Beam-element-based analysis of locally and/or distortionally buckled members: Theory.

The paper describes the derivation of a beam finite element method which enables the effects of local and/or distortional buckling deformations to be accounted for. The development of local/distortional buckling reduces the rigidity of the section against axial straining, minor and major axis flexure, as well as twisting. The reduction in rigidity can be determined by an analysis performed prior to the frame analysis, in which tangent rigidities (( EA ) t , ( EI z ) t , ( EI y ) t , ( EI w ) t ) and other tangential stiffness terms are obtained for increasing values of generalised strains ( ε x , κ z , κ y , κ x ). The paper sets out the tangential stiffness matrix for a locally/distortionally buckled element and discusses in detail the determination of tangent rigidities. The companion paper [1] shows that close agreement can be obtained between the beam analysis which incorporates reduced rigidities and analysis using full shell finite element discretisation. The paper is limited in scope to doubly-symmetric open sections. [ABSTRACT FROM AUTHOR]