Civil Engineering Reference
In-Depth Information
Q
2
Q
1
Q
1
L/L
1
=0.5
Q
2
Q
1
Q
1
L/L
1
=2
Column loads
Q
1
Figure 8.12
Elastic flexural-torsional buckling loads of portal frames.
ofthisprogramtocalculatetheelasticflexural-torsionalbucklingloadofaframe
will simplify the design problem considerably.
Interactiondiagramsfortheelasticflexural-torsionalbucklingloadsofanumber
of frames have been determined [45], and two of these (for the portal frames
shown in Figure 8.11) are given in Figure 8.12. These diagrams show that the
region of stability is convex, as it is for the in-plane buckling of rigid frames
(Figures8.6and8.7)andfortheflexural-torsionalbucklingofcontinuousbeams
(Figure6.23b).Becauseofthisconvexity,linearinterpolations(asinFigure6.24)
made between known buckling load sets are conservative.
Althoughtheelasticflexural-torsionalbucklingofrigidframeshasbeeninves-
tigated,therelatedproblemsofinelasticbucklinganditsinfluenceonthestrength
ofrigidframeshavenotyetbeensystematicallystudied.However,advancedcom-
puterprogramsfortheinelasticout-of-planebehaviourofbeam-columns[49-54]
have been developed, and it seems likely that these will be extended in the near
future to rigid-jointed frames.
8.3.8 Out-of-plane design of frames
Whilethereisnogeneralmethodyetavailableofdesigningforflexural-torsional
buckling in rigid frames, design codes imply that the strength formulations for
isolated beam-columns (see Section 7.3.4) can be used in conjunction with the
moments and forces determined from an appropriate elastic in-plane analysis of
the frame.
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