Frames - The Behaviour and Design of Steel Structures to EC3 - page 368

should also satisfy the member buckling requirements of Clause 6.3.1 in which
the effective length is used.
Ifthememberimperfectionsorfictitiousforcesarealsoincludedintheanalysis,
then the frame members are satisfactory when their axial forces and moments
satisfy the section resistance requirements of Clause 6.2 of EC3.

8.3.6.4 Strength design using second-order elastic analysis

When an unbraced frame has high slenderness so that

3 >α cr = F cr / F Ed (8.30)

thenEC3requiresamoreaccurateanalysistobemadeofthesecond-ordereffects
(Section 8.3.5.5) than by amplifying the moments determined by a first-order
elastic analysis. EC3 requires the equivalent global imperfections or fictitious
forcestobeincludedintheanalysis.Theframemembersareadequatewhentheir
axialforcesandmomentssatisfythesectionresistancerequirementsofClause6.2
of EC3 and the member buckling resistance requirements of Clause 6.3.3. EC3
allowsthememberlengthtobeusedinClause6.3.3,butthismayoverestimatethe
resistances of members for which compression effects dominate. It is suggested
thatinthiscase,themembershouldalsosatisfythememberbucklingrequirements
of Clause 6.3.1 in which the effective length is used.
Ifthememberimperfectionsorfictitiousforcesarealsoincludedintheanalysis,
thentheframemembersaresatisfactorywhentheaxialforcesandmomentssatisfy
the section resistance requirements of Clause 6.2 of EC3.

8.3.6.5 Strength design using first-order plastic collapse analysis

When a braced or unbraced frame has low slenderness so that

α cr = F cr / F Ed ≥ 15 (8.31)

thenEC3allowsalltheequivalentimperfectionsorfictitiousforcestobeignored
and a first-order rigid plastic collapse analysis (Section 8.3.5.7) to be used. This
limit is reduced to α cr ≥ 10 for clad structures, and to α cr ≥ 5 for some portal
frames subjected to gravity loads only.
All members forming plastic hinges must be ductile so that the plastic moment
capacitycanbemaintainedateachhingeoverarangeofhingerotationssufficient
toallowtheplasticcollapsemechanismtodevelop.Ductilityisusuallyensuredby
restrictingthesteeltypetoonewhichhasasubstantialyieldplateauandsignificant
strain-hardening(Section1.3.1)andbypreventingreductionsinrotationcapacity
by local buckling effects (by satisfying the requirements of Clause 5.6 of EC3,
including the use of Class 1 sections), by out-of-plane buckling effects (by sat-
isfying the requirements of Clause 6.3.5 of EC3, including limiting the unbraced