In-plane bending of beams - The Behaviour and Design of Steel Structures to EC3 - page 154

In-plane bending of beams
5.1Introduction
Beamsarestructuralmemberswhichtransferthetransverseloadstheycarrytothe
supports by bending and shear actions. Beams generally develop higher stresses
thanaxiallyloadedmemberswithsimilarloads,whilethebendingdeflectionsare
much higher.These bending deflections of a beam are often, therefore, a primary
designconsideration.Ontheotherhand,mostbeamshavesmallsheardeflections,
and these are usually neglected.
Beamcross-sectionsmaytakemanydifferentforms,asshowninFigure5.1,and
these represent various methods of obtaining an efficient and economical mem-
ber. Thus most steel beams are not of solid cross-section, but have their material
distributed more efficiently in thin walls. Thin-walled sections may be open, and
whilethesetendtobeweakintorsion,theyareoftencheapertomanufacturethan
the stiffer closed sections. Perhaps the most economic method of manufacturing
steel beams is by hot-rolling, but only a limited number of open cross-sections is
available. When a suitable hot-rolled beam cannot be found, a substitute may be
fabricated by connecting together a series of rolled plates, and this has become
increasinglycommon. Fabricatingtechniquesalsoallowtheproductionofbeams
compoundedfromhot-rolledmembersandplates,andofhybridmembersinwhich
theflangematerialisofahigheryieldstressthantheweb.Taperedandcastellated
beams can also be fabricated from hot-rolled beams. In many cases, a steel beam
is required to support a reinforced concrete slab, and in this case its strength may
be increased by connecting the steel and concrete together so that they act com-
positely. The fire resistance of a steel beam may also be increased by encasing it
in concrete. The final member cross-section chosen will depend on its suitability
for the use intended, and on the overall economy.
The strength of a steel beam in the plane of loading depends on its section
properties and on its yield stress f y .When bending predominates in a determinate
beam, the effective ultimate strength is reached when the most highly stressed
cross-section becomes fully yielded so that it forms a plastic hinge. The moment
M p atwhichthisoccursissomewhathigherthanthefirstyieldmoment M y atwhich
elastic behaviour nominally ceases, as shown in Figure 5.2, and for hot-rolled