Civil Engineering Reference
In-Depth Information
member is to be checked, then the cross-section should first be classified as
Class 1, 2, 3, or 4. This allows the effective section modulus to be determined
and the design section moment resistance to be compared with the maximum
design moment. Following this, the lateral bracing should be checked, then the
web shear resistance, and then combined bending and shear should be checked
at any cross-sections where both the design moment and shear are high. Finally,
the web bearing resistance should be checked at reactions and concentrated load
points.
Whenthemembersizeisnotknown,thenatargetvaluefortheeffectivesection
modulusmaybedeterminedfromthemaximumdesignmoment,andatrialsection
chosenwhoseeffectivesectionmodulusexceedsthistarget.Theremainderofthe
design process is then the same as that for checking a specified member. Usually
the moment resistance governs the design, but when it doesn't, then an iterative
process may need to be followed, as indicated in Figure 5.32.
The following sub-sections describe each of the EC3 checking processes sum-
marised in Figure 5.32. A worked example of their application is given in
Section 5.12.15.
5.6.1.2 Section classification
The moment, shear, and concentrated load bearing resistances of beams whose
plate elements are slender may be significantly influenced by local buckling con-
siderations (Chapter 4). Because of this, beam cross-sections are classified as
Class1,2,3,or4,dependingontheabilityoftheelementstoresistlocalbuckling
(Section 4.7.2).
Class 1 sections are unaffected by local buckling and are able to develop and
maintain their fully plastic resistances until a collapse mechanism forms. Class 2
sectionsareabletoformafirstplastichinge,butlocalbucklingpreventssubsequent
momentredistribution.Class3sectionsareabletoreachtheyieldstress,butlocal
bucklingpreventsfullplastificationofthecross-section.Class4sectionshavetheir
resistances reduced below their first yield resistances by local buckling effects.
Sectionsareclassifiedbycomparingtheslenderness
λ
=
(
c
/
t
)
√
(
f
y
/
235
)
ofeach
compressionelementwiththeappropriatelimitsofTable5.2ofEC3.Theselimits
depend on the way in which the longitudinal edges of the element are supported
(eitheroneedgesupportedasfortheflangeoutstandofanI-section,ortwoedges
supported as for the internal flange element of a box section), the bending stress
distribution(uniformcompressionasinaflange, orvaryingstressesasinaweb),
and the type of section, as indicated in Figures 5.33 and 4.29.
Thesectionclassificationisthatofthelowestclassificationofitselements,with
Class 1 being the highest possible and Class 4 the lowest.All UB's in S275 steel
are Class 1, all UC's are Class 1 or 2, but welded I-section members may also be
Class 3 or 4.
Worked examples of section classification are given in Sections 5.12.15 and
4.9.2-4.9.4.
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