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Anchor Checking Using ACI 318 Appendix D

Anchor Checking Using ACI 318 Appendix D. Rob Abernathy CASE Center GTSUG June, 2011 Delray Beach, FL. What is “Anchoring to Concrete”.

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Anchor Checking Using ACI 318 Appendix D

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  1. Anchor Checking Using ACI 318 Appendix D Rob Abernathy CASE Center GTSUG June, 2011 Delray Beach, FL

  2. 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. GTSUG June, 2011

  3. 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. GTSUG June, 2011

  4. 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. GTSUG June, 2011

  5. 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. GTSUG June, 2011

  6. 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. GTSUG June, 2011

  7. 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. GTSUG June, 2011

  8. 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 Ffactors based on governing sections • Calculate interaction value GTSUG June, 2011

  9. 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. GTSUG June, 2011

  10. 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. GTSUG June, 2011

  11. 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. GTSUG June, 2011

  12. 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. GTSUG June, 2011

  13. 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 GTSUG June, 2011

  14. 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. GTSUG June, 2011

  15. 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. GTSUG June, 2011

  16. 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. GTSUG June, 2011

  17. 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 GTSUG June, 2011

  18. 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 GTSUG June, 2011

  19. 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. GTSUG June, 2011

  20. 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 GTSUG June, 2011

  21. 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) GTSUG June, 2011

  22. 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. GTSUG June, 2011

  23. 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 n = number of loaded anchors Ase = cross-section area from do or manufacturer specified Use resultant shear for 6.1 GTSUG June, 2011

  24. 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 GTSUG June, 2011

  25. D.6.2 (con’t) AVco = max. single anchor shear breakout area = 4.5*ca12 (D-23) (D-24) GTSUG June, 2011

  26. 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. GTSUG June, 2011

  27. 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 GTSUG June, 2011

  28. 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 GTSUG June, 2011

  29. 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. GTSUG June, 2011

  30. 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” GTSUG June, 2011

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