Anchor Checking Using ACI 318 Appendix D by h1st7S4

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 Anchor Checking Using ACI
      318 Appendix D



           Rob Abernathy
           CASE Center
           GTSUG
           June, 2011 Delray Beach, FL
GTSTRUDL

              What is “Anchoring to Concrete”

 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 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 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?

 • 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?
 • 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 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.
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                     The 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
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                        Anchor 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.
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       Anchor Characteristics: Cast-in anchors

 • Headed bolt
 • Hooked bolt
 • Headed stud
 Optional data
 Np 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 bolts
 Washer – headed anchors, increases hef.
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  Anchor 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.




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  Anchor Characteristics: Post-installed anchors
                 Optional 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.

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                     Anchor Configuration and
                      Base 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



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                     Concrete 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.
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                Tension 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 Anchor

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

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

                     n      = number of loaded anchors
                     Ase    = 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 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)


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




   ANco       = Maximum area of single breakout cone
              = 9.0*hef2

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                D.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 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 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.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 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

 Use resultant            n      = number of loaded anchors
 shear for 6.1            Ase    = cross-section area
                                 from do or manufacturer specified
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             D.6.2 Concrete breakout in shear

                     This is the most complicated provision,
                     but only applies to anchors “near” an
                     edge, such that 4.5*ca13.5*sqrt(f’c)
                     approaches Ase*futa (D-19).




 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

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                         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 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
Determine governing failure mode:
If steel element governs,
         Ductile: F = 0.75 for tension, 0.65 for shear
         Brittle: 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 anchors
         F = 0.75 for tension, cast-in & post-installed Cat 1
                  -0.10 for Category 2, -0.20 for Category 3
If concrete governs but not the above conditions
         F = 0.70 for shear, all anchors
         F = 0.70 for tension, cast-in
         F = 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
Determine governing failure mode:
If steel element governs,
         Ductile: F = 0.80 for tension, 0.75 for shear
         Brittle: 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,
         F = 0.85 for shear, all anchors
         F = 0.85 for tension, cast-in & post-installed Cat 1
                  -0.10 for Category 2, -0.20 for Category 3
If concrete governs and no supplementary reinforcement,
         F = 0.75 for shear, all anchors
         F = 0.75 for tension, cast-in & post-installed Cat 1
                  -0.10 for Category 2, -0.20 for Category 3

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                Seismic Loads, Section D.3.3
  D.3.3.3
  If 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.3.3.4
  Nn and Vn must be governed by ductile steel elements or

  D.3.3.5
  The “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
  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, untorqued
        6*do (or cover) for cast-in, torqued
        6*do for post-installed UC
        8*do for post-installed TC
        10*do for post-installed DC

  Minimum concrete thickness for post-installed anchors
        t ≥ 1.5*hef
        t ≥ hef+4”
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