as the ones due to bending-torsion or local-shear coupling effects. For validation purposes, some of the above results are also compared with values recently reported in the literature and obtained by means of numerical implementations of analytical models developed independently by several researchers - with a single exception (addressed in detail in the paper), an excellent correlation was found between the buckling load values (the differences are always below 2%).
Composite I-section columns local buckling mode and variation of global buckling loads with fibre angle θ.