slip critical connections (friction type connections) (1)
DESCRIPTION
Engineering School Lecture on Slip Critical jointsTRANSCRIPT
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Steel ConnectionsSteelConnections
Alecturepreparedp pBy
A P D E B MAssis. Prof. Dr Ehab Boghdadi Matar
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AcknowledgementAcknowledgement
k l d h f d i hi l h Iacknowledgephotosfoundinthislecturetothescientificteachingaidsfoundindifferentsources,
i llespecially AISCdigitallibrary ESDEPlecturenotes AISC connection teaching toolkitAISC connectionteachingtoolkit PersonalphotostakeninGermany,Holland,Austria and EgyptAustriaandEgypt.
Ehab matarAssis. Prof. of steel structural
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ObjectivesObjectives
Throughthefollowing3lectures,weshallstudytogetherthesteelconnections.Ourmainobjectiveswillbe:
1. Identify the different types of steel connections1. Identifythedifferenttypesofsteelconnections2. Understandingtheforcetransferthroughsteel
ticonnections3. Practicingthedesignofboltedandwelded
connectionsthroughneatselfexplainedcalculationsandfulldwgs details.g
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PretensionedHighstrengthboltedconnections (SlipCritical Joints) (AISC)connections(Slip CriticalJoints)(AISC)
In a slipcritical joint the bolts must be fully pretensioned to cause a clamping forcebetween the connected elements
This force develops frictional resistance between the connected elements
The frictional resistance allows the joint to withstand loading without slipping intobearing against the body of the bolt, although the bolts must still be designed for
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g g y , g gbearing
The faying surfaces in slipcritical joints require special preparation (AISC)
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WhentoUseSlipCriticalJoints
Per the RCSC Specification (2000), Slipcritical joints are only required in the followingapplications involving shear or combined shear and tension:
1. Joints that are subject to fatigue load with reversal of the loading direction (notapplicable to wind bracing)
2 J i h ili i d h l2. Joints that utilize oversized holes
3. Joints that utilize slotted holes, except those with applied load approximatelyperpendicular to the direction of the long dimension of the slot
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p p g
4. Joints in which slip at the faying surfaces would be detrimental to the performance ofthe structure
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SnugtightInstallation
Snugtight is the tightness attained with a few hits of an impact wrench or the full effort ofan ironworker using an ordinary spud wrench to bring the connected plies into firm
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contact
(RCSC 2000)
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TurnofNutInstallation
Installation beyond snugtight is called pretensioning
Turnofnut pretensioning involves several steps:p g p
1. The bolt is snugtightened
2. Matchmarks are placed on each nut, bolt, and steel surface in a straight line
3. The part not turned by the wrench is prevented from turning
4. The bolt is tightened with a prescribed rotation past the snugtight condition
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The specified rotation varies by diameter and length (between 1/3 and 1 turn)
(RCSC 2000, AISC)
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CalibratedWrenchInstallation
Calibrated Wrench pretensioning uses an impact wrench (above left) to tighten theb l ifi d ibolt to a specified tension
A SkidmoreWilhelm calibration device (above right) is used to calibrate the impactwrench to the torque level which will achieve the specified tension
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q p
A sample of bolts representative of those to be used in the connections are tested toverify that the correct tension will be achieved (RCSC 2000, AISC)
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ASTMF1852Installation(AISC)
F1852 bolts are twistofftype tensioncontrolbolts
These bolts must be pretensioned with a twistofftype tensioncontrol bolt installation wrenchthat has two coaxial chucks
The inner chuck engages the splined end of thebolt
Th t h k th t The outer chuck engages the nut
The two chucks turn opposite to one another totighten the bolt
9 The splined end of the F1852 bolt shears off at
a specified tension (AISC 2003)
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ASTMF959DirectTensionIndicatorsDTIsDTI s
Another way to try to ensure proper pretensioning of a bolt is through the use ofdi t t i i di t (DTI )
FeelerGages
direct tension indicators (DTIs)
These washers have protrusions that must bear against the unturned element
As the bolt is tightened the clamping force flattens the protrusions and reduces the As the bolt is tightened the clamping force flattens the protrusions and reduces thegap
The gap is measured with a feeler gage
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When the gap reaches the specified size the bolt is properly pretensioned
(AISC & NISD 2000)
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InstallationofDTIs
(Adapted from Figure C 8 1 RCSC 2000)
It is essential that direct tension indicators be properly oriented in the assembly
a) The bolt head is stationary while the nut is turned DTI under bolt head
(AdaptedfromFigureC8.1RCSC2000)
a) The bolt head is stationary while the nut is turned DTI under bolt head
b) The bolt head is stationary while the nut is turned DTI under nut (washer required)
c) The nut is stationary while the bolt head is turned DTI under bolt head (washer required)
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d) The nut is stationary while the bolt head is turned DTI under nut
(RCSC 2000)
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Pre tensioning force T Torque MPretensioningforceT,TorqueMaxAxfT 70 sybxAxfT 7.0
TdM 20Where:
TdMa ..2.0fyb= yield point stress of the bolt material = 6.4t/cm2 for bolts grade 8.8 and = 9t/cm2 for bolts grade 10.9A = stress area in the table given laterAs= stress area in the table given later.d= bolt diameter
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Thedesignshearstrengthofasingleboltinslipcriticalconnections
TmPs
boltin theforcetensioningpreTplansshear of no. m
:Where
sliptoregardhfactor witsafety=tcoefficienfriction =
bolt in theforcetensioning-pre= T
structure steelordinary in loading II case1.05for = structure steelordinary in loading I casefor 1.25 =
slip toregardh factor witsafety
cranes and bridgesin loading II casefor 1.35 = cranes and bridgesin loading I casefor 1.6 =
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Coefficient of friction Coefficientoffriction Thefrictioncoefficientisofprimeimportanceforfrictiontypeconnection.
Itdependsontheconditionandpreparationofthesurfacestobejoined.SurfacetreatmentsareclassifiedintothreemainclassesA,BandCasfollows:
ClassA(=0.5):( ) Surfacesareblastedwithshotorgritwithanylooserustremoved,no
painting.f bl d h h d l d h l Surfacesareblastedwithshotorgritandspraymetalizedwithaluminum.
Surfacesareblastedwithshotorgritandspraymetalizedwithazincbasedcoating.g
ClassB(=0.4): Surfacesareblastedwithshotorgritandpaintedwithalkalizincsilicate
d h k fpaintingtoproduceacoatingthicknessof5080m. ClassC(=0.3): Surfaces are cleaned by wire brushing or flame cleaning with any looseSurfacesarecleanedbywirebrushingorflamecleaningwithanyloose
rustremoved.
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Tablefortheallowableshearforce/bolt(grade10.9)Noteforgrade8.8,70%ofthesevaluesistobeused
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Bolts Subjected to shearBoltsSubjectedtoshear
Q/bPs=mT/Q/b s /
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Bolts subjected to TensionBoltssubjectedtoTension
0 6* T/b0.6*T
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Bolts subjected to shear and tensionBoltssubjectedtoshearandtensionbolteachinforcesActing
T
FQFT ddext cos.&sin.bolteach in forces Acting
QnTT extbext /
nQQ b/
ditiS f t TT bext / 8.0
conditionsSafety
T
TmPQ bextsb // 1
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Bolts subjected to Shear and MomentBoltssubjectedtoShearandMoment
bolteach in forces ActingM
/TT
hMCT
bext
/
/
nQQ
n
b
tbext
8.0conditionsSafety
/
mPsQTT bext
h
bolts ofnumber total=n moment todueonly tension tosubjected bolts ofnumber = n t
/ mPsQ b
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Boltssubjectedtoshear,tensionandbendingmoment
bolteachinforcesActing bolteach in forcesActing
hMCT
/ nTT
h
tbMext
/ nTT extbext
t
h
ditiS f t
/ nQQ b
8.0conditionsSafety
//
TTPTT bMextbext
connection ypefriction tin 1 //
T
TmPQ bextsb
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Prying forces (P)Pryingforces(P)I h b di f h .InthebendingoftheflangesoftheTee,thebolts act as a pivot pointboltsactasapivotpointsothatthereisacompressivereaction(Q)betweentheouteredgesoftheflanges,whichisdefined as the PryingdefinedasthePryingForce.Thetensioninducedinthebolts,for,equilibrium,isthusFb =F+Q.
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ValueofPryingforcesy g
4
,,,
2
4
2
4
.
...30.
144
3...30
.21
bextMbextp
s
p
orTT
AbatW
baa
AbatW
P
s area stressBolt = A bolts. ofcolumn one r.t.breadth w. stub Tee Flange =W
cm.in stub teestem thew.r.t.dimension bolt inner and overhangouter Bolt = ba,...3044 sAbab
Mb,ext,
b,ext,
M/dCTcouplesoppositeandequaltwoby Mmoment applied theoft replacemen the todueor Text = T
force tension externalan either toduecolumn bolt oneon force tension external Applied = T
bbb M/d=C=Tcouples opposite andequal two