Lateral-torsional buckling strength of castellated beams.

A review of the research carried out on the subject of
castellated beams revealed that one subject had remained largely
untouched — lateral stability. This was despite the fact that the
fabrication process had increased their strength and rigidity about the
,
plane of loading at the expense of lateral stiffness.
This emphasis on in—plane behaviour stemmed from the need to
catalogue the failure modes particular to castellated beams. However
because of the high number of parameters necessary to describe a
castellated beam and their high degree of internal redundancy, little
that can be regarded as firm design recommendations and no provisions
against lateral buckling have been included in national codes of
practice, particularly in the present British codes.
This is why the draft of the new British code for structural
steelwork 8/20 to be published as BS 5950 suggests the use of the simple
Vierendeel analogy for in—plane behaviour and has adopted the
conservative approach of 0..11.3.2 to the prevention of lateral buckling
in which the contribution of the web and tension flange are ignored.
The work undertaken herein had the aim of mainly providing the
missing quantitative data on the lateral —torsional buckling strength of
castellated sections currently available in the U.K. Eight full size
castellated beams were tested. The results of these plus the few cases
reported in the literature were used as a basis for a critical
evaluation of several design approaches.
Comparisons between the test results and the strength
predicted by B/20 were found to be generally acceptable provided cross—
sectional properties at a castellation were used in the calculations. Similar results were obtained for the two — stage procedure of BS 449 and
BS 153 whereas the use of Table 3 in the former was shown to lead to
rather low load factors.
A preliminary test programme on small scale beams showed the
negligible effect that the holes had on lateral buckling behaviour.
Finally a computer program which was used to calculate a value of
elastic critical load confirmed that the web post did not distort when
the beams failed.