ASRAnetStructural Reliability&Risk Assessment4-8 July 2016Wuhan, ChinaLecture 19: Reliability Based Code Developmentand TutorialProfessor Purnendu K. DasB.E., M.E., PhD, CEng, CMarEng, FRINA, FIStructE, FIMarEST
Structural Reliability & Risk Assessment 4 – 8 July 2016 Wuhan, China Lecture 19: Reliability Based Code Development and Tutorial Professor Purnendu K. Das B.E., M.E., PhD, CEng, CMarEng, FRINA, FIStructE, FIMarEST 1
OverviewPrinciplesofcodecalibrationStructuralCodesandRequirementsReliabilitydifferentiationPrinciplesof limitstatedesignCodecalibrationprocedure2
Overview Principles of code calibration Structural Codes and Requirements Reliability differentiation Principles of limit state design Code calibration procedure 2
IntroductionTime-varyingquantitiesRepresentation in normal engineering designConsidering extreme values of quantities?Designforcombinationsoftime-varyingquantitiesLoadsactinginoppositedirections..3
Introduction Time-varying quantities Representation in normal engineering design Considering extreme values of quantities? Design for combinations of time-varying quantities Loads acting in opposite directions . . . 3
Principlesof codecalibrationAcodeformatisaformalsystemofvariablestogetherwithasetof rulesTherulesdefinetherelationsbetweenthesevariablesandtheir relationstothe mechanical modelingofthe structureandtheactionsonthestructureForacodeformatofthepartialsafetyfactortype,thevariablesarecharacteristicvalues,partialcoefficients,andloadreductionfactorsCalibrationlevelsJudgementalvalueassignmentcalibrationtoexistingdesignpractice,orcodeoptimization4Ref: 8
Principles of code calibration A code format is a formal system of variables together with a set of rules The rules define the relations between these variables and their relations to the mechanical modeling of the structure and the actions on the structure For a code format of the partial safety factor type, the variables are characteristic values, partial coefficients, and load reduction factors Calibration levels Judgemental value assignment calibration to existing design practice, or code optimization 4 Ref: 8
Principlesof codecalibration..Calibrationbyjudgmentwasthepredominantwayofvalueassignment until about 1960Acodethatgivessatisfactoryexperiencesof safedesignthrough several years of practical usewas consideredtohavereasonablycorrectvaluesoftheparametersDrawbacksofjudgementalcalibrationuseofnewmaterials,newstructuralprinciples,anddevelopmentsinnewfieldslackof rationalanalysisleadtounnecessarilysafeandexpensivestructures5Ref:8
Principles of code calibration . . . Calibration by judgment was the predominant way of value assignment until about 1960 A code that gives satisfactory experiences of safe design through several years of practical use was considered to have reasonably correct values of the parameters Drawbacks of judgemental calibration use of new materials, new structural principles, and developments in new fields lack of rational analysis lead to unnecessarily safe and expensive structures 5 Ref: 8
Principlesofcodecalibration.:Calibrationtoexistingdesignpracticeisoftenusedasavalueassignmentmethod in situations wherea changeof thecodeformatisactualasimplercodea superiorgoal of obtaining national or international harmonizationof differentcodesThehighestlevelofcodecalibrationconsistsof abestfitoptimizationaimingatapproximatingtheresultsofasuperiorreliability analysis modelSeveralcodesmadesinceabout1975arebasedonmoreorlessextendedprinciplesofbestfitoptimization6
Principles of code calibration . . . Calibration to existing design practice is often used as a value assignment method in situations where a change of the code format is actual a simpler code a superior goal of obtaining national or international harmonization of different codes The highest level of code calibration consists of a best fit optimization aiming at approximating the results of a superior reliability analysis model Several codes made since about 1975 are based on more or less extended principles of best fit optimization 6
StructuralCodesAllowablestressdesignelasticstress<allowablestressIntheearliestdesignstheallowablestresswasselectedbytheengineer, taking into consideration the dispersion anduncertainty in loads,strength and analysisTobroaden the judgment base embodied in each design,themethod of analysis and the safety factor were standardized andlaid down in design codes during the nineteenth centuryAsDemploysonlyonesafetyfactor andlacksthe flexibilitytoadjust the safety margin according to the differences in loaddispersion,likelihood of load combinations, consequences offailure, and uncertainty in analysis etc.7
Structural Codes 7 Allowable stress design : elastic stress < allowable stress In the earliest designs the allowable stress was selected by the engineer, taking into consideration the dispersion and uncertainty in loads, strength and analysis To broaden the judgment base embodied in each design, the method of analysis and the safety factor were standardized and laid down in design codes during the nineteenth century ASD employs only one safety factor and lacks the flexibility to adjust the safety margin according to the differences in load dispersion, likelihood of load combinations, consequences of failure, and uncertainty in analysis etc
StructuralCodesLimitstatedesign:RYrQ≤YmMultiple-factorformatswereintroducedtogivethedesiredflexibilityinsafetymarginsEachbasicrandomvariableisrepresentedinthedesigninequalitiesbyacharacteristicvalue,multiplied bya load oraresistancefactorasappropriateThedesigninequalitiesseparatedemandononesidefromcapacityontheotherThedesign inequalities further contain factors orterms thatrelatetoparticularcircumstancesofthelimitstate,suchastolerancesorimportancefactorsPresentlyelementbehaviourisonlyconsidered8
Structural Codes 8 Limit state design : Multiple-factor formats were introduced to give the desired flexibility in safety margins Each basic random variable is represented in the design inequalities by a characteristic value, multiplied by a load or a resistance factor as appropriate The design inequalities separate demand on one side from capacity on the other The design inequalities further contain factors or terms that relate to particular circumstances of the limit state, such as tolerances or importance factors Presently element behaviour is only considered f m R Q
StructuralCodesLimitstatedesignG:R-Q≤0fa(q), fr(r)q,rO
Structural Codes . . . 9 Limit state design : q, r fQ (q), fR (r) G R Q : 0
Basicstructuralrequirements Structures and structural elements shall be designed,constructed and maintained in such a waythat theyare suited fortheir use during the design working life and in an economic wayIn particular they shall, with appropriate levels of reliabilityfulfil thefollowingrequirements:Theyshall remainfitfortheuseforwhichtheyarerequired (SLS:serviceabilitylimitstaterequirement)They shall withstand extremeand/orfrequently repeated actions occurringduring their constructionand anticipated use (ULS:ultimate limit staterequirement)Durabilityreguirements10Ref: 4,7,5
Basic structural requirements 10 Structures and structural elements shall be designed, constructed and maintained in such a way that they are suited for their use during the design working life and in an economic way In particular they shall, with appropriate levels of reliability, fulfil the following requirements: They shall remain fit for the use for which they are required (SLS: serviceability limit state requirement) They shall withstand extreme and/or frequently repeated actions occurring during their construction and anticipated use (ULS: ultimate limit state requirement) Durability requirements Ref: 4, 7, 5