Local and overall instabilities of the steel beam occur in the hogging-moment region in a
continuous composite beam (Figure 1) and these
forms of buckling have been recognised to be
highly interactive. The overall mode of buckl
ing in composite steel-concrete tee-beams in
regions of negative bending is what is herein te
rmed restrained-distortio
nal buckling (RDB). In
negative bending the slab restrains the tension re
gion of the steel and the neutral axis is not
located at the mid-height of the web. The neutra
l axis is shifted towards the top/tension flange,
and in negative bending the steel region is subjected to predominantly compressive loading. In
addition, the web usually carries
proportionally higher shear loads than in ordinary steel beams.
The lateral-distortional buckling resistance of th
e steel portion in continuous composite beams is
therefore dependent on the extent to which the web can provide a restraining action to the
unstable compression flange. Although buckling of plain steel beams in both the elastic and
inelastic ranges of structural response has been
studied extensively and a great deal of research
work has been devoted to the understanding of their buckling modes, and codes of practice for
the design of structural steelwork contain relevant
clauses that presently are considered to be
quite accurate, buckling of the steel component in
composite beams still repr
esents a grey area in
structural engineering research and is much less well documented. The bubble augmented spline
finite strip method of analysis, developed by the authors, is thus employed herein to study
extensively the issue of RDB in composite tee-beams in hogging bending regions.