89

2.4 SEISMICREHABILITATIONOFEXISTINGREINFORCEDCONCRETEANDMASONRYBUILDINGSWITHMETAL-BASEDSOLUTIONS

Romania is acountryofmoderate tohighseismicrisk.The firstcompulsoryseismicdesigncodewasissuedin1963.TheRCstructuresbuiltbefore1963,weredesignedtoresistonlygravityloads,mainly. Later, new codes were drafted (e.g. 1978, 1992, 2006) the last one being aligned withEurocode 8. Practically, almost all the buildings located in severe seismic zones and gravity loaddesignedmustbeevaluatedandstrengthened.

2.4.1 STRENGTHENINGOFR.C.FRAMESWITHBUCKLINGRESTRAINEDBRACES

A "typical" RC frame designed and built according to the technical regulation of 1950-s is firstevaluatedand,after,strengthenedwithaninvertedVBRBsystem.TheBRBsystemisappliedinthemiddlespansoftheframe.Additionally,FRPlocalconfinementofthecolumnswasconsidered(seeFigure 101). The confinement was applied only on the columns from the first two stories. TheanalysiswasperformedonRCframestrengthenedbymeansofBRBwith/withoutfiberreinforcedpolymers(FRP)confinement.BRBaredesignedtoresistandprovidethenecessaryductilitytotheframe.InordertochecktheireffectivenessandthecorrelationwithnumericalmodelassumptionstheBRBmembershavebeentested.TheframegeometryandcrosssectionsarepresentedinFigure101.

(a)

(b)

Figure101.Framegeometry,characteristicbeamandcolumncross-sections(a)andstrengthenedsolutionsappliedontheframe(b).

ConsideringaninappropriatedetailingofRCelements,concretewastakenasunconfinedaccordingtoFEMA356(2000).ThematerialmodelKent&ParkfromPark&Paulay(1975)wasconsideredas an unconfined material with linear softening of rigidity and no tension. Due to the pooranchorage length of the bottom longitudinal reinforcement in the beams, the equivalent yieldstrength of the steel was used (FEMA 356, 2000). The reinforcing steel was defined as uniaxialbilinearmaterialofstrainhardeningaccordingtoEurocode3.

A A

6Ø14

25 [cm]

25 [cm]

4Ø14

25 [cm]

25 [cm]

B B

2Ø12

2Ø10

1Ø12

beff = 100 [cm]

35 [cm]

10 [cm]

25 [cm]

20 [cm]

Ø6/25

2Ø10

3Ø12

Ø6/25

B B

2Ø16

Ø6/25

C C

2Ø10

1Ø12

2Ø12

2Ø10

Ø6/25

2Ø12

Ø6/20

A A2Ø16

D D

2Ø10

2Ø12

Ø6/25

EE

B

A B C D E D C B A

3.6

[m]

A B C D E D C B A

B

A

B

B

B

B

B

A

B

B

B

3.0 [m]

3.0 [m]

3.0 [m]

4.0 [m] 4.0 [m] 4.0 [m]

Ø6/20

90

The effective stiffness of the members, corresponding to cracked cross-section, was reducedaccording to FEMA 356 (2000). For plastic analysis, beams and columns were modelled usingconcentratedplasticityattheends,definedbyarigidplasticbilinearmoment-rotationrelationship.Theplastichingelength(Lp)wascomputedaccordingtoPaulayandPriestley(1992).

BRB’s were considered pinned at the ends. Inelastic behaviour was modelled by concentratedplasticity. The material used for BRB is S235 grade mild carbon steel. In order to obtain theadjustment of the design strengths (maximum compression strength Cmax and maximum tensionstrengthTmax)theAISC(2005)formulaswereapplied.BRBmemberbehavesaccordingtoabilinearforce-deformation relationship with strain hardening. In Figure 102 is presented the BRBbehaviourmodelforallthreestoreys.

Figure102.BRBbehaviormodel. Figure103.EffectofconfinementbyFRPonthemoment-curvaturerelationshipcorrespondingtoan

axialforceof389.6kNfromseismicloadcombination

In order to enhance ductility of reinforced concrete columns, their strengthening with FRP wasconsidered. The fabric was applied in horizontal layers, its effect being confinement of concrete.The effect of confinement by FRP was determined according to FIB Bulletin 14 (2001), andconsistedinanincreaseofconcretecompressionstrength(from12.5N/mm2to40.8N/mm2)andultimate strain (from 0.005 to 0.02). A more favourable behaviour of the confined columns isresulting(Figure103).

Pushover analysis and time history analysis were applied in order to evaluate the differencesbetween the original frame (MRF) and the retrofitted ones. Performance of the structure wasevaluatedintermsofdisplacementdemandscorrespondingtoattainmentofinelasticdeformationcapacitiesattheultimatelimitstate(ULS).Developmentofplasticmechanismwasalsoobserved.Figure 104 to Figure 106 summarize themain results of these analyses. Details are presented inDubinaetal.(2007)andBordeaetal.(2008).

The analysis showed the inelastic demands in beams and BRB’s are significant. From pushoveranalysis ultimate plastic deformations in bracings and beams are attained at top displacementslower than the displacement demand at the ultimate limit state. However, from time historyanalysisitcanbeseenaverygoodimprovementoftheseismicbehaviourasthetopdisplacementisreduced significantly (Table 20) and no plastic hinge in columns occurred at the ultimate limitstate;theplastichingesareinitiatedfirstinBRB,afterfollowedbythoseinbeams.Table21showstheductilitydemandsforBRB,astheyresultedfromtheanalysis.

BRB Modeling

-80

-60

-40

-20

0

20

40

60

80

-0.035 -0.025 -0.015 -0.005 0.005 0.015 0.025 0.035

Displacement Δ [m]

Fo

rce

(C

om

pre

sio

n,T

en

sio

n)

[K

N]

2'nd Floor

2'nd Floor

Ground Floor

1'st Floor

Ground Floor

1'st Floor

M - Φ relationship

0

10

20

30

40

50

60

70

0

0.0

1

0.0

2

0.0

3

0.0

4

0.0

5

0.0

6

0.0

7

0.0

8

0.0

9

0.1

0.1

1

0.1

2

Φ [1/m]

M [

KN

m]

RC+FRP

RC

91

Figure104.Pushovercurvesoftheanalysedframes.

Figure105.TopdisplacementtimehistoryandplastichingesthatreachultimaterotationforRCframestrengtheningwithBRB.

Figure106.TopdisplacementtimehistoryandplastichingesthatreachultimaterotationforRCframestrengtheningwithbothBRBandFRP.

Table20.Topdisplacementof2DRCframeduringnonlinearanalysis.

Pushover(EC8)-targetdisplacementdemand,m

Time-historyanalysis-maximumtopdisplacement,m

(-)FRP* (+)FRP** (-)FRP* (+)FRP**0.224 0.222 0.067 0.057

*(-)FRP–withoutFRPcontribution(confinementofthecolumns)**(+)FRP–withFRPcontribution(confinementofthecolumns)

Pushover Curves

0

20

40

60

80

100

120

140

160

180

0 0.1 0.2 0.3 0.4

Top Displacement [m]

Sh

ea

r B

ase

Fo

rce

[K

N]

BRB-ULS Beam-ULS Column-ULS N2-Target Displacement

MRF

MRF+FRP

MRF+BRB

MRF+FRP+BRB

MRF+BRB

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0 5 10 15 20 25 30 35 40

Time [s]

Top

Dis

plac

emen

t [m

]

Column-ULS Maximum Top Displacement

MRF + FRP +BRB

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0 5 10 15 20 25 30 35 40

Time [s]

Top

Dis

plac

emen

t [m

]

Maximum Top Displacement plastic hinges which didn't reach ultimate deformations

pl

92

Table21.DefromationdemandsandFEMAacceptabilitycriteriaforBRBsatLifeSafetyperformanceobjective(mm).

Pushover TimeHistory FEMA356(-)FRP 62.0 24.4 28(+)FRP 56.0 16.6 28

The maximum displacement of the BRB elements correspond to the ultimate limit state (ULS)deformation.PushoverandtimehistoryanalyseswereappliedaccordingtotheRomanianseismicloading(BucharestspectrumandVranceaaccelerogram1977onNSdirectionwithapeakgroundaccelerationof1.949m/s2).ItcouldbeconsideredasanapproximationofULSdeformationwithLS(lifesafety)fromFEMA356(2000).InordertocheckthecapacityofBRBmemberstocomplywiththesedemandsanexperimentalprogramwascarriedout.

Todesigntheexperimentalprogramthemiddlespanfromthegroundflooroftheanalysedframewasisolatedandconsideredtobepinnedatthesupports(seeFigure107).

Figure107.Proposedexperimentalreinforcedconcreteframe.

Figure108.ProposedBRBsystem.

TheBRBdesignfollowsthegeneralrulesofEN1998-1forsteelconcentricallybracedsystems,andthespecificrulesofAISC2005,respectively.AsteelplatecoreisinsertedinasteeltubefilledwithconcreterepresentstheBRBsystem.Thesteelplateresiststheloadsanddissipatesseismicenergyby yielding while the steel tube and the concrete restrain the buckling of the core plate (Figure108).InordertostudythebehaviourofBRBelement,asubassemblytestwaspreparedaccordingtoAISC2005specifications,asitcanbeseeninFigure109.Thefollowingtypesofmaterialswereused: for thecore -steelS275 (fy = 275 N/mm2, fu = 400N/mm2, A% = 34%); three differentasunbondingmaterialsi.e.polyethylenefilm,rubber,asphalticbitumenandconcreteC40/50asinfillmaterial.Twomonotonic(tensionandcompression)andtwocyclictestsforeachBRBunbondingtypehavebeenconsidered.

93

Figure109.Testingset-up.

The following parameters have been monitored during monotonic and cyclic tests, conducted indisplacementcontrol:steelcoredisplacement,pinnedendconnectiondisplacement(tochecksteelplatebearing)andglobalrotationoftheBRB.Theyielddisplacement(Dy)andcorrespondingyieldforce (Fy) have been evaluated from monotonic tests. The results are summarized in Figure 110.The reference values of Fy and Dy were determined as the average of the whole results(Dy=1.91mmandFy=128kN).

Figure110.ThemonotonicbehaviouroftheBRBtests(compressionvs.tension).

Twoloadingprotocolswereappliedonduringcyclictests.ForthefirstspecimenthecyclicloadingwasappliedaccordingtoAISC(2005),characterizedbyarepetitionoftwocyclesatDy,followedbygroupsoftwocyclesinincrementsof0.5Dbm–untilthecumulativeinelasticdeformationreaches200Dyatleast.Forthesecondspecimen,themodifiedECCScyclicloadingprotocolwasapplied;itischaracterizedbyasingleloadingatDy/4;2Dy/4;3Dy/4andDy,followedbythreerepetitionsat4Dy,8Dy until a cumulative inelastic deformation of 200Dy is reached at the end of the protocol. Onenoticethecumulativeplasticdisplacementof200Dywaskeptinbothprotocolsinordertohaveacommonbasisofreference.ThetwoprotocolsarecomparativelyrepresentedinFigure111.

ItcanbeobservedthatcomparedtoAISC,theECCSprotocolproducesmorefatigueeffectsduetoanincreasingnumberofcyclesleadingtoathecollapseofBRBmember,atsmallerdisplacements(Table22).TheBRBofpolyethylenefilmunbondingmaterialappearstobethebestone.

0

25

50

75

100

125

150

175

200

225

250

275

300

0 10 20 30 40 50 60 70 80

Fo

rce

[K

N]

Deformation [mm]

Asphaltic Bitumen t = 2 mm Fy(C) = 124.73 KN ;Dy(C) = 1.89 mm Fy(T) = 126.79 KN ;Dy(T) = 1.87 mm

Rubber t = 3 mm Fy(C) = 129.72 KN ;Dy(C) = 2.11 mm Fy(T) = 129.12 KN ;Dy(T) = 0.97 mm

PE foil t = 1 mm Fy(C) = 129.85 KN ;Dy(C) = 2.24 mm Fy(T) = 128.66 KN ;Dy(T) = 1.44 mm

(C)

(T)

(C)

(T)

(C)

(T)

94

Figure111.TheAISCandECCSloadingprotocolsforcyclictests.

Table22.Summaryofcyclictests.

Unbondingmaterial

Dy,

mm Fy,kN

AISC ECCSno.ofcycles

Du(+),mm

Du(-),mm

Dcum,mm

no.ofcycles

Du(+),mm

Du(-),mm

Dcum,mm

Polyethylenefilm

C 1.89 124.710 45.8 46.1 448 16 30.5 31.4 432

T 1.87 126.8AsphalticBitumen

C 2.11 129.77 35.0 34.3 196 13 22.8 22.9 252

T NA NA

RubberC 2.24 129.9

9 45.6 45.6 356 14 30.4 30.4 312T 1.44 128.7

Du(+)/(-)–max/minplasticdisplacementDcum–cumulativedisplacement

Figure112.CyclicresponseofBRBswithPEfoil.

ThecyclicbehaviourintensionandcompressionofBRB,usingpolyethylenefilm,showninFigure112,provesastablehysteresisloops,almostthesamecapacityincompressionandtensionandthesamerigidity.

Figure 113 shows one of the tested specimen after test. One observes the sine plastic bucklingshapewhichenabledforstableandhighlydissipativeloopsduringcyclictest.

AISC vs ECCS Loading History

-45

-30

-15

0

15

30

45

1 11 21 31 41 51 61 71 81

Dis

pla

cem

en

t [m

m]

ECCS AISC

-350

-300

-250

-200

-150

-100

-50

0

50

100

150

200

250

-50 -40 -30 -20 -10 0 10 20 30 40 50Fo

rce

[k

N]

Displacement [mm]

PE foil - AISC

PE foil - ECCS

95

Figure113.BRBuncoveringandsteelcoredeformations(localbuckling).

Table23.UltimatedeformationofBRBsundercyclicload.

Unbondingmaterial AISC ECCS

Polyethylenefilm +/-46 +/-31AsphalticBitumen +/-34 +/-22

Rubber +/-45 +/-30

Inorder tocompare thedemandedplastic displacementcapacity forBRB,resulted from analysis(see Table 21), in Table 23 were extracted the experimental values of ultimate displacementsobtained during the cyclic tests. If the AISC protocol is considered, all BRB types convenientlysatisfythedemandaswellastheFEMArequirements.IncaseofasphalticbitumenunbondingBRB,theECCSprotocolleadstoalowervaluethanthedemandedone.However,sincethreerepetitionsatmaximumamplitudeofseismicmotionhavebeenneverrecordedduringhistoricalearthquakes,oneconsiderstheAISCprotocolhastobetakenasreferenceinsuchakindoftests.

TheeffectivenessofBRBsystemsusedforstrengtheningandprovideenergydissipationcapacityofapoorreinforceconcreteframehasbeenanalysed.NumericalanalysisshowedthattheuseofBRBsystemhastobeassociatedwithlocalFRPconfinementofcolumnsatleast(confinementofbeamswouldbebeneficial, too).BRBspecimens havebeendesignedaccordingto thedemands resultedfromtheanalysisandaccordingtoFEMA356criteria.Usingthesamesteelcore,threetypesofBRBhave been prepared, using three different unbonding materials i.e. polyethylene film, asphalticbitumenandrubber.Bothmonotonicandcyclictestshavebeenperformed.CyclictestshavebeencarriedoutaccordingtobothAISC2005andECCSRecommendations.AllthethreeBRBsolutionssatisfiedthedemand,howevertheoneusingpolyethylenefilmprovedabetterbehaviour.

96

2.4.2 STRENGTHENINGOFMASONRYWALLSWITHMETAL-BASEDTECHNIQUES

Masonry buildings are widely spread in Europe. Most of these structures represent historicalconstructions with symbol value for many towns or countries. Their functionality is diverse,includingresidentialhouses,hospitals,schoolsandotheressentialfacilities.Therefore,thesetypesofstructuresareimportantfrommanypointsofview:lifesafety,economicalaspectsandculturalheritage preservation. Erected in a period when design methods where poor or missing, and theknowledge regarding seismic action was almost inexistent, these buildings need a structuralupgradeinordertorespectsafetycriteriaofmoderncodes.

Poorbehaviourofmasonrystructuresunderseismicactionisduetothelackofresistance,tensilestress mainly, small deformation capacity and low ductility. Moreover, under seismic action themasonry,becauseitisstiffandheavy,attractssignificantinertialforces.Commondamagepatternsfor masonry buildings recorded during earthquakes can be classified in the following fourcategories: Out-of-planedamageorcollapseofwalls; In-planeshearorflexuralcrackingofwalls; Lossofanchorageofwallstofloororroofdiaphragms; Damageorcollapseofcorners.

Out-of-plane failure modes, e.g. falling down, can be a result of: load capacity exceeded due toinertial seismic forces, excessive deflection imposed on walls from diaphragm action, lack ofanchorage,poorpossibilityoftransferringdeflectionandinertialforcestohorizontalelements.In-plane damage can be a result of: diagonal cracking through masonry units due to excessiveprincipal stress (tensile stress), shear sliding along bed joints, excessive toe compressive stresscausing crushing (sliding shear), or tensile cracking normal to bed joints resulting in rocking(bending).Theinteractionofin-planeandout-of-planeforceshasasconsequencefailureofcorners.

This research focused on strengthening techniques aiming to improve the in-plane behaviour ofmasonrypanel.However,theyobviouslyenhancetheout-of-planeresistance,too.

The objective of traditional consolidation techniques was mainly the local repair of damagedelementswithoutageneralstrategyrelatedtotheglobalbehaviourofthestructure.Atpresent,notonlytheimpactoflocalstrengtheningontheglobalresponseofthestructurehastobeconsidered,but also the reversibility of the used techniques and compatibility between materials, the addedandexistingones(e.g. the"mixed"action)havetobeanalysedandevaluated.Thereversibilityisvery important because it offers the possibility to remove a solution when more advancedtechnologywillbeavailable.Theuseof"mixedmaterialbasedtechnology"enablestooptimisetheperformanceofretrofittedstructure.

Forthisreasons,combiningmetalsheeting,whichisresistantandductile,withmasonry,providingaproperconnectingsystem,seemstobeasuitablesolution.Theuseof"dry"connectionenableseasy removal of metallic elements. Additionally, this solution offers the advantage of highmechanical properties, e.g. strength and ductility, without changing too much the initial rigidity.This technique can provide a stable post-cracking behaviour to the masonry wall. Moreover, aperformancebasedstrengtheningmethodologycouldbedeveloped.

Two strengthening solutions were proposed and investigated within the research program. Thesolutionsusesteel(SSP)oraluminium(ASP)sheetingplates(seeFigure114),andsteelwiremesh(SWM),respectively(seeFigure116).

Connectionofthemetalsheetsplatestothemasonrywallisrealisedintwoways:chemicalanchors(CA)andprestressedties(PT),placedat200-250mm.Thewiremeshisgluedusingepoxyresin.Bothsystemscanbeappliedononesideorbothsidesofthepanel.Itisexpectedthatthesystemwith metallic elements on both sides to perform better, but it isn’t always possible due to

97

architecturalreasons.Suchatypeofsolutioncanbesuccessfullyappliedincaseofmasonrywalls,butitisnotappropriateincaseofmasonryvaultsandarches.

Observing the behaviourof a masonry wall with openings it is easy to identify the weak regionsthatneedstrengtheningwithmetalplates(SP)orwiremesh(WM)(seeFigure115).

Theapplicationtechnologyisrathersimple.Inthecaseofmetallicplatestheymustbepreviouslydrilled. Afterwards the plate is placed on the wall, anchor holes are drilled in the masonry wallthroughtheplateholes.Thedustisblownawayfromtheholes,followedbyinjectionofepoxyresinandfixingofchemicalanchors(seeFigure116).Prestressedtiesareappliedsimilarly,butnoresinisused,andthetiesaretightenedusingatorquecontrolwrench.

Figure114.Proposedsolution.

Figure115.WeakareaonmasonryfaçadeandlocationofSPorWM.

Figure116.Wiremeshgeometryandtextureandchemicalanchor.

Themeshisproducedeitherasgalvanisedsteelorstainlesssteelbidirectionalfabric.Spacingofthemesh is between 0.05 and 16 mm, while wire diameter is between 0.03 and 3.0 mm. Tensilestrengthreaches650-700N/mm2,whileelongationisabout45-55%inthecaseofstainlesssteelwires.Forgalvanisedsteelwire,tensilestrengthisusuallyintherangeof400-515N/mm2.

Applicationofwiremesh(seeFigure116)requiresapreviouspreparationofthewallstoobtainasmoothsurface.ThepreparationofresinissimilartotheoneusedforFiberReinforcedPolymers(FRP).Theresinisappliedintwosteps:afluidlayerisappliedfirst,andafteritisdried,asecond

CHEMICAL ANCHOR

METALL SHEATING

MASONRY WALL

PRESTRESSED TIE

METALL SHEATING

MASONRY WALL

98

thickfluidlayerisappliedtoembedthemesh.Forlargesurfacesthemeshshouldbefixedtothewallwithnailsinordertokeepplainitssurface.Itisimportanttomentionthat,byheatingtheresinlayer,thewiremeshcanberemoved.

Inordertovalidatethetwosolutions,anexperimentalprogramwascarriedout.Itincluded: Materialtests; Preliminarytestson500x500mmspecimens; Fullscaletestson1500x1500mmspecimens,bothundermonotonicandcyclicloading.

Somesimplenumericalcalculationshavebeenperformedtodeterminethethicknessofsteelshearplate in order to obtain a rational behaviour. On this purpose, three preliminary design criteriaexpressed in terms of stiffness, stability and strength have been used. First material tests wereperformedinordertoestablishstrengthandstiffnessparameters.TheyaresummarizedinTable24.

Table24.Summaryofmaterialtests

Masonrycomponent

ElasticmodulusofmasonryCompressiontestonbrick

CompressiontestonmortarTensiontestonmortar

SteelwiremeshTensiletestonwireTensiletestonmesh

Connectors TensiletestontiesTensiletestonsteelplates

Tensiletestonaluminiumplates

Firstcriterionisusedtoobtaincomparablestiffnessofthemetallicsheetingplateswithmasonrypanel,inordertoprovideauniformdistributionofstressesbetweenwallandsheeting.Toevaluatetherigidityofthewallandsheetingplatethefollowingformulashavebeenused:

3

1m

eff eff

m g v m

kh h

E I A G

(22)

wherekm=stiffnessofmasonrypanel;heff=effectivewallheight;Em=longitudinalelasticmodulusof masonry; Ig = moment of inertia; Av = shear area; and Gm = transversal elastic modulus ofmasonry.

1

plateeff

v s

kh

A G

(23)

wherekplate=stiffnessofsteelplate;heff=heightofplate;Av=sheararea,andGs=transversalelasticmodulusofsteel(Astaneh-Asl,2001).

Consideringknownallmaterialparametersandbyequatingthetworelations,a2.16mmthicknessdemandforthesteelsheetingwasobtained.

Secondcondition follows toobtainacompactplate inorder toprevent localbucklingandassuredissipationofenergythroughplasticbearingworkinconnectingpointsonly.Toestablishthe"non-compact"behaviourdomainthefollowingcriterionwasused:

99

1.10 1.37v v

yw w yw

K H h K H

F t F (24)

whereKv=platebucklingcoefficient;H=horizontalloadofthepanel;Fyw=yieldingstressofsteel;h=distancebetweenconnectors(imposedbymasonrytexture);andtw=steelplatethickness.

Fromequation(24),thecompactnesscriterionresultsastw2.27(mm).

A more complex methodology, to evaluate the resistance of each component of the system,proposedbytheproducerofchemicalanchorcanbeused.Threecomponentsgovernthebehaviourof the chemical connection, e.g. the matrix (masonry with epoxy resin), steel anchor and steelplates.Itisbelievedthatthemostdesirablefailuremodeisthebearingofthesteelhole(e.g.intheconnectingpoints).Inordertoobtainthisfailuremode,thebearingresistanceshouldbelessthantheminimumbetweentheshearresistanceofconnectorandcrushingresistanceofmatrix.

min( , )bearing masonry conectorN N N (25)

Forchemicalanchors,thedesignmethodologysuggestedbyproducer(Hilti-Catalogue,2005)hasbeenadaptedformasonrymatrix:

0, , ,Rd c Rd c BV V AR VV V f f f (26)

where VRd,c = matrix edge resistance; VRd,c0 = basic matrix edge resistance; fBV = matrix strength

influence;fV=loaddirectioninfluence;andfAR,V=spacingandedgecoefficient.

Two cases were considered: ø8 and ø10 connector diameter. Corresponding plate thicknessamountedto2.20and2.48mm.Itwasdecidedtousea3mmthicknesssteelplateofS235gradewhen applied on one side and 2 mm thickness plate of S235 grade when applied on both sides.Alternatively,5mmaluminiumplateswereused(99.5%Al1050H14-Rp0.2=105N/mm2).

Due to the inherent approximations in design assumptions and the poor accuracy of analyticalapproachbasedonavailableformulas,itwasdecidedtoperformaseriesoftestonsmallspecimensin order to validate and calibrate the proposed techniques. The tests on small specimens aresummarizedinTable25.

Table25.Testsonsmallspecimens

Preliminary Masonrypanel

ConnectionChemicalanchor(CA)

ø8ø10

Prestressedties(PT)ø10–0%

ø10–100%

DiagonaltensiontestSteelwiremesh(SWM)

Steelshearpanel(SSP)ChemicalanchorPrestressedties

Somepreliminarytestswerecarriedoutonunreinforcedmasonrypanels(brickunitstrengthof10N/mm2andmortarstrengthof13N/mm2)toobtainreferencevalues.

Connection tests were performed in order to establish the connector diameter and to assess theinfluenceofprestresslevelofsteelties.Theexperimentalset-upispresentedinFigure117.

Chemicalanchorsø8andø10diametersgr.5.8havebeentested.Thefailuremodeforø8wastheshear of connector and for ø10 the shear of connector and crushing of masonry. For the largespecimentests,aø10connectorwaschosen,duetothemoreefficientbehaviourandresistance.

100

Figure117.Experimentalset-upandtestingmachineforconnectors.

Two prestressing levels have been applied for the ø10 ties gr.5.8 (i.e. snug tightened ties (0%prestress) and full prestress (100%)). The failure mode was shear of ties, masonry specimensremainingalmostintact.Itwasnotedthattheprestresslevelincreasestheresistanceofconnectionduetoconfinementofmasonry.Incomparisonwithchemicalanchors,prestressedtiesledtomoreresistantandmorerigidspecimens.

Systemtestswerecarriedoutinordertovalidatetheanalyticalassumptionincaseofshearplatesandtochooseapropersteelwiremesh.Theexperimentalset-uponsmallspecimensandasampletestonunreinforcedmasonrypanelarepresentedintheFigure118.

Figure118.Experimentalset-upforsplittest.

Steelshear plates S235 grade of2 mm thickness on both sides and 3mm thickness on oneside,connectedwithchemicalanchorsandprestressedtiesweretested.

There are no analytical procedures to design the steel wire mesh reinforced masonry, thereforecalibrationwasbasedonexperimentaltest.Thepurposeoftestswastoselecttheappropriateresinand wire mesh to be applied on large specimens. In the first step six types of wire mesh weretested.

Compared to FRP technique a thicker fluid resin was selected. In order not to change too manyparameters and based on the experimental results, the following wire meshes were chosen: zinccoated(ZC)0.4x1.0(DxW),stainlesssteel(SS)0.4x0.5and0.4x1.0.ThefailuremodesareshowninFigure119: WM3–suddenwiremeshrupturesimultaneouswithmasonrycrack–strengthimprovement

(weakWM); WM5–debondingofwiremesh,ruptureinresin–strengthimprovement,energydissipation

duetothesuccessivedebonding(strongWM); WM6–wiremeshyield–improvementofresistanceandductility(optimal).

LOAD

LOAD

WIRE MESH

MASONRYPANEL

101

(a)

(b)

(c)

Figure119.FailuremodeforSMWonbothsidesa)ZC0.4x1.0b)SS0.4x0.5andc)SS0.4x1.0.

Table26.Testsonlargespecimens.

Mo

no

ton

ic Referencemasonrywalltest REF

SteelshearpanelChemicalanchor SSP-CAPrestressedties SSP-PT

AluminumshearpanelChemicalanchor ASP-CAPrestressedties ASP-PT

Steelwiremesh SWM

Cy

clic

Referencemasonrywalltest REF-c

SteelshearpanelChemicalanchor SSP-CA-cPrestressedties SSP-PT-c

AluminumshearpanelChemicalanchor ASP-CA-cPrestressedties ASP-PT-c

Steelwiremesh SWM-c

Based on these observations, the stainless steel wire mesh 0.4x1.0 was chosen to be applied onlargespecimens.

TheexperimentalprogramonlargespecimensissummarisedinTable26.Thetestswerecarriedoutintwodifferentexperimentalframes,oneformonotonicloadingandoneforcyclicloading.Thetestsset-upispresentedinFigure120.

Loadingwasappliedusingdisplacementcontrol,withlateraldriftofthepanelbeingusedascontrolparameter.Incaseofcyclicloadingthefollowingloadingprotocolwasused:onecycleat±0.5mm,±1.0mm,±1.5mm,±2.0mm,±3.0mm,±5.0mm,±7.0mm,±9.00mm,±11.00mm,etc.The"yield"displacement, ey, was considered when significant stiffness degradation was observed. After"yielding",threecyclesatey,1.5ey,2ey,etc.wereapplied,untilthefailureofspecimenoccurred.

Figure120.Testingframesformonotonic/cyclicloading.

102

Diagonal failure mode was observed for all specimens, both undermonotonic andcyclic loading.Duetoflexibilityoftestingframeusedforcyclicloading,amoresubstantialdamageatthecornersofpanelwasobservedincomparisonwithmonotonictests.However,forcyclicloadedspecimensthe characteristic failurewas also thediagonal shear, but with asmall influence due to eccentriccompression.Asignificantimprovementintermsofultimatedisplacement(thatshowssignificantimprovementinductility),andalsotheincreaseinstrength,withaslightincreaseinstiffnesswererecorded for all specimens. An overview of qualitative performance in terms of strength andductilityoftestedspecimens,relatedtounreinforcedmasonry,ispresentedinTable27.

Table27.Largespecimens’qualitativeresults

SpecimenMonotonic Cyclic

Resistance Ductility Resistance DuctilityASP–CA-1

ASP–CA-2

ASP-PT-1

ASP-PT-2

SSP–CA-1

SSP–CA-2

SSP-PT-1

SSP-PT-2

SWM-1

SWM-2 Legendslight moderate largeincrease1-oneside; 2-bothsides

Fortheonesidesheetingundercyclicloadingasignificantoutofplanedeformationwasobserved.Theforce-displacementrelationshipsarepresentedforASP-PT-2,monotonicandcyclicspecimens(seeFigure122andFigure124),aswellastheirfailuremodes(seeFigure121andFigure123).

Figure121.FailuremodeforASP-PT-2m Figure122.Monotonictestonaluminiumshearpanel

Due to large in-plane stiffness of masonry walls, the strengthening solution does not avoidcompletelydamagetomasonry.Alimitedamountofdamagetomasonryhastobeallowedinorderto take benefit from ductility of the metal used for sheeting. Aluminium is believed to beparticularlysuitableinthiscase,duetoamorefavourablestrength-to-stiffnessratiothansteel.

Itcanbeobservedthat,despitestrengthening,themasonrypanelcracksatalmostthesameforceand displacement as reference panel. The mixed masonry-metallic plate system is activated only

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after masonry cracking. This can be observed also by the fact that the initial stiffness of bothstrengthenedandreferencepanels does notchange.This is anadvantage forglobalbehaviourofretrofittedbuilding.

Themonotoniccurves(seeFigure122)showanimportantincreaseintermsofresistance,butthemainadvantageofthissystemseemstobetheverylargeultimatedisplacementthatassuresaverystablepost-crackingbehaviourandalargeductility.Also,forcyclicloadingthissystemhasprovedhis validity by increasing the resistance and obtaining a good hysteretic behaviour despite ofsignificantpinching(seeFigure124).

Figure123.FailuremodeforASP-PT-2C Figure124.Cyclictestonaluminiumshearpanelconnectedwithprestressedties

The proposed strengthening solutions are an alternative to FRP technology enabling to obtain aductileincreaseofstrength,butwithoutincreasingthestiffnessofthewall.Itcanbeconcludedthatsteel plates increases mainly the ductility, while wire mesh increases the resistance. Bothtechniquesaremoreefficientwhenappliedonbothsides.Theprestressedtieconnectionsseemtobe more appropriate and the specimens sheeted with aluminium plates have shown a betterbehaviourthanonessheetedwithsteel.

2.4.3 PUBLICATIONS

[1] T. Arangelovski, G. Altay Askar, F. Aras, C. Arion, D. Beg, A. Birouk, A. Bozer, G. Brando, L.Calado,M.Cherraj,B.Chesca,F.Colanzi,D.Cotofana,C.Daescu,D.Dan,M.Eleni Dasiou, M.D'Aniello,G.DellaCorte,G.DeMatteis,J-F.Demonceau,D.Diaconu,A.Dogariu,D.Dubina,A.El Hammoumi, A. El Mouraouah, V. Esposito, B. Faggiano, L. Fiorino, A. Formisano, K.Gramatikov, R. Gonçalves, M. Rosaria Grippa, K. Gueraoui, A. Iben Brahim, M. Jovanovski, J.Josifovski, J-P. Jaspart, M. Kasmi, M. Kerroum, S. Kourkoulis, P. Kozlevcar, L. Krstevska, R.Landolfo,E.Loznica,K.Marinelli,A.Marzo,F.M.Mazzolani,H.P.Mouzakis,T.Nagy-György,A.S.Panao,S.Panico,S-A.Papanicolopulos,K.Papantonopoulos,M.Pavel,L.Pavlovcic,V.Popa,F.Portioli,J.M.Proença,I.N.Psycharis,F.Sinur,P.Skuber,V.Stoian,A.Stratan,L.Tashkov,ElArbi Toto, R. Vacareanu, I. Vayas (2012). "Seismic Protection of historical buildings:experimental activity (FP6 PROHITECH project: Volume 3)", Federico M. Mazzolani(coordinator),PolimetricaPublisher,Italy,ISBN978-8876991738,780pag.

[2] AltayAskar,Gülay;Arion,Cristian;Barrecchia,Enrico;Beg,Darko;Bordea,Sorin;Bozer,Ali;Brando, Giuseppe; Calado, Luis; Daescu, Cosmin; Dan, Daniel; D'Aniello, Mario; Della Corte,Gaetano; De Matteis, Gianfranco; Demonceau, Jean-François; Dogaru, Adrian; Dubina, Dan;Fiorino, Luigi; Florut, Codrut; Formisano, Antonio; Gioncu, Victor; Nagy-Gyorgy, Tamas;Jaspart, Jean-Pierre;Kourkoulis,Stavros;Landolfo,Raffaele;Ly,Lam;Lungu,Dan;Mandara,

ASP-PT-2c

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-400

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Alberto; Marinelli, Aikaterini; Mazzolani, Federico Massimo; Mosoarca, Marius; Panao, AnaSofia;Panico,Simeone;Papanicolopulos,Stefanos-Aldo;Pavlovcic,Luka;Portioli,Francesco;Proença, Jorge Miguel; Ramundo, Felicita; Spina, Gerardo; Stoian, Valeriu; Stratan, Aurel;Thanopoulos, Pavlos; Vayas, Ioannis (2012). "Seismic protection of historical buildings:calculation models (FP6 PROHITECH project: Volume 5)", Federico M. Mazzolani(coordinator),PolimetricaPublisher,Italy,ISBN978-8876991776,308pag.

[3] Dubina,D.,Bordea,S.,StratanA.(2008)."Seismicupgradingofreinforcedconcretemoment-resistingframewithdissipativebucklingrestrainedsteelbraces".ActaTechnicaNapocensis.Section: Civil Engineering – Architecture. Nr. 51, vol. 1, 2008, ISSN 1221-5848, p. 353-360.ProceedingsoftheInternationalConference–CONSTRUCTIONS2008,9-10May2008,Cluj-Napoca.

[4] Dubina, D., Bordea, S., Stratan, A. (2009). "Performance Based Evaluation of a RC FramestrengthenedwithBRBSteelBraces".Proc.oftheInternationalConferenceonProtectionofHistorical Buildings PROHITECH 09, Rome, Italy, 21-24 June 2009. Ed. F.M. Mazzolani. CRCPress,ISBN978-0-415-55803-7,p.1741-1746.

[5] Dubina,D.,Dogariu,A.,Stratan,A.,Stoian,V.,Nagy-Gyorgy,T.,Dan,D.andDaescu,C.(2007)."Masonrywallsstrengtheningwithinnovativemetalbasedtechniques".Proc.ofthe3rdIntl.Conf.onSteelandCompositeStructures(ICSCS07),Manchester,UK,30July-1August2007,Eds.WangandChoi.Taylor&Francis,ISBN:978-0-415-45141-3,pp.1071-1077.

[6] Dubina,D.,Stratan,A.,andBordea,S.(2007)."Seismicretrofitofr.c. frameswithhystereticbracing systems". Proc. of the 3rd Intl. Conf. on Steel and Composite Structures (ICSCS07),Manchester,UK,30July-1August2007,Eds.WangandChoi.Taylor&Francis,ISBN:978-0-415-45141-3,pp.833-839.

[7] Bordea,S.,Stratan,A.,Dogariu,A.andDubina,D.(2007)."Seismicupgradeofnon-seismicr.c.framesusingdissipativebraces".ProceedingsofWorkshopinPrague30-31/3/2007"Urbanhabitatconstructionsundercatastrophicevents".COSTActionC26.Ed.Wald,F.,Mazzolani,F.,Byfield,M.,Dubina,D.,Faber,M.,ISBN978-80-01-03583-2,pp.211-220.

[8] Grecea, D., Bordea, S., Stratan, A., Dogariu, A., Dubina, D. (2008) "Soluţii moderne pentruconsolidarea şi reabilitarea clădirilor amplasate în zone seismice". Structuri metaliceamplasate în zone seismice - preocupări actuale. Ed. D. Dubina, V. Ungureanu. EdituraOrizonturiUniversitare,Timişoara.ISBN978-973-638-377-9,p.141-156.

[9] Bordea,S.,Stratan,A.,Dubina,D.(2008)."Performancebasedevaluationofanon-seismicRCframe strengthened with buckling restrained braces", Proceedings of the InternationalSymposium"UrbanHabitatConstructionsunderCatastrophicEvents",Malta,22-23October2008, COST Action C26, Editors: Mazzolani, Mistakidis, Borg, Byfield, De Matteis, Dubina,Indirli,Mandara,Muzeau,Wald,Wang,p.241-246.ISBN978-99909-44-42-6

[10] Dogariu, A., Stratan, A., Dubina, D., Nagy-Gyorgy, T., Daescu, C. and Stoian, V. (2007)."Strengthening of masonry walls by innovative metal based techniques". Proceedings ofWorkshopinPrague30-31/3/2007"Urbanhabitatconstructionsundercatastrophicevents".COSTActionC26.Ed.Wald,F.,Mazzolani,F.,Byfield,M.,Dubina,D.,Faber,M.,ISBN978-80-01-03583-2,pp.201-210.

[11] Dubina, D., Dogariu, A., Stratan, A. (2005). "Dissipative steel bracing systems forstrengtheningofexistingreinforcedconcreteframes".A3-aconferinţănaţionalădeinginerieseismică.Bucureşti,9decembrie2005,VolumulII.pp.183-196.

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2.5 VALIDATIONOFTHETECHNICALSOLUTIONFORBRACESWITHTRUEPINCONNECTIONS

Circular hollow section braces with "true pin" connections were adopted in the design of a 29storeybuildinglocatedinBucharest,Romania.Thebraceusesconnectionswithgussetplatesandpin. One of the brace connections has an eccentric pin, allowing for variation of the pin-to-pinlength,whichfacilitateserectionononehand,andallowscompensationforaxial forcesinbracesduetogravityloadsontheotherhand.Highstrengthsteelwasusedforgussetsandpin,inordertokeepconnectiondimensionstoaminimum.Finiteelementanalysesandcyclicexperimentaltestswereperformedinordertovalidatetheseismicperformanceofthebraceanditsconnection.Fourtestswereperformedonascaledmodelofthebrace,fortwodifferentpin-to-pinlengths.

In-plandimensionsofatypicalfloorofthebuildingare52.0x25.6m,whilethetotalheightamountsto117.6m.Thestructureusessteelframingforresistinggravityforces.Inthetransversaldirectionthemainlateralforceresistingsystemiscomposedoftworeinforcedconcretecores,whileinthelongitudinalonethecoresaresupplementedbysteelbraceslocatedinthefacadeofthebuilding.ThebracesareplacedinXconfigurationdevelopedovertwostoreys.Thisreducesthenumberofbraceconnectionsandhelpsincomplyingwithcodelimitationsonslenderness.Bracesarerealisedfrom hot-finished Circular Hollow Sections (CHS) and have connections with pins. The structurewasdesignedaccordingtoEN1993-1-1(2005)andP100-1(2006)–theRomanianseismicdesigncode,whichisverysimilartoEN1998-1(2004).TheconnectionsweredesignedaccordingtoEN1993-1-8(2005).

Therearetwobraceconfigurations:developedovertwostoreys,of9300mmpintopinlength(seeFigure125a),anddevelopedoveronestorey,withanadditionalconnectionatthebeamfromtheintermediatestorey,of4200mm pin topin length(seeFigure125b).The initialdesignusedthefollowingcross-sections:D244.5x25,D244.5x20,D219.1x20,D219.1x16andD219.1x10,allinS355steel. One of the pins ofeach of the brace features an eccentricity of 5 mm, allowing a +/-5mmadjustmentofthepintopinlengthofthebrace.Thisallowsformorerelaxederectiontolerancesononehand,andreducesgravity-inducedaxialforcesinthebraceontheotherhand,astheeccentricpinismountedaftercastingofreinforcedconcretefloors.

(a)

(b)

Figure125.Braceconfigurationsintheanalysedstructure:developedovertwostoreys(a)andonestorey(b).

In order to have a validation of the seismic performance of the braces, a series of numericalsimulationsandexperimentaltestswereperformed,whicharedescribedinthefollowingsections.

Thenon-dimensionalslendernessᵪ�ofbracesusedin thestructurevariedbetween1.53and1.72

forlongbraces(9300mm),andbetween0.69and0.78forshortbraces(4200mm).Thebucklinglengthasconsideredtobethedistancebetweenthepins.Allbraceswereofclass1accordingtoEN1993-1-1(2005).Duetoconstraintsimposedbythesizeofthetestingplatformandactuatorstroke

106

andcapacity,reducedscaleexperimentalmodelswereadopted.Theexperimentalspecimenswerechosensoastoreproducethenon-dimensionalslendernessandcross-sectionclassofbracesusedinthedesignedstructure.Asaresult,aclass1crosssectionof139.7x6.3wasadopted,withthepin

topinbracelengthof5900mmand2700mm,havingᵪ�valuesof1.64and0.75respectively.The

samesteelgradeasinthebracesfromthestructurewasused–S355J0H.

2.5.1 PRE-TESTFINITEELEMENTANALYSES

Pre-test finite element analyses were performed with the general purpose finite element codeAbaqus. Firstly a connection model was analysed, followed by a complete model of the braceassembly.

Theconnectionofthe219.1x10bracesizewasusedforadetailedanalysis.Itconsistsofacentralgussetconnected throughapin to twoexternalgussets,whicharewelded to theendplateusingfull-penetrationwelds.TheinitialconnectiondesignusedS690Qgradesteelforthepin,andS460Ngradesteelfortheendplateandgussets.Atoleranceof1mmondiameterwasusedbetweenthepinandthegussets.Duetobearingresistancerequirements,thecentralgussetresultedquitethick(68 mm). In order to reduce the weight of the structural steelwork and avoid disproportionatethicknessesattheweldedconnectionbetweenthecentralgussetandthebeam/column,thecentralgusset was locally reinforced with two welded plates (see Figure 126a). An alternative solutionaimingatreducingboththethicknessandtheworkmanshipwasalsoanalysed,byadoptingS690Qgrade steel for the gussets. In this way, the central gusset could be realised using a single piece,withoutreinforcements(seeFigure126b).

(a)

(b)

Figure126.Connectiongeometry:initialdesign(a)andmodifiedone(b).

The numerical model of the connection consisted of the central gusset, pin, washers, externalgussets,endplateandashortportionoftheCHSbrace.Itwasdiscretizedusingthree-dimensionalcontinuum linear hexahedral elements with reduced integration and hourglass control – typeC3D8R. Normal and tangential (with a coefficient of friction of 0.3) behaviour was defined at allsurfacesincontact.Stress-strainrelationshipsbasedonnominalmaterialcharacteristicswereused

forallcomponents,withtheexceptionofthebrace,forwhichanoverstrengthov=1.25wasappliedtotheyieldstrength.ThemodelwassubjectedtotensileforceequaltothedesignforceaccordingtoEN1998-1(2004):plasticresistanceofthebraceamplifiedbyoverstrengthandstrainhardening

(1.1∙ovNpl,Rd).Theexplicitsolutionmethodwasusedinallcases.

Three models were analysed: the initial one (S460N gussets and reinforced central gusset), witheccentric pin (model 219x10-ecc) and constant pin (219x10-ct), and the modified one (S690Qgussets and unreinforced central gusset) with constant pin (model 219x10-ct-690) – see Figure127.

34 34

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Figure127.DistributionofvonMisesstresses(a)andequivalentplasticstrains(b).

Table28.MaximumvaluesofvonMisesstressesandequivalentplasticstainsinconnectionmodels.

ModelPin Externalgusset

Centralgussetandreinforcements

VM[N/mm2]

pVM

[N/mm2]p

VM[N/mm2]

p

219x10-ecc 646.4 0.00527 464.0 0.00720 465.2 0.00938219x10-ct 639.5 0.00399 464.7 0.00800 471.2 0.01402

219x10-ct-690 638.9 0.00304 464.6 0.00772 706.2 0.00430

Minorplasticstrainswereobservedinthepinandinthegussetplatesofallmodels(seeFigure127and Table 28). However plastic strains are local only, as they do not extend over the full cross-section. Thus, there is always a large portion of the pin or gusset that stays in the elastic range,providingthenecessarystrengthoftheconnection.Thisbehaviourisinagreementwiththedesignfollowed,asthepinwasdesignedasnon-replaceableaccordingtoEN1993-1-8(2005).

Theinitialmodelwitheccentricpin(219x10-ecc)showedslightlylargerplasticstrainsinthepin,close ones in the external gussets and smaller ones in the central gusset and reinforcements, in

108

comparisonwith thecorrespondingmodelwithconstantpin(219x10-ct).Ontheotherhand, themodelthatusedasingle-thicknesscentralgussetandhigherstrengthsteelingussets(219x10-ct-690)showedslightlysmallerplasticstrainsinthepinandintheexternalgussets,andconsiderablysmaller ones in the central gusset, in comparison with the initial design (219x10-ct). This isattributed to the beneficial effect of reduced bending in the shorter pin of the modified model(219x10-ct-690).Consideringtheaboveobservations,themodifiedmodelusingS690Qgussetswasadoptedforthebrace.

Full-size numerical models of the experimental specimens were analysed as well. Each braceassembly consisted of a connection with constant pin, the CHS brace and a connection witheccentric pin. The same parameters as in the connection modelling were adopted, with theexceptionof thebrace,whichwasmeshedusingshell linearquadrilateralelementswithreducedintegrationandhourglasscontroloftypeS4R.Consideringthefactthatbehaviourintensionisnotmuch different than the on observed on connection model, in the following the behaviour incompression is described. Residual stresses in the brace member were ignored, as they are verylowinhot-finishedtubes(Ziemian,2010).Aninitialimperfectionofthememberequalto1/500ofthe pin to pin length (the maximum delivery tolerance allowed per EN10210-2, 2006) wasconsidered,intheformofacirculararc.Duringpreliminarynumericalsimulations,itbecameclearthatthebraceassemblyissusceptibletoout-of-planebuckling(outofplaneoftheconnection).Inthisparticularcaseoutofplanedeformationsaredetrimentalmainlyduetotworeasons:(1)therotationintheconnections isperpendiculartothe intendedone, leadingtostressconcentrationsandpossiblefailurenotaccountedforinitsdesign,and(2)largeoutofplanedeformationsofthebrace would damage non-structural components (building facade). In order to account for the

worstsituation,memberimperfectionwasorientedpredominantlyout-of-plane(7.5withrespecttothedirectionperpendiculartotheplaneofthegussets).

Severalmodelsofbraceassembliesweresubjectedtocompressionindisplacementcontrol,uptoadisplacementof30mm,wellintothepost-bucklingrange.Figure128ashowsthedeformedshapeatthemaximumdisplacementofthebraceassemblywithapintopindistanceof2700mm,withnominal geometry and initial imperfections as described above (SP27N-C model). Lateraldeformations follow the initial member imperfection and are predominantly out of plane. In aneffort to understand the causes of this behaviour, two other models were derived from thereferenceone:withlateraldeformationsforcedinplane(SP27N-CI)andoutofplane(SP27N-CO),throughsomerigidfrictionlesssurfaces.AscanbeseenfromFigure128b,thebucklingstrengthoftheSP27N-CImodelislargerthanoftheSP27N-CO,theresponseofthereferencemodel(SP27N-C)beingbasicallyidenticaltotheoneofthelatter.

Thereasonforoutofplanebucklingofthebrace,apartfromthequiteunfavourableorientationoftheinitialimperfections,istheconnectionitselfandthecircularshapeofthecross-section.Thein-plane connection rotation is not totally free due to friction between its components (pin andgussets).Ontheotherhand,outofplanebehaviouroftheconnectionisveryclosetoaperfectpinatsmall rotations, due to the clearance between the pin and the holes in the gussets (1 mm ondiameter),aswellasbetweenthecentralandlateralgussets(1.5mm).Intheconditionsdescribedabove, at initial stages of loading lateral deformations develop mainly in the direction of initialimperfections (out of plane). Once the wedging of the gussets and the pin occurs, out of planerotationof theconnectionisnotfreeanymore, thoughits in-planerotations isalsorestrainedtosome extent due to additional friction between the gussets and the pin. With reference to theSP27N-C series of models (see Figure 128b), even if the post-buckling strength of the SP27N-CImodel is smaller than the one of the SP27N-CO one, "switching" to in-plane buckling at largedeformationsisrestrainedbythefrictiondevelopedbetweenthegussetsandthepin,asaresultofoutofplanerotationsoftheconnection.

109

(a)

(b)

Figure128.DeformedshapeoftheSP27N-Cmodelatthemaximumdisplacement(a)andtheforce-displacementrelationshipforSP27N-Cseriesofmodels(b).

(a)

(b)

Figure129.DeformedshapeoftheSP27NE-Cmodelatthemaximumdisplacement(a)andtheforce-displacementrelationshipforSP27NE-Cseriesofmodels(b).

Apossiblesolutionwassoughtintheformofadesigninitialconnectioneccentricityspecifiedintheplane of the connection (SP27NE-C series of models, see Figure 132a). The magnitude of theeccentricity (4 mm) was chosen close to the maximum initial bow imperfection of the member(L/500=5.4mm).AscanbeseeninFigure129a,thebehaviourofthebraceimprovedsomehow,lateraldeformationsofthebraceinthepost-bucklingrangebeingpredominantlyintheplaneoftheconnection,thoughwithimportantcomponentintheoutofplanedirection.Thein-planebucklingstrength(modelSP27NE-CI,seeFigure129b)isnowonlyslightlyhigherthantheoutofplaneone(modelSP27NE-CO).Thoughthereferencemodel(SP27NE-C)bucklesinitiallyoutofplane,inthepost-bucklingrangelateraldeformationsdeveloppredominantlyin-plane.

A further improvement of the response of the assembly was obtained by welding two 14x14squaresalongthetubeperpendiculartotheplaneoftheconnection(seeFigure130a).Itincreasestheoutofplanestiffnessofthetube,decreasingtheoutofplanelateraldeformationofthebraceinfavourofthein-planeones.Thebucklingstrengthofthemodelwithlateraldeformationsforcedinplane(SP27NES-CI)isnowveryclosetotheoneofthemodelwithlateraldeformationsforcedoutofplane(SP27NES-CO),seeFigure130b.Furthermore,thepost-bucklingstrengthofthereferencemodel (SP27NES-C) is now very close to the one of SP27NES-CI. Additionally, this in followingmodelusedstrengthenedwasherssecuringthepin(seeFigure132b).

Themodelof the longerbrace(5900pin topin length)used thesameconnectioneccentricity(4mm), but no longitudinal stiffeners. In the post-buckling range lateral deformations arepredominantly in the plane of the connection (see Figure 131a), though the in-plane bucklingstrengthisslightlylargerthantheoutofplaneone(seeFigure131b).TheSP59NE-Cmodelinitiallydevelopsmainlyoutofplanelateraldeformations,but,atabout12mmaxialdeformation,whenthe

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in-planemomentimposedontheconnectionbecomeslargerthantheresistingoneduetofriction,asudden switching to in-plane buckling takes place, which becomes the governing post-bucklingbehaviour.

(a)

(b)

Figure130.DeformedshapeoftheSP27NES-Cmodelatthemaximumdisplacement(a)andtheforce-displacementrelationshipforSP27NES-Cseriesofmodels(b).

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Figure131.DeformedshapeoftheSP59NE-Cmodelatthemaximumdisplacement(a)andtheforce-displacementrelationshipforSP59NE-Cseriesofmodels(b).s

(a)

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Figure132.Connectioneccentricity(a)andimprovedconnectiondetailing(b).

2.5.2 EXPERIMENTALINVESTIGATION

Theexperimentalprogramincluded4bracessubjectedtocyclic loading,overviewedinTable29.SpecimenSP27-1usedtheconnectionconfigurationshowninFigure126b,withweakwashers.Allother specimens used longer pins with strong washers securing the pins (see Figure 132b). TheSP27-2specimenhadtwo14x14squaresweldedalongthetubeinaplaneperpendiculartotheone

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111

of the connection. A design connection eccentricity of 4 mm in the plane of the connection wasrequired for all specimens. After fabrication of specimens, imperfections were measured. Theyrevealedthatinitialmemberimperfectionswerequitesmall:1.19,0.91,2.28,2.21mmforSP27-1,SP27-2,SP59-1andSP59-2respectively.However,measuredconnectioneccentricitieswerelarge:between-1.58mmand+2.41mmintheoutofplanedirection(+/-0.0specified),andbetween-5.67mm and +4.07 mm in the plane of the connection (+4.0 specified). This did not allow fullexperimental assessment of the effectiveness of design connection eccentricity. All 4 specimenswere subjected to cyclic loading according to ECCS (1985) protocol. The cyclic tests consisted offourcyclesintheelasticrange(±0.25Dy,±0.5Dy,±0.75Dyand±1.0Dy),followedbygroupsofthreecycles at amplitudes multiple of 2Dy (3x±2Dy, 3x±4Dy, 3x±6Dy, etc.). The yield displacement Dywas determined from numerical simulations using mechanical properties of materials obtainedfromtensiletests.Theloadingwasappliedquasi-statically,indisplacementcontrol.

Table29.Experimentalprogram.

SpecimenPintopin

length[mm]Cross-section

Crosssection

class Loadingprotocol

SP27-1 2700 D139.7x6.3 1 0.75 Cyclic,firstcycleintension

SP27-2 2700D139.7x6.3

withreinforcements

1 0.68Cyclic,firstcyclein

compression

SP59-1 5900 D139.7x6.3 1 1.64 Cyclic,firstcycleintension

SP59-2 5900 D139.7x6.3 1 1.64Cyclic,firstcyclein

compression

TheSP27-1specimenbuckledoutofplane(seeFigure133)inthefirstcycleof2Dy.Thewashersused to keep the pins into position where fixed to the pin using M6 screws. Due to out of planebuckling,thescrewsbroke,thewashersfelloff,lettingexteriorgussetplatesbendoutofplane.Thiscausedpartial lossofcontactbetweenthepinandtheoutergussetplates,withrapidlossof loadbearingcapacityofthespecimen(seeFigure136).

Due to favourable initial imperfections and connection eccentricities, as well as due to theimprovedconnectiondesign(strongwasherssecuringthepin)andthetwo14x14squaresweldedalong the tube, the SP27-2 specimen buckled in the plane of the connection (see Figure 134a).Failuretookplaceduringthe first tensioncycleat6Dydueto fractureof thecrosssectionwhichexperiencedlocalbucklinginpreviouscompressioncycles(seeFigure136).

Figure133.FailuremodeoftheSP27-1specimen.

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(a)

(b)

Figure134.FailuremodeoftheSP27-2(a)andSP59-1(b)specimens.

(a)

(b)

(c)

Figure135.SpecimenSP59-2:outofplanebucking(a),followedbyin-planebuckling(b),andruptureofthecross-section(c).

Figure136.Force–axialdeformationcurvesoftheexperimentalspecimens.

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The SP59-1 specimen buckled in the plane of the connection, failure taking place at significantplastic deformations – 16Dy (see Figure 136). Failure was caused by fracture in tension due toprogressive local buckling of the brace in compression (see Figure 134b). The SP59-2 specimenexperiencedsimilarlevelofplasticdeformations–16Dy(seeFigure136).However,itfirstbuckledout of plane (see Figure 135a). However, starting with the 4Dy cycles, the buckling changedprogressively to in-plane one (see Figure 135b), failure taking place similar to the previousspecimen(seeFigure135c).

Welds between the tube and end plate, as well between end plate and gussets performedadequately in all cases. The cyclic response of the tubular member was similar to the onecharacteristicforstructuralhollowsections.Thetotalductilityatfracture,µF(Tremblay,2002)wasconsiderablylargerforlongerbraces(amountingto4.3forSP27-1,9.6forSP27-2,26.6forSP59-1,and28.3forSP59-2specimens).Somepinchingwascausedbyslipintheconnectionwithpinsandrotationoftheeccentricpin,additionallytotheoneexperiencedduetobracebuckling.Connectiondeformation amounted roughly to 17% and 6% of the total one for short and long specimensrespectively.

Duetoconnectiondetailingandtubularshapeofthecross-section,thebraceassemblywasshowntobesensitivetooutofplanebuckling,leadingtofailureoftheconnectioninabrittleway.Firmlyfixingthewasherstothepinhelpspreventingbrittlefailureoftheconnection,evenwhenbucklingtakesplaceoutoftheplaneoftheconnection.

Themaincausesofoutofplanebucklingare(1)thefreeoutofplanerotationsoftheconnectionatsmalldeformationsduetothetolerancebetweenthepinandtheholeingussets,(2)frictionthatrestrainstosomeextentin-planerotationsoftheconnectionand(3)initialmemberandconnectionimperfections.In-planebucklingofthebraceassemblyisfavouredbythefollowing:(1)designin-plane connection eccentricity, (2) reduction of out of plane rotation of the connection throughsmaller tolerances at the pin or larger spacing between gussets, (3) lower friction at the pin –gussetsinterface,(4)slenderbraces,and(5)bracecrosssectionwithdifferentmomentsofinertiaaboutthetwoprincipalaxes(elliptical,RHS,wideflange).

2.5.3 PUBLICATIONS

[1] Gabor, R., Vulcu, C., Stratan, A., Dubina, D., Voica, F., Marcu, D., Alexandrescu, D. (2012)."Experimental and numerical validation of the technical solution of a brace with pinnedconnections for seismic-resistant multi-story structures", Proceedings of the 15th WorldConference on Earthquake Engineering, September 24 - 28, 2012, Lisbon, Portugal, paper4431.

[2] Stratan,A.,Dubina,D.,Gabor,R.,Vulcu,C.,Marginean,I.(2012)."Experimentalvalidationofabrace with true pin connections", Proceedings of the 7th International Workshop onConnections in Steel Structures, May 30 - June 2, 2012, Timisoara, Romania, ECCS, DanDubinaandDanielGrecea(Eds),ISBN978-92-9147-114-0,pp.535-546.

[3] Dubina, D., Florea, D., Stratan, A., Marcu, D., Voica F. (2012). "Proiectarea asistata deexperimentastructurilorcomplexe".AICPSReview,nr.1-2/2012,ISSN:2067-4546,pp.108-122.

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2.6 SEISMICPERFORMANCEOFMULTI-STOREYSTEELSTRUCTURESWITHFRICTIONDAMPERS

Thegeneralaimoftheresearchprogramwastoestablishtheseismicperformanceofmultistoreysteel concentrically braced structures equipped with strain hardening friction damper in thebraces.Recentearthquakesaroundtheglobeprovedthatthecurrentdegreeofseismicprotectionisunsatisfactoryandbuildingssufferextensivedamageorevencollapsewhensubjectedtosevereorevenmoderateearthquakeactivity.Asaconsequencethebuildingdesigncodesincreaseseismicdemands and aim to improve structural response capacity through accuracy of design andenhanced technical solutions. In current practice there are three efficient strategies to reduceseismic risk (Rai, 2000): (1) reduce seismic forces, (2) appropriate the structural response toseismicdemandand(3)enhancestructuraldamping.

Reducingseismicforcesleadstocapacitydesignofstructures.Dissipativestructuresaredesignedtoconsumetheenergy inducedby theseismicmotion in thestructureby allowingsomespecificelements to enter plastic domain. These dissipative elements act as fuses for the structureconsumingenergy,whiletherestoftheelementsthatareconsiderednon-dissipativearedesignedtoremaininelasticdomain.

Enhancing damping strategy will imply base isolation and introduction of energy dissipationdevices in the structural system. For structures isolated from seismic action and those withsupplemental damping the structure is conceived as not to undergo plastic deformations byimplementingdeviceswhichcanabsorbtheseismicenergyandcanmodifytheperiodofvibrationofthestructuretomorefavourablevaluesforglobalbehaviour.Ingeneralthesedevicescanbeofthreetypes:(1)seismicisolationdevices,(2)passiveenergydissipationdevicesand(3)activeandsemi-activeenergydissipationdevices.

Passivesystemsaredesignedtobeusedbothfornewstructuresandforseismicretrofitofexistingstructures. In general these devices function on principles such as friction between surfaces,yieldingofcomponents,andphasetransformationofsteelalloys,viscoelasticdeformationofsolidsorfluidscombinedwiththecontroloftheflowofliquid.

Active/hybrid/semi-active control systems are an evolution of passive devices that have sensorsandrealtimecontrolandevaluationsystemsthatmodifypartiallyorcompletelythepropertiesofthedampingdevicesduringtherecordedgroundmotioninordertoobtainanoptimalbehaviourofthe structure. A general classification of passive dampers might be done in reference to theirgoverningparameterasfollows: Velocitydependentdevices-thesedevicesaredependentofthevelocityofapplicationofthe

load.Theymodifytheirhystereticbehaviouraccordingtovelocity.Asanexamplewecanmentionherefluidviscousdampersandfluidspringdampers.

Displacementdependentdevices-inthecategoryenterdeviceswithnon-linearbehavioursuchas:steelhystereticdampers,shapememoryalloydevices,andwithlinearbehavioursuchas:elastomericviscoelasticdevices.

The research program described in the following presents the evaluation of the behaviour of aparticulartypeoffrictiondamper.Forthispurposeexperimentalandnumericalanalyseshavebeenconducted. Based on the experimental data numerical models were calibrated and applied toevaluatetheperformanceofconcentricallybracedframesequippedwithsuchdevicesinthebraces.

The damper studied herein is also a friction damper but has a completely different behaviourconcept.Thedamperstudiedisastrainhardeningfrictiondamperwith2distinctzones.Astartingzone with low stiffness, aimed at increasing the period of vibration of the structure, andsubsequentlyreducingtheseismicforcesbyconductingitonthedescendingpathofthespectrum

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(T>TC)andasecondzonewithincreasedstiffnessconceivedtolimitdisplacementsforhighvaluesofseismicaction(Figure137).

Theconceptofthisfrictiondamperisdifferentfrom"classical"frictiondampers,andwhatismoreimportantinvolvesadifferentapproachforthephilosophyofdissipativestructures.Thiswasthereasonwhichmotivatedthepresentstudy.

The damper dissipates energy through the elongation of a set of prestressed circular steel coilsaroundacentralsteelcore.Thedamperischaracterisedby4mainparameters: Slip Stiffnessofstrengtheningbranch Maximumforce Maximumstroke

Thedeviceworkslikeakindofmechanicaltelescopicdevicethatensuresanincreaseinflexibilityofthestructureandallowsenergydissipationevenatsmalldisplacementsavoidingtheformationofplastichinges.

Figure137.HystereticbehaviourofSERBfrictiondamper(Panaitetal,2007).

ThestudieddamperisaNonLinearElasticDevice(NLED)accordingtoEN15129,2009thathasitsnon-linearbehaviourbasedongeometricalnon-lineareffectsduetothepeculiarshapeofitscoreelements,inthiscaseasetofsteelringsslidingaroundasteelcore,andtheaddedfrictionbetweentheseelements.TheincreaseofstiffnessofthesecondbranchclassifiesthisdamperasaHardeningDevice(Figure138).Thesedevicescanproduceanincreaseoftheinitialperiodofvibrationofthestructuralsystemduetolowstiffnessofthefirstbranchbutcanalsolimitdisplacementsinthecaseof earthquakes with increase in force. In addition the shape of the force displacement behaviourcurveforthedampersuggestsagoodre-centringcapacity.

Figure138.Hardeningdevice(EN15129,2009).

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Thebehaviourofadisplacementdependentdevice is identifiedbytheeffectivestiffnessKeffb, theeffectivedampingζeffb,firstbranchstiffnessK1andsecondbranchstiffnessK2,designforceVbdanddesign displacement dbd. Force displacement capacity of the device should be able to sustain amaximumdisplacementor load,whichever isreachedfirst,amplifiedbythereliability factorandpartial factorsthat takeintoaccountactioneffectsotherthanseismic,whichcanaffect theinitialconfigurationofthedevice(EN15129,2009).

2.6.1 EXPERIMENTALPROGRAM

Theaimoftheexperimentalprogramwastoevaluateandcharacterisethedamper,inafirststep,andthebehaviourofthebrace-damperassembly,inthesecondstep.Thefirstsetofexperimentaltestswasconductedonthetwodamperswith1000kNand1500KNcapacity(Table30),inordertovalidate their hysteretic behaviour and to ensure that the devices function in the desiredparametershavingasymmetricbehaviourintensionandcompressionwithstablehystereticloops.

Table30ParametersfortheSERBdamperstested:

Parameters SERB1 SERB2Slip[mm] 2 2

Stiffness[kN/m] 2x105 2x105Max.force[kN] 1000 1500

Max.stroke[mm] +/-15 +/-20

Dataacquisitionwasdonedirectlythroughthecontrolandacquisitionstationofthemachineitselfwithoutanyotheradditionalmeasuringinstruments.Monitoredparametersweretotalforceinthedamperanddamperstroke.Tomeasurethe influenceof theconnectionsonthebehaviourof thedevice the experimental test setup included the bolted end connections of the damper (Figure139a).Acyclicloadprotocolwithaprogressiveincreaseofforcelevelateachstepwasused.

(a)

(b)

Figure139Experimentaltestsetupfordampertests(a)anddamperbehaviourdeterminedexperimentally(b).

ThehystereticbehaviouroftheSERBdamperobtainedexperimentallyispresentedinFigure139b.Bothdeviceshadasimilarbehaviourwithstablehystereticbehaviourintensionandcompression.

Thesecondsetofexperimentaltestswasconductedonthebracewithdamperassemblyusingtwodifferentconcepts.Afirstconceptistodesignthebracestoremaininelasticdomaincontrollingtheresponse of the structure solely through the friction dampers. In this case the structure has no

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ductile elements and is designed with a behaviour factor corresponding to low dissipativestructuresof1<q<2andbenefitsfromthereductionofdesignseismicforcesduetotheincreaseinglobal damping. However, introducing supplemental damping in the structure leads to a muchsmaller reduction of design seismic loads compared to the reduction that comes from using ahigherbehaviourfactorvaluethatcorrespondstoadissipativedesignapproachinwhichthebraceitselfisthemainenergyconsumingelement.Forexampleanincreaseofdampinginthestructureto15%criticaldampingleadstoareductionoftheloadswithonly35%.Furthermorethesetypesofdampershaveabrittlefailurethatmustbeavoidedinallconfigurations.Alltheabovementionedleadtoaseconddesignconceptinwhichthedamperhassufficientoverstrengthcomparedtothebracetoassurethatthebracehasdeformationintheplasticdomainandistheweakerelementintheconfiguration.Thisconceptshouldbenefitintheoryfromboththeenergydissipationcapacityofthebraceandthesupplementaldampingfromthedevice,andthefailurewilloccurinthebraceandnotinthedevice.Forseismicmotionlevelscorrespondingtoultimatelimitstatethebraceisthe "active" element according to the dissipative design concept and for service limit state thedamperisthe"active"elementensuringthatthebraceremainsinelasticdomainandprovidesanoveralldampingincrease.AccordingtoP100/2006therelativestorydriftcriteriaforSLSis0.008h,where h is the story height. For the structure analysed here this corresponds to a drift value of28mmwhichleadstoadisplacementof20mminthebrace.ThedampingdeviceswereselectedtosatisfythisdisplacementcriteriacorrespondingtoSLS.Bothdesignconceptspresentedabovewillbeusedin theconfigurationof theexperimental tests thatwillbepresented furtheron.Thetwodesignconceptswillbereferredtoasthe"Strong"braceconceptand"Weak"braceconcept.Theexperimentalconfigurationwasdesignedstartingfromthegeneralgeometryandconsistsofhalfofthebeamandoneofthecentralbracesinatriangularconfiguration,hingedatbothends.Thebraceandbeamassemblywererotated90degreesfromtheirpositioningintheframetofacilitateloadapplicationonthebracewhichwasdonethrougha1000kNhydraulicactuatorwhichwasfixedonthepre-existentexperimentaltestframe.Inadditiontotheinitialconfigurationpresentedaboveasecondaryframewasconstructedaroundthespecimentopreventoutofplanedeformation.Twoprofiles were attached to the front and back of the vertical beam element that ensure a 4 pointcontactwiththetwoguidancebeamstopreventanyoutofplanedisplacements(Figure140).Thesametestsetupisusedbothfortheexperimentaltestonbracesaloneandforexperimentaltestsonbracewithdamperassemblieswiththedamperconnectedatthebaseofthebracethroughboltedendplates(Figure141).

Figure140Experimentaltestsetupforbracetests

Figure141Experimentaltestsetupforbracewithdamperassemblytests

For the experimental tests on single brace and brace with damper configurations recordedparameters were: total applied force, total displacement of the specimen, relative displacementbetweencertainpredeterminedpointsrelevantforeachtypeoftest.Aloadprotocolrecommended

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by European Convention for Constructional Steelworks was used (ECCS, 1985) was used. Theprotocol consists of a monotonic test to determine the force-displacement relationship of thespecimen.Withthedeterminedvaluesofyielddisplacement(ey)thedisplacementbasedcyclicloadprotocolisconstructedwithonecycleateachelasticstepof0.25ey,0.5ey,0.75ey,1eyand3cyclesateachloadstepmultipleof2ey(2ey,4ey,6ey,8ey,etc.).Thecrosssectionsusedforthebracereflectthesetwodesignconceptsasfollows:"strong"braceconcept:HEA240and"weak"braceconcept:circularhollowsectionD133x5andHEA100profile.

Experimentaltestsfor"weak"designconceptrepresentthemainpointfortheresearchandwillbedetailedinthefollowing.TheresultsobtainedforCHSandHEAbraceswereverysimilaranditisforthisreasonthatwewillpresentonlytheexperimentaltestsfortheHEAbraceswithandwithoutdamper configurations. All braces without dampers were first testedmonotonically todetermineyielddisplacementandyieldforceneededtoestablishtheECCScyclicloadprotocolthatwaslaterused forcyclic tests.Theexperimental testsetuphad thesamegeneralconfiguration forall testsandthebraceswerepositionedwiththeirweakaxis intheplaneof thetest frametoensurethatbucklingoccursintheverticaldirection.

ForcedisplacementcurvesfortheHEA100bracewithoutdampers,obtainedfrommonotonictests,are presented in Figure 142. The behaviour of the HEA brace under cyclic load was very similarwiththebehaviourrecordedfortheCHSbrace.Attensionloadcyclesthebraceexhibitedastrengthdecayofapproximately20%forthesecondandthirdcycleateachloadstep.Thebraceexhibitedsignificantstiffnessdegradationforeachsuccessivetensioncycles.Bucklingofthebraceoccurredfor compression cycles with the formation of a plastic hinge in the middleof the brace. The firstbucklingofthebracewasrecordedataforcelevelofapproximately0.7timestheyieldforceofthebraceFyandthevaluesofthebucklingforcedroppedcontinuouslyforthesuccessivecompressioncyclesthatfollowed.Thetestwasstoppedwhenthevaluesofthecompressionforcedroppedwithmore than 50%of maximum compression forcereached.The force displacement curve recordedforcyclictestsontheHEA100bracewithoutdamperarepresentedinFigure143.

Figure 142 Force displacement curves for theHEA100braceobtainedfrommonotonictests

Figure143Forcedisplacementcurverecordedfor cyclic tests on the HEA100 brace withoutdamper

Inordertoanalysetheinfluenceofthedamperontheglobalbehaviourofthebracethehystereticbehaviourofthebracewithoutdamperistakenasreferencecurve.Thebehaviourofthebracewithdamperobtainedforthetwodesignconceptsof"weak"and"strong"braceisthereforecomparedwiththehystereticbehaviourofthebracewithoutdamper.

Forthe"strong"bracedesignconcepttheglobalbehaviourof thesystemofbraceanddamperiscompletelygovernedbytheconstitutivelawofthedamperanditsproperties.Thesystemdoesnotsuffer any degradation in terms of strength and stiffness these being strictly dependent on thedamper properties. The system will continue to take on load until the maximum capacity of the

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device is reached, with the brace remaining in elastic range. This high load carrying capacitywithoutstrengthandstiffnessdegradationrepresentstheadvantageofthistypeofdesignconceptbutcanalsoleadtoanincreaseoftheloadlevelsinthebeamsandcolumnsofthebracedframeduetothepseudo-elasticbehaviourofthedamper.Furthermorefailureofthistypeofsystemisabrittleoneduetofailureofthedeviceandmustbeavoided.

Forthe"weak"bracewithdamperthebraceisallowedtohaveplasticdeformationandtheglobalbehaviourofthedamperbracesystemisamixedone.Theweakelementinthisconfigurationisthebrace which will ultimately fail. The behaviour of this system is presented in Figure 144 incomparison with the behaviour of the same brace, under the same load protocol but withoutdamper. Inbothconfigurationstheforceleveldropssignificantlyafterthefirstcycleateachloadstep and the next two cycles of the same deformation step. The brace with damper has a higherflexibilityandyieldsatthesameloadstepbutatadisplacementofapproximately50%higher.Forthissystemuptoalevelof2eytheglobalbehaviourisgovernedbythebehaviourofthedamperandby thebehaviourof thesimple braceathigher loadsteps.Thedifferencebetweenthese twosystemscanbeobservedmorecloselyuptoaleveloftwotimesyielddeformationey(Figure145)

Figure144Hystereticbehaviourofthebracewithdamper(HBDY)andwithoutdamper(HB-C).

Figure145Hystereticbehaviourat2dyofthebracewithdamper(HBDY)andwithout

damper(HB-C).

Uptothislevelthebehaviouristhatgivenbythedamperparameters.Attensioncyclesthebraceremainsinelasticdomainandtheloadlevelinthesystemissignificantlysmallerthanthatofthebrace without damper with a higher overall flexibility. For compression cycles the brace withdamper buckles at the same load level as the one without damper but has a higher deformationcapacityduetothedamperproperties.Thetestwasstoppedwhenthevaluesofthecompressionforcedroppedwithmorethan50%ofmaximumcompressionforcereached.

The experimental results are in agreement with the two design concepts considered. For thestartingloadlevelsofupto2eythebraceremainsinelasticdomainandhasalowerlevelofenergydissipationbutthereisasignificantdecreaseinloadlevelduetothedamperandalsoanincreaseinflexibility.Afterthislevelthehystereticbehaviourofthesystemisverysimilartothatofthebracewithoutdamper,withenergydissipationduetotheformationofaplastichingeinthebrace.Failureinthisdesignconceptisrepresentedbythefailureofthebraceincompression.

2.6.2 SEISMICPERFORMANCEOFMULTISTOREYFRAMESWITHSERBDAMPERS

Themainissueswithmodellingthebehaviourofthedamperarethepinchingeffectofhystereticcurve,stiffeningbehaviour(K2>K1)andlackofdegradationoftheloops.FormodellingofdampingdevicesSEISMOSTRUCTsoftwareofferstheuseoflinkelementsthathavethepossibilityofdefiningdifferenthystereticbehaviourforeachofthe6degreesoffreedom.Severalhystereticbehaviours

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were tested in an attempt to model the behaviour of the SERB damper. These behaviours weredefined for the degree of freedom corresponding to axial deformation having a linear elasticbehaviourdefinedfortheother5withsufficientstiffnesstoensuretheirrestraint.Somethetrialhystereticbehaviourmodelswereusedinthefirststepusingcommonbehaviourcurvesforfrictiondampers. A conclusion of these trials was that to model the behaviour of the SERB damper acombinationof two linkelements was neededinorder toobtaina largerstiffness for thesecondbranchofthedevice.Thefinaldampermodelwasconstructedusingatwolinkelementsworkinginparallelnamelyabilinearsymmetricbehaviourtypelink(Figure146)combinedwithagap-hookelementthatisemployedtomodelthepinchingofthecurve(Figure147).Thecoupledstiffnessofthesetwoelementsinparallelyieldedthedesiredstiffnessforthedevice.

The combined behaviour of the two hysteretic behaviours presented above is shown in (Figure148).Thedampermodelwascomparedwiththebehaviourobtainedexperimentally(Figure149).

Figure146.Gap-Hooklinkbehaviourfordampermodel(Seismosoft,n.d.)

Figure147.BilinearSymmetriclinkbehaviourfordampermodel(Seismosoft,n.d.)

Figure148.Behaviourofdampermodel. Figure149.Comparisonbetweenthedamperbehaviourofthemodelandthedamper

behaviourobtainedexperimentally.

UsingasreferencetheexperimentalbehaviouroftheHEA100braceanumericalmodelthatcouldmodelwithsufficientaccuracythecyclicbehaviourofthebracewasdeveloped.Themainissuethatarises with brace modelling is the accurate modelling of brace buckling. For the numericalsimulation SEISMOSTRUCT version5.5 Build 10 software (Seismosoft, n.d.) was used, a finiteelementpackagethatusesfibreformulation.ThebucklingbehaviourofbracewasmodelledusinggeometricimperfectionscomputedaccordingtoEN19931-1.Thebraceelementwasdividedintosegmentswitheachpointhavingcorrespondingvaluesoftheimperfectionscomputedbasedona

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parabolic shapeof thedeflectionwith thevalueof the imperfectioncomputedatmidpointof theelemente0=26.5mm.ThematerialmodelusedforthesteelwasMenegotto-Pintosteelmodelwithisotropichardening,withparametersobtainedexperimentally fromtensiletestsonsteelsamplesfromthebraceandcalibratedparametersasfollows:A1=17,A2=0.1,A3=0.025,A4=8.

A parametric study was conducted to determine the optimum number of elements in which thebrace is to be divided and the value of the imperfections to be adopted comparing the cyclicbehaviourofthebracewiththebehaviourobtainedfromexperimentaltests.Thebracewasdividedin2and4elementsandforeachofthetwomodels4valuesoftheimperfectionswereconsidered:e0,e0/2,e0/3,e0/4andlengthoftheplastichingeof16.66%,20%and25%(Table31).

Table31.Parametricstudytodetermineoptimumnumberofelementsandplastichingelength.

No.ofelements Imperfection Plastichingelength2 e0 e0/2 e0/3 e0/4

16.66%4 e0 e0/2 e0/3 e0/42 e0 e0/2 e0/3 e0/4

20%4 e0 e0/2 e0/3 e0/42 e0 e0/2 e0/3 e0/4

25%4 e0 e0/2 e0/3 e0/4

Figure150.Comparisonbetweencyclicbehaviourofbracefromthenumericalmodelwiththeoneobtainedexperimentally.

Figure151.Comparisonbetweennumericalandexperimentalbehaviourofbracewithdamper.

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Thebestresultswereobtainedforthe2elementbracewithavalueofimperfectionatmidpointofe0/2andplastichingelengthof20%.ThebehaviourofthisbracemodelispresentedinFigure150,incomparisonwiththebehaviourof thesamebraceobtainedexperimentally.ParametricstudiesconductedbyLandolfoetal.(2010)alsorecommendedtheuseof2elementdivisionformodellingcyclicbehaviourofbrace.

The two numerical models detailed above, calibrated on experimental test data for damper andbracewerethencombinedtoobtainthebrace-damperbehaviour.Theresultsfromthenumericalmodelwerecomparedtotheexperimentalresults(Figure151).

Thenumericalmodelpresents thesameglobalbehaviourastheoneobtainedfromexperimentaldata with a damper governed behaviour up to 2ey and a brace governed behaviour afterwards,reaching the same peak values of force for each tension cycle and with sufficiently accuratemodellingofslidingof thedamperatzeroforcepointtransition.Thesetwomodels forthebraceandforthedamperaspresentedaboveareemployedintheoverallassessmentofthebehaviourofthefullframebyimplementingthemforstudiedconcentricallybracedframes.

The numerical model calibrated as detailed in the previous chapter is used to determine theperformanceofthissystemcoupledwithconcentricallybracedframes.Thestructureanalysedisa5storeyplaneframewithanundergroundlevelextracted froma3x3layout(Figure152)with3spansof6mwithchevronbracinginthemid-spanandastoreyheightof3.5m(Figure153).Theframewasdesignaccordingtoen1993-1-1andEN1998-1withsomespecialconsiderationsfromtheRomanianseismicdesigncodeP100/2006consideringthedesignspectraforBucharestwithacornerperiodofTC=1.6sandpeakgroundaccelerationag=0.24g.

Figure152.Planlayout. Figure153.Selectedframegeometry.

Extensive time-history analyses were conducted using two sets of seismic motions recordingsscaledtothedesignspectraas follows:7semi-artificialseismicmotioncharacteristicforsoftsoiltype(Bucharest)and7artificiallygeneratedseismicmotionscharacteristicforstiffsoil(ClassBsoilaccordingtoEN1998-1)bothwithandwithoutdampers.Thetwotargetspectrawerescaledtothefundamentalperiodofvibrationof theanalysedstructure,soas toyieldroughlythesamedesignseismicforces(Figure154).

Performance based evaluation was performed using acceptance criteria for plastic axialdeformation in the braces and plastic rotation for beams and columns according to FEMA356.Three performance levels were considered for each seismic motion having an accelerationmultiplier of 0.5 (30 years return period), 1.0 (100 years return period), 1.5 (475 years return

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period) corresponding to serviceability limit state (SLS), ultimate limit state (ULS) and collapseprevention(CP): SLS:ag,SLS=0.5ag,ULS ULS:ag,ULS=0.24g CP:ag,CP=1.5ag,ULS

Maximum drift levels (Figure 155), maximum drift at each storey (Figure 156) and topdisplacement for the structure (Figure 157) without dampers are presented as mean values ofrecordedvaluesforall7seismicmotionsatlevelscorrespondingtoSLS,ULSandCPincomparisonwiththesamevaluesrecordedforthestructurewithdampersinthebraces.

Figure154.Elasticspectraforsoftsoiltype(TC=1.6s)andstiffsoiltype(TC=0.5s).

Figure155.Maximumdriftvaluesforthestructurewithandwithoutdampers(stiffsoil).

Figure156.Maximumdriftateachstoreyforthestructurewithandwithoutdampers.

Figure157.Topdisplacementforthestructurewithandwithoutdampers.

Attheendofeachseismicrecordingusedthestructurewaslefttovibratefreelyfor10s.Recordedvalues of permanent displacement at top of the structure are presented as mean values for all 7recordingsinFigure158.

For all 7 seismic motions characteristic for stiff soil type used the results showed that for allperformance levels thebuildingwithdampersexhibitedanincrease indrift forall5storeys.The

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structurewithdampershaslowervaluesofpermanentdisplacementatthetopofthestructureatSLSandCP.

At SLS the structure with dampers avoids almost completely the formation of plastic hinges inbraces. At CP the structure with dampers has higher values of plastic deformation/rotation inelements.Allplasticdeformations/rotationssatisfytheacceptancecriteriaatalllevels.

Figure158.Permanenttopdisplacementforthestructurewithandwithoutdampers.

Figure159.PlastichingeformationforCBFwithoutdampersatSLS

Figure160.PlastichingeformationforCBFwithdampersatSLS

Figure161.PlastichingeformationforCBFwithoutdampersatULS

Figure162.PlastichingeformationforCBFwithdampersatULS

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AtULSbothframeswithandwithoutdampersformplastichingesinbraces(Figure161andFigure162). The structure with dampers has lower values of axial plastic deformation in braces incompressionbutwithslightlyhighervalues for thebraces in tension.At this level thestructureshaveasimilarbehaviourwithsimilarvaluesofplasticdeformation/rotationinelements.Noplasticrotationsofthecentralbeamsarerecordedforeitherstructure.Allplasticdeformationssatisfytheacceptancecriteriacorrespondingtolifesafety(LS)fromFEMA356.Thestructurewithdampershaslowervaluesofpermanenttopdisplacementthenthestructurewithoutdampers.

At CP both frames with and without dampers form plastic hinges in braces and central beams.Structurewithdampershaslowervaluesofplasticaxialdeformationforcompressionbracesandslightlyhigher for tensionbraces thenthestructurewithoutdampers.Noplastic rotations of thecentral beams are recorded for either structure with slightly lower values of permanent topdisplacementforthestructurewithdampers(Figure165andFigure166).Allplasticdeformationssatisfytheacceptancecriteriacorrespondingtocollapseprevention(CP)fromFEMA356.

Figure163.PlastichingeformationforCBFwithoutdampersatULS

Figure164.PlastichingeformationforCBFwithdampersatULS

The structures with dampers were more flexible in all cases with drift levels and values ofmaximum top displacement higher than the ones of the structures without dampers at allperformancelevels.Theintroductionofthedamperinthebracesleadstoareductionofpermanentdrift values for structure. Plastic hinge formation in elements and the values of plastic axialdeformationsandplasticrotationswereasfollows: AtSLSplastichingesappearexclusivelyinthebraces.Thedamperavoidsalmostcompletelythe

formationofplastichingesinbraceskeepingthestructuresinelasticdomain. AtULSplastichingesarelimitedtobracesforbothstructures.Thestructureswithandwithout

dampershaveasimilarbehaviourwithvaluesofallplasticaxialdeformationthatsatisfytheacceptancecriteriaforLS.

AtCP,fortheCBFframe,plastichingesformonlyinbraceswithonlyafewexceptionswhenplasticrotationsarerecordedincentralbeamsbutwithverylowvalues.

Asaconclusionthistypeofdamper isefficient inreducingtheseismicresponseofabuildingforearthquakescharacterizedbyshortcornerperiodTC=0.5s(stiffsoil)bypreventingtheformationofplastichingesatSLSandreducingthepermanentdisplacementof thestructure.Forearthquakescharacterized by long corner period TC=1.6s (soft soil) this type of damper is not effective inimproving thebehaviourof thestructure.Under this typeofseismicmotions thestructures withdampers form plastic hinges in non-dissipative elements with values that exceed the acceptancecriteriaforthecorrespondingperformancelevels.

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2.6.3 PUBLICATIONS

[1] Filip-Vacarescu, N., Stratan, A., and Dubina, D. (2014). "Experimental validation of a strainhardening friction damper". Proceedings of the Romanian Academy Series a - MathematicsPhysicsTechnicalSciencesInformationScience,15(1),60-67.

[2] Filip-Vacarescu, N., Stratan, A., and Dubina, D. (2014). "Seismic performance of multistoreysteelframeswithstrainhardeningfrictiondampers".ProceedingsoftheRomanianAcademySeriesa-MathematicsPhysicsTechnicalSciencesInformationScience,15(2),174-181.

[3] Filip-Vacarescu, N., Stratan, A., and Dubina, D. (2011). “Behavior of concentrically bracedframeswithfrictiondampers.”PollackPeriodica,6(1),59–71.PaperISSN:1788-1994,OnlineISSN:1788-3911.

[4] Norin Filip-Vacarescu, Aurel Stratan, Dan Dubina, Application of strain hardening frictiondampersinconcentricallybracedframes,BULLETINOFTHETRANSILVANIAUNIVERSITYOFBRASOV, CNCSIS B+, Brasov, ISSN 2065-2127, 2012, CD, CD, Bulletin of the TransilvaniaUniversity of Brasov, Series I Engineering Sciences, International Scientific ConferenceCIBv2012.

[5] Norin Filip-Văcărescu, Aurel Stratan, Dan Dubină (2011). “Cadre metalice contravântuitecentricdotatecuamortizoricufrecare”,AICPSReview,nr.1-2/2011,p.184-197.ISSN:2067-4546

[6] Filip-Vacarescu N., Stratan A., Dubina D. (2011). "Behavior of concentrically braced frameswith friction dampers". COMPDYN 2011, 3rd ECCOMAS Thematic Conference onComputationalMethodsinStructuralDynamicsandEarthquakeEngineeringM.Papadrakakis,M.Fragiadakis,V.Plevris(eds.)Corfu,Greece,25–28May2011,pp.2793-2807.

[7] Filip-Văcărescu, N., Stratan, A., Dubină, D. (2013). "Sisteme structurale disipative cucontravântuiri cu amortizori cu frecare: studiu de souție și comparație cu sistemeleconvenționale". A XIII-a Conferinţă Naţională de Construcţii Metalice, Bucureşti, 21–22noiembrie2013,Editori:DanielBîtcășiPaulIoan,pp.141-156,ISBN978-973-100-306-1.

[8] Filip-Vacarescu, N., Stratan, A., and Dubina, D. (2011). "Behaviour of Concentrically BracedFrameswithFrictionDampers",EUROSTEEL2011,August31-September2,2011,Budapest,Hungary,p.987-992.ISBN:978-92-9147-103-4.

[9] Filip-Văcărescu, N., Stratan, A., Dubina, D. (2010). "Cadre metalice contravântuite centricdotatecuamortizoricufrecare".Lucrărileceleide-a12-aConferinţeNaţionaledeConstrucţiiMetalice"Realizărişipreocupăriactualeîningineriaconstrucţiilormetalice",Timişoara,26-27noiembrie2010,Dubina,Ungureanu,Ciutina(Ed.),EdituraOrizonturiUniversitare,p.221-232.ISBN:978-973-638-464-6.

[10] Filip-Vacarescu,N.,Stratan,A.,Dubina,D.(inprint)."Evaluationofdissipativeeffectivenessofa hybrid system composed by a buckling restrained brace with a magneto rheologicaldamper", 8th International Conference on Behavior of Steel Structures in Seismic AreasSTESSA2015,Shanghai,China,July1-3,2015.Paperno.121.

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2.7 PREQUALIFICATIONOFBOLTEDBEAMTOCOLUMNJOINTSWITHHAUNCHES

Theuseofpre-qualified joints isacommonpracticeinU.S.andJapan.Nevertheless, thestandardjoints pre-qualified according to codified procedures in U.S. and Japan cannot be extended toEurope, due to differences in materials and section shapes. Moreover, the beam-to-column jointtypesusuallyadoptedinU.S.andJapanarenotcommonlyusedinEurope.Asaresult,theexistingscientificandtechnicalbackgroundonpre-qualificationmaynotbedirectlyextendedtoEuropeancontext.Forthisreason,aEuropeanresearchprojectentitledEuropeanpre-QUALifiedsteelJOINTS(EQUALJOINTS), is currently underway and aims at seismic pre-qualification of several beam-to-column connection typologies common in the European practice, and which are illustrated inFigure 165. In particular, the research activities will focus on the standardization of design andmanufacturing procedures based on different geometric and mechanical parameters of selectedjoint typologies.Hence,a largeexperimentalprogrammesupportedby theoreticalandnumericalanalysesisproposedtoprovidereliablestandardjointsthatcanbeeasilyusedbydesigners.

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Figure165.Connectiontypologieswith:(a)haunches,(b)stiffenedend-plate,(c)un-stiffenedend-plate.

2.7.1 EXPERIMENTALPROGRAM

TheaimoftheexperimentalprogramatthePolitehnicaUniversityofTimisoaraistoinvestigatethebehaviour of bolted connections with haunches under monotonic and cyclic loading. A set of 24beam-to-columnjointassemblieswillbetestedinordertoevaluatedifferentparametersthatarespecificforthistypeofconnection.Singlesidedandrespectivelydoublesidedjointswereselectedfrommomentresistingframes(MRF)asillustratedinFigure166.

Figure166.Single-sidedanddouble-sidedjointsselectedfromMRFstructure.

Theexperimentalprogram,summarizedinTable32,containsthreegroupsofspecimens: Group1:fullstrength&rigidconnection,shallowhaunch(35°angle),strongwebpanel; Group2:fullstrength&rigidconnection,steephaunch(45°angle),strongwebpanel; Group3:fullstrength&semi–rigidconnection,shallowhaunch(35°angle),balancedweb

panel.

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Table32.Experimentalprogramonboltedbeam-to-columnjointswithhaunches.

Group1:single-sidedjoints Group2:single-sidedjoints Group3:double-sidedjoints

EH1-TS-35IPE360HEB280

EH1-TS-45IPE360HEB280

EH1-XB-35IPE360HEB340

EH2-TS-35IPE450HEB340

EH2-TS-45IPE450HEB340

EH2-XB-35IPE450HEB500

EH3-TS-35IPE600HEB500

EH3-TS-45IPE600HEB500

Group1andGroup2serveforqualifyingtwoalternativehaunchgeometries(lowerandupperlimitof reasonable haunch angle) for considered range of beam size. Group 3 investigates joints withbalanced panel zone strength, but which are semi–rigid connections. Two supplementary webplatesareusedforthe jointsofGroup1andGroup2,while forGroup3onlyonesupplementaryweb plate is used. Additionally, larger column depth increases the range of prequalified columnsizes.Thecompleteparametersconsideredwithintheexperimentalprogramare:loadingprotocol(monotonicandcyclic),membersize,single-sidedanddoublesidedconnections,strongpanelzone/ balanced panel zone, strong beam and haunch geometry. It is to be mentioned that theexperimentalprogramwillbeextendedthroughnumericalsimulationswiththeaimtoinvestigateadditionalparametersaswell.

2.7.2 PRE-TESTNUMERICALSIMULATIONS

Thepre–testnumericalsimulationswererealisedwiththeaimtoobtainanaccuratepredictionforresponse of the beam-to-column joint assemblies, and in particular considering the followingobjectives: (i) to avoid unacceptable failure during the tests, (ii) to validate the analytical designprocedure.

ThenumericalsimulationswereperformedwiththefiniteelementmodellingsoftwareAbaqus.Inbrief, themodellingprocedurewasrealisedconsidering:solid linearhexahedralelementsof typeC3D8R,dynamicexplicittypeofanalysis(duetolargecontactsurfaces),contactbetweenend-plateandcolumnflangedefinedthroughanormal(hardcontact)andtangential(penaltywithfriction)interactionlaw,andtheloadwasappliedthroughadisplacementcontrol.Inaddition,thematerialproperties for bolts were calibrated based on data from a past project (Dubina et al., 2015), inwhichextensiveexperimentalinvestigationswereperformedonT-stubs.

Figure167ashowsthematerialmodelconsideredforboltsandforthelongitudinalandtransversalplatesoftheinvestigatedT-stub.Itistobenotedthatthematerialmodelisrepresentedintermsoftruestress–truestrainrelationship.Inaddition,Figure167bshowsthecomparisonbetweentestandsimulationforaT-stubconfigurationwhichwasdesignedconsideringaMode2closetoMode3failure,inwhichMode2ischaracterizedbythefailureoftheboltsandyieldingoftheflange,andMode3ischaracterizedbythefailureofthebolts.Ascanbeobserved,thenumericalmodeloftheT-stub was able to reproduce with good accuracy the response of the tested T-stub in terms offorce–displacementcurve.Consequently,theinitialstiffnessandtheultimatecapacitywereingood

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agreementwiththetest,aswellasthelossofcapacityat8.75mmdisplacement,correspondingtofailureofthebolts.

ThecalibratedmaterialmodeloftheboltswasfurtherusedintheFEjointmodelsdevelopedwiththeaimtoinvestigatetheresponseofthedesignedboltedbeam-to-columnjointassembliesundermonotonic(saggingandhoggingbendingmoment)andcyclicloading.

(a)

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Figure167.CalibrationofnumericalmodelofT-stub:(a)materialmodel,i.e.truestress–truestrainrelationshipforbolts(calibrated)andforlongitudinalandtransversalplates(frommaterialsampletests),(b)comparisonbetweentestandsimulation(T-stub)intermsofforce-displacement

relationship.

Theanalyticaldesignprocedureoftheboltedconnectionswithhauncheswasdevelopedbasedonthe component method implemented in EN 1993-1-8. In brief, the design of the joints wasperformedconsideringtheformationoftheplastichingeinthebeam(neartothehaunch).Further,thecomponents of the jointweredesignedand/orcheckedsoas tocompriseanequalorhighercapacityincomparisontothefullyyieldedandstrainhardenedplastichinge(Marisetal.,inprint).It istobenotedthattheconventionsofthecomponentmethodassumethatbasiccomponentsoftheconnection,i.e.end-plate,webpanel,beamflangeandcolumnflange,possesscertainductilityandforthisreasonaplasticdistributionofinternalforcescanbeassumed.Inaddition,acommonassumptionofthemethodconsidersthecentreofcompressionlocatedinthemiddleofthebeamflange (sagging bending moment), and respectively in the middle of the haunch flange (hoggingbendingmoment).Consequently,animportantissuewastodeterminetheactualvalueofforcesinactiveboltrowsandtheleverarmcorrespondingtoeachboltrow.

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Figure168.(a)EH1-TS35joint,(b)activeboltsunderhoggingmoment,(c)vonMissesstressdistributioninthebolts.

Numericalmodelsweredevelopedforeachbeam-to-columnjointconfigurationoftheexperimentalprogram (see Table 32). Further, the joint models were analysed under monotonic loading,consideringbothsaggingandhoggingbendingmomentinthebeam.Theresultsofthenumericalsimulationsconfirmedtheintendedfailuremode,i.e.formationoftheplastichingeinthebeam(seeFigure171).Inaddition,itwasfoundthattheactiveboltrowswerethosesituatedneartheflangeintensionascanbeobservedinFigure168forEH1-TS35joint.Figure168bshowsacomparison

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betweenthecapacitiesofthespecificboltrowscomputedbasedonEN1993-1-8codeprovisions,and the actual forces which developed in thecorresponding bolt rows. As can be observed, eventhoughtheboltrownumber3and4hasasignificantcapacityaccordingtothecodeprovisions,intheFEmodelthesearenotactive.

Theactualdistributionof forces within theboltrowscanbe justifiedby thestiffnessof theend-plate and the small plastic deformation within bolts (i.e. plastic strain in amount of 2.5%).Furthermore,itwasobservedthatthelocationofthecompressioncentrewasshiftedinsideoftheconnection.Figure169ashowsthepressureareasattheinterfacebetweenend-plateandcolumnflange. It was found that the compression centre was located at a distance equal to 60% of thehaunchdepthmeasuredfromthebottomflangeofthebeamascanbeseeninFigure169b.

(a)

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Figure169.Compressionsurfaces(a),forcedistributioninboltsandpositionofcentreofcompression(b).

Basedontheoutcomesof theFEsimulations, thedesignprocedurewasadjustedconsideringtheactual position of the compression centre and the active bolt rows. As a result, each beam-to-column joint configuration of the experimental program (see Table 32) was re-designed andinvestigatedthroughFEM.

Thepre-testnumericalsimulations carriedouton thedesignedbeam-to-column jointassembliesallowed investigating the influence of several parameters, i.e. influence of: member size, haunchgeometry,thicknessofhaunchflange,columnwebpanelstrength,andcyclicloading.Accordingly,the pre-test numerical program consisted of extensive investigations for which only a briefpresentationispossibleherein.

Theinfluenceofmembersizewasinvestigatedconsideringthreecross-sectionsforbothmembersofthejointassembliesaccordingtotheexperimentalprogramdescribedinTable32.Consequently,the numerical simulations evidenced higher values of equivalent plastic strain (PEEQ)corresponding to beams of larger cross-sections and at the same rotation (i.e. 40 mrad). Figure170ashowsthemoment–rotationcurveforthethreejointconfigurationsofGroup1(seeTable32).Inaddition,itwasobservedthatthecentreoftheplastichingewassituatedatadistanceequalwith20%ofthebeamdepthmeasuredfromtheendofthehaunch(seeFigure170b).Itistobenotedthat initial imperfections were not considered within joint models. For this reason the ultimatecapacity of the beams are higher than the expected bending capacity of the beams. Table 33summarizesthePEEQvaluesforeachjointconfigurationofGroup1andGroup2,correspondingto40 mrad rotation obtained under hogging and sagging bending moment.The beams aremade ofS355steelgrade,andthecross-sectionsareclass1.Itistobenotedthatthematerialmodelusedinsimulations was based on the nominal properties of the steel grade, which were affected(multiplied)bytheoverstrengthfactor(γov=1.25),justifiedbytheactualpropertiesofsteelgrades(higherthannominalvalues).

The influence of haunch geometry was investigated considering various angles ranging between30°and50°.Theoptimumresultswereobtainedforanglesbetween35°and45°.Thedepthofthehaunchincludedvaluesof45%upto55%ofthebeamdepth.Thestudyrevealedthatthecapacityandtheinitialstiffnessoftheconnectionincreasedforuppervaluesofthehaunchangleanddepth.

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However,fromarchitecturalreasonsandfromtheeconomicalpointofview,theminimumhaunchdepthandasmoothslopearerecommended.Theinfluenceofthecolumnwebpanelstrengthisoneoftheparametersoftheexperimentalprogram(seeTable32).Accordingly,Group1andGroup2are characterised by a strong web panel, while the joint configurations within Group 3 have abalancedwebpanelzone.BasedonFEMresults,Figure171showstheratiobetweenvonMissesstress and yield strength, measured within the column web panel corresponding to a 40 mradrotation.Ascanbeobserved,thesinglesidedconnections,i.e.EH1-TS35andEH2-TS35,havea25%strengthreserve,whilethedoublesidedconnections,i.e.EH1-XB35andEH2-XB35,exceedby5%the yield strength. However, corresponding to a rotation of 65 mrad (equivalent to the expectedbendingcapacityofthebeam),theplasticdeformationofthecolumnwebpanelishigherfordoublesidedjoints.Itistobenotedthatthedoublesidedjointsweredesigntobefullstrengthandsemi-rigidjointswithbalancedcolumnwebpanel.Consequently,plasticdeformationsweredevelopedinthepanelzoneandthetotalrotationofthejointassemblywasaffected,butinalowamount.

(a)

(b)

Figure170.Moment-rotationcurves(a),PEEQforEH2-TS-45hoggingmoment(b).

Table33.Equivalentplasticstrain(PEEQ).

Jointconfiguration EH1-TS35 EH2-TS35 EH3-TS35 EH1-TS45 EH2-TS45 EH3-TS45Hoggingmoment 3.8% 4.3% 5.6% 3.4% 4.3% 5.3%Saggingmoment 3.6% 4.3% 5.4% 3.6% 4.3% 5.1%

Figure171.RatiobetweenvonMissesStressandyieldstrengthfy=355MPa.

According to EN 1998-1 the contribution of the panel zone to the total joint rotation must belimitedto30%oftotaljointrotation.Consequently,theaimofthisinvestigationwastoanalysetheinfluenceofthepanelzonetothetotaljointrotation.Figure172showsthemomentrotationcurvesand equivalent plastic strain corresponding to EH2-TS35 and EH2-XB35 joints. An increase ofstrengthcanbeobservedforEH2-XB35connectionduetothechangeofthecolumncrosssectionfromHEB340toHEB500.Theweakestcomponent for theconnectionwas thecolumn flangeinbending. For double sided connection the PEEQ deformations in the beams are smaller due todistributionoftheplasticstrainincolumnwebpanel.

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Figure172.MomentrotationcurvesandplasticdeformationscorrespondingtoEH2-TS35andEH2-XB35joints.

In addition to the previous parameters, pre-test numerical simulations included also theinvestigationofthejointresponseundercyclicloadingconditions.Onthismatter,itistobenotedthat the difference between the monotonic and cyclic investigation of a joint model is mainlyrelated to the loadingprocedureandthematerialmodel (correspondingto thedissipativezone).Regardingtheloadingprocedure,asmoothcyclicloadingpatternwasusedbasedontheANSI/AISC341loadingprotocol,whichwascharacterizedbytwocyclesforeachofthefollowingamplitudes:10,15,20,30,40,50mrad.

Inrelationtothematerial,severalmodelsareavailableinAbaqus,i.e.withisotropic,kinematic,orcombinedisotropic-kinematichardeningmodel.Undermonotonicloadingconditions,theisotropicmodelcanbeadoptedforsteelcomponents.Forthispurpose,thetruestress–truestraincurvecanbe used as input. Under cyclic loading conditions, the combined isotropic-kinematic hardeningmodelismoresuitable,andtherefore,itwasusedinthecurrentstudy.Theinputforthematerialmodelwasrepresentedbytheyieldstrengthofthesteelpart,andinadditionthecyclichardeningparametersasgivenbyDuttaetal.(2010),i.e.C1=42096,γ1=594.45,Q∞=60,b=9.71.

The results from the cyclic analysis of the EH2-TS-35 joint configuration are shown in terms ofmoment-rotation curve computed at column centreline, von Misses stress distribution andequivalentplasticstrainwithinthejointassembly(seeFigure173).

(a)

(b)

(c)

Figure173.CyclicresponseforEH2-TS-35:(a)moment-rotationcurve(monotonicandcyclicloadingconditions),(b)vonMissesstressdistribution,(c)equivalentplasticstrain.

As can be observed, under cyclic loading conditions, large plastic deformations developed in thebeam (dissipative zone),while the bolts were characterised by elastic response with localminorplasticdeformations.Inaddition,themomentrotationcurveevidencedanincreaseofcapacity(seecycles of 20 and 30 mrad amplitude), compared to the monotonic response, and further thecapacitydecreased(seecyclesof40,50and60mradamplitude).

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Theseismicperformanceofthejointwasevaluatedbyconstructinganenvelopecurvethroughthepeakvaluesofthefirstcycleforeachamplitude,andfurthertherotationofthejointassemblywasobtainedfromtheintersectionoftheenvelopecurvewiththehorizontallinerepresentingthe20%reduction of the capacity with reference to the maximum bending moment. Consequently, arotation of 41 mrad was obtained, which is higher than the 35 mrad limit from EN 1998-1, andthereforetheseismicperformanceofthejointwasconsideredacceptable.

Thepre-testnumericalsimulationscarriedoutforthejointassembliesdesignedbasedonEN1993-1-8confirmedtheintendedfailuremode,i.e.formationoftheplastichingeinthebeamclosetothehaunch ending. However, other assumptions considered in design were not confirmed, inparticular:forcedistributionintheboltrows,andpositionofthecompressioncentre(middleofthecompressedhaunchflange).Incontrast,theactiveboltrowswerethosesituatedneartheflangeintension,andthecompressioncentrewas locatedatadistanceequal to60%of thehaunchdepthmeasuredfromthebottomflangeofthebeam.Asaresult,thedesignprocedurewasadjustedandthebeam-to-columnjointconfigurationswerere-designedandanalysed.

Theparametricstudyallowedinvestigatingtheinfluenceof:membersize,haunchgeometry,webpanel strength, and cyclic loading. Consequently, the study evidenced higher strain in members(beams)oflargercross-section,forthesamejointrotation.Inaddition,thecapacityandtheinitialstiffnessoftheconnectionincreasedforhighervaluesofhaunchangleanddepth,butforeconomicandarchitecturalreasons,hauncheswithminimumdepthandasmoothslopearerecommended.

Finally,thejointresponsewasevaluatedundercyclicloadingconditions,whichleadtoanincreaseofcapacitycorrespondingtothecyclesof20and30mradamplitudes.Forhigheramplitudes,thecapacity decreased compared to the monotonic curve due to local buckling of flanges and webundercompression.Inaddition,theseismicperformancewasevaluatedconstructinganenvelopecurvethroughthepeakvaluesofthefirstcycleforeachamplitude,andfurthertherotationofthejointassemblywasobtained fromthe intersectionof theenvelopecurvewith thehorizontal linerepresenting 20% reduction of the maximum capacity. Consequently, a rotation of 41 mrad wasobtained, which is higher than the 35 mrad limit from EN 1998-1, and therefore the seismicperformanceofthejointwasconsideredacceptable.

2.7.3 PUBLICATIONS

[1] Dubina,D.,Stratan,A.,Maris,C.,Marincu,A.(2014)."Precalificareaîmbinărilorstructurilorîncadremetaliceamplasateînzoneseismice–proiecteuropean".AICPSReview,nr.1-2/2014,ISSN:2067-4546,pp.207-216.

[2] Maris,C.,Vulcu,C.,Stratan,A.,andDubina,D.(2014).“NumericalSimulationsofBoltedBeamtoColumnConnectionswithHaunchesinSteelMomentFrames.”ProceedingsoftheSecondInternational Conference for PhD Students in Civil Engineering and Architecture, UTPRESS,Cluj-Napoca,Romania,109–116.

[3] Maris, C., Vulcu, C., Stratan, A. and Dubina, D. (in print). "Experimental program andnumerical simulations of bolted beam to column joints with haunches", 8th InternationalConferenceonBehaviorofSteelStructures inSeismicAreasSTESSA2015,Shanghai,China,July1-3,2015.Paperno.110.

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3 PROFESSIONALDEVELOPMENTPLAN

3.1 SCIENTIFICDEVELOPMENTPLAN

Asshowninsection1.1,thesubjectof"re-centringeccentricallybracedframes"hasacentralplacein the past achievements of the author. It gradually evolved from a conceptual idea developedwithinthePhDthesisintoasoliddevelopmentsupportedbyextensivenumericalsimulationsandlarge-scaletests.Onelaststepismissing:implementationintopractice.Asashort-termobjective,theauthor intends tocoordinatedevelopmentofdesignguidelines, inacooperativeeffortby theresearchteamfromthePolitehnicaUniversityofTimisoara(Romania),JointResearchCentre(JRC)inIspra(Italy),andUniversityofNaplesFedericoII(Italy).Inordertopromotetheoutcomesofthepursued research, it is intended to organise a workshop at the JRC Ispra. The medium-termobjective is to implement this design concept into a practical application, and promote theinnovative design approach to structural designers. The long-term objective is to extend theconcept of "re-centring structures" to other structural typologies, such as dual Buckling-RestrainedFrames(BRBFs)andSteelPlateShearWalls(SPSWs).

The Laboratory of Structures from the department of Steel Structures and Structural Mechanicsoffersalargesetofhigh-endtestingequipment.Large-scaletestingiscurrentlysupportedbyfourquasi-static actuators, a reaction frame, a reaction wall and a strong floor. Upgrading of testingfacilities is currently underway with two high-capacity quasi-static actuators, two dynamicactuators,hydraulicpowersupplyandashakingtable,amongothers.Thevastmajorityofseismic-relatedstructuraltestingthathasbeenperformeduntilnowwasquasi-statictestsoncomponents(e.g. beam to column joints), using standardised and pre-determined cyclic loading protocols.Though technically possible, pseudo-dynamic testing, that combines experimental testing withcomputersimulationofdynamicresponsehasnotbeenimplementeduptodateinthelaboratoryofSteelStructures.Thisexperimentaltechniqueofferstheadvantageofrealisticseismicsimulationonlarge-scale structural models, when strain-rate effects are not important. Therefore,"implementation of pseudo-dynamic testing technique" represents the next target of theauthor. At the same time, development of "testing techniques for shaking table tests" will beaddressed, in order to fully utilize the experimental potential of the laboratory. Though thedimensionsoftheshakingtable(1.5x1.5m)willallowonlyreduced-scaledynamictests,theyaremandatory for testing systems that use-strain-rate or velocity dependent devices, such viscousdampers,magneto-rheologicaldevices,etc.

Anotherresearchdirectionthatistargetedbytheauthoris"Buckling Restrained Braced Frames(BRBFs)". They represent a promising structural typology that has been extensively used in theUnitesStatesandJapan,buthasaverylimitedapplicationinEurope.Oneofthemainimpedimentsis lack of design guidelines, "know-how", and the need for experimental qualification of thedissipative devices. Design guidelines for BRBFs have been recently introduced in the Romanianseismicdesigncodes,theotherproblemsstillremain.Aprojectcoordinatedbytheauthor(contractno.99⁄2014,PN-II-PT-PCCA-2013-4-2091:2014-2016"Implementareaînpracticadeproiectareanti-seismicădinRomâniaacontravântuirilorcuflambajîmpiedicat–IMSER–ImplementationintoRomanianseismicresistantdesignpracticeofbucklingrestrainedbraces"iscurrentlyunderway.ItwillsupportthedevelopmentoftwotypesofBRBdevices:classicaland"dry",prequalificationofasetofcommon-capacitydevicesanddevelopmentofdesignguidesforfabricatorsanddesigners.

A further research direction that the author intends to develop is the "seismic protection of buildings using passive and semi-active control".Aparticulartypeofsemi-activedevice,basedonmagneto-rheologicalfluidsiscurrentlyunderinvestigationbytheauthoraspartoftheresearch

135

teamofthecontractno.77⁄2014PN-II-PT-PCCA-2013-4-1656:2014-2016"Protectiaseismicăastructurilor cu sisteme de contravantuiri disipative echipate cu amortizoare cu fluid nano-micromagnetoreologic – SEMNAL-MRD – Seismic protection of engineering structures throughdissipative braces of nanomicro magnetorheological fluid dampers", coordinated by PolitehnicaUniversityofTimisoara(coordinatorDanDubina).Thisparticularresearchcontract is supportedbyastrongindustrialinterest,whichmotivatesanapplicativeoutputoftheresearch.Semi-activecontrol has an important potential in seismic protection of buildings, as it can adapt to thecharacteristicsofthefutureseismicevents,unknownatthedesignstage.Moreover,thedevicecanadapt itscharacteristics totherequirements imposedbydifferentdesignsituation(serviceabilityvs. ultimate limit state, earthquake vs. wind loading). Semi-active structural control has a stronginterdisciplinary character. This kind of research opens the possibility of enlarging the researchnetwork, and accessing new funding sources. It is intended in the future to enlarge the researchpartnership and apply for a Horizon 2020 project, in order to extend the obtained results fromproof-of-concepttoapplication-readystage.

"Improved seismic design criteria for steel structures in case of ground motions with high frequency content in the long-period range"isanothersubjectthatdeservesattention.Groundmotions characterised by large control periods TC impose substantially larger demands onstructuresrespondingintheinelasticrangethantheoneswithsmallcontrolperiods.Intheformercase conventional design should be based on reduced behaviour factors. Though this is a well-knownfact,modernseismicdesigncodesdisregardit.TheproblemisespeciallyimportantinthecaseofRomania,wherespecificcharacteristicsofVranceaseismicsourceandsoftsoilconditionsgenerategroundmotionswithlargevaluesofcontrolperiods(TC=1,6sec)inBucharestarea.Thissituation is quite unusual, and therefore did not receive much attention globally. A systematicnumericalprogramisnecessaryinordertoevaluatetheseismicperformanceforarelevantsetofbuildings and derive behaviour factors to be introduced in future revisions of national seismicdesigncodeandEurocode8.

Seismic performance of steel structures depends to a large extent on the behaviour of itsconnections. A great deal of research was devoted to analysis of cyclic response of varioustypologies of beam to column connections in moment resisting frames worldwide. Designguidelines for prequalified beam to column connections exist in United States and Japan. Acomprehensive research project with the same target is underway in Europe (EQUALJOINTSproject,seesection0).However,connectionsinsteelconcentricallybracedframesreceivedmuchless attention. Design guidelines for brace connections in seismic resistant construction arevirtuallynon-existentinEurope.AEuropean(RFCS)projectdevotedtothissubjectcanprovidetheframeworkforfillingthisgap."Design criteria for connections in concentrically braced frames and prequalification of typical European connections" is thus anothersubject that theauthorwishestodevelopinthefuture.

Allofthesubjectsaddressedtodatebytheauthorarestronglybiasedtowardtheapplicativeanddeterministic components of seismic engineering. Though future research directions in view willremainfocusedonthisapproach,thelargeuncertaintywhichcharacterisesseismicactionandtheuncertaintiesinmodellingofstructuralresponseaskforaprobabilistictreatmentoftheproblem.Therefore, "seismic vulnerability and risk assessment" will complete the list of researchdirectionsthattheauthorwishestoexploreinthefuture.

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3.2 PROFESSIONALDEVELOPMENTPLAN

Thecomplexityoftoday'sresearch,itsmultidisciplinarycharacter,andthedemandforapplicativeoutputsaskforacollaborativeapproachinitsimplementation.Thereforeoneofthemainaspectsof the professional development plan consists in strengthening existing connections withinternationalandnationalresearchpartners.AttheEuropeanlevel,theauthorintendstocontinueanddevelophisscientificcollaborationwithUniversityofNaplesFederico II,TechnicalResearchCentre of Finland VTT, University of Ljubljana, Joint Research Centre in Ispra, University ofCoimbra, Imperial College London. Initiation of new collaborations with Budapest University ofTechnologyandEconomics,andUniversityofPortoisalsoenvisaged.

At the same time, collaboration at the national level is considered to be important. Existingconnections with Technical University of Civil Engineering in Bucharest and the TechnicalUniversity of Cluj-Napoca will be maintained and developed. It is intended to create newconnections at the Gheorghe Asachi Technical University of Iasi. The experimental infrastructurefrom the Politehnica University of Timisoara, specialised in quasi-static tests and potentiallypseudo-dynamic ones is complementary to the shaking table testing available at TechnicalUniversityofIasi.Collaborationbetweenthetworesearchfacilitiesisbelievedtobesynergeticandmutuallybeneficial,andthereforeisonthelistoffuturedevelopmentsoftheauthor.

Collaborationwith industrialpartners willbemaintainedanddevelopedas well.Pastexperiencewith design companies and steel producers (Popp & Asociatii, Bucharest and Hidromatic system,Timisoara) has shown that this collaboration is effective in identifying innovative solutions topracticalproblemsandattractingfundingforresearch.

AmoreactiveengagementintheactivitiesofthetechnicalcommitteesoftheEuropeanConventionfor Constructional Steelwork (ECCS), the European Committee for Standardization (CEN) andRomanian Standards Association (ASRO) is also envisaged. Currently CEN/TC250 received amandate for developing the second generation of structural Eurocodes. The author intends tocontributetothisprocess,bothbyapplyingfortheroleofprojectteammemberintasksrelatedtoseismic design of steel structures, as well as through the reviewing activity of CEN and ASROtechnicalcommittees.

Theauthorwillcontinuetoseekfundingoftheresearchactivitiesthroughmoreactiveinvolvementingrantapplications.Europeanfundingopportunities,liketheResearchfundforCoalandSteelandHorizon2020willbeemphasised.

Improvementoftheresearchinfrastructure(experimental,computingandsoftware)willcontinueto be addressed, through allocating resources within research grants, but also by seekinginfrastructure-specific funding opportunities. At the same time, the internal organisation of thelaboratorywillbeimproved,bysettingupasetofproceduresforperformingtests,maintenanceofequipment,andarchivingofexperimentalresults.

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3.3 ACADEMICDEVELOPMENTPLAN

Academic activities are regarded as an important part of the personal development plan. TheprimaryobjectiveinthisareaconsistsinearningtherighttoconductPhDtesis,andsubsequentlyto effectively coordinate PhD students. The subjects to be investigated would be those stated insection3.1"Scientificdevelopmentplan",andwillbesupportedbyresearchgrants.

Anotherdirectionfollowedwillbetheimprovementoftheteachingmethodsandaids.Thoughtheauthor has already prepared lecture notes for the courses he teaches, some of them can benefitfrom better teaching materials. In particular, it is intended to write two books on "Basis ofstructuraldesign"and"Performancebasedearthquakeengineering",supportingthecorrespondinglectures. On the other hand, the teaching methods will be improved by involving the studentsthrough interactive learning techniques. In particular, laboratory activities demonstrating thedynamic response of structures using the shaking table will be organised with studentsparticipatingtothecourseof"Structuraldynamicsandearthquakeengineering".Atthesametime,the author intends to develop web-based applications demonstrating different dynamicphenomena, which will increase attractiveness of the course and improve student'scomprehension.

Furtherdevelopmentof the internationalcooperationat theacademic level isalso targeted.Asaconcrete measure, the author intents to coordinate new student and teaching staff exchangethroughtheERASMUS+scheme.Asafirststep,anapplicationforacooperationagreementwiththeTechnicalUniversityofMoldovaisintendedtobeestablished.

Lastbutnotleast,thecontinuingeducationisanareathatisrelativelyunderdevelopedcurrentlyand which has a good potential for improvement.Considering the fact thatdesign codes are inapermanentprocessofimprovementandrevision,andthatresearchoutcomesshouldfindtheirwayinto practice, there is an increasing demand for short-term training courses for practicingengineers. The author intends to play an active role in organisation such courses in cooperationwiththeRomanianAssociationofStructuralEngineering(AICPS).

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