-------------- | -------------------------------------------------- | ---------------------------------------------------- | | Philosophy | Single resistance factor (phi) on nominal strength | Partial safety factor (gamma_M) on material strength | | Bolt shear | phi = 0.75 | gamma_M2 = 1.25 | | Bolt tension | phi = 0.75 | gamma_M2 = 1.25 | | Bolt bearing | phi = 0.75 | gamma_M2 = 1.25 | | Net section fracture | phi_t = 0.75 | gamma_M2 = 1.25 | | Block shear | phi = 0.75 | gamma_M2 = 1.25 | | Weld strength | phi = 0.75 | gamma_M2 = 1.25 | | Effective divisor | 1 / 0.75 = 1.33 | 1.25 directly |

In AISC, the design strength is phi * Rn (nominal strength multiplied by phi). The effective safety margin on the nominal value is 1 / phi = 1.33. Under EN 1993, the design resistance is Rk / gamma_M2 where Rk is the characteristic resistance. The effective safety margin is gamma_M2 = 1.25, slightly less than AISC for the same nominal capacity.

The practical implication: for a bolt of identical material strength, AISC 360 gives a design strength approximately 6% more conservative than EN 1993 on the resistance side alone. However, this is partially offset by differences in how nominal strengths are calculated.

Bolt Grades: A325/A490 vs Grade 8.8/10.9

Bolt material specifications differ in both designation system and mechanical properties.

Grade 8.8 is the closest metric equivalent to A325, with Fu = 800 MPa vs A325 at 827 MPa. Grade 10.9 sits between A325 and A490 in tensile strength (1000 MPa vs 1034 MPa for A490), making it a near-match for A490 in most applications.

A critical difference: EN 14399 preloaded bolts (HR or HV systems) require a specific tightening method and assembly verification per EN 1090-2. AISC slip-critical bolts are installed to a minimum pretension per Table J3.1 regardless of the tightening method, provided the method is calibrated.

Nominal Shear Strength

Both codes express bolt shear strength as the product of a reduction factor, the tensile strength, and the bolt cross-sectional area, but the specific formulas differ.

AISC 360-22 (LRFD):

Rn = Fnv * Ab

where:

• Fnv = nominal shear stress from Table J3.2
  • A325-N (threads included): Fnv = 54 ksi (372 MPa)
  • A325-X (threads excluded): Fnv = 68 ksi (469 MPa)
  • A490-N (threads included): Fnv = 68 ksi (469 MPa)
  • A490-X (threads excluded): Fnv = 84 ksi (579 MPa)
• Ab = nominal unthreaded body area of bolt
• Design strength = phi _ Rn = 0.75 _ Fnv * Ab

EN 1993-1-8 Table 3.4:

Fv,Rd = alpha_v * fub * A / gamma_M2

where:

• alpha_v = 0.6 for grades 4.6, 5.6, 8.8 (shear plane through threaded portion)
• alpha_v = 0.5 for grades 6.8, 10.9 (shear plane through threaded portion)
• alpha_v = 0.6 for all grades (shear plane through unthreaded portion)
• A = tensile stress area As (threads intercepted) or gross shank area A (threads not intercepted)
• gamma_M2 = 1.25

Note that EN 1993 uses the tensile stress area As when threads are in the shear plane, while AISC uses the nominal body area Ab regardless. For an M20 8.8 bolt: As = 245 mm^2 vs Ab = 314 mm^2, a 22% difference that partly offsets the different safety factors.

Bolt Shear Comparison — M20 / 3/4 in. Example

Consider a single bolt in single shear with threads excluded from the shear plane:

AISC 360 — 3/4 in. A325-X:

• Ab = pi * (0.75)^2 / 4 = 0.442 in^2 = 285 mm^2
• Fnv = 68 ksi = 469 MPa
• Rn = 68 * 0.442 = 30.1 kip = 134 kN
• phi _ Rn = 0.75 _ 30.1 = 22.6 kip = 100.5 kN

EN 1993 — M20 8.8, shear plane through unthreaded portion:

• A = pi * (20)^2 / 4 = 314 mm^2
• alpha_v = 0.6, fub = 800 MPa
• Fv,Rd = 0.6 _ 800 _ 314 / 1.25 = 120,576 N = 120.6 kN

Comparison: EN 1993 gives ~20% higher design shear resistance for the same nominal bolt size. This gap narrows to approximately equal values when threads are in the shear plane because EN 1993 uses the smaller tensile stress area (As = 245 mm^2 for M20) while AISC continues to use the body area.

Hole Sizes and Clearances

AISC standard holes add 1/16 in. to the bolt diameter for bolts up to 1 in., then d + 1/16 in. for larger diameters. EN 1993-1-8 Table 3.3 specifies d + 1 mm for M12–M14, d + 2 mm for M16–M24, and d + 3 mm for M27 and above. For the common M20 / 3/4 in. bolt, AISC clearance is 1/16 in. (1.59 mm) per side while EN 1993 clearance is 1 mm per side. AISC is slightly looser.

Oversized holes in AISC add 5/16 in. (7.9 mm) to the bolt diameter but are restricted to slip-critical connections. EN 1993 does not have an exact "oversized" category but permits slotted holes with similar restrictions in EN 1993-1-8 Table 3.3.

Combined Shear and Tension

When a bolt carries shear and tension simultaneously, the interaction check differs fundamentally.

AISC 360-22 J3.7 — Elliptical Interaction:

(Vu / phi*Vn)^2 + (Tu / phi*Tn)^2 ≤ 1.0

where Vn = Fnv _ Ab and Tn = Fnt _ Ab (Fnt from Table J3.2, modified per J3.7 when tension and shear coexist).

EN 1993-1-8 Table 3.4 — Linear Interaction:

Fv,Ed / Fv,Rd + Ft,Ed / (1.4 * Ft,Rd) ≤ 1.0

where Ft,Rd = k2 _ fub _ As / gamma_M2 and k2 = 0.63 for countersunk bolts, 0.9 otherwise.

The AISC elliptical interaction is generally more generous than the EN 1993 linear interaction for intermediate shear-to-tension ratios. At Vu/Vn = 0.5, AISC allows Tu/Tn = 0.866, while EN 1993 allows approximately Ft,Ed/Ft,Rd = 0.70 × 1.4 = 0.98. However, the definitions of Vn and Tn differ, so direct comparison requires converting both to absolute forces.

Worked Example: Combined Shear + Tension

Consider a 3/4 in. A325-N / M20 8.8 bolt with:

• Applied shear: Vu = 5 kip (22.2 kN) per bolt
• Applied tension: Tu = 8 kip (35.6 kN) per bolt

AISC 360 — 3/4 in. A325-N:

• phi*Vn = 0.75 * 54 ksi * 0.442 in^2 = 17.9 kip
• phi*Tn = 0.75 * 90 ksi * 0.442 in^2 = 29.8 kip
• Check: (5/17.9)^2 + (8/29.8)^2 = 0.078 + 0.072 = 0.150 ≤ 1.0 ✓ OK

EN 1993 — M20 8.8 (threads in shear plane):

• As = 245 mm^2, fub = 800 MPa
• Fv,Rd = 0.6 _ 800 _ 245 / 1.25 = 94,080 N = 94.1 kN
• Ft,Rd = 0.9 _ 800 _ 245 / 1.25 = 141,120 N = 141.1 kN
• Check: 22.2/94.1 + 35.6/(1.4*141.1) = 0.236 + 0.180 = 0.416 ≤ 1.0 ✓ OK

Both codes pass comfortably at these load levels. The controlling condition would typically be bearing or tear-out on the connected ply, not bolt shear or tension.

Bearing and Tear-Out

At standard spacing and edge distances, both codes produce similar bearing capacities for the same connected material. The key difference is that EN 1993 combines bearing and tear-out into a single check via the alpha_b and k1 coefficients, while AISC treats them as separate limit states and takes the minimum.

Slip-Critical Connections

Both codes define slip resistance from pretension, friction coefficient, and number of slip planes. AISC distinguishes between Class A (unpainted mill scale, Class A coating, mu = 0.30) and Class B (blast-cleaned with Class B coating, mu = 0.50). EN 1993 uses surface preparation classes A through D with corresponding slip factors.

The EN 1993 Category C (slip-resistant at SLS) is roughly equivalent to AISC slip-critical at the serviceability level, while Category B (slip-resistant at ULS) is a higher requirement with no direct AISC equivalent.

Which Code Is More Conservative?

There is no universal answer. Conservatism depends on the specific limit state:

For general structural bolted connections, an AISC 360-designed connection with A325 bolts will typically be 5–15% more conservative than an EN 1993-designed connection with Grade 8.8 bolts of the same diameter, depending on the controlling limit state.

Frequently Asked Questions

Can I use A325 bolts in a Eurocode design? Not directly, because EN 1993-1-8 design equations require bolt grades per EN 15048 or EN 14399. However, if the A325 mechanical properties (Fu = 120 ksi = 827 MPa) are verified, you can use the Grade 8.8 design rules as an approximation since Fu values are comparable (800 MPa vs 827 MPa). A project-specific deviation or engineering judgment is required, and the responsible engineer must confirm equivalence.

Which code gives thinner bolt groups for the same load? At identical bolt count and diameter, AISC 360 typically produces a slightly lower utilization ratio because of the phi = 0.75 factor (effective 1.33x margin) versus gamma_M2 = 1.25. However, EN 1993 often allows smaller edge distances (1.2d0 vs 1.5d per typical AISC minimums), which can lead to physically smaller connection geometry.

Is bolt pretension different between the two codes? The minimum pretension levels themselves are similar (both target ~70% of tensile strength), but the installation verification requirements differ. AISC requires pretension verification by one of four methods: turn-of-nut, calibrated wrench, twist-off-type tension-control bolts, or direct tension indicators. EN 1090-2 requires similar methods but with specific k-class (k1 or k2) torque coefficient testing for the HR system.

Do both codes allow bearing-type connections in seismic applications? No. AISC 341 (Seismic Provisions) requires slip-critical connections or prequalified moment connections in the seismic force-resisting system (SFRS). EN 1998-1 (Eurocode 8) similarly restricts connections in dissipative zones depending on the ductility class (DCM or DCH). Both codes allow bearing-type connections outside designated seismic-resisting elements per the specific seismic design requirements.

How do I specify bolt holes when a US-fabricated connection is installed in Europe? Use the hole sizes required by the design code governing the project, not the fabrication location. If the project is governed by EN 1993, specify EN 1993-1-8 Table 3.3 clearances regardless of where the steel is fabricated. The fabricator must adjust their CNC or drilling equipment accordingly. Conflicts arise when a US fabricator defaults to AISC Table J3.3 standard holes and the European erection team finds the holes oversized relative to the EN specification.

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See Also

• AISC Bolt Hole Sizes — Table J3.3
• Bolt Capacity Tables
• Bolt Grades Reference
• Bolted Connection Worked Example
• EN 1993 Connection Design Guide
• Bolt Spacing and Edge Distance
• Slip-Critical Connection Guide
• Steel Connection Types Overview
• Steel Design Codes Compared
• How to Verify Calculations

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be independently verified against the applicable building code and project specifications by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in construction. The site operator disclaims liability for any loss arising from the use of this information. Results are PRELIMINARY -- NOT FOR CONSTRUCTION.