deflection limit state

ABeginner'sGuidetotheSteelConstructionManual,13thed.(old)

Chapter8BendingMembers2006,2007,2008T.BartlettQuimby

Introduction

Flexure

FlexuralLimitStateBehaviorDeterminingApplicableLimitStatesFlexuralYieldingLimitStateLateralTorsionalBucklingLimitStateFlangeLocalBucklingLimitState

Shear

ShearBehaviorShearStrengthLimitState

Deflection

DeflectionBehaviorDeflectionLimitState

Misc.LimitStates

WebLocalYieldingWebCrippling

BeamDesign

Section8.4.2

DeflectionLimitStateLastRevised:11/04/2014

Intheabsenceofmorespecificcriteria,criteriaforstructureswithbrittlefinishes(asfoundincodedocumentsforyears)isfrequentlyused.Thissimplisticcriteriaputsalimitofthespandividedby360ontheincrementaldeflectionduetolive(ortransient)loadonlyandalimitofthespandividedby240ondeflectionundertotalload.Theselimitstatesaremathematicexpressedas:

LL

Figure8.4.2.1FrozenScupper

SelectingSectionsCoverPlatesTransverseStiffenersforShearBearingPlatesTransverseStiffenersforConcentratedLoadsContinuousBeams

ChapterSummary

ExampleProblems

HomeworkProblems

References

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FloorBeams Ordinaryusagea L/360 L/240Highwaybridgestringers L/200toL/300 Railwaybridgestringers L/300toL/400 aOrdinaryusageclassificationforfloorsisintendedforconstructioninwhichwalkingcomfortandminimizedplastercrackingarethemainconsiderations.Theserecommendeddeflectionlimitsmaynoteliminateallobjectionstovibrationssuchasinlongspansapproachingthemaximumlimitsorforsomeofficeandinstitutionalapplicationswhereincreasedfloorstiffnessisdesired.Fortheseusages,thedeflectionslimitsoftable8.4.2.2havebeenfoundtoprovideadditionalstiffness.

Table8.4.2.2AITCDeflectionLimitsforUsesWhereIncreasedFloorStiffnessisDesired

UsedwithPermission

UseClassification AppliedLoadOnlyAppliedLoad+DeadLoada

FloorBeamsCommercial,Office&InstitutionalFloorJoists,spansto26ftb

LL

pondingwater.Pondingisaprogressiveevent.Themorewaterontheroof,themoredeflectionyouget,whichmeansthatevenmorewatercanberetained,whichleadstomoredeflection,etc...Ifthebeamisstiffenough,thenpondingcanbeminimized.

Thebestsolutiontothepondingproblemisarchitectural.Itisstronglyrecommendedthatsufficientslopebegiventoroofsystems(aminimumof1/4"perfoot)topreventponding.Appropriatedrainagemustalsobeprovided.

Roofsincoldregionsthatusescupperstodrainarooflocatedbehindaparapetmaybecomepluggedwithice,resultinginunintentionalponding,leadingtodisastrousresults.Figure8.4.2.1showssuchafrozenscupper.Scuppersmustalsobemadelargeenoughtoallowwatertoescapeduringadelugeandtominimizeblockagebydebris.

TolerancesofAttachedElementsorNonStructuralElementsBelow

Oftentimes,nonstructuralelementshavespecificdeflectiontolerancesthataremorerestrictivethanthegeneralcriteriagivenabove.Thesetolerancesgenerallyareexpressedintermsasamaximumdeflectionvalueandmustbeconsideredindesign.

Forexample,afloorgirderspanning36ftmaydeflectupto1.2inchesunderaliveloadonlydeflectionlimitofL/360.Anynonstructuralpartitionunderthebeammustbeabletoaccommodatethisdeflection.However,ifitcannot,thentheamountofliveloaddeflectionthatcanbeaccommodatedbecomesthenewdeflectioncriteriaforthisbeam.

Vibrations

Certainvibrationshavebeenfoundtobeobjectionableinmostoccupancyclassifications.Vibrationsareoftenlumpedtogetherwithdeflectionsincebotharestiffnessrelated.Vibrationsareafunctionof

stiffnessandmass.Thefrequencyofthevibrationsisofmoreconcernthantheamplitude.Thetreatmentofvibrationsisbeyondthecurrentscopeofthistext.

SelectionofCriteria

Thechoiceofdeflectioncriteriaisaprojectdependent.Othercriteriamaybeencounteredthathavebeendevelopedforspecialstructuresand/orsituations.Thesemaybeconsideredasneeded.

Fortheproblemsinthistext,theequationslistedatthestartofthissectionwillbeusedunlessotherwisespecified.