Introduction - The Behaviour and Design of Steel Structures to EC3 - page 4

structuremayprovideshelterandprotectionagainsttheenvironmentbyenclosing
space, as in buildings; or it may provide access for people and materials, as in
bridges; or it may store materials, as in tanks and silos; or it may form part of a
machine for transporting people or materials, as in vehicles, or for operating on
materials. The design requirement of effectiveness is paramount, as there is little
point in considering a structure which will not fulfil its purpose.
Thesatisfactionoftheeffectivenessrequirementdependsonwhetherthestruc-
ture satisfies the structural and other requirements. The structural requirements
relate to the way in which the structure resists and transfers the forces and loads
actingonit.Theprimarystructuralrequirementisthatofsafety,andthefirstcon-
siderationofthestructuralengineeristoproduceastructurewhichwillnotfailin
itsdesignlifetime,orwhichhasanacceptablylowriskoffailure.Theotherimpor-
tantstructuralrequirementisusuallyconcernedwiththestiffnessofthestructure,
which must be sufficient to ensure that the serviceability of the structure is not
impaired by excessive deflections, vibrations, and the like.
The other design requirements include those of economy and of harmony. The
cost of the structure, which includes both the initial cost and the cost of mainte-
nance,isusuallyofgreatimportancetotheowner,andtherequirementofeconomy
usually has a significant influence on the design of the structure. The cost of the
structure is affected not only by the type and quantity of the materials used, but
also by the methods of fabricating and erecting it. The designer must therefore
givecarefulconsiderationtothemethodsofconstructionaswellastothesizesof
the members of the structure.
The requirements of harmony within the structure are affected by the relation-
ships between the different systems of the structure, including the load resistance
and transfer system (the structural system), the architectural system, the mechan-
ical and electrical systems, and the functional systems required by the use of the
structure.Theserviceabilityofthestructureisusuallydirectlyaffectedbythehar-
mony,orlackofit,betweenthesystems.Thestructureshouldalsobeinharmony
withitsenvironment,andshouldnotreactunfavourablywitheitherthecommunity
or its physical surroundings.
1.2.2 The design process
Theoverallpurposeofdesignistoinventastructurewhichwillsatisfythedesign
requirementsoutlinedinSection1.2.1.Thusthestructuralengineerseekstoinvent
a structural system which will resist and transfer the forces and loads acting on
it with adequate safety, while making due allowance for the requirements of ser-
viceability,economy,andharmony.Theprocessbywhichthismaybeachievedis
summarised in Figure 1.4.
The first step is to define the overall problem by determining the effectiveness
requirementsandtheconstraintsimposedbythesocialandphysicalenvironments
andbytheowner'stimeandmoney.Thestructuralengineerwillneedtoconsultthe
owner; the architect, the site, construction, mechanical, and electrical engineers;