Concrete Fastener Design Summary (ACI 318-19) -- revised
- Adisorn O.
- Apr 17
- 3 min read
Adisorn Owatsiriwong, D.Eng.
alpsdev
Introduction
Concrete fasteners (anchors) are used to transfer loads between structural and non-structural components into concrete. Design of anchors must account for different failure modes, load combinations, and embedment conditions, as per ACI 318-19 Chapter 17.
Types of Anchors
- Cast-in-place anchors
- Post-installed mechanical anchors (e.g., expansion, screw)
- Post-installed adhesive anchors
Post-installed anchor
Failure Modes
Direction | Failure Mode | Governing ACI Equation Section |
Tension | 1. Steel strength (anchor rupture) | §17.4.1.2 |
2. Concrete breakout | §17.4.2.1 | |
3. Pullout of anchor | §17.4.3.1 | |
4. Side-face blowout (thin members) | §17.4.4.1 | |
Shear | 5. Steel shear strength | §17.5.1.2 |
6. Concrete breakout in shear | §17.5.2.1 | |
7. Pryout of anchor | §17.5.3.1 |
Anchor design is governed by multiple possible failure modes. According to ACI 318-19, the following strength checks must be evaluated for both tension and shear loads. The lowest capacity governs.
Tension Failure Modes
2.1 Steel Strength (N_sa):
N_sa = A_se × f_u
- A_se: Effective tensile area of steel anchor
- f_u: Ultimate tensile strength of steel (MPa)
Example: A_se = 201 mm², f_u = 600 MPa → N_sa = 201 × 600 = 120.6 kN
2.2 Concrete Breakout Strength (N_cb):
N_cb = k1 × √f'c × hef^1.5 × ψ_ed,N × ψ_c,N × (A_nc / A_nco)
- k1 ≈ 10 (units adjusted)
- hef: Effective embedment depth (mm)
- ψ_ed,N: Edge distance reduction factor
- ψ_c,N: Cracking condition factor
- A_nc: Projected concrete cone area (actual)
- A_nco: Projected area of full cone for 1 anchor
Example: hef = 100 mm, f'c = 30 MPa → N_cb_single ≈ 10×√30×100^1.5 = 54700 N = 54.7 kN
2.3 Pullout Strength (N_p):
N_p = 3 × A_br × √f'c × ψ_c,N
- A_br: Bearing area under anchor head
Shear Failure Modes
3.1 Steel Shear Strength (V_sa):
V_sa = 0.6 × A_se × f_u
Example: A_se = 201 mm², f_u = 600 MPa → V_sa = 0.6 × 201 × 600 = 72.4 kN
3.2 Concrete Breakout in Shear (V_cb):
Two methods are available. Use the lower:
Method 1 (Empirical): V_cb = 7 × √f'c × c_edge^1.5 / (1 + hef / c_edge)
Method 2 (Area Based): V_cb = V_cb_basic × (A_vc / A_vco)
- V_cb_basic = shear breakout for single anchor away from edge
Modifiers: ψ_ed,V (edge), ψ_c,N (cracked concrete) apply
Example: f'c = 30 MPa, c_edge = 100 mm → V_cb ≈ 7×√30×100^1.5 / (1 + 100/100) ≈ 96.2 kN
Hint:
Both tension and shear breakout fomula can be written in the unified format
Strength = Coeffients*Basic_Strength_single x (A_group/A_single)
where (A_group/A_single)<-- Breakout Geometry ratio
This gives sense of the strength of group anchor at the final,
Pryout and Other Considerations
4.1 Concrete Pryout Strength (V_cp):
V_cp = 1.0 × N_cb (if hef/d_anch > 4), or 2.0 × N_cb otherwise
Depends on embedment depth and anchor head profile.
4.2 Side-Face Blowout (N_sb):
- Occurs in thin members when cone cannot develop
- Typically checked via detailing or software
Strength Reduction Factors (φ) and Modifiers (ψ)
φ_sa = 0.75 → Steel (Tension/Shear)
φ_cb = 0.65–0.75 → Concrete breakout
ψ_c,N = 0.75 (cracked), 1.0 (uncracked)
ψ_ed,N / ψ_ed,V = 0.7–1.0 → Based on proximity to edge
Tension Generated from Moment (Uniaxial Mxx)
When moment acts about the anchor group centroid:
- N_moment = M × y_max / Σ(y²)
- N_total = Nua / n + N_moment
Where:
- y_max = furthest anchor from group centroid
- Σ(y²) = sum of squared distances from centroid
Interaction of Tension and Shear
When both tension and shear exist, combined effects must be checked:
Linear: (Nu / φN) + (Vu / φV) ≤ 1.0
Quadratic (RMS): √[(Nu / φN)^2 + (Vu / φV)^2] ≤ 1.0
Strength Reduction Factors (φ)
Typical values:
- φ_steel = 0.75
- φ_concrete breakout = 0.65-0.75 (use 0.65 for adhesive anchor)
- φ_pullout = 0.70 (bond-controlled)
Example
Calculation Sheet (.docx):