Frames - The Behaviour and Design of Steel Structures to EC3 - page 369

lengths), and by in-plane buckling effects (by using equation 8.31 to limiting the
member in-plane slendernesses and axial compression forces).
The frame members are adequate when

α p = F p / F Ed ≥ 1 (8.32)

in which F p is the plastic collapse load. It should be noted that the use of reduced
full plastic moments (to allow for the axial forces) will automatically ensure that
Clause 6.2.9 of EC3 for the bending and axial force resistance is satisfied.

8.3.6.6 Strength design using advanced analysis

EC3permitstheuseofasecond-orderplasticanalysis.Whentheeffectsofresidual
stresses and geometrical imperfections are allowed for through the use of global
and local equivalent imperfections or fictitious forces, and when local and lateral
bucklingareprevented(Clauses5.6and6.3.5ofEC3),thenthisprovidesamethod
of design by advanced analysis.
The members of the structure are satisfactory when the section resistance
requirements of Clause 6.2 of EC3 are met. Because these requirements are auto-
matically satisfied by the prevention of local buckling and the accounting for
inelastic behaviour, the members can be regarded as satisfactory if the analysis
showsthatthestructurecanreachanequilibriumpositionunderthedesignloads.

8.3.6.7 Serviceability design

Because the service loads are usually substantially less than the factored loads
used for strength design, the behaviour of a frame under its service loads is
usually closely approximated by the predictions of a first-order elastic analysis
(Section 8.3.5.2). For this reason, the serviceability design of a frame is usually
carried out using the results of a first-order elastic analysis. The serviceability
design of a frame is often based on the requirement that the service load deflec-
tions must not exceed specified values.Thus the member sizes are systematically
changed until this requirement is met.

8.3.7 Out-of-plane behaviour of frames with rigid joints

Most two-dimensional rigid frames which have in-plane loading are arranged so
thatthestifferplanesoftheirmemberscoincidewiththatoftheframe.Suchaframe
deformsonlyinitsplaneuntiltheout-of-plane(flexural-torsional)bucklingloads
arereached,andifthesearelessthanthein-planeultimateloads,thenthemembers
and the frame will buckle by deflecting out of the plane and twisting.
Insomecases,theactionofoneloadedmemberdominates,anditselasticbuck-
lingloadcanbedeterminedbyevaluatingtherestrainingeffectsoftheremainder
of the frame. For example, when the frame shown in Figure 8.11 has a zero beam