Lateral buckling of beams - The Behaviour and Design of Steel Structures to EC3 - page 227

Lateral buckling of beams
6.1Introduction
In the discussion given in Chapter 5 of the in-plane behaviour of beams, it was
assumedthatwhenabeamisloadedinitsstifferprincipalplane,itdeflectsonlyin
that plane. If the beam does not have sufficient lateral stiffness or lateral support
toensurethatthisisso,thenitmaybuckleoutoftheplaneofloading,asshownin
Figure 6.1. The load at which this buckling occurs may be substantially less than
the beam's in-plane load resistance, as indicated in Figure 6.2.
Foranidealisedperfectlystraightelasticbeam,therearenoout-of-planedefor-
mations until the applied moment M reaches the elastic buckling moment M cr ,
whenthebeambucklesbydeflectinglaterallyandtwisting,asshowninFigure6.1.
These two deformations are interdependent: when the beam deflects laterally, the
appliedmomenthasacomponentwhichexertsatorqueaboutthedeflectedlongi-
tudinalaxiswhichcausesthebeamtotwist.Thisbehaviour,whichisimportantfor
longunrestrainedI-beamswhoseresistancestolateralbendingandtorsionarelow,
iscalledelasticflexural-torsionalbuckling,(referredtoaselasticlateral-torsional
buckling in EC3). In this chapter, it will simply be referred to as lateral buckling.
The failure of a perfectly straight slender beam is initiated when the additional
stresses induced by elastic buckling cause the first yield. However, a perfectly
straight beam of intermediate slenderness may yield before the elastic buckling
moment is reached, because of the combined effects of the in-plane bending
stresses and any residual stresses, and may subsequently buckle inelastically, as
indicated in Figure 6.2. For very stocky beams, the inelastic buckling moment
may be higher than the in-plane plastic collapse moment M p , in which case the
moment resistance of the beam is not affected by lateral buckling.
Inthischapter,thebehaviouranddesignofbeamswhichfailbylateralbuckling
and yielding are discussed. It is assumed that local buckling of the compression
flangeoroftheweb(whichisdealtwithinChapter4)doesnotoccur.Thebehaviour
anddesignofbeamsbentaboutbothprincipalaxes,andofbeamswithaxialloads,
are discussed in Chapter 7.