Frames - The Behaviour and Design of Steel Structures to EC3 - page 372

A rational design method has been developed [55] for the columns of
two-dimensionalbuildingframesinwhichlaterallybracedhorizontalbeamsform
plastic hinges. Thus, when plastic analysis is used, columns which do not par-
ticipate in the collapse mechanism may be designed as isolated beam-columns
against flexural-torsional buckling. In later publications [56, 57] the method was
extendedsothattheoccurrenceofplastichingesatoneorbothendsofthecolumn
(instead of in the beams) could be allowed for. Because the assumption of plastic
hinges at all the beam-column joints is only valid for frames with vertical loads,
the analysis for lateral loads must be carried out independently.Aframe analysed
by this method must therefore have an independent bracing or shear wall system
to resist the lateral loads.
Anumberofcurrentresearcheffortsarebeingdirectedtowardstheextensionof
advancedmethodsofanalysingthein-planebehaviourofframes(Section8.3.5.8)
to include the out-of-plane flexural-torsional buckling effects [52-54]. The
availabilityofasuitablecomputerprogramwillgreatlysimplifythedesignoftwo-
dimensional frames in which local buckling is prevented, since such a frame can
beconsideredtobesatisfactoryifitcanbeshownthatitcanreachanequilibrium
position under the factored design loads.
8.4 Three-dimensional flexural frames
Thedesignmethodscommonlyusedwheneithertheframeoritsloadingisthree-
dimensional are very similar to the methods used for two-dimensional frames.
The actions caused by the design loads are usually calculated by allowing for
second-order effects in the elastic analysis of the frame, and then compared with
the design resistances determined for the individual members acting as isolated
beam-columns. In the case of three-dimensional frames or loading, the elastic
frameanalysisisthree-dimensional, whilethedesignresistancesarebasedonthe
biaxial bending resistances of isolated beam-columns (see Section 7.4).
A more rational method has been developed [55, 56] of designing three-
dimensionalbracedrigidframesforwhichitcouldbeassumedthatplastichinges
form at all major and minor axis beam ends. This method is an extension of the
methodofdesigningbracedtwo-dimensionalframes([55-57]andSection8.3.8).
Forsuchframes,allthebeamscanbedesignedplastically,whilethecolumnscan
againbedesignedasifindependentoftherestoftheframe.Themethodofdesign-
ingthesecolumnsisbasedonasecond-orderelasticanalysisofthebiaxialbending
of an isolated beam-column. Once again, the assumption of plastic hinges at all
the beam ends is valid only for frames with vertical loads, and so an independent
design must be made for the effects of lateral loads.
Inmanythree-dimensionalrigidframes,theverticalloadsarecarriedprincipally
bythemajoraxisbeams,whiletheminoraxisbeamsarelightlyloadedanddonot
developplastichingesatcollapse,butrestrainandstrengthenthecolumns.Inthis
case, an appropriate design method for vertical loading is one in which the major
axisbeamsaredesignedplasticallyandtheminoraxisbeamselastically.Thechief