The economic use of cold-formed steel members means that buckling and the possible loss
of effectiveness it produces are important features of design. Cross-sectional instabilities in
laterally-restrained cold-formed steel beams include local and distortional buckling. The
prediction of the true buckling behaviour of cold-formed steel beams accounting for all
governing features such as geometrical imperfections, spread of yielding, postbuckling etc.
has been possible with the development of advanced numerical modelling. In this thesis, the
finite element (FE) method (ABAQUS) has been used to develop numerical analyses to study
the buckling behaviour of laterally-restrained cold-formed steel lipped Z-section beams. The
FE models were verified against a series of four-point bending tests available from previous
research, with special references to material and geometrical nonlinearities. Two sets of
analyses have been conducted: FE analyses allowing for both local and distortional buckling
and ones allowing for local buckling while distortional buckling is restrained using
appropriate boundary conditions. For the former, the controlling buckling mode (local,
distortional or combined) at different stages of loading (up to, at and beyond maximum load)
has been realized. Comparing the results of two sets of analyses, the effect of distortional
buckling on performance for different geometric proportions has been studied.
The effect of the lip size, flange width, angle of inclination of the edge stiffener (lip), size
and position of the intermediate stiffener and material strength as well as the interaction
between them on both the ultimate strength and the buckling of cold-formed Z-section beams
has been investigated. Limits for optimum design of the section were proposed. Depending on
the geometric properties and material strength of the section, transitions between local,
distortional and combined local/distortional buckling were observed. The lip/flange
interaction including the interaction between the edge stiffener (lip) and the intermediate
stiffener was the key governing feature of behaviour.
The effect of the linear moment gradient and sharply varying bending moment on both the
ultimate strength and the buckling of cold-formed Z sections was investigated. The latter
occurred in two-span continuous beams subject to uniformly distributed loading. The results
of moment gradient cases were compared with those of pure bending cases.
The suitability of the design treatments available in Eurocode 3 (EC3) for local, combined
local/distortional and distortional buckling of cold-formed Z-section beams was assessed.
Overall, the EC3 predictions for cross-sectional bending resistances were unconservative.
Shortcomings were identified and some suggestions for improvements were made. This
included improvements in plate buckling factors for edge-stiffened compression flanges.