b_belev_weimar_2011_part_1.pdf

8/14/2019 B_BELEV_Weimar_2011_Part_1.pdf

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Dr.Dr. BorislavBorislav BelevBelev

Dept. of Steel and Timber StructuresDept. of Steel and Timber StructuresUACEG, Sofia, BulgariaUACEG, Sofia, Bulgaria

Model Validation and Simulation Model Validation and Simulation BAUHAUS Summer School BAUHAUS Summer School

August 2011August 2011

Implementation of capacity designImplementation of capacity designrules to steel structures in seismicrules to steel structures in seismic

regionsregions


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22Model Validation and Simulation,Model Validation and Simulation,

BAUHAUS Summer School, Aug. 2011BAUHAUS Summer School, Aug. 2011

Lecture overviewLecture overview1.1. IntroductionIntroduction

2.2. Basic concepts in seismic designBasic concepts in seismic design3.3. Capacity design principles (CDP)Capacity design principles (CDP)4.4. Major seismicMajor seismic --resisting systems in steelresisting systems in steel------------------------------------------------------------------------------------------------------------------------------5.5. Application of CDP toApplication of CDP to MRFsMRFs6.6. Application of CDP toApplication of CDP to CBFsCBFs7.7. Evolution of capacity design philosophyEvolution of capacity design philosophy8.8. Concluding remarksConcluding remarks


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33Model Validation and Simulation,Model Validation and Simulation,Bauhaus Summer School 2011Bauhaus Summer School 2011

1. Introduction1. IntroductionCapacity Design Philosophy in Seismic Design:Capacity Design Philosophy in Seismic Design:

Originates from New Zealand (Originates from New Zealand ( ~ 1970)~ 1970) ;;Is also termed Failure mode controlIs also termed Failure mode controlApplicable to all major structural materials and systems;Applicable to all major structural materials and systems;Is a design approach, not an analysis technique;Is a design approach, not an analysis technique;Serves as a tool for providing more reliable andServes as a tool for providing more reliable andpredictable seismic response of the structures;predictable seismic response of the structures;Already accepted and embedded in most modern designAlready accepted and embedded in most modern design

codes;codes;Assumes that adequate ductility can be achievedAssumes that adequate ductility can be achievedthrough proper detailing of the potential plastic zonesthrough proper detailing of the potential plastic zones


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History of developmentHistory of development


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Related design codesRelated design codesEN 1998-1: General rules, seismic actions and

rules for buildings (Eurocode 8, Part 1)ANSI/AISC 341-05: Seismic provisions forstructural steel buildings (AISC Seismic 2005)

Notations according to Eurocode 3 andEurocode 8


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The StructuralThe Structural EurocodesEurocodesEN 1990 Eurocode 0 : Basis of structural designEN 1991 Eurocode 1 : Actions on structures

EN 1992 Eurocode 2 : Design of concrete structuresEN 1993 Eurocode 3 : Design of steel structuresEN 1994 Eurocode 4 : Design of composite steel and

concrete structuresEN 1995 Eurocode 5 : Design of timber structuresEN 1996 Eurocode 6 : Design of masonry structuresEN 1997 Eurocode 7 : Geotechnical designEN 1998 Eurocode 8 : Design of structures for

earthquake resistanceEN 1999 Eurocode 9 : Design of aluminium

structures


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2. Basic conceptsTypical seismic response of structuresDuctility and overstrengthEnergy balance and energy dissipation

Seismic demand vs. seismic capacityFailure modes: ductile vs. brittle

Failure mechanisms: global vs. localGuiding principles for conceptual design


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Typical seismic response ofstructures


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DuctilityThe ability of the material, members/joints andThe ability of the material, members/joints and

structure as a whole to sustain large inelasticstructure as a whole to sustain large inelasticdeformations without essential decrease ofdeformations without essential decrease of

their load their load - - bearing capacity bearing capacity

Ductility at material levelDuctility at material levelDuctility at crossDuctility at cross --sectional levelsectional levelDuctility of members and joints (local ductility)Ductility of members and joints (local ductility)Ductility of the structural systemDuctility of the structural system(global ductility of 4 to 6 is desirable)(global ductility of 4 to 6 is desirable)


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Effect of globalEffect of global ductility on seismicon seismicresponseresponse

Response spectra for a g = 0.25g and soil type C

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Period of vibration T (s)

S p e c

t r a l a c c e

l e r a

t i o n

( m / s 2 )

Elastic response spectrum

Design response spectrum for q=2

Design response spectrum for q=4

Lower bound of the design spectra


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OverstrengthActual resistances at the plastic regions Actual resistances at the plastic regions well above their nominal/design valueswell above their nominal/design values

may endanger the global ductility and resistance may endanger the global ductility and resistance Typical sources ofTypical sources of overstrengthoverstrength ::

MaterialMaterial overstrengthoverstrengthStrain hardeningStrain hardeningStrain rate effectsStrain rate effects

Design of members governed by nonDesign of members governed by non --seismicseismiccombinations of loads or noncombinations of loads or non --ULSULS(serviceability) criteria(serviceability) criteria


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Energy balance and energydissipation

Seismic input energy E i = E k + E s + E + E h


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Performance in terms of energyPerformance in terms of energydissipationdissipation

The structures differ in the way they manage andThe structures differ in the way they manage anddistribute the total input seismic energydistribute the total input seismic energy E E i i

Conventional structuresConventional structures ::energy dissipation through cyclic plastic deformationenergy dissipation through cyclic plastic deformation

ductile response means damage and lossesductile response means damage and lossescodecode --based design does not explicitly evaluatebased design does not explicitly evaluate E E h h / / E E i i the structure may remain vulnerable to aftershocksthe structure may remain vulnerable to aftershocks

Structures with damping systems:Structures with damping systems:energy dissipation performed byenergy dissipation performed by specialized partsspecialized parts primary structure/frame has mainly gravity loadprimary structure/frame has mainly gravity loadsupporting function and resupporting function and re --centering functioncentering function


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Seismic demandSeismic demand vsvs capacitySeismic demand: typically expressed by peak values ofSeismic demand: typically expressed by peak values ofresponse parameters: (forces, displacements,response parameters: (forces, displacements, interstoreyinterstoreydrifts, ductility, etc.), but also by integral parameters:drifts, ductility, etc.), but also by integral parameters:(energy input, damage index, etc.)(energy input, damage index, etc.)General requirement:General requirement:

Seismic demandSeismic demand seismic capacity, e.g.seismic capacity, e.g.Force demandForce demand ResistanceResistance

Ductility demandDuctility demand Ductility capacityDuctility capacityThe accuracy of estimation of seismic demands dependsThe accuracy of estimation of seismic demands dependson the analysis method andon the analysis method and modellingmodelling assumptions (e.g.assumptions (e.g.with or w/o inclusion ofwith or w/o inclusion of nonstructuralnonstructural infill walls)infill walls)


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Failure modes: ductile vs. brittleFailure modes: ductile vs. brittleBolted connections:Bolted connections:

Shearing of boltsShearing of boltsTensile fracture at net cross sectionsTensile fracture at net cross sectionsBlock shearBlock shear

Welded connections:Welded connections:

See next slide for typical observed failuresSee next slide for typical observed failures

Steel members and joints:Steel members and joints:Instability (mainly premature local buckling)Instability (mainly premature local buckling)

Conclusion: Conclusion: The presence of brittle components is unavoidableThe presence of brittle components is unavoidable

even in structures made of highly even in structures made of highly - - ductile material ductile material


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Failures of welded connections inFailures of welded connections inNorthridge (Northridge (1994 ) quake) quake


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Failure mechanisms:Failure mechanisms:global vs. localglobal vs. local

Local ductility demand much more severe for theLocal ductility demand much more severe for thecolumncolumn --sway (softsway (soft --storey) mechanismstorey) mechanismPP -- effects may result in global instabilityeffects may result in global instability

Beam-sway mechanism column-sway mechanism


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Guiding principles for conceptualGuiding principles for conceptualdesign of seismicdesign of seismic --resistant buildingsresistant buildingsstructural simplicity

uniformity, symmetry and redundancybi-directional resistance and stiffness

torsional resistance and stiffnessdiaphragmatic behaviour at storey leveladequate foundation

Regularity in plan and in elevation helpsbuildings to survive in major earthquakes!


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3. Capacity design principles3. Capacity design principlesExperience from past earthquakes: Experience from past earthquakes:

The design quake cannot be predicted inThe design quake cannot be predicted interms of PGA, frequency content, strongterms of PGA, frequency content, strong --motion duration, etc.motion duration, etc.The inelastic response of structures duringThe inelastic response of structures duringstrong quakes isstrong quakes is inavoidableinavoidable ;;

The structures with good ductilityThe structures with good ductilityperformed much better than the nonperformed much better than the non --ductileductile


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Capacity design principles (contCapacity design principles (cont d)d)The structures shall be madeThe structures shall be made insensitiveinsensitive to theto theuncertain parameters of ground shakinguncertain parameters of ground shakingThe locations of dissipative zones that willThe locations of dissipative zones that willexperience large plastic strains should beexperience large plastic strains should be prepre --selectedselected so that they do not endanger theso that they do not endanger the

overall stabilityoverall stabilityThese plastic regions require special care inThese plastic regions require special care inorder to maintain the desired ductile failureorder to maintain the desired ductile failuremode during a seismic eventmode during a seismic eventFor maximizing the global ductility, the otherFor maximizing the global ductility, the other(non(non --dissipative) parts of structuredissipative) parts of structure must bemust besecuredsecured against premature (brittle) failureagainst premature (brittle) failure


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Capacity design principles (contCapacity design principles (cont d)d)Illustration via Prof. Paulays chain

Option 1: Brittlelinks weaker

Option 2: Brittlelinks stronger

(Capacity Design)

Brittle links Ductile link Brittle links


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Capacity design steps:Capacity design steps:1.1. Choose a target plastic mechanism for which theChoose a target plastic mechanism for which the

required global ductility can be developed with therequired global ductility can be developed with thesmallest local ductility demandssmallest local ductility demands

2.2. Identify the potential dissipative zones (criticalIdentify the potential dissipative zones (criticalregions, plastic hinges)regions, plastic hinges)

3.3. Design these zones for stable ductile cyclic responseDesign these zones for stable ductile cyclic responsewith careful detailing. Suppress the undesirable (brittle)with careful detailing. Suppress the undesirable (brittle)failure modes in the dissipative zonesfailure modes in the dissipative zones

4.4. Protect the other parts of structure (members andProtect the other parts of structure (members and

joints) from failure through providing extra joints) from failure through providing extra --strength,strength,taking into account all possible sources oftaking into account all possible sources of overstrengthoverstrengthin the dissipative zonesin the dissipative zones


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Approach of the design codesApproach of the design codesEurocodeEurocode 8:8:

materialmaterial overstrengthoverstrength factorfactor ovov = [1.25]= [1.25]crosscross --sectionsection overstrengthoverstrength factorfactor iisystemsystem overstrengthoverstrength factorfactor =min{=min{ ii}}

system redundancy insystem redundancy in ee --pp stagestage uu / / 11typical combination for design actions on nontypical combination for design actions on non --dissipativedissipativemembers: Emembers: E dd == EE d,Gd,G ++ 1.11.1 ovov EE d,Ed,Estrength limit for the dissipative zones:strength limit for the dissipative zones: ff

y,maxy,max 1.11.1

ovovff

yy

Major drawback: the ultimate strength of the structure is notMajor drawback: the ultimate strength of the structure is notknown if only linear elastic analysis is performed known if only linear elastic analysis is performed


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Approach of the design codes (contd)Approach of the design codes (contd)AISC Seismic 2005:AISC Seismic 2005:

materialmaterial overstrengthoverstrength factorfactor RR yy = 1.1= 1.1 1.61.6crosscross --sectionsection overstrengthoverstrength factorfactor n.an.a ..systemsystem overstrengthoverstrength factorfactor 00 = 2= 2 3 (empirical)3 (empirical)

system redundancy insystem redundancy in ee --pp stagestage typical combination for design actions on nontypical combination for design actions on non --dissipative members: Edissipative members: E dd == EE d,Gd,G ++ 00 EE d,Ed,E

((note:note: 0 0 is applied to forces induced byis applied to forces induced byhorizontal component of the seismic action only horizontal component of the seismic action only ))


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Role of advancedRole of advanced modellingmodelling

TheThe overstrengthoverstrength (redundancy) beyond first plastic hinge is design(redundancy) beyond first plastic hinge is design --sensitivesensitiveand systemand system --dependentdependentIt may increase a lot with the magnitude of lateral displacementIt may increase a lot with the magnitude of lateral displacement demanddemand(displacement(displacement --dependent)dependent)Think Think InelasticallyInelastically and apply at least Nonlinear Static Pushover Analysis and apply at least Nonlinear Static Pushover Analysisfor verification of system performance!!!for verification of system performance!!!The plastic regions shall beThe plastic regions shall be modelledmodelled with their expected materialwith their expected materialoverstrengthoverstrength and actual crossand actual cross --section sizes and propertiessection sizes and properties


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4. Major seismic4. Major seismic --resisting systemsresisting systemsin steel: MRFin steel: MRF


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SeismicSeismic --resisting systems: CBFresisting systems: CBF


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SeismicSeismic --resisting systems: EBFresisting systems: EBF


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SeismicSeismic --resisting systems: Invertedresisting systems: Invertedpendulum structurespendulum structures


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SeismicSeismic --resisting systems:resisting systems:mixed and dualmixed and dual


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Reference values ofReference values of qq for steel structuresfor steel structures


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End of Part 1End of Part 1

Thank you for your attention !Thank you for your attention !Questions or comments?Questions or comments?

b_belev_weimar_2011_part_1.pdf

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