Civil Engineering Reference
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whatever the stress level is. Internal forces and moments may be calculated
according to elastic global analysis even if the resistance of a cross section is
based on its plastic resistance. Elastic global analysis may also be used for cross
sectioning the resistances of which are limited by local buckling. On the
other hand, plastic global analysis allows for the effects of material nonlinear-
ity in calculating the action effects of a structural system. The behavior
should be modeled by elastic-plastic analysis with plastified sections and/
or joints as plastic hinges, by nonlinear plastic analysis considering the partial
plastification of members in plastic zones, or by rigid plastic analysis neglect-
ing the elastic behavior between hinges. Plastic global analysis may be used
where the members are capable of sufficient rotation capacity to enable the
required redistributions of bending moments to develop. Also, plastic global
analysis should only be used where the stability of members at plastic hinges
can be assured. A bilinear stress-strain relationship may be used for different
structural steel grades. Rigid plastic analysis may be applied if no effects of the
deformed geometry (e.g., second-order effects) have to be considered.
The role of cross-sectional classification is to identify the extent to which
the resistance and rotation capacity of cross sections is limited by its local
buckling resistance. Four classes of cross sections are defined in EC3
[2.11]: class 1 cross sections, which are sections that can form a plastic hinge
with the rotation capacity required from plastic analysis without reduction of
the resistance; class 2 cross sections, which are sections that can develop their
plastic moment resistance, but have limited rotation capacity because of local
buckling; class 3 cross sections, which are sections in which the stress in the
extreme compression fiber of the steel member assuming an elastic distribu-
tion of stresses can reach the yield strength, but local buckling is liable to
prevent the development of the plastic moment resistance; and, finally, class
4 cross sections, which are sections in which local buckling will occur before
the attainment of yield stress in one or more parts of the cross section. In class
4 cross sections, effective widths may be used to make the necessary allow-
ances for reductions in resistance due to the effects of local buckling. The
classification of a cross section depends on the width to thickness ratio of
the parts subject to compression. Compression parts include every part of
a cross section that is either totally or partially in compression under the load
combination considered. The various compression parts in a cross section
(such as a web or flange) can, in general, be in different classes. A cross sec-
tion is classified according to the highest (least favorable) class of its compres-
sion parts. The limiting proportions for class 1, 2, and 3 compression parts are
specified in EC3. A part that fails to satisfy the limits for class 3 should be
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