Frames - The Behaviour and Design of Steel Structures to EC3 - page 367

frames, and may be ignored in the analysis (their effects on member resistance
are included in Clause 6.3 of EC3). More generally, the global (frame) imperfec-
tions or fictitious forces are included in the analysis, but not the local (member)
imperfectionsorfictitiousforces.Itisraretoincludeboththeglobalandthelocal
imperfections or fictitious forces in the analysis.
The EC3 methods used for the analysis and design of frames with rigid joints
are discussed in the following sub-sections.

8.3.6.2 Strength design using first-order elastic analysis

When a braced or unbraced frame has low slenderness so that

α cr = F cr / F Ed ≥ 10 (8.28)

inwhich F Ed isthedesignloadingontheframeand F cr istheelasticbucklingload,
thenEC3allowsthemomentsandaxialforcesintheframetobedeterminedusing
a first-order elastic analysis of the frame with all the equivalent imperfections or
fictitiousforcesignored.Theframemembersareadequatewhentheiraxialforces
andmomentssatisfythesectionandmemberbucklingresistancerequirementsof
Clauses 6.2 and 6.3.3 of EC3.
When a braced frame does not satisfy equation 8.28, Clause 5.2.2(3)b of EC3
appearstoallowafirst-orderelasticanalysistobeusedwhichneglectsallsecond-
ordereffectsandtheimperfectionsassociatedwithmembercrookedness,provided
the member axial forces and moments satisfy the buckling resistance require-
mentsofClause6.3.3.Itissuggested,however,thatamplifiedfirst-orderanalysis
should be used for frames of moderate slenderness which satisfy equation 8.29
(Section8.3.6.3),andthatsecond-orderanalysisshouldbeusedforframesofhigh
slenderness which satisfy equation 8.30 (Section 8.3.6.4).

8.3.6.3 Strength design using amplified first-order elastic analysis

When an unbraced frame has moderate slenderness so that

10 >α cr = F cr / F Ed ≥ 3 (8.29)

then EC3 allows the moments in the frame to be determined by amplifying
(Section 8.3.5.6) the moments determined by a first-order elastic analysis, pro-
vided the equivalent global imperfections or fictitious forces are included in the
analysis. The frame members are adequate when their axial forces and moments
satisfy the section resistance requirements of Clause 6.2 of EC3 and the member
buckling resistance requirements of Clause 6.3.3. EC3 allows the member length
tobeusedinClause6.3.3,butthismayoverestimatetheresistancesofmembersfor
whichcompressioneffectsdominate. Itissuggestedthatinthiscase, themember