Anchor Failure Modes per EN 1992-4

EN 1992-4 identifies six distinct failure modes for anchors in concrete. The design resistance is the minimum of all applicable modes. This is the fundamental principle: the weakest failure mode governs.

Concrete Cone Breakout — Clause 7.2.3

Single Anchor — Characteristic Resistance

The characteristic resistance of a single anchor against concrete cone failure in uncracked concrete:

N_Rk,c = N_Rk,c⁰ × (A_c,N / A_c,N⁰) × ψ_s,N × ψ_re,N × ψ_ec,N × ψ_M,N

Where:

• N_Rk,c⁰ = k_1 × sqrt(f_ck) × h_ef^1.5 — basic resistance of a single anchor
• k_1 = 7.7 for cracked concrete, k_1 = 11.0 for uncracked concrete (cast-in headed studs)
• h_ef = effective embedment depth (mm)
• A_c,N / A_c,N⁰ = geometric ratio accounting for edge and spacing effects
• ψ_s,N = factor for edge disturbance of stress distribution
• ψ_re,N = shell spalling factor (only relevant for h_ef < 100 mm)
• ψ_ec,N = eccentricity factor for the tensile load on the anchor group
• ψ_M,N = factor for compression reinforcement presence

The reference projected area for a single anchor: A_c,N⁰ = s_cr,N² = (3 × h_ef)²

Anchor Group — Projected Area

For an anchor group, the actual projected concrete area A_c,N is limited by:

• Member edges (c_1, c_2 — distance to edge in each direction)
• Spacing between anchors (s_1, s_2)
• The critical spacing s_cr,N = 3 × h_ef

For a typical 4-anchor base plate with anchors spaced at s_1 × s_2 and edge distance c_1 from the concrete edge:

A_c,N = (c_1 + s_1 + 0.5 × s_cr,N) × (c_2 + s_2 + 0.5 × s_cr,N), limited to ≤ n × A_c,N⁰

Where n is the number of anchors in the group.

Design Resistance

N_Rd,c = N_Rk,c / γ_Mc

With γ_Mc = 1.5 (partial factor for concrete, tension) for cast-in anchors per ETAG 001 or relevant ETA.

Pull-Out Resistance — Clause 7.2.4

Pull-out failure occurs when the anchor head (or deformed bar ribs) pulls through the concrete cone without breaking the full cone. The pull-out resistance depends on the bearing area of the anchor head:

N_Rk,p = k_2 × A_h × f_ck

Where:

• k_2 = 7.5 for cracked concrete, k_2 = 10.5 for uncracked concrete (headed studs)
• A_h = bearing area of the anchor head = π/4 × (d_h² − d²)
• d_h = diameter of the anchor head
• d = diameter of the anchor shank (bolt diameter)

For a head diameter ratio d_h/d ≥ 1.5, the head area provides adequate bearing for typical concrete strengths. Smaller head ratios require careful pull-out verification.

Steel Failure (Tension) — Clause 7.2.1

The steel failure resistance in tension is straightforward:

N_Rd,s = A_s × f_uk / γ_Ms

Where:

• A_s = tensile stress area of the bolt (A_s = 0.78 × π × d²/4 for metric threads)
• f_uk = ultimate tensile strength of the anchor steel
• γ_Ms = 1.2 × f_uk/f_yk ≥ 1.4 (minimum partial factor for steel under tension)

For a typical M24 Class 8.8 bolt (A_s = 353 mm², f_ub = 800 MPa):

N_Rd,s = 353 × 800 / (1.2 × 800/640) = 353 × 800 / 1.5 = 188.3 kN

Shear Resistance — Clause 7.3

Steel Failure in Shear

For anchors with the shear plane in the bolt shank (stand-off, grout layer):

V_Rd,s = 0.6 × A_s × f_uk / γ_Ms (single shear plane)

With lever arm (stand-off): V_Rd,s,M = α_M × M_Rk,s / (γ_Ms × l_a), where l_a is the lever arm and α_M = 2.0 for full restraint.

Concrete Edge Breakout — Clause 7.3.3

The characteristic shear resistance for concrete edge failure:

VRk,c = V_Rk,c⁰ × (A_c,V / A_c,V⁰) × ψ_h,V × ψ_s,V × ψ_ec,V × ψα,V

Where:

• V_Rk,c⁰ = k_9 × d_nom^α × h_ef^β × sqrt(f_ck) × c_1^1.5
• k_9 = 1.7 for cracked concrete applications
• α = 0.1 × (h_ef / c_1)^0.5, β = 0.1 × (d_nom / c_1)^0.2
• c_1 = edge distance in the direction of shear load (mm)

This failure mode governs when anchors are close to a free edge (c_1 < 10 × h_ef or c_1 < 60 × d).

Pry-Out — Clause 7.3.4

Pry-out failure is concrete cone breakout reversed — the anchor group rotates out under shear, mobilising the concrete cone in compression behind the back anchors:

V_Rk,cp = k_8 × N_Rk,c

Where k_8 = 1.0 for h_ef < 60 mm, and k_8 = 2.0 for h_ef ≥ 60 mm.

Combined Tension + Shear — Clause 7.4

When an anchor or anchor group is simultaneously loaded in tension and shear, the interaction must be verified:

For steel failure: (N_Ed / N_Rd,s)² + (V_Ed / V_Rd,s)² ≤ 1.0 (quadratic interaction)

For concrete failure modes (cone, edge, pry-out): (N_Ed / N_Rd,c)^1.5 + (V_Ed / V_Rd,c)^1.5 ≤ 1.0 (tri-linear interaction for concrete)

For a combined loading case with N_Ed / N_Rd = 0.6 and V_Ed / V_Rd = 0.4:

Steel: 0.6² + 0.4² = 0.36 + 0.16 = 0.52 ≤ 1.0 — OK Concrete: 0.6^1.5 + 0.4^1.5 = 0.465 + 0.253 = 0.718 ≤ 1.0 — OK

Worked Example — M24 Headed Stud in C30/37

Step 1 — Steel Failure (Tension)

N_Rk,s = 353 × 800 = 282.4 kN

N_Rd,s = 282.4 / 1.5 = 188.3 kN

Utilisation = 85 / 188.3 = 0.45 — OK

Step 2 — Concrete Cone Breakout (Single Anchor)

N_Rk,c⁰ = 7.7 × sqrt(30) × 200^1.5 = 7.7 × 5.48 × 2828 = 119.4 kN

A_c,N⁰ = (3 × 200)² = 600² = 360,000 mm²

Since c_1 = 300 mm > 0.5 × s_cr,N = 300 mm, edge effect does not reduce the projected area. A_c,N / A_c,N⁰ ≈ 1.0 for interior anchor away from edges.

ψ_s,N = 0.7 + 0.3 × c_1 / (1.5 × h_ef) = 0.7 + 0.3 × 300 / 300 = 1.0

N_Rk,c = 119.4 × 1.0 × 1.0 = 119.4 kN

N_Rd,c = 119.4 / 1.5 = 79.6 kN

Utilisation = 85 / 79.6 = 1.07 — FAILS. Increase embedment to h_ef = 250 mm or use anchor group.

Step 3 — Anchor Group Option (4 anchors, interior)

With 4 × M24 anchors in a square pattern, N_Ed per anchor = 85 / 4 = 21.3 kN.

N_Rd,c per anchor = 79.6 kN. Utilisation = 21.3 / 79.6 = 0.27 — OK.

Step 4 — Pull-Out

A_h = π/4 × (36² − 24²) = 0.785 × (1296 − 576) = 565 mm²

N_Rk,p = 7.5 × 565 × 30 = 127.1 kN

N_Rd,p = 127.1 / 1.5 = 84.8 kN per anchor > 21.3 kN — OK

Step 5 — Shear (Steel)

V_Rd,s per anchor = 0.6 × 188.3 = 113.0 kN (single shear plane)

V_Ed per anchor = 45 / 4 = 11.3 kN. Utilisation = 11.3 / 113.0 = 0.10 — OK

Step 6 — Combined Tension + Shear

Steel: (21.3/188.3)² + (11.3/113.0)² = 0.113² + 0.100² = 0.0228 ≤ 1.0 — OK

Concrete: (21.3/79.6)^1.5 + (11.3/V_Rd,c)^1.5 — concrete shear edge breakout not critical for this configuration.

Frequently Asked Questions

When does concrete cone breakout govern anchor design instead of steel failure?

Concrete cone breakout governs when the embedment depth is shallow (h_ef < 10d), the concrete strength is low (C20/25 or less), or when anchors are placed near free edges (c_1 < 1.5 × h_ef). In typical practice, for M20-M30 anchors in C30/37 concrete with h_ef ≥ 8d, steel failure governs in tension. However, for anchor groups loaded in tension, concrete cone breakout of the group (rather than individual anchor) may govern when the cone overlap between anchors reduces the effective projected area.

How does EN 1992-4 differ from the superseded CEN/TS 1992-4 (ETAG approach)?

EN 1992-4:2018 formalised the design methods previously in CEN/TS 1992-4 into a full Eurocode standard. Key changes include: (1) explicit partial factors γ_Mc = 1.5 (γ_Mc = 1.8 was used in some national versions of the TS); (2) clearer distinction between cracked and uncracked concrete k-factors; (3) harmonised k_8 factor for pry-out with a step change at h_ef = 60 mm; (4) updated ψ_re,N shell spalling factor dependence on reinforcement; and (5) explicit provisions for post-installed anchors referencing the relevant EAD (European Assessment Document). The design philosophy of checking all six failure modes remains unchanged.

What is the role of supplementary reinforcement in anchor design?

Per EN 1992-4 Annex B, supplementary reinforcement (hairpins, edge reinforcement, surface reinforcement) can significantly increase concrete breakout resistance. For tension, reinforcement placed perpendicular to the concrete cone surface and anchored on both sides of the potential crack can carry the tension force that would otherwise cause cone breakout. For shear, edge reinforcement (U-bars or hairpins near the edge) can increase edge breakout resistance. The reinforcement must be designed to EN 1992-1-1 with adequate anchorage length l_bd on both sides of the failure surface. In practice, supplementary reinforcement is often used to justify higher loads on anchors in thin slabs or near edges where unreinforced concrete would govern.

Design Resources

• EN 1993 Base Plate Design — Column Base to EN 1993-1-8
• EN 1993 Column Design — Buckling per Clause 6.3
• EN 1993 End Plate Connection Design
• EN 1993 Bolt Grade Selector — 4.6, 5.6, 8.8, 10.9
• EN 1993 Bolt Pretension — Slip-Resistant Categories B/C
• All European Reference Guides →

Reference only. Anchor bolt design must be verified against the current edition of EN 1992-4:2018 and the applicable National Annex. Anchor products must hold a valid European Technical Assessment (ETA) for the specific concrete condition (cracked/uncracked). All anchor designs must be independently verified by a licensed Structural Engineer. This guide is for educational purposes only and does not constitute professional engineering advice.