Civil Engineering Reference
In-Depth Information
Frames
8.1Introduction
Structural frames are composed of one-dimensional members connected together
in skeletal arrangements which transfer the applied loads to the supports. While
mostframesarethree-dimensional,theymayoftenbeconsideredasaseriesofpar-
allel two-dimensional frames, or as two perpendicular series of two-dimensional
frames. The behaviour of a structural frame depends on its arrangement and
loading, and on the type of connections used.
Triangulated frames with joint loading only have no primary bending actions,
and the members act in simple axial tension or compression. The behaviour and
design of these members have already been discussed in Chapters 2 and 3.
Frames which are not triangulated include rectangular frames, which may be
multi-storey, or multi-bay, or both, and pitched-roof portal frames. The members
usuallyhavesubstantialbendingactions(Chapters5and6), andiftheyalsohave
significant axial forces, then they must be designed as beam-ties (Chapter 2) or
beam-columns (Chapter 7). In frames with simple connections (see Figure 9.3a),
themomentstransmittedbytheconnectionsaresmall,andoftencanbeneglected,
andthememberscanbetreatedasisolatedbeams,oraseccentricallyloadedbeam-
tiesorbeam-columns. However, whentheconnectionsaresemi-rigid(referredto
assemi-continuousinEC3)orrigid(referredtoascontinuousinEC3),thenthere
are important moment interactions between the members.
When a frame is or can be considered as two-dimensional then its behaviour is
similartothatofthebeam-columnsofwhichitiscomposed.Within-planeloading
only,itwillfaileitherbyin-planebending,orbyflexural-torsionalbucklingout
ofitsplane. If, however, theframeoritsloadingisthree-dimensional, thenitwill
fail in a mode in which the individual members are subjected to primary biaxial
bending and torsion actions.
In this chapter, the in-plane, out-of-plane, and biaxial behaviour, analysis,
and design of two- and three-dimensional frames are treated, and related to the
behaviouranddesignofisolatedtensionmembers,compressionmembers,beams,
and beam-columns discussed in Chapters 2, 3, 5, 6, and 7.
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