Frames - The Behaviour and Design of Steel Structures to EC3 - page 366

(a) No-sway sub-assemblage (b) Sway sub-assemblage

Figure 8.10 Sub-assemblages for multi-storey frames.

More recent research [41] has concentrated on extending computer methods of
analysis so that they allow for residual stresses and geometrical imperfections as
well as for second-order effects and inelastic behaviour. Such methods can dupli-
cate design code predictions of the strengths of individual members (through the
incorporationofallowancesforresidualstressesandgeometricalimperfections)as
wellasthebehaviourofcompleteframes.Twogeneraltypesofanalysishavedevel-
oped,calledplasticzoneanalysis[42,43]andconcentratedplasticityanalysis[44].
Whileplasticzoneanalysisisthemoreaccurate,concentratedplasticityisthemore
economical of computer memory and time. It seems likely that advanced meth-
ods of analysis will play important roles in the future design of two-dimensional
rigid-jointed frames in which local and lateral buckling are prevented.

8.3.6 In-plane design of frames with rigid joints

8.3.6.1 Residual stresses and geometrical imperfections

The effects of residual stresses are allowed for approximately in EC3 by using
enhancedequivalentgeometricalimperfections.Theseincludebothglobal(frame)
imperfections (which are associated with initial sway or lack of verticality) and
local (member) imperfections (which are associated with initial bow or crooked-
ness).EC3alsoallowstheequivalentgeometricalimperfectionstobereplacedby
closed systems of equivalent fictitious forces. The magnitudes of the equivalent
geometrical imperfections and equivalent fictitious forces are specified in Clause
5.3.2 of EC3.
Insomelowslendernessframes,theequivalentimperfectionsorfictitiousforces
have only small effects on the distributions of moments and axial forces in the