In-plane bending of beams - The Behaviour and Design of Steel Structures to EC3 - page 189

The ability of a beam web to transmit concentrated loads and reactions is
discussed in Section 4.6.2. The web should be designed as in Section 4.7.6 using
the design loads and reactions calculated by plastic analysis.
Aworked example of the plastic design of a beam is given in Section 5.12.17.
5.7 Serviceability design of beams
The design of a beam is often governed by the serviceability limit state, for
which the behaviour of the beam should be so limited as to give a high proba-
bility that the beam will provide the serviceability necessary for it to carry out its
intended function. The most common serviceability criteria are associated with
the stiffness of the beam, which governs its deflections under load. These may
need to be limited so as to avoid a number of undesirable situations: an unsightly
appearance;crackingordistortionofelementsfixedtothebeamsuchascladding,
linings, and partitions; interference with other elements such as crane girders; or
vibration under dynamic loads such as traffic, wind, or machinery loads.
Aserviceabilitydesignshouldbecarriedoutbymakingappropriateassumptions
for the load types, combinations, and levels; for the structural response to load;
andfortheserviceabilitycriteria.Becauseofthewiderangeofserviceabilitylimit
states,designrulesareusuallyoflimitedextent.Therulesgivenareoftenadvisory
rather than mandatory because of the uncertainties as to the appropriate values to
be used.These uncertainties result from the variable behaviour of real structures,
and what is often seen to be the non-catastrophic (in terms of human life) nature
of serviceability failures.
Serviceabilitydesignagainstunsightlyappearanceshouldbebasedonthetotal
sustained load, and so should include the effects of dead and long-term imposed
loads.Thedesignagainstcrackingordistortionofelementsfixedtoabeamshould
be based on the loads imposed after their installation, and will often include the
unfactored imposed load or wind load, but exclude the dead load. The design
against interference may need to include the effects of dead load as well as of
imposed and wind loads. Where imposed and wind loads act simultaneously, it
may be appropriate to multiply the unfactored loads by combination factors such
as 0.8. The design against vibration will require an assessment to be made of the
dynamic nature of the loads.
Most stiffness serviceability limit states are assessed by using an elastic anal-
ysis to predict the static deflections of the beam. For vibration design, it may be
sufficient in some cases to represent the dynamic loads by static equivalents and
carryoutananalysisofthestaticdeflections.Moregenerally,however,adynamic
elastic analysis will need to be made.
Theserviceabilitydeflectionlimitsdependonthecriterionbeingused.Toavoid
unsightlyappearance,avalueof L / 200( L / 180forcantilevers)mightbeusedafter
making allowances for any pre-camber. Values as low as L / 360 have been used
fordesignagainstthecrackingofplasterfinishes,whilealimitof L / 600hasbeen