Anchor Checking Using ACI 318 Appendix D

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Anchor Checking Using ACI 318 Appendix D

Anchor Checking Using ACI 318 Appendix D

Rob AbernathyCASE Center GTSUG June, 2011 Delray Beach, FL

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What is “Anchoring to Concrete”What is “Anchoring to Concrete”

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Appendix D, first added to 318 in 2002, “provides design requirements for anchors in concrete used to transmit structural loads…” This presentation is limited to the provisions that are used in anchoring base plates to a bearing surface, although Appendix D can be used in some situations of anchoring to structural members.

This presentation is based on ACI 318-05, and includes figures copied from the Appendix D commentary. Many of the formulas and techniques found in Appendix D are based on the paper “Concrete Capacity Design (CCD) …”, ACI Structural Journal, 1995, which is reference D.9 in ACI 318-05 and -08.

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Anchoring to Concrete: Terms and ConceptsAnchoring to Concrete: Terms and Concepts


F is a reduction factor applied to calculated capacity.y is a modification factor applied when determining capacity. It may be greater than 1.0.

hef is the effective embedment depth of an anchor.

Anchor groups are sets of anchors with spacing ≤ 3hef

and having “approximately equal” hef.

Supplementary reinforcement is “reinforcement proportioned to tie a potential concrete failure prism to the structural member”, although the rules for proportioning are not defined in Appendix D.

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Terms and Concepts: Breakout/Failure ConeTerms and Concepts: Breakout/Failure Cone


The major concern of Appendix D is the failure of the concrete via a “breakout” cone. Tension can cause breakout from the top surface of the concrete as shown to the left or at an edge. Shear can cause breakout on the surface through “pryout” or at an edge. Note that the area involved

extends 1.5hef from the anchor

center, hence the 3hef (1.5 + 1.5)

spacing definition of an anchor group.

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What does Appendix D cover?What does Appendix D cover?

• Anchor + concrete capacity only – no base plate provisions

• Limited to anchors 2” or less diameter and hef (effective length) of 25” or less.

• f’c of 10,000 psi for cast-in anchors and

8,000 psi for post-installed anchors.

• “Regular” arrangement of similar anchors. Not a formal requirement, but only examples of such are given.

• Anchor + concrete capacity only – no base plate provisions

• Limited to anchors 2” or less diameter and hef (effective length) of 25” or less.

• f’c of 10,000 psi for cast-in anchors and

8,000 psi for post-installed anchors.

• “Regular” arrangement of similar anchors. Not a formal requirement, but only examples of such are given.


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What is not covered?What is not covered?

• Plate failures – overstressing of plate material, weld failures, etc.

• Cyclic (fatigue) or impact loads• Through bolts• “Irregular” anchor arrangements.• Shear lugs and embedded plates.• Bearing surface crushing.• Individual anchor capacity! Since anchors are

evaluated as ‘groups’, the failure of an individual anchor is not necessarily detected.

• Plate failures – overstressing of plate material, weld failures, etc.

• Cyclic (fatigue) or impact loads• Through bolts• “Irregular” anchor arrangements.• Shear lugs and embedded plates.• Bearing surface crushing.• Individual anchor capacity! Since anchors are

evaluated as ‘groups’, the failure of an individual anchor is not necessarily detected.


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Capacity CriteriaCapacity Criteria

• Seismic reduction (Section D.3.3)• Check tension (N)

– FNn ≥ Nua (D-1)

• Check shear (V)– FVn ≥ Vua (D-2)

• Check interaction if Nua > 0.2FNn and Vua >

0.2FVn:

Nua/FNn + Vua/FVn ≤ 1.2 (D-31)

• Spacing checks– D.8 requirements unless supplemental

reinforcement exists.

• Seismic reduction (Section D.3.3)• Check tension (N)

– FNn ≥ Nua (D-1)

• Check shear (V)– FVn ≥ Vua (D-2)

• Check interaction if Nua > 0.2FNn and Vua >

0.2FVn:

Nua/FNn + Vua/FVn ≤ 1.2 (D-31)

• Spacing checks– D.8 requirements unless supplemental

reinforcement exists.GTSUG June, 2011GTSUG June, 2011 7

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The checking processThe checking process

• Get characteristics of anchors, base plate and bearing surface concrete plus the location of anchors and edges.

• Loadings: Section 9.2 or Appendix C load factors

• Check edge distance and spacing if supplementary reinforcement does not exist.

• Create groups based on anchor geometry.

• Check anchor capacity for shear and tension – reduce if seismic requirement

• Determine F factors based on governing sections

• Calculate interaction value

• Get characteristics of anchors, base plate and bearing surface concrete plus the location of anchors and edges.

• Loadings: Section 9.2 or Appendix C load factors

• Check edge distance and spacing if supplementary reinforcement does not exist.

• Create groups based on anchor geometry.

• Check anchor capacity for shear and tension – reduce if seismic requirement

• Determine F factors based on governing sections

• Calculate interaction value


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Anchor characteristicsAnchor characteristics

• Cast-in or post-installed

• Embedment length hef

• Diameter do

• Ductility – ductile or brittle, based on test elongation or area reduction.

• Tensile strength futa

– The use of futa instead of fya is to match with

AISC LRFD design.

• Ase – effective cross-section area; manufacturer’s

value if not pdo2.

• Cast-in or post-installed

• Embedment length hef

• Diameter do

• Ductility – ductile or brittle, based on test elongation or area reduction.

• Tensile strength futa

– The use of futa instead of fya is to match with

AISC LRFD design.

• Ase – effective cross-section area; manufacturer’s

value if not pdo2.


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Anchor Characteristics: Cast-in anchorsAnchor Characteristics: Cast-in anchors

• Headed bolt

• Hooked bolt

• Headed stud

• Headed bolt

• Hooked bolt

• Headed stud


Optional dataNp pullout strength in cracked concrete

Abrg bearing area of head if Np not specified,

for headed bolts or studs.eh width of hook or L if Np not specified, for

hooked boltsWasher – headed anchors, increases hef.

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Anchor Characteristics: Post-installed anchorsAnchor Characteristics: Post-installed anchors

• Type: UC (undercut), TC (torque-controlled), DC (displacement-controlled)

• Pullout strength Np

• Category: 1, 2 or 3 – low to high sensitivity.

• Type: UC (undercut), TC (torque-controlled), DC (displacement-controlled)

• Pullout strength Np

• Category: 1, 2 or 3 – low to high sensitivity.


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Anchor Characteristics: Post-installed anchorsOptional data

Anchor Characteristics: Post-installed anchorsOptional data

• cac: Manufacturer’s critical edge distance

or taken from D.8.3.

• Vsa: Shear strength in lieu of D-20

• kc and yc,N – Concrete breakout strength

coefficient + cracked concrete modification factor. If kc from the

manufacturer is used, the specified yc,N must also be used.

• cac: Manufacturer’s critical edge distance

or taken from D.8.3.

• Vsa: Shear strength in lieu of D-20

• kc and yc,N – Concrete breakout strength

coefficient + cracked concrete modification factor. If kc from the

manufacturer is used, the specified yc,N must also be used.


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Anchor Configuration andBase Plate Conditions

Anchor Configuration andBase Plate Conditions

• Anchor and edge locations– In general, edges > 1.5*hef may be ignored

– Without supplement reinforcement, other criteria should be checked and may be up to 4.0*hef for post-installed anchors.

• Grout pad under plate – affects y factors

• Anchor and edge locations– In general, edges > 1.5*hef may be ignored

– Without supplement reinforcement, other criteria should be checked and may be up to 4.0*hef for post-installed anchors.

• Grout pad under plate – affects y factors


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Concrete CharacteristicsConcrete Characteristics

• f’c• Cracked/Uncracked• Supplementary reinforcement

– Outside the scope of Appendix D in ’05. Note: ’08 adds “Anchor reinforcement” with specific design guides.

• Thickness– In general, > hef*1.333 or hef+4” for post-

installed without supplemental reinforcement.

• f’c• Cracked/Uncracked• Supplementary reinforcement

– Outside the scope of Appendix D in ’05. Note: ’08 adds “Anchor reinforcement” with specific design guides.

• Thickness– In general, > hef*1.333 or hef+4” for post-

installed without supplemental reinforcement.


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Tension capacity: Section D.5Tension capacity: Section D.5


Tension capacity is determined using “Section D.5 – Design requirements for tensile loading”, with four types of strength measures specified in sections D.5.1 to D.5.4. The lowest value of Nn calculated is used in

equation D-1 and its type will be used when determining F for tension.

Processing must be done on a load-by-load basis, since not all anchors will be in tension for all loads and this will affect capacity calculations, especially yec,N, the

modification factor for loading eccentricity.

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D.5.1 Steel Strength of AnchorD.5.1 Steel Strength of Anchor


The simplest check!For all loaded anchors in each load case:

Nsa = n*Ase*futa (D-3)

n = number of loaded anchorsAse = cross-section area

from do or manufacturer specified

Note that the calculated value is for the sum of all loaded anchors, without a specified check on individual anchors.

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D.5.2 Concrete BreakoutD.5.2 Concrete Breakout

Ncb = (ANc/ANco)*yec,N*yed,N*yc,N*ycp,N*Nb

yec,N: Eccentricity factor from D-9 (single anchor = 1.0)

yed,N: Edge proximity factor

yc,N: Cracked concrete factor

ycp,N: Post-installed cac reinforcement factor (cast-in = 1.0)

Nb = Concrete breakout for a single anchor, no edges

From D-7, D-8 or manufacturer’s specs

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D.5.2 Concrete Breakout: (ANc/ANco) D.5.2 Concrete Breakout: (ANc/ANco)


ANco = Maximum area of single breakout cone

= 9.0*hef2

ANc , the total projected

concrete failure area of an anchor group, is affected by anchor spacing and edge distance.

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D.5.2 Concrete Breakout: yec,ND.5.2 Concrete Breakout: yec,N


If eccentricity exists in both X and Y, yec,N is the product of yec,N_X

and yec,N_Y.

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D.5.3 Pullout StrengthD.5.3 Pullout Strength


For each anchor in tension – no grouping:

Npn = yc,P*Np

Cast-in anchors: Np from D-15 or D-16

Np = 8*Abrg*f’c (D-15)

Np = 0.9*f’c*eh*do (D-16)

Post-installed anchors: Np specified

yc,P = 1.0 for cracked concrete

= 1.4 for uncracked concrete

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D.5.4 Concrete side-face blowoutD.5.4 Concrete side-face blowout


Only cast-in, headed anchors with ca1 < 0.4*hef:

Nsb = 160*ca1*sqrt(Abrg)*sqrt(f’c)

If there are multiple anchors, Nsb is modified by: 1.0 + s/(6*ca1)

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Shear capacity: Section D.6Shear capacity: Section D.6


Shear capacity is determined using “Section D.6 – Design requirements for shear loading”, with three types of strength measures specified in sections D.6.1 to D.6.3. The lowest value of

Vua calculated is used in equation D-2 and its section will be used

when determining F.Processing must be done on a load-by-load basis, even

though all anchors tend to share the shear load, since yec,N, the

modification factor for loading eccentricity, will vary.If edges are present, processing for shear becomes very

complicated, since ca1, the minimum edge distance, is measured

“in the direction of the applied shear”, and may in fact be different for every anchor if in-plane rotation exists. One solution is to separate the shear into (X, Y) components and process separately, but ACI does not address this issue directly. Non-orthogonal edges would complicate the problem even more.

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D.6.1 Steel shear strengthD.6.1 Steel shear strength


For all loaded anchors in each load case,

For cast-in headed studs:Vsa = n*Ase*futa (D-19)

For cast-in headed and hooked bolts, plus post-installed anchors without sleeves in the shear plane:

Vsa = n*0.6*Ase*futa (D-20)

Else: Manufacturer’s specified value

n = number of loaded anchorsAse = cross-section area

from do or manufacturer specified

Use resultantshear for 6.1

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D.6.2 Concrete breakout in shearD.6.2 Concrete breakout in shear


Vcb = (AVc/AVco) *yec,V*yed,V*yc,V*Vb (D-22)

yec,V = Eccentricity factor, = 1.0 for single anchors

yed,V = Edge proximity factor

yc,V = Cracked concrete factor

Vb = Single anchor breakout strength in cracked concrete

This is the most complicated provision, but only applies to anchors “near” an edge, such that 4.5*ca1

3.5*sqrt(f’c)

approaches Ase*futa (D-19).

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D.6.2 (con’t)D.6.2 (con’t)


AVco = max. single anchor shear breakout area

= 4.5*ca12 (D-23)

(D-24)

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D.6.3 Concrete pryoutD.6.3 Concrete pryout


Vcp = kcp*Ncb (D-29)

Vcp = kcp*Ncbg (D-30)

kcp = 1.0 if hef < 2.5 inches

2.0 if hef ≥ 2.5 inches

Note the back-reference to Ncb – hopefully

D.5.2.1 (concrete breakout strength) was not limited by nN*ANco (nN = # of loaded anchors

in tension), or D.5.2.1 will need to be re-calculated for the shear group.

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Determining F values: Section D.4.4, loads from Section 9.2

Determining F values: Section D.4.4, loads from Section 9.2


Determine governing failure mode:If steel element governs,

Ductile: F = 0.75 for tension, 0.65 for shearBrittle: F = 0.65 for tension, 0.60 for shear

If concrete governs, but not D.5.3 or D.6.3, and supplementary reinforcement exists,

F = 0.75 for shear, all anchorsF = 0.75 for tension, cast-in & post-installed Cat 1

-0.10 for Category 2, -0.20 for Category 3If concrete governs but not the above conditions

F = 0.70 for shear, all anchorsF = 0.70 for tension, cast-inF = 0.65 for tension, post-installed Category 1

-0.10 for Category 2, -0.20 for Category 3

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Determining F values: Section D.4.5, loads from Appendix C

Determining F values: Section D.4.5, loads from Appendix C


Determine governing failure mode:If steel element governs,

Ductile: F = 0.80 for tension, 0.75 for shearBrittle: F = 0.70 for tension, 0.65 for shear

If concrete governs, but not D.5.3 or D.6.3, and supplementary reinforcement exists,


-0.10 for Category 2, -0.20 for Category 3If concrete governs and no supplementary reinforcement,


-0.10 for Category 2, -0.20 for Category 3

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Seismic Loads, Section D.3.3Seismic Loads, Section D.3.3


D.3.3.3If seismic loads are used and in regions of moderate to high seismic risk, an additional reduction factor of 0.75 must be applied to FNn and FVn.

0.75FNn ≥ Nua

0.75FNn ≥ Nua

D.3.3.4Nn and Vn must be governed by ductile steel elements or

D.3.3.5The “attachment” must undergo ductile yielding at loads below failure of the anchors. No guidance is offered as to making this determination.

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Section D.8 – Edge distances, spacings and thickness Section D.8 – Edge distances, spacings and thickness


If no supplementary reinforcement exists:

Min c-to-c spacing: 4*do for cast-in, untorqued anchors

6*do for all other anchors

Minimum edge distance:Cover requirements for cast-in, untorqued6*do (or cover) for cast-in, torqued6*do for post-installed UC8*do for post-installed TC10*do for post-installed DC

Minimum concrete thickness for post-installed anchorst ≥ 1.5*hef

t ≥ hef+4”

Anchor Checking Using ACI 318 Appendix D

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