Introduction - The Behaviour and Design of Steel Structures to EC3 - page 25

anduncertaintiesinhisorheranalysisofthestructuralbehaviourandestimationof
theloadsacting,andeveninthestructuralrequirementsthemselves.Thedesigner
usuallysimplifiesthistaskbyusinganumberofdesigncriteriawhichallowhimor
hertorelatethestructuralbehaviourpredictedbyhisorheranalysistothestructural
requirements. Thus the designer equates the satisfaction of these criteria by the
predicted structural behaviour with satisfaction of the structural requirements by
the actual structure.
In general, the various structural design requirements relate to corresponding
limit states , and so the design of a structure to satisfy all the appropriate require-
mentsisoftenreferredtoasa limit states design .Therequirementsarecommonly
presented in a deterministic fashion, by requiring that the structure shall not fail,
orthatitsdeflectionsshallnotexceedprescribedlimits.However,itisnotpossible
to be completely certain about the structure and its loading, and so the structural
requirements may also be presented in probabilistic forms, or in deterministic
formsderivedfromprobabilisticconsiderations.Thismaybedonebydefiningan
acceptablylowriskoffailurewithinthedesignlifeofthestructure,afterreaching
somesortofbalancebetweentheinitialcostofthestructureandtheeconomicand
humanlossesresultingfromfailure.Inmanycasestherewillbeanumberofstruc-
tural requirements which operate at different load levels, and it is not unusual to
requireastructuretosuffernodamageatoneloadlevel,buttopermitsomeminor
damage to occur at a higher load level, provided there is no catastrophic failure.
The structural design criteria may be determined by the designer, or he or she
may use those stated or implied in design codes. The stiffness design criteria
adopted are usually related to the serviceability limit state of the structure under
theserviceloads,andareconcernedwithensuringthatthestructurehassufficient
stiffness to prevent, excessive deflections such as sagging, distortion, and settle-
ment, and excessive motions under dynamic load, including sway, bounce, and
vibration.
The strength limit state design criteria are related to the possible methods of
failure of the structure under overload and understrength conditions, and so these
designcriteriaareconcernedwithyielding,buckling,brittlefracture,andfatigue.
Also of importance is the ductility of the structure at and near failure: ductile
structures give a warning of the impending failure and often redistribute the load
effectsawayfromthecriticalregions,whileductilityprovidesamethodofenergy
dissipationwhichwillreducethedamageduetoearthquakeandblastloading.On
the other hand, a brittle failure is more serious, as it occurs with no warning of
failure, and in a catastrophic fashion with a consequent release of stored energy
and increase in damage. Other design criteria may also be adopted, such as those
related to corrosion and fire.
1.7.2 Errors and uncertainties
Indeterminingthelimitationsprescribedbydesigncriteria,accountmustbetaken
ofthedeliberateandaccidentalerrorsmadebythedesigner,andoftheuncertainties