Introduction - The Behaviour and Design of Steel Structures to EC3 - page 16

(d) Axial force
and bending

(a) Axial force (b) Bending (c) Shear

Figure 1.13 Structural load-transfer actions.

Alternatively,themembersmaysupporttransverseloadswhicharetransferredby
bendingandshearactions.Usuallythebendingactiondominatesinstructurescom-
posed of one-dimensional members, such as beams and many single-storey rigid
frames (Figure 1.13b), while shear becomes more important in two-dimensional
plate structures (Figure 1.13c). The members of many structures are subjected
to both axial forces and transverse loads, such as those in multistorey buildings
(Figure1.13d).Theload-transferringactionofthemembersofastructuredepends
onthearrangementofthestructure,includingthegeometricallayoutandthejoint
details, and on the loading arrangement.
In some structures, the loading and joints are such that the members are effec-
tively independent. For example, in triangulated structures with joint loads, any
flexural effects are secondary, and the members can be assumed to act as if
pin-jointed, while in rectangular frames with simple flexible joints the moment
transfersbetweenbeamsandcolumnsmaybeignored.Insuchcases,theresponse
of the structure is obtained directly from the individual member responses.
More generally, however, there will be interactions between the members, and
the structure behaviour is not unlike the general behaviour of a member, as can
be seen by comparing Figures 1.14 and 1.12. Thus, it has been traditional to
assume that a steel structure behaves elastically under the service loads. This
assumption ignores local premature yielding due to residual stresses and stress
concentrations, but these are not usually serious. Purely flexural structures, and
purely axial structures with lightly loaded compression members, behave as if
linear (curve 1 in Figure 1.14). However, structures with both flexural and axial
actions behave non-linearly, even near the service loads (curve 3 in Figure 1.14).
This is a result of the geometrically non-linear behaviour of its members (see
Figure 1.12).
Most steel structures behave non-linearly near their ultimate loads, unless they
fail prematurely due to brittle fracture, fatigue, or local buckling. This non-linear
behaviour is due either to material yielding (curve 2 in Figure 1.14), or member
or frame buckling (curve 4), or both (curve 5). In axial structures, failure may