Title:
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On the lateral stability of beams
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The problem of the lateral instability of deep beams under
the action of bending loads applied in tiieir planes of greatest
flexural rigidity has been a subject of investigation by
engineers since the introduction of the first rolled sections,
over a century ago. The elementary theories for the
determination of the bending stresses sufficient to cause
failure of a beam by buckling have been evolved by a number
of mathematicians, but there has been little experimental
verification of these results by tests under prop@rly controlled
conditions.
Although the buckling of beams has been compared with, the
failure of columns under end loads, the possibility of such
beam failure in practice has been of far less importance than
that of stanchion buckling--due,, primarily, to the fact that in
most cases considerable restraints are applied to a beam under
working conditions which invariably increase the loads necessary
to cause instability. Also the materials in general use- have
low specific strengths with limiting design stresses usually
lower than the critical buckling stresses for beams of normal
proportions. With the introduction as structural materials
of high strength steels, and also of aluminium alloys having
low elastic modulii, the problem becomes more crucial from
the design aspect, and recently it has been found desirable to
include design data, based on the fundamental theory, in new
codes of Structural Engineering practice. The recent draft
Code of Practice for Structural use of Steel in Buildings l
contains clauses relating to the lateral stability of steel
beams in which design formulae, deduced from the theoretical
results for beams under idealised conditions, are suggested
as alternatives to the existing empirical formulae in terms
of the slenderness ratio L. This improved design data will
give estimates of the permissible stresses in a beam which
will generally be in error on the safe side and in many
cases of beams used as floor joists or frame members it can
be shown that failure due to lateral buckling cannot occur
at flange stresses lower than the yield stress of the
material, snd even then only as a secondary form of failure.
The qualitative analysis of the influence of the various
factors influencing the stability of beams under practical
conditions may prove of great assistance to designers who
at present sacrifice economy in their materials due to
reduction of the allowable stresses as a precaution against
buckling, -which may prove unnecessary when the actual
conditions of loading and support are considered.
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