Anchorage to Concrete Per ACI 318-08 Appendix D and IBC 2006

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ACI 318-08, Appendix DIBC 2006 Section 1912

Anchorage to Concrete

Mark Bartlett, PEField Engineer

Simpson Anchor Systems

Presentation Topics

• Brief History of Anchor Design• ACI 318-08, Appendix D

• Design Equations• Phi (Ф) Factors• Interaction Equation• Seismic Provisions• Reinforcement to Prevent Breakout• Other Issues• Edge Distances, Thicknesses & Spacings

• When to design per App. D• IBC 2006• Adhesive Anchors and Concrete

Screws• The Future of Anchor Design

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Prior to ACI 318-02

• Cast-In-Place anchors covered by:– PCI / ACI 349– UBC / IBC codes listed allowable

stress capacities for CIP bolts

Prior to ACI 318-02

• Design of Post-Installed anchors:– Individual manufacturers supplied

load values based on testing

– Values found in catalogs and ICBO/ICC reports

– Methodology was allowable stressand assumed an uncracked and unreinforced section.

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ACI 318-08, Appendix D

ACI 318, Appendix D

• Strength design method for anchorage to concrete (i.e. Nua ≤ ΦNn or Vua ≤ ΦVn)– Cast-In-Place (CIP) anchors– Post-Installed (PI) anchors

• Undercut anchors• Torque-controlled anchors• Deformation-controlled anchors

– PI anchors must be prequalified per ACI 355.2

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Appendix D Design Equations & Failure Modes

• Design equations check 5 different failure modes– Steel capacity

• Tension and Shear– Concrete breakout capacity

• Tension and Shear– Pullout/Pull-through capacity

• Tension only– Concrete Pryout

• Shear only– Concrete side-face blowout

• Tension and CIP only.

Appendix D Design Equations

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Design Equations

Tension CapacitiesNsa = nAse,NfutaNcb = ANc/ANco(Ψec,NΨed,N Ψc,N Ψcp,NNb)Npn = Ψc,PNpNsb = (160ca1√Abrg)λ√f’c

Shear CapacitiesVsa = n 0.6 Ase,V futaVcbg = AVc/AVco(Ψec,VΨed,V Ψc,V Ψh,VVb)Vcpg = kcpNcbg

Steel Strength in Tension

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Steel Strength In Tension – D.5.1

Nsa = nAse,Nfuta(Eq. D-3)– Nsa – Nominal tensile

strength of an anchor group– n – Number of anchors– Ase,N – Effective cross

sectional area of anchor in tension

– futa – Specific ultimate tensile strength of anchor

Concrete Breakout Strength in Tension

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Concrete Breakout In Tension – D.5.2

Ncb=ANc/ANco(Ψec,NΨed,NΨc,NΨcp,NNb) (Eq. D-5)

• Ncb – Concrete breakout strength in tension


Ncb=ANc/ANco(Ψec,NΨed,NΨc,NΨcp,NNb)

• ANc – Projected failure area of group • ANco = 9 hef

2 Projected failure area of one anchor (Eq. D-6)

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Ncb=ANc/ANco(Ψec,NΨed,N Ψc,NΨcp,NNb)

Modification for eccentric loadΨec,N = 1/[1+(2e’N/3hef)] (Eq. D-9)

T3 T2 T1

N

Resultant tension load

e’N

Centroid of anchors



Modification for edge effects

If ca,min > 1.5hef then: Eq. D-10 Ψed,N = 1.0

If ca,min < 1.5hef then: Eq. D-11 Ψed,N = 0.7 + 0.3 (ca,min / 1.5hef )

ca

9



Modification for crackingΨc,N =1.4 for uncracked section if kc = 17 in eq. (D-7)Ψc,N per evaluation report (ER) if kcfrom ER used in eq. (D-7)Ψc,N =1.0 for cracked section


Ncb=ANc/ANco(Ψec,NΨed,NΨc,NΨcp,NNb)Ψcp,N – Modification for Post-Installed anchors

Uncracked concreteNo supplemental reinf. to control splitting

If ca,min > cac then: Ψcp,N = 1.0 (Eq. D-12)

If ca,min < cac then: Ψcp,N = ca,min/cac (Eq. D-13)

Where cac= 2.5 hef (undercut anchors)4 hef (wedge anchors)

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Ncbg=ANc/ANco(Ψec,NΨed,NΨc,NΨcp,NNb)

• Basic concrete breakout strength• Nb=kc λ √f’c hef

1.5 (Eq. D-7)– kc – Coefficient for basic concrete

breakout strength• Found in either App. D or per product ER

– λ – Modification factor for lightweight concrete

– f’c – Concrete compressive strength – hef – Effective embedment depth

• Tested hef found in manufacturer’s catalog or product ER

Pullout Strength in Tension

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Pullout Strength In Tension – D.5.3

Npn = Ψc,PNp (Eq. D-14)

• Npn – Nominal pullout strength• Ψc,P – Modification for cracking

– 1.0 for cracked– 1.4 for uncracked

• Np – Pullout strength in tension

Pullout Strength In Tension – D.5.3

Npn = Ψc,P Np (Eq. D-14)

• Np – Pullout strength in tension

• For PI anchors Np based on ACI 355.2 test results

• For CIP anchors, Np based on:– Np = 8 Abrgf’c (Eq. D-15) headed bolts– Np = 0.9f’cehda (Eq. D-16) hooked bolts

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Side-Face Blowout Strength in Tension

Side-Face Blowout Strength – D.5.4

Nsb = (160ca1√Abrg)λ√f’c (Eq. D-17)

• Nsb – Side-face blowout strength (headed anchors only)

• ca1 – edge distance

• Abrg – Net bearing area of the head of anchor

• λ – Modification factor for lightweight concrete

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Steel Strength in Shear

Steel Strength In Shear – D.6.1

• Vsa = n Ase,V futa (eq. D-19) CIP HSA

• Vsa = n 0.6 Ase,V futa (eq. D-20)

– n – number of anchors– Ase,V – effective cross sectional

area of a single anchor in shear– futa – specified tensile strength of

anchor steel

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Steel Strength In Shear – D.6.1

• Vsa may also be based on the results of tests performed and evaluated according to ACI 355.2

Concrete Breakout Strength in Shear

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Concrete Breakout Strength In Shear –D.6.2

Vcbg = AVc/AVco(Ψec,VΨed,V Ψc,V Ψh,VVb) (Eq. D-22)

• Vcbg – Concrete breakout strength in shear

Vcbg = AVc/AVco(Ψec,VΨed,VΨc,V Ψh,VVb)

– AVc – projected concrete failure area of a group of anchors

AVc = (1.5ca1 + s1 + ca2) ha


ca2s11.5ca1

c a1

ha

V

AVc

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– AVco – maximum projected concrete failure area of a single anchor

1.5ca1

c a1

1.5c1

V

1.5ca1

AVco = 4.5 ca12 (Eq. D-23)

AVco

Concrete Breakout In Shear – D.6.2


Ψec,V – Modification for eccentric load (Eq. D-26)

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Ψed,V – Modification for edge effectsIf ca2 > 1.5ca1

then Ψed,V = 1.0 (Eq. D-27)

If ca2 < 1.5ca1

then Ψed,V = 0.7 + 0.3ca2/1.5ca1 (Eq. D-28)

V

ca1

ca2



Ψc,V Modification factor for cracking

Ψc,V = 1.4 for anchors located in a region where analysis indicates no cracking at service loads

Who is currently doing this analysis?

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Ψc,V = 1.0 for anchors in cracked concrete with no supplemental reinforcement or edge reinforcement smaller than a #4 bar V

<#4



Ψc,V = 1.2 for anchors in cracked concrete with reinforcement of a #4 bar or greater between the anchor and the edge

V

>#4

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Ψc,V = 1.4 for anchors in cracked concrete with reinforcement of a #4 bar or greater between the anchor and the edge, and with the reinforcement enclosed within stirrups spaced at not more than 4”.

V

≥#4

#4@4”



Ψh,V – Modification factor for shear strength of anchors located in concrete members with ha < 1.5ca1

Ψh,V = √1.5ca1/ha but not less than 1.0

When ha < 1.5ca1, AVc is reduced. However, breakout strength is not directly proportional to member thickness. Ψh,V adjusts for this.

c a1

1.5c1

V

ha

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• Vb=(7(ℓe /da)0.2√da)λ√f’c (ca1)1.5 (Eq. D-24)

– ℓe – load bearing length of anchor• Same as hef if there is no sleeve on anchor• Per manufacturer if there is a sleeve

– da – outside diameter of anchor– λ – adjustment for lightweight concrete– f’c – concrete compressive strength– ca1 – edge distance

Pryout Strength in Shear

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Concrete Pryout Strength In Shear –D.6.3

Vcpg = kcpNcbg (Eq. D-30)

• kcp = 1.0 for hef < 2.5”• kcp = 2.0 for hef > 2.5”

• Ncbg – Nominal concrete breakout strength in tension

– Always do tension calcs first

Phi (Ф) Factors

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Phi (Φ) factors

• Nua ≤ ΦNn or Vua ≤ ΦVn

• Phi (Ф) factors are applied to nominal capacities before comparing with factored forces

• Based on:– Supplemental reinforcement– Failure mode– Load type– Anchor property

Phi (Φ) factors D.4.4

0.700.45Cat. 3

0.700.55Cat. 2

0.700.65Cat. 1

0.700.70

Use Condition B

CIP

Pryout

0.700.45Cat. 3

0.700.55Cat. 2

0.700.65Cat. 1

0.700.70

Use Condition B

CIP

Pullout

0.700.450.750.55Cat. 3

0.700.550.750.65Cat. 2

0.700.650.750.75Cat. 1

0.700.700.750.75CIP

Breakout

0.700.700.750.75CIPSide Face Blowout

0.600.65Brittle

0.650.75Use Condition B

DuctileSteel

ShearTension ShearTension

Condition BCondition A

Ф FactorAnchor

PropertyFailure Mode

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Supplemental Reinforcing D.4.4

• Condition A– Applies where supplementary

reinforcement is present except for pullout and pryout strengths.

• Condition B– Applies where supplementary

reinforcement is not present, and for pullout or pryout strength.

Supplemental Reinforcing

• Supplemental Reinforcement– Reinforcement that acts to restrain

the potential concrete breakout but is not designed to transfer the full design load from the anchors into the structural member.

– Refer to sections D.5.2.9 and D.6.2.9 for full design load transfer requirements

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Interaction of Tension and Shear

Interaction of Tension and Shear – D.7

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Interaction of Tension and Shear – D.7

• If Vua≤0.2ΦVn full tension allowed– Ignore Shear

• If Nua≤0.2ΦNn full shear allowed– Ignore Tension

• OtherwiseNua + Vua < 1.2ΦNn ΦVn

Appendix D Seismic Provisions

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Seismic Provisions

D.3.3 – When anchor design includes earthquake forces for structures assigned to Seismic Design Category C, D, E, or F, the additional requirements of D.3.3.1 through D.3.3.6 shall apply.

D.3.3.1 – The provisions of Appendix D do not apply to the design of anchors in plastic hinge zones of concrete structures under earthquake forces.

D.3.3.2 – Post-installed structural anchors shall be qualified for use in cracked concrete and shall have passed the Simulated Seismic Tests in accordance with ACI 355.2. Pullout strength Np and steel strength of the anchor in shear Vsa shall be based on the results of the ACI 355.2 Simulated Seismic Tests.

Seismic Provisions

D.3.3.3 – The anchor design strength associated with concrete failure modes shall be taken as 0.75φNnand 0.75φVn, where φ is given in D.4.4 or D.4.5, and Nn and Vn are determined in accordance with D.5.2, D.5.3, D.5.4, D.6.2, and D.6.3, assuming the concrete is cracked unless it can be demonstrated that the concrete remains uncracked.

• 0.75 reduction to concrete capacity in Seismic Design Category C – F

• Impractical to prove concrete remains uncracked

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Seismic Provisions

D.3.3.4 – Anchors shall be designed to be governed by the steel strength of a ductile steel element as determined in accordance with D.5.1 and D.6.1, unless either D.3.3.5 or D.3.3.6 is satisfied.

D.3.3.5 – Instead of D.3.3.4, the attachment that the anchor is connecting to the structure shall be designed so that the attachment will undergo ductile yielding at a force level corresponding to anchor forces no greater than the design strength of anchorsspecified in D.3.3.3.

D.3.3.6 – As an alternative to D.3.3.4 and D.3.3.5, it shall be permitted to take the design strength of the anchors as 0.4 times the design strength determined in accordance with D.3.3.3. For the anchors of stud bearing walls, it shall be permitted to take the design strength of the anchors as 0.5 times the design strength determined in accordance with D.3.3.3.

Seismic Provisions

• Summary– Seismic Design Category C, D, E & F– No anchors in plastic hinge– PI anchors must pass Simulated

Seismic Test– Design strength reduced by 25%– Ductile steel failure of anchors shall

control, or...– Ductile yielding of attachment, or...– Anchor capacity reduced by 60%

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Seismic Provisions

• Seismic and edge effects– At small edge distances, concrete

breakout (non ductile failure mode) will often control

– If attachment will not experience ductile yielding before breakout occurs, then 40% anchor capacity reduction unless...

– Reinforce section to prevent breakout from occurring

Reinforcement to Prevent Concrete Breakout

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D.4.2.1 – The effect of reinforcement provided to restrain the concrete breakout shall be permitted to be included in the design models used to satisfy D.4.2. Where anchor reinforcement is provided in accor-dance with D.5.2.9 and D.6.2.9, calculation of the concrete breakout strength in accordance with D.5.2 and D.6.2 is not required.

D.5.2.9 – Where anchor reinforcement is developed in accordance with Chapter 12 on both sides of the breakout surface, the design strength of the anchor reinforcement shall be permitted to be used instead of the concrete breakout strength in determining φNn. A strength reduction factor of 0.75 shall be used in the design of the anchor reinforcement.


• Refer to Commentary RD.5.2.9 for more information

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D.6.2.9 – Where anchor reinforcement is either developed in accordance with Chapter 12 on both sides of the breakout surface, or encloses the anchor and is developed beyond the breakout surface, the design strength of the anchor reinforcement shall be permitted to be used instead of the concrete breakout strength in determining φVn. A strength reduction factor of 0.75 shall be used in the design of the anchor reinforcement.

Plan Section

• Refer to Commentary RD.6.2.9 for more info


Bar

s ef

fect

ive

as

anch

or re

info

rcem

ent

Plan

Edge Reinforcement

Anchor Reinforcement

Section

• Refer to Commentary RD.6.2.9 for more info

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• Per Commentary RD.5.2.9 and RD.6.2.9:– “As a practical matter, use of anchor

reinforcement is generally limited to cast-in-place anchors.”

• What about post-installed anchors?– At small edge distances, anchor

capacity will be greatly reduced for seismic design.

Other Appendix D Issues

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Capacity Adjustments

• PI anchor pullout capacity– Tested values of Np are done in 2500 psi

concrete– Pullout capacities increase for higher f’c– Adjustment equations in ER

• Grout pads– 20% reduction in shear strength (D.6.1.3)– App. D makes no mention to grout pad

thickness• Shear load parallel to concrete edge

– Breakout capacity doubled per D.6.2.1(c).

Triple Edge Conditions

D.5.2.3 – Where anchors are located less than 1.5hef from three or more edges, the value of hef used in Eq. (D-4) through (D-11) shall be the greater of ca,max/1.5 and one-third of the maximum spacing between the anchors within the group.

D.6.2.4 – Where anchors are influenced by three or more edges, the value of ca1 used in Eq. (D-23) through (D-29) shall be the greatest of ca2/1.5 in either direction, ha/1.5; and one-third of the maximum spacing between the anchors within the group.

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Triple Edge Condition in Tension

Triple Edge Condition in Shear

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Corner Condition D.6.2.1(d)

(d) For anchors located at a corner, the limiting nominal concrete breakout strength shall be deter-mined for each edge, and the minimum value shall be used.

V

ca1

ca2

V

ca2

ca1

Shear Near an Edge D.6.2.1

Where anchors are located at varying distances from the edge and the anchors are welded to the attach-ment so as to distribute the force to all anchors, it shall be permitted to evaluate the strength based on the distance to the farthest row of anchors from the edge. In this case, it shall be permitted to base the value of ca1 on the distance from the edge to the axis of the farthest anchor row that is selected as critical, and all of the shear shall be assumed to be carried by this critical anchor row alone.

V

ca1

0.5V

0.5V

ca1

Anchors welded to

plate

Anchors not welded to plate

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Shear Near an Edge D.6.2.1

Vca1

• Increase ca1 without welding to plate– Slot holes closest to edge

Required Edge Distances, Spacings, and Thicknesses

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Section D.8

Minimum spacings and edge distances for anchors and minimum thicknesses of members shall conform to D.8.1 through D.8.6, unless supplementary reinforce-ment is provided to control splitting. Lesser values from product-specific tests performed in accordance with ACI 355.2 shall be permitted.

D.8.1 – Unless determined in accordance with D.8.4, minimum center-to-center spacing of anchors shall be 4da for untorqued cast-in anchors, and 6da for torquedcast-in anchors and post-installed anchors.

D.8.2 – Unless determined in accordance with D.8.4, minimum edge distances for cast-in headed anchors that will not be torqued shall be based on specified cover requirements for reinforcement in 7.7. For cast-in headed anchors that will be torqued, the minimum edge distances shall be 6da.

Section D.8

D.8.3 – Unless determined in accordance with D.8.4, minimum edge distances for post-installed anchors shall be based on the greater of specified cover requirements for reinforcement in 7.7, or minimum edge distance requirements for the products as deter-mined by tests in accordance with ACI 355.2, and shall not be less than 2.0 times the maximum aggregate size. In the absence of product-specific ACI 355.2 test information, the minimum edge distance shall be taken as not less than:

Undercut anchors..................................................6daTorque-controlled anchors.....................................8daDisplacement-controlled anchors.........................10da

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Section D.8

D.8.4 – For anchors where installation does not produce a splitting force and that will remain untorqued, if the edge distance or spacing is less than those speci-fied in D.8.1 to D.8.3, calculations shall be performed by substituting for da a smaller value d’a that meets the requirements of D.8.1 to D.8.3. Calculated forces applied to the anchor shall be limited to the values corresponding to an anchor having a diameter of d’a.

D.8.5 – The value of hef for an expansion or undercut post-installed anchor shall not exceed the greater of 2/3 of the member thickness and the member thickness minus 4 in.

Section D.8

D.8.6 – Unless determined from tension tests in accordance with ACI 355.2, the critical edge distance, cac, shall not be taken less than:

Undercut anchors...............................................2.5hefTorque-controlled anchors....................................4hefDisplacement-controlled anchors..........................4hef

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Limitations of Appendix D

• Applies for CIP and some Post-Installed anchors– Specialty inserts, through bolts,

adhesive anchors, screw anchors, PAT fasteners outside scope of Appendix D

– ACI Commentary: “Adhesive anchors are widely used and can perform adequately. At this time…outside the scope.”


• NW Concrete and LW Concrete only– Reductions in capacity in LW

– CMU and Concrete on metal deck outside scope of App. D

• Grouted CMU will still use existing post-installed anchor products

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• Limits to:– Diameter (≤2”)

– Embedment depth (≤25”)

– Concrete compressive strength (≤8000 psi PI; <10000 psi CIP).

When to Use Appendix D

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When to use Appendix D

• Per ACI 318-08, D.2.1– “…anchors in concrete used to

transmit structural loads by means of tension, shear, or a combination of tension and shear between (a) connected structural elements; or (b) safety-related attachments and structural elements.”

– What is a “safety-related attachment”?

When to use Appendix D

• Per ACI 318-08, RD.2.1– Commentary lists examples for safety-

related attachments.– “…safety-related attachments that are

not part of the structure (such as sprinkler systems, heavy suspended pipes, or barrier rails) are attached to structural elements.”

– Will sprinkler systems be attached with cracked concrete anchors?

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When to Use Appendix DIBC 2006

IBC 2006, Section 1911

Anchorage To Concrete –Allowable Stress Design

1911.1 Scope. The provisions of this section shall govern the allowable stress design of headed bolts, and headed stud anchors cast in normal-weight concrete for purposes of trans-mitting structural loads from one connected element to the other. These provisions do not apply to anchors installed in hardened concrete or where load combinations include earth-quake loads or effects. The bearing area of headed anchors shall be not less than one and one-half times the shank area. Where strength design is used, or where load combinations include earthquake loads or effects, the design strength of anchors shall be determined in accordance with Section 1912.Bolts shall conform to ASTM A 307 or an approved equivalent.

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IBC 2006, Section 1912

Anchorage To Concrete – Strength Design1912.1 Scope. The provisions of this section shall govern the strength design of anchors installed in concrete for purposes oftransmitting structural loads from one connected element to the other. Headed bolts, headed studs and hooked (J- or L-) bolts cast in concrete and expansion anchors and undercut anchors installed in hardened concrete shall be designed in accordance with Appendix D of ACI 318 as modified by Section 1908.1.16, provided they are within the scope of Appendix D.

The strength design of anchors that are not within the scope of Appendix D of ACI 318, and as amended above, shall be in accordance with an approved procedure.

Exception: Where the basic concrete breakout strength in tension of a single anchor, Nb, is determined in accordance with Equation (D-7), the concrete breakout strength requirements of Section D.4.2.2 shall be considered satisfied by the design procedures of Sections D.5.2 and D.6.2 for anchors exceeding 2 inches (51mm) in diameter or 25 inches (635mm) tensile embedment depth.

IBC 2006, Section 1908Modifications to ACI 318

1908.1.16 ACI 318, Section D.3.3. Modify ACI 318, section D.3.3.2 through C.3.3.5, to read as follows:

D.3.3.2 – In structures assigned to Seismic Design Category C, D, E or F, post-installed anchors for use under D.2.3 shall have passed the Simulated Seismic Tests of ACI 355.2.

D.3.3.3 – In structures assigned to Seismic Design Category C, D, E or F, the design strength of anchors shall be taken as 0.75φNn and 0.75φVn, where φ is given in D.4.4 or D.4.5, and Nn and Vn are determined in accordance with D.4.1.

D.3.3.4 – In structures assigned to Seismic Design Category C, D, E or F, anchors shall be designed to be governed by tensile or shear strength of a ductile steel element, unless D.3.3.5 is satisfied.

D.3.3.5 – Instead of D.3.3.4, the attachment that the anchor is connecting to the structure shall be designed so that the attachment will undergo ductile yielding at a load level corresponding to anchor forces no greater than the design strength of anchors specified in D.3.3.3, or the minimum design strength of the anchors shall be at least 2.5 times the factoredforces transmitted by the attachment.

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Adhesive Anchors and Concrete Screws

Adhesives Anchors and Concrete Screws

• IBC 2006, Section 1912– “The strength design of anchors that

do not within the scope of Appendix D of ACI 318…shall be in accordance with an approved design procedure.”

• What design procedures are approved?

• Who decides?

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• IBC 2006, Section 104.11104.11 Alternative materials, design and methods of construction and equipment. The provisions of this code are not intended to prevent the installation of any material or to prohibit any design or method of construction not specifically prescribed by this code, provided that any such alternative has been approved.An alternative material, design or method of construction shall be approved where the building official finds that the proposed design is satisfactory and complies with the intent of the provisions of this code, and that the material, method or work offered is, for the purpose intended, at least the equivalent of that prescribed in this code in quality, strength, effectiveness, fire resistance, durability and safety.

104.11.1 Research reports. Supporting data, where necessary to assist in the approval of materials or assemblies not specifically provided for in this code, shall consist of valid research reports from approved sources.


• IBC 2006, Section 104.11 – The building official has the ability to

approve a material if it is not specifically referenced in the code

– Adhesive anchors and screw anchors fall into this category

– Caution: Most building officials are still learning about strength design provisions of anchors

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• Many engineers are still designing adhesive anchors and screw anchors per ASD

• Strength design code reports for adhesives and screws are just starting to come online


• ICC ES AC 193– Expansion anchors– Undercut anchors– Screw anchors

• ICC ES AC 308– Adhesive anchors

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Code Reports

The Future of Anchors

• Reliance on software for anchor design

• Many new post-installed anchors

• Confusion among engineers, contractors and building officials

• Lengthy transition period

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The Future of Anchors

• What changes will the IBC 2009 and ACI 318-11, Appendix D bring?

– Clearer provisions for adhesives and concrete screws?

Questions

Anchorage to Concrete Per ACI 318-08 Appendix D and IBC 2006

Documents

face blowout

prevent concrete

small edge

prevent concrete

np pullout

minimum edge

strength reduction

seismic design