Introduction - The Behaviour and Design of Steel Structures to EC3 - page 24

1 First-order elastic analysis

Load

4

Elastic stability analysis

3 Second-order

elastic analysis

Strength load
Service load

First-order plastic
analysis
Advanced
analysis

2

5

Deformation

Figure 1.15 Predictions of structural analyses.

Someestimateoftheimportanceoftheseintheabsenceofflexuraleffectscanbe
obtained by making an elastic stability analysis .A second-order elastic analysis
accountsforbothflexureandinstability,butthisisdifficulttocarryout,although
computer programs are now generally available [44, 45]. EC3 permits the use
of the results of an elastic stability analysis in the amplification of the first-order
moments as an alternative to second-order analysis.
The analysis of statically indeterminate structures near the ultimate load is
further complicated by the decisive influence of the material and geometrical
non-linearities. In structures without material non-linearities, an elastic stability
analysis is appropriate when there are no flexural effects, but this is a rare occur-
rence. On the other hand, many flexural structures have very small axial forces
and instability effects, in which case it is comparatively easy to use a first-order
plastic analysis , according to which a sufficient number of plastic hinges must
form to transform the structure into a collapse mechanism.
More generally, the effects of instability must be allowed for, and as a first
approximationthenominalfirstyieldloaddeterminedfromasecond-orderelastic
analysismaybeusedasaconservativeestimateoftheultimateload.Amuchmore
accurate estimate may be obtained for structures where local and lateral buckling
is prevented by using an advanced analysis [46] in which the actual behaviour is
closely analysed by allowing for instability, yielding, residual stresses, and initial
crookedness. However, this method is not yet in general use.

1.7 Design of steel structures

1.7.1 Structural requirements and design criteria

Thedesigner'staskofassessingwhetherornotastructurewillsatisfythestructural
requirementsofserviceabilityandstrengthiscomplicatedbytheexistenceoferrors