5-81 ANCHORAGE TO CONCRETE 1 LRFD BRIDGE DESIGN AIDS 5-81 • JANUARY 2012 5-81 ANCHORAGE TO CONCRETE 1. Introduction Steel-to-concrete or concrete-to-concrete connections can be accomplished through the use of several types of anchorage systems. This design aid describes the concrete anchor systems that are most widely used on Caltrans’ jobs and assists the designer in selecting the system that is best suited for a particular application. 2. Anchor Systems and Design Criteria There are two broad types of anchor systems – post-installed anchors and cast-in-place (CIP) anchors. The design recommendations for post-installed anchors in this aid conform to the requirements in the California Amendments to AASHTO LRFD Bridge Design Specifications. For CIP anchors, the design recommendations are based on ACI 318-08 as well as California Amendments to AASHTO LRFD Bridge Design Specifications. The designer should determine the loading combinations and the corresponding load factors for each application. 3. Post-Installed Anchors This type of an anchor is installed in a hole that is drilled in hardened concrete. There are two main types of post-installed anchors – Mechanical Expansion Anchors (MEA) and Bonded Anchors. 3.1 Mechanical Expansion Anchors MEAs are inserted in pre-drilled holes. These anchors expand and bear against the concrete surface and are placed using any of the following techniques: a) Hammering the anchor (deformation controlled) b) Tightening a nut (torque controlled) c) Expanding into an undercut (expanding into a notched opening at the bottom of a hole).
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5-81AnchorAge to concrete 1
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Bridge design Aids 5-81 • JAnuAry 2012
5-81 AnchorAge to concrete
1.IntroductionSteel-to-concrete or concrete-to-concrete connections can be accomplished through theuseofseveraltypesofanchoragesystems.ThisdesignaiddescribestheconcreteanchorsystemsthataremostwidelyusedonCaltrans’jobsandassiststhedesignerinselectingthesystemthatisbestsuitedforaparticularapplication.
The design recommendations for post-installed anchors in this aid conform tothe requirements in the California Amendments to AASHTO LRFD Bridge Design Specifications.
For CIP anchors, the design recommendations are based on ACI 318-08 as well asCalifornia AmendmentstoAASHTO LRFD Bridge Design Specifications.
3.1MechanicalExpansionAnchorsMEAsare inserted inpre-drilledholes.These anchors expandandbear against theconcretesurfaceandareplacedusinganyofthefollowingtechniques:
a) Hammeringtheanchor(deformationcontrolled)b) Tighteninganut(torquecontrolled)c) Expandingintoanundercut(expandingintoanotchedopeningatthebottomof ahole).
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MEAsarefrequentlyusedtoanchorminorortemporaryattachmentssuchassigns,brackets, inspection ladders, safety railing, utility pipes and light fixtures to hardened concrete.MEAshavethefollowingadvantages:
Materialsand installationmethodsofMEAsmustcomplywith the requirementsofSection 75 of the Standard Specifications. Designers should refer to the Caltrans’“Authorized Materials List”foralistofapprovedMEAs.
BasedontestsperformedattheTransportationLaboratory(Translab) by the Office of StructureMaterials,METS,thefollowingtwotypesofMEAsarecurrentlyapprovedforuseonCaltrans’projects[JohnP.DuselandCraigN.Harrington,1986]:
a) Shellexpansionanchorwithinternalthreads– Thisanchorrequiresanindependentstud,nutandwasher,andisstrongerin tensionthanothertypesofshellanchors.
b) Integralstudanchor(wedgetypeandexternalplug)– Thisanchor,whichisfurnishedwithnutsandwashers,iseasiertoinstallina multi-holebaseplateandisstrongerinshear.
3.1.2 Design and Detail GuidelinesTheDesignersshouldconsiderthefollowingissueswhenMEAsareselected:
a) ThePlansshouldshowthesizeofMEArequired,butnotshowthedepthorthediameterofthehole–thesizeoftheholeisaddressedintheStandard Specifications.Figure3.1.2-1showsatypicaldetailforMEAstobeusedinthePlans.
b) The thicknessofconcrete,whereananchor isembeddedshouldbeadequate toresist driving forces during anchor seating – typically, the minimum concretethicknessshouldbe1.5timesthedepthofembedment.
c) ToensureproperseatingofshelltypeMEAs,thetopoftheshellbodyistypicallyrecessedbetween1/2" and1" belowtheconcretesurface,andan independentthreadedstudratherthanaheadedboltisrequired.
d) As the design strength of shell and stud type MEAs is governed by creepconsiderations,themaximumanchorsizeislimitedto3/�inch.
c) DrillandEpoxyBondDowel:Bulkepoxyisusedasthebondingagent.
This method of bonding anchors to concrete is no longer used in structuralapplications as several bulk epoxies exhibit high creep characteristics undersustainedtensileloads.Inadditiontheseepoxiesmayrequireexactmixratiosandaresensitivetofreeze/thawconditions.
Thechemicaladhesivesindrillandbondapplicationsmustbepre-approvedandlistedunderthe“Authorized Material List”priortouseonCaltrans’projects.
Bonded anchors provide a simple, effective, economical and preferred system forattaching metal fixtures or new concrete to existing/hardened concrete. In this system, bar reinforcement dowels or threaded rods are placed in drilled holes filled with either grout or a bonding material.Typically, bonded anchors are used for attaching newbridgebarriers,signframesorelectroliersontoexistingbridgedecks,wideningbridgeabutments and bridge decks, and in seismic retrofits.
3.2.1 Applications Typical applications of different types of bonded anchors are discussed in thefollowing:
a) Ingeneral,drillandgrout,drillandbondordrillandepoxybonddowelsshouldbeusedonly inholesdrilledatadownwardangleofat least20degrees to thehorizontal–generallydetailedasa3:1slopeonPlans.
b) Drill and Bond (Chemical Adhesives) dowels should be used when a drilledholeata3:1slopecannotbeachieved.Inhorizontalholes,chemicaladhesivesshould always be used since the adhesives have a higher viscosity (example:indeckoverhangswherehorizontalholesmaybe theonlyoptiondue to smalldeckthickness).Thisanchoragesystemisnotrecommendedforuseinoverheadapplications.
c) Ingeneral,drillandbonddowelsshouldbeusedonlyinapplicationswheretensionistheprimaryforceintheanchor;grouting,withbondingasanoption,shouldbeusedwhenshearistheprimaryanchorforce.
Note: For the design parameters shown in Tables 3.2.2-1 and 3.2.2-2, the tensilestrengthsofrebar(bothgroutedandbonded)aswellasthoseforgroutedanchorrodsare controlledby anchor slipwhile the tensile strengthsofbondedanchor rods arecontrolledbyyieldingofanchor.
c) EdgedistancethatissmallerthantheminimumedgedistanceshowninTables 3.2.2-1and3.2.2-2isnotpermitted.
d) Whenananchorissubjectedtocombinedtensionandshearloading,thefollowingequation should be satisfied:
1≤+
resistancetensileFactored
loadtensileFactored
resistanceshearactoredF
loadshearFactored
Table 3.2.2-1 Factored Resistance (Design Strength) for ASTM A 706 Rebars (Grade 60)
Rebar Size
Minimum Edge
Distance-de(inches)
Minimum Embedment
Depth-le(inches)
Hole Diameter (inches)
Design Strength for Minimum de and le
(kips)
Design Shear Strength
(kips) (de ≥10 db)Tension Shear
Grout Bond Grout BondGroutand
Bond
Grout and
Bond
#5 3 5 7/8 11/8 3.1 10.5 2.5 8.2
#6 � 6 1 11/� �.� 1�.8 �.5 11.6
#7 � 7 11/8 13/8 6.0 20.3 �.5 15.8
#8 5 8 11/� 11/2 7.9 26.7 7.0 20.9
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Table 3.2.2-2 Factored Resistance (Design Strength) for ASTM A 307 Threaded Rods
Anchor Size
(inches)
Minimum Edge
Distance-de(inches)
Minimum Embedment
Depth-le(inches)
Hole Diameter (inches)
Design Strength for Minimum de and le
(kips)
Design Shear
Strength (kips)
(de ≥10 db)Tension Shear
Grout Bond Grout BondGrout and
Bond
Grout and
Bond
5/8 3 5 7/8 11/8 2.3 6.1 2.5 3.6
3/� � 6 1 11/� 3.3 9.0 �.5 5.3
7/8 � 7 11/8 13/8 �.6 12.5 �.5 7.3
1 5 8 11/� 11/2 6.1 16.3 7.0 9.6
3.2.3 Design and Detail GuidelinesTheDesignersshouldconsiderthefollowingissueswhendrillandbonddowelsareselected:
a) ThePlansshouldshowthetypeoftheanchor[e.g.,DrillandBondDowel(ChemicalAdhesive)],theanchorsizeandtheembedmentdepthrequired.ThePlansshouldnotshowthediameterofthehole–thisinformationisaddressedintheStandard Specifications.Figure3.2.3-1showstypicaldetailsforadrillandgroutdowelthatcanbeusedinthePlans.
Drill and grout#6 x 1’-6”@24
6”
8”
Figure 3.2.3-1 Typical Detail for Grouted Anchor
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b) The embedment depths listed inTables 3.2.2-1 and 3.2.2-2 are not necessarilysufficient to develop the yield strength of the anchor. Deeper embedments may provide the required ductility under certain conditions. Generally, holes havingtwotimestheminimumembedmentdepthforreinforcementbarsoroneandone-halftimestheminimumembedmentdepthforthreadedrodswilllikelydeveloptheyieldstrengthofananchor.Otherpotentialfailuremodessuchasconcreteconefailureorconcreteshearfailurehavetobeconsideredwhenadeeperembedmentisprovided.
c) For anchors spaced closer than two times the embedment length, the concretestrengthmaycontrolthedesignduetooverlappingofthefailurecones(seeFigure3.2.3-2).Thedesignstrength,basedontheconcretestrengthandthesurfaceareaofthefailureconeiscalculatedasfollows:
Failure plane whenanchors are closer thanthe minimum spacing
Failure cone
ElevationFailure cone overlap
Potential failure plane
Plan
Figure 3.2.3-2 Effect of Anchor Group
3.3ResinCapsuleAnchorsAResinCapsuleAnchor (RCA) isa special typeofabondedanchorused tobondathreadedrodorarebarintohardenedconcrete.RCAsaretypicallyusedin-lieuofMEAs. They can also be used in-lieu of “Drill and Bond (ChemicalAdhesives)”anchors only if they conform to all the test requirements specified for a chemical adhesiveasdiscussedinSection3.2.1.
The RCA system is composed of a two-chambered glass/foil capsule that containstheresinandthecatalyst(hardener).Thesizeofacapsuledependsonthesizeoftheanchor.Thecapsuleisinsertedinaclean,pre-drilledholeofpropersize.Next,achisel-pointedthreadedsteelrodorrebarisattachedtoaroto-hammerandpowerdriventothebottomofthehole.Thisprocesswillbreakthecapsuleandmixitscontents.Themixedresincompoundformsastrongwaterproofbondwithboththeembeddedanchorandconcrete.
RCAscanbeloadedsoonercomparedtootherbondedanchorsbecauseoftheirshortcuretimes.IngeneralRCAscanbeusedinawidevarietyofapplicationsincludingin corrosive environments, or where dynamic loading is present. They are notrecommended for use under water or where fires are likely to occur.
Unless approved by the State Bridge Engineer, the use of RCA in an overheadapplicationisnotpermitted.
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3.3.1 Resistance (Design Strength)The design strengths of RCAs are based on limited pullout tests performed at theTranslab (AbidA. Mir and John P. Dusel, 1993) as well as data furnished by themanufacturersinthetestingprogram.
a) They are controlled by creep deformations (for creep limits, see Section 75,Standard Specifications),andincludearesistancefactor(strengthreductionfactor)of0.33,bothfortensionandforshear.
b) TheyarebasedontestsusingASTMA307threadedrodsasanchors;thesestrengthsmayalsobeusedforothertypesofanchorsthathaveahigherstrength.
c) The anchors are assumed to be subjected to static load conditions only. Whendynamicloading(movingloads,vibratoryloadsetc)governs,theresistanceslistedinthetableshouldbemultipliedby0.5.
e) Theyarevalidforconcretetemperaturesupto70°F.Atatemperatureof120°F,thedesign strengths reduce to approximately75%of those shown in the table.Forintermediatetemperatures,thedesignstrengthsshouldbecomputedthroughlinear interpolation. However, RCAs are not recommended where the ambienttemperatureislikelytoexceed120°F.
f) They are applicable when the center-to-center distance (spacing) between theanchorsisatleastequaltoitsstandardembedmentlength.SpacingcanbereducedtohalfthestandardembedmentdepthonlyifthedesignstrengthintheTableismultipliedby0.5.Forintermediatespacing,linearinterpolationmaybeused.
Ifnecessary,thedesignermayrefertotheCaltrans’“Authorized Materials List”andreviewmanufacturer’sbrochurestogetanupdatedestimateofdesignstrengths.Thecapacities should not exceed the strength shown on the manufacturer’s data sheetmultipliedbyareductionfactorof0.33.
Table 3.3.1-1 Factored Resistance (Design Strength) for Resin Capsule Anchors
AnchorSize
(inches)
Minimum Embedment
Depth(inches)
HoleDiameter(inches)
Design Strength for ASTM A 307 Threaded Rods (kips)
Tension Shear
3/8 31/2 7/16,15/32 2.1 1.7
1/2 �1/� 9/16 3.9 2.1
5/8 5 11/16,3/� 5.8 2.7
3/� 65/8 7/8 8.6 3.1
7/8 65/8 1 10.8 8.2
1 81/� 11/8 13.0 9.6
11/� 101/� 11/2 22.8 15.8
3.3.2 Design and Detail GuidelinesTheDesignersshouldconsiderthefollowingissueswhenRCAsareselected:
a) ThePlans should call out the anchor asResinCapsuleAnchor and specify theanchordiameter.ThesizeoftheholeandanchorembedmentareaddressedintheStandard Specifications.
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b) RCAsshouldbeinstalledinhardenedconcretethatisatleast28daysold.
c) RCAsshouldbeinstalledindryholesonly.Theinstallationtemperatureshouldnotexceedthemaximuminstallationtemperaturerecommendedbythemanufacturer.
d) Typically, the anchor embedment as specified by the manufacturer will develop thestrengthsshowninTable3.3.1-1.However,todeveloptheyieldstrengthoftherodandensureamoreductileperformance,thedepthofembedmentshouldbeincreasedtoabouttwicethatshowninthemanufacturer’sdata(andthisincreaseddepthshouldbeshownonPlans).Tworesincapsulesforbondinglongeranchorsindeeperholesmayberequired.Insuchcases,thestrengthofananchorbasedonconcrete failure should be verified. Deep holes that require more than two standard capsules to fill should not be used.
Figure�-1showstheengineeringdetailsofarockboltanchor.Thesedetailsarebasedontests performed by the Office of Structural Materials, METS at the Translab(J.P.Duseletal, 1979). This figure shows an expansion shell type anchored rock bolt where the bolt is placedinsideacoredholeandthenrotated.Therotationpullsawedgeintoanexpansionshell,whichexpandsagainstandintothewallofthecoredhole.Thevoidbetweentheboltandthecoredholeisthengrouted.
Yield TensileStrength = 37 kips
Ultimate TensileStrength = 53 kips
Stressed Area ofBolt = 0.544 sq. in.
6" min
12" m
in
Edge ofconcrete
12" m
in
1" hollow core galvanizedrock bolt. Torque to 250 ft lbs.before grouting.
Mechanical expansion headassembly (long or short coneand shell).
Figure 5.1-2 Typical Partial Embedment Failure Planes and Reinforcement
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5.1.1 Resistance (Design Strength) ThedesignstrengthsofCIPanchorsaresummarizedinTable5.1.1-1andarederivedusingACI318-08,AppendixD.Table5.1.1-1alsoliststhedesignparameters,includingtheminimumrecommendededgedistance,theminimumembedmentdepthtodevelopthe ultimate strength of a single anchor, and the tensile and shear design strengthsbasedonthefullembedmentcondition.
a) Thedesignstrengthofconcreteisassumedas3600psi.ThedesignstrengthsofboltsshowninTable5.1.1-1mayalsobeusedforhigherconcretestrengths,unlessmoreupdatedstrengthsarerequired.
b) The tensileandshearstrengthsareshownforsingleanchors. Foranchors inagroup,groupeffectsshouldbeconsideredperACI318-08.
c) Case1andCase2valuesforthetensilestrengthofboltsapplytobolttypesotherthan“J”bolts.Case3 applies to “J”bolts.For shear strength, there isno suchdistinctionbetween“J”boltsandothertypesofbolts.
e) TheanchorstrengthsinTable5.1.1-1arebasedoncrackedconcrete.Ifthedesignengineer determines that the concrete in which the anchors are installed willremain uncracked during service conditions, then the design strength shown inTable5.1.1-1maybemultipliedby1.�0.
Table 5.1.1-1 Factored Resistance (Design Strength) for Cast-In-Place ASTM A 307 Anchor Bolts
BoltDiameter(inches)
Minimum Edge
Distance(inches)
MinimumEmbedment
Depth(inches)
Tensile Strength(kips)
Shear Strength (kips)
Case 1 Case 2 Case 3 Case 1 Case 2
1/2 21/2 � 3.3 7.3 2.3 0.8 1.1
5/8 3 5 �.5 11.3 2.8 1.1 1.7
3/� 3 6 5.3 1�.8 5.1 1.2 1.8
7/8 � 7 7.3 18.7 5.9 2.0 3.0
1 � 8 8.1 22.8 9.1 2.1 3.2
5.1.2 Design and Detailing Guidelinesa) ThePlansshouldshowthediameteroftheanchorbolt,itsembedmentdepthand
thedesirednut/washercombinationifrequired.
b) Where possible, supplementary “hairpin” or tie back reinforcement should bedetailedonPlans(Figure5.1-2).Thisreinforcementwillhelptopreventsuddenconcreteconeorsideblow-outfailures.
Wherelargersizeboltsunderlateralloadingareused,suchassteelsuperstructuretoconcretesubstructureconnections,refertothepublication“Lateral Resistance of Anchor Bolts Installed in Concrete”(R.A.Swirskyetal,1977).
5.2Cast-In-PlaceInsertsCIP Inserts are prefabricated metal devices with female threads which are specifically made for attachment of bolted connections (Figure 5.2-1). Such inserts, which arecommerciallyavailable,aresimpleandeasytouseinconstruction.Otheradvantagesincludethefollowing:
CIP inserts may be used for either temporary or permanent applications. Suchapplications include overhead signs and fixtures, safety railings, inspection ladders andfalseworktoconcreteconnections.CIPinsertsshouldmeettherequirementsofSection75oftheStandard Specifications.TensiledesignstrengthsforvarioussizesofinsertsarelistedinTable5.2-1andincludearesistancefactorof0.7.
6. Caltrans,(2008&2011).California AmendmentstoAASHTO LRFD Bridge Design Specifications, 4th Edition, California Department of Transportation, Sacramento,CA.