An article about analysis of structural behavior of saddles for stay cables, including standards comparison and laboratory testing.

1. Fromaperformancepointofview,threemainkeyaspectsshouldbeaddressed.First
isfriction,whichinfluencesforcetransmissionfromthestaycabletothetowerand,
strictlyspeaking,mayalsoaffectthestructuralefficiencyofthesaddledevice.Asarule
ofthumb,thehigherthefriction,thegreateristhehorizontalloadthatistransferredto
thetower.Hence,thefrictioncoefficientwhichdevelopsattheinterfacebetweenthe
strandsandthepipesbecomesanimportantdesignparameter.
Regardingsaddlefrictionvalidation,FIBBulletin30doesnotmakespecific
requirements;thePTIrecommendationhoweverexplicitlyrequiresmanufacturersto
assessthefrictioncoefficientthroughtesting,althoughthedetailsandconditionsof
suchtestsarenotspecified.Inparticular,itseemsreasonabletoworkoutthefriction
coefficientasameanvalue,averagedacrossallthetubesforanyspecificsaddledevice;
butnoguidanceisprovidedinthatregard.Tothatend,Eytelwein’sformulaisusedto
evaluatethefrictioncoefficient,usingspecificationsinEN1993-3-1-11.
Furthermore,therecommendationsdonotgiveanyguidanceastowhetherorhow
fatiguemightaffectthefrictioncoefficient.Itseemsreasonabletoexpectaslight
reductioninfrictionasaresultofsystemusage.
Additionalinformationshouldbeprovidedaboutthetestingconditions,forinstance
thenumberofstrands,saddledeviationanglesandloadlevels.Theindustryrecognises
theneedforimprovedrecommendationsinthisregard,anddiscussionsarecurrently
underwaytodevelopmoredetailedguidanceonthissubject,whichwillbeincludedin
revisedversionsofbothinternationaldocuments.
Thesecondaspectisthefatiguebehaviourthataffectsstay-cablesystemresponse
attheserviceabilitylimitstate.BothFIBandPTIrequirethemanufacturertocarry
outatensilefatiguetestonatestspecimenwithafreelengthofatleast2m,installed
withaninclinationofabout30°fromthehorizontalandsubjectedtoa10mradangular
I
n recent years, saddlesystemshavebecomeamorecommonsolutionforcable-
stayed bridges. Theydelivernotablematerialsavingsandmakeitpossibleto
reduce the size of bridgetowers,creatingarchitecturalconfigurationsthatare
more elegant and aestheticallypleasing.Astay-cablesaddleisastructuraldevice
that allows continuousdeviationofthetensileelementsfromthedeckthrough
the tower and back to thedeck,aswellastransferringthestay-cableforceintothe
bridge structure, eliminatingtheneedforanchorsinthetowers.Nevertheless,themain
international recommendationsrequirethesameperformancebothforastandardstay-
cable configuration, anchoredatthetowers,andaconfigurationwithsaddlesystems.
The two systems mustthusbeequivalentnotonlyintermsoftheirmechanical
performance but also functionalperformance;saddlesystemsmustensurethestrands
have proper corrosion protectionandmustincorporatethepossibilityofreplacingeach
tensile element individually.
Stay-cable saddles arecomplexsystemsthatneedcarefuldesignandinstallation.The
most advanced systemsavailablethesedaysaremulti-tubesaddles,whereeachstrand
runs through an individualtubeanddifferentialstay-cableforcesarebornebyfriction.
This solution also accommodatestheoptionofreplacingeachtensileelement,hence
ensuring effective maintenancesolutions.
As the analysis of thestructuralbehaviourofaspecificsaddledeviceisfairly
difficult, involving severalco-dependentfactors,testingplaysakeyroleinassessingthe
performance of these systems.Design,testingandinstallationofsaddlesareusually
performed according toeitherFIBBulletin30(2005)orPTIDC45.1-12(2012),whichare
the two main internationalcodesgoverningstay-cablesystems.
The behaviour of thesaddledependscruciallyonthelevelofbendingtensions
which develop as a consequenceofthegeometricconfigurationofthesystem.This
stress distribution is in additiontotheuniformtensilestressproducedbypermanent
and live loads acting onthebridge,makingthetensionsinthestaycablemoresevere.
Such bending stresses arefairlydifficulttoproperlypredictbecauseoftheintrinsic
complexity of the interactionbetweenthewires.
TheFIBrecommendationdefinessmallerdesignminimumvaluesfortheradiusof
curvature of the saddlesand,asaconsequence,potentiallytakesintoaccounthigher
bending stress states whicharebasicallyhigherthanthoseconsideredbyPTI.
Another fundamentalaspectofsaddledesign,essentiallylinkedtodamagefrom
fretting fatigue, is to eliminateanyunintentionalcontactbetweenthestrandsandthe
saddle exit areas which mayleadtosuddenbrittlefractureofstrands.Propersaddle
detailing is, therefore, required.
As regards the complexityofsaddletesting,FIBBulletin30allowstestresultstobe
extrapolated to larger orsmallersaddleswithasimilardesign,basedonthegeometrical
similarity between the radiusofcurvatureandthenumberoflayersoftensileelements
inthe saddle used in thetestandthosevaluesintendedtobeusedonaproject.
64 www.bridgeweb.com Bd&e | ISSUE 85 | 2016
n CABLE TECHNOLOGY
IN THE
SADDLE
Analysis of the structural behaviour of saddle
systems for stay cables is complex and involves
several co-dependent factors. Tommaso
Ciccone, Francesco Moraglia and
Andrea Castiglioni di Caronno explain
The test rig included the anchorage element of a stay-cable system

2. correspondedtotheendofthestrokeforthehydraulicjacksonthetestrig,hencethe
loadlevelcouldnotbeincreasedanyfurtherandthetestwasstopped.
Attheendofthestatictest,noevidenceofdamagetothesaddlesystemorthe
anchorageswasdetected.Afteralmostonemonthofdirectexposuretoatmosphere,no
corrosionwasnoticedoveranycomponentofthetestsystem.
Thehighperformanceofthesystemobservedduringthetensilefatigueandstatic
testsprovedthatsaddledevicesareperfectlycapableofcarryingfatigueloading—
correspondingtotrafficloadsonthebridge—and,moreover,thattheydonotaffectthe
strengthorductilityofthecable.
Finally,otherfrictiontestswerecarriedoutinthefinalstageofthetestprogrammeto
assesstheimpactonthefrictioncoefficientoffatiguecyclingandthelargestrainofthe
strands.Frictioncoefficientsrangingbetween0.24and0.39weremeasured.
Thetensilefatigueandstatictestsproducedapredictable,smallusageofthesystem,
iebothpipesurfaceandinternalcover.Thefrictioncoefficientsthatweremeasured
shouldbeconsideredasminimumvaluesforultimatelimitstates;thatis,thelower
boundsoftheactualfrictioncoefficientsthatthesystemexhibitsinitsinitialconditions.
Thus,predictedvaluesofthefrictioncoefficientjustafter thefatiguetestmay
certainlybeconsideredtobearound0.4.
Thistestingcampaignprovidedawide-rangingandcomprehensiveinsightinto
thestructuralandfunctionalbehaviourofsaddledevices,takingaccountofarange
ofworkingconditions.Inparticular,theTSS-Tsaddlesystemmatchedalltheexisting
recommendationsrequirements;itsperformancewasequivalenttowhatwouldbe
expectedfromatraditionalstay-cablesystem,anditprovidedsatisfactoryresponsesto
theconditionsnotfullydescribedanddetailedincurrentrecommendationsn
TommasoCicconeistechnicaldirectorofpost-tensioning&stay-cablesystemsatTensa;
FrancescoMoragliaandAndreaCastiglionidiCaronnoareengineersinthetechnical
departmentofTensa
deviation at saddle exits.Thesaddlemovesverticallyfor2millioncyclesbetweena
lower configuration, producingastressequalto45%GUTS-Δσ inthestay-cable,andan
upper configuration, inducingastressequalto45%GUTSinthestaycable.Althoughthe
general test procedure isexactlythesame,FIBrequirementsarealittlemorestringent.
Asa matter of fact, FIB specifiesastressrangeforparallelstrands,ofthestaytype,
equal to 200MPa, greaterthanthatstatedbyPTI,thelatterbeingequalto110MPaplus
35MPa through vertical movementofthesaddle.Acceptancecriteria—2%maximum
wire fractures — are the sameforbothstandardswhile,forsmallcables,PTIaccepts
three wire breaks ratherthanFIB’stwo.Henceifastay-cablesaddlemeetstheFIB
fatigue test requirementsitautomaticallyfulfilsthePTIrequirementsaswell.
The last aspect is staticbehaviour,whichgovernsthestay-cablesystem’sresponse
atultimate limit state. FIBBulletin30requiresthatatensilestatictestiscarriedout
after the fatigue test, wherethestaycableissymmetricallyloadedinordertoreachthe
maximum stress level thatstrandsmayexhibit.Thestaycablemustreachaminimum
tensile force equal to 95%GUTSor92%AUTS,whicheverisgreater.
The Tensa multi-tubesaddlesystemTSS-Tisaweldedboxmemberwithseveral
parallel steel pipes, surroundedbyahigh-strengthcementmortarwhichproduces
proper confinement andstrengtheningeffects.Theinnersurfaceofeachtubeiscovered
with a special compoundprovidingincreasedfrictionproperties.
To investigate the behaviouroftheTSS-Tsaddle,anextendedtestingcampaignwas
carried out at the structuralengineeringlaboratoryofthePolytechnicUniversityof
Milan, Italy, following therecommendationsdescribedaboveinadditiontoadditional
improvements in testingprotocols.Beforeanyfatigueorstatictestwascarriedout,
friction tests were performedoverseveralTSS-Tsaddlesystems,whichvariedbothin
their geometrical configurationandloadingboundaryconditions.
The geometric configurationofthesaddleisstrictlydependentonthedeviation
angle provided to the staycable,whichdefinestheradiusofcurvature.Toaccountforall
the different values of inclinationrequiredbystaycablesonrealbridges,saddleswere
tested considering deviationanglesrangingbetween30°and150°.
Stay-cables were subjectedtoarangeofloadswhichinitiallywerelowerthan50%
GUTS, the maximum designworkingloadofthestaycable.Althoughthetensionina
stay cable is usually known,itcanvarybothduringbridgeconstructionand,laterinthe
bridge’s lifetime, as a consequenceofhighertrafficloads.Aspredicted,testsproved
that neither saddle deviationanglenorinitialstay-cabletensionsignificantlyaffectthe
friction, and friction coefficientsrangingbetween0.38and0.48weremeasured.
To further investigateaddlebehaviour,full-scaletensilefatigueandstatictestswere
performed on a seven-strandsaddlesystem.Toenablethistobecarriedout,acustom-
designed steel frame weighingalmost10twasdesigned,producedandassembledinthe
laboratory in order to carryalltheloadspredictedforthetest,andensuringthestay-
cable system configurationwouldbeascloseaspossibletotheoneactuallyadoptedon
site. Costly, specially-builtequipmentwasusedtostressthestaycableandtoimpose
the vertical movement ofthesaddle.Inaddition,alltheelementsusuallypresentinthe
transition, free length andanchoragezoneofthestay-cablesystem,suchasdeviator
discs, wax boxes and so on,wereincludedinthesample.Themulti-tubesaddlewas
embedded in a concreteblocktosimulatetherealconditionsofthebridgetower.
As with the TSS-T system,thesaddlewasfilledwithasoftprotectivecompoundto
provide an adequate levelofcorrosionprotectiontothestrands.
The fatigue test lastedabout20daysandwascarriedoutat1.2Hzaveragefrequency.
There were no wire fracturesordamageinthesaddleorinanycomponentoftheanchor
heads during testing; thiswasconfirmedbythenetworkofaccelerometersfixedtothe
test sample. In particularthetestfullysatisfiedtheacceptancecriteriarequiredbyFIB
Bulletin30and the systemshowedoutstandingperformanceasregardsfatigueloading.
During the subsequentstatictensiletest,theseven-strandstaycablewasloadedup
toa force which totally satisfiedtheFIBBulletin30requirements;infact,theloadwas
97.5% of GUTS and 96.2%ofAUTS.
In the final test stagesthestay-cableelongationreachedapproximately1.9%,which
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Full-scale tensile fatigue and static tests were carried out on a 7-strand saddle system
Cutaway of the saddle arrangement