AISC 360 vs EN 1993 Bolt Design — Complete Comparison

Structural steel bolted connections are designed under fundamentally different reliability frameworks depending on whether you work in the United States (AISC 360) or Europe (EN 1993-1-8). Understanding the differences is critical for engineers working on international projects, reviewing designs from other jurisdictions, or transitioning between North American and European practice. This page covers every major difference: reliability philosophy, material grades, shear and tension capacities, hole sizes, combined loading formulas, and bearing design.

PRELIMINARY — NOT FOR CONSTRUCTION. All information is for educational and reference use only. Must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in any project.

Reliability Framework — LRFD vs Partial Factor Method

The most fundamental difference between AISC 360 and EN 1993 is the reliability philosophy.

AISC 360 uses the Load and Resistance Factor Design (LRFD) framework. Resistance is multiplied by a resistance factor phi (less than or equal to 1.0) to account for material strength variability, fabrication tolerances, and modeling uncertainty. Loads are amplified by load factors (1.2D + 1.6L for the basic gravity combination).

EN 1993-1-8 uses the partial factor method prescribed by EN 1990. Characteristic resistance values are divided by partial factors (gamma_M) to obtain design values. Loads are factored per EN 1990 Annex A1 (1.35G + 1.5Q for persistent/transient design situations). The partial factor for bolt resistance is gamma_M2, with a recommended value of 1.25.

Practical impact: For an identical bolt under identical loads, the AISC design strength is approximately 0.75 / 0.80 = 0.9375 times the EN 1993 design strength — about 6% lower by the resistance factor alone. However, the characteristic/nominal strengths also differ, so the final comparison is case-specific.

Bolt Material Grades and Properties

AISC and EN 1993 use different bolt material specifications with different mechanical properties.

AISC Bolt Materials

AISC nominal shear strength (Section J3.6):

• Threads excluded from shear plane (N): F_nv = 0.563 F_u
• Threads included in shear plane (X): F_nv = 0.450 F_u

AISC nominal tensile strength (Section J3.6): F_nt = 0.75 F_u

EN 1993 Bolt Materials

EN 1993 design shear strength (Table 3.4):

• Shear plane through unthreaded shank: Fv,Rd = (alpha_v * fub * A) / gamma_M2
  • alpha_v = 0.6 for grades 4.6, 5.6, 8.8
  • alpha_v = 0.5 for grades 6.8, 10.9
• Shear plane through threaded portion: Fv,Rd = (alpha_v * fub * A_s) / gamma_M2
  • alpha_v = 0.6 for grades 4.6, 5.6, 8.8
  • alpha_v = 0.5 for grades 6.8, 10.9

EN 1993 design tensile strength (Table 3.4):

• Ft,Rd = (k_2 * fub * A_s) / gamma_M2
  • k_2 = 0.63 for countersunk bolts, 0.9 otherwise

Cross-Code Equivalent Grades

Key difference: EN 1993 bolt grades encode both tensile and yield strength: Grade 8.8 means f_ub = 800 MPa and f_yb = 0.8 * 800 = 640 MPa (first digit x 100 = f_ub, product of two digits x 10 = f_yb). AISC grades use a letter-and-number designation. The European convention provides instantly readable mechanical properties from the grade alone.

Shear Strength Comparison — F_nv vs alpha_v * f_ub

AISC 360-22 Shear Strength

Per Section J3.6, the design shear strength of a single bolt is:

phi _ R_n = phi _ F_nv * A_b

Where:

• phi = 0.75
• F_nv = 0.563 F_u (threads excluded, N condition) or 0.450 F_u (threads included, X condition)
• A_b = nominal unthreaded body area of the bolt

For an A325 3/4 in. bolt (A_b = 0.442 in², F_u = 120 ksi), threads excluded:

• phi _ R_n = 0.75 _ (0.563 _ 120) _ 0.442 = 22.4 kips per shear plane

For threads included:

• phi _ R_n = 0.75 _ (0.450 _ 120) _ 0.442 = 17.9 kips per shear plane

EN 1993-1-8 Shear Strength

Per Table 3.4, design shear resistance per shear plane:

Fv,Rd = (alpha_v * fub * A) / gamma_M2 (unthreaded plane)

Where:

• gamma_M2 = 1.25
• alpha_v = 0.6 (grades 4.6, 5.6, 8.8) or 0.5 (grades 6.8, 10.9)
• A = gross cross-section area (unthreaded) or A_s = tensile stress area (threaded)

For an M20 8.8 bolt (A = 314 mm², f_ub = 800 MPa), unthreaded shear plane:

• F*v,Rd = (0.6 * 800 _ 314) / 1.25 = 120.6 kN = 27.1 kips

For threaded shear plane (A_s = 245 mm²):

• F*v,Rd = (0.6 * 800 _ 245) / 1.25 = 94.1 kN = 21.2 kips

Direct Comparison — Equivalent Sizes

The EN 1993 bolt shear strength tends to be 10%–18% higher for equivalent bolt sizes because the partial factor 1/gamma_M2 = 0.80 is slightly less conservative than phi = 0.75, and because EN 1993 uses f_ub directly while AISC applies an additional reduction (0.563 F_u or 0.450 F_u) accounting for the difference between tensile and shear behavior.

Tensile Strength Comparison

AISC 360-22 Tensile Strength

Per Section J3.6:

phi _ R_n = phi _ F_nt * A_b

Where F_nt = 0.75 F_u and phi = 0.75.

For A325 3/4 in. bolt (A_b = 0.442 in², F_u = 120 ksi):

• phi _ R_n = 0.75 _ (0.75 _ 120) _ 0.442 = 0.75 _ 90 _ 0.442 = 29.8 kips

EN 1993-1-8 Tensile Strength

Ft,Rd = (k_2 * fub * A_s) / gamma_M2

Where k_2 = 0.9 (non-countersunk) and gamma_M2 = 1.25.

For M20 8.8 bolt (A_s = 245 mm², f_ub = 800 MPa):

• F*t,Rd = (0.9 * 800 _ 245) / 1.25 = 141.1 kN = 31.7 kips

Tension Capacity Comparison

Combined Shear and Tension — Elliptical vs Linear Interaction

This is one of the most significant formula-level differences between the two codes. When a bolt is subjected to simultaneous shear and tension (common in moment connections, bracing connections, and end plates), the interaction check uses different mathematical forms.

AISC 360-22 — Elliptical Interaction (Section J3.7)

AISC uses an elliptical interaction curve for combined shear and tension. The available tensile strength must satisfy:

When f_rv (required shear stress) is present:

F'_nt = 1.3 Fnt - (F_nt / (phi * Fnv)) * f_rv, with F'_nt <= F_nt

Where:

• F'_nt = nominal tensile stress modified for shear
• f_rv = required shear stress (V_u / A_b)
• phi = 0.75

The design check is: T*u <= phi * F'_nt _ A_b

This produces an elliptical reduction in tensile capacity as shear demand increases. At zero shear, full tensile capacity is available. As shear approaches the limit, the available tension reduces but does not reach zero — the curve is elliptical, not a straight line.

Example: For an A325 3/4 in. bolt with 50% of shear capacity used (f*rv = 0.5 * 0.75 _ F_nv):

• F'_nt = 1.3 _ 90 - (90 / (0.75 _ 0.563 _ 120)) _ (0.5 _ 0.75 _ 0.563 * 120)
• F'_nt = 117 - (90 / 50.67) * 25.34 = 117 - 45 = 72 ksi
• Reduced tensile capacity: 0.75 _ 72 _ 0.442 = 23.9 kips (80% of pure tension capacity)

EN 1993-1-8 — Linear Interaction (Table 3.4)

EN 1993 uses a simple linear interaction:

F_v,Ed / F_v,Rd + F_t,Ed / (1.4 * F_t,Rd) <= 1.0

Where:

• F_v,Ed = applied shear force
• F_v,Rd = design shear resistance
• F_t,Ed = applied tensile force
• F_t,Rd = design tensile resistance

The 1.4 factor on tension in the denominator accounts for the observation that combined loading is not simply additive — bolts can sustain more simultaneous shear and tension than a straight-line interaction would suggest.

Example: For an M20 8.8 bolt with 50% of shear capacity used (F_v,Ed = 0.5 * F_v,Rd):

• 0.5 + F_t,Ed / (1.4 * F_t,Rd) <= 1.0
• F_t,Ed / (1.4 * F_t,Rd) <= 0.5
• F_t,Ed <= 0.7 * F_t,Rd = 70% of pure tension capacity

Interaction Comparison

At 50% shear utilization, the AISC elliptical interaction permits approximately 80% of pure tension capacity compared to approximately 70% for the EN 1993 linear interaction. This means AISC 360 is generally less conservative than EN 1993 for combined shear and tension at intermediate loading levels. At low shear (below 20% utilization), the codes produce similar results. At very high shear (above 80%), the AISC curve drops sharply and the codes converge again.

Bolt Hole Sizes and Clearance

AISC 360-22 — Table J3.3

AISC specifies bolt hole dimensions based on the nominal bolt diameter with clearance measured in 1/16 in. increments.

Rule: Standard holes are bolt diameter + 1/16 in. (up to 1 in. diameter). Oversized holes add 3/16 in. for bolts up to 7/8 in.; 5/16 in. for 1 in.; and 5/16 in. above 1 in.

EN 1993-1-8 — Table 3.8

EN 1993 specifies clearance in mm with categories of "normal" and "oversize." Clearances are larger for larger bolts.

Key differences in hole sizing:

• Smaller bolts (<= 14 mm): EN 1993 clearance = d + 1 mm; roughly equivalent to AISC d + 1/16 in. (1.6 mm).
• Medium bolts (16–24 mm): EN 1993 clearance = d + 2 mm (~d + 5/64 in.), slightly larger than AISC d + 1/16 in. (1.6 mm).
• Larger bolts (>= 27 mm): EN 1993 clearance = d + 3 mm (~d + 1/8 in.), substantially larger than AISC d + 1/16 in. (1.6 mm).

This means EN 1993 allows progressively larger clearances for larger bolts, while AISC maintains a constant 1/16 in. clearance for standard holes regardless of bolt size. The larger clearances in EN 1993 facilitate erection at the cost of slightly lower slip resistance.

Bolt Bearing on Steel Plates

AISC 360-22 Bearing Strength (Section J3.10)

phi _ R_n = phi _ (1.2 _ L_c _ t _ F_u) <= phi _ (2.4 _ d _ t * F_u)

Per bolt, for standard, oversized, and short-slotted holes loaded parallel to the slot. For long-slotted holes loaded perpendicular to the slot, use phi = 0.75 and reduce the coefficient from 2.4 to 2.0. For end distances less than required, bearing strength is reduced proportionally to L_c.

EN 1993-1-8 Bearing Strength (Table 3.4)

Fb,Rd = (k_1 * alphab * f*u * d _ t) / gamma_M2

Where:

• alpha_b = min(e_1 / (3d_0), p_1 / (3d_0) - 0.25, f_ub / f_u, 1.0)
• k_1 = min(2.8 * e_2 / d_0 - 1.7, 2.5) for edge bolts
• k_1 = min(1.4 * p_2 / d_0 - 1.7, 2.5) for inner bolts
• gamma_M2 = 1.25

The EN 1993 bearing formula is more complex, explicitly accounting for end distance (e_1), edge distance (e_2), pitch (p_1/p_2), and the ratio of bolt to plate strength (f_ub / f_u) through the alpha_b parameter.

Bearing Comparison

For a typical connection with adequate end distance (e_1 >= 1.5 d_0, edge distance e_2 >= 1.5 d_0), both codes produce similar bearing strengths. The differences emerge with substandard edge distances, where EN 1993 reduces bearing strength through alpha_b and k_1 while AISC reduces it through the 1.2 L_c t F_u term.

Slip-Critical and Preloaded Connections

Both codes recognize two categories of bolted connections: bearing-type (where bolts bear against the connected material) and slip-critical/friction-grip (where bolts clamp plates together and force is transferred by friction).

Both codes use similar friction/slip theory, but EN 1993 distinguishes between Category B (slip-resistant at serviceability) and Category C (slip-resistant at ultimate), while AISC uses a single design check at the required strength level.

Practical Cross-Code Comparison Table

Frequently Asked Questions

1. Why does AISC use phi = 0.75 while EN 1993 uses gamma_M2 = 1.25?

These reflect different calibration approaches. AISC LRFD was calibrated to a target reliability index beta = 3.0 for connections using test data primarily on ASTM A325 and A490 bolts. The phi factor accounts for variability in material strength, fabrication, and the professional factor (model error). EN 1993 was calibrated to European reliability targets (beta = 3.8 for RC2 structures), using a different database of bolt tests conducted primarily on ISO 8.8 and 10.9 bolts. The partial factor gamma_M2 = 1.25 was selected to achieve the target reliability given the European material and fabrication variability data.

2. Can I use AISC tension values for an EN 1993 bolt of equivalent strength?

No. AISC design strength formulas reference ASTM specifications with specific F_u minimum values. EN 1993 bolt strengths reference ISO specifications with f_ub values. Even when the ultimate strengths are nominally similar (e.g., A325 F_u = 120 ksi ~ 830 MPa and 8.8 f_ub = 800 MPa), the different phi/gamma_M factors, different tensile stress areas (AISC uses A_b = gross area while EN 1993 uses A_s = tensile stress area), and different combined-loading rules produce different design values.

3. Which code has stricter edge distance requirements?

EN 1993-1-8 Table 3.8 specifies minimum edge distance of 1.2 d_0 for rolled edges and 1.5 d_0 for sheared or hand-flame-cut edges. AISC 360-22 Table J3.4 specifies minimum edge distance as a function of bolt diameter (not hole diameter), ranging from 3/4 in. for 1/2 in. bolts to 3 in. for 3 in. bolts. For medium structural bolts (3/4 in. / M20), AISC requires 1-1/4 in. (31.8 mm) minimum edge distance; EN 1993 requires 1.5 * 22 mm = 33 mm for a sheared edge — very similar.

Try it now: Check your bolt design with our free Bolted Connection calculator →

Related Pages

• Bolt Torque Calculator — Free online bolt torque calculator with k-factor selection
• Bolt Hole Sizes — AISC Table J3.3 — Complete hole size reference for all bolt diameters
• EN 1993-1-8 Steel Connection Design — Reference Guide — Eurocode connection design fundamentals
• AS 4100 vs AISC 360 Column Design — Australian vs US column design comparison
• Steel Design Codes — AISC vs EN 1993 vs AS 4100 vs CSA S16 — Master comparison table across all four codes
• AISC Minimum Weld Size — Table J2.4 — Fillet weld sizing by material thickness
• Steel F_y and F_u — AISC Table 2-4 — Steel grade properties for US shapes
• Free Steel Calculators — All calculators for beam, column, connection design

Disclaimer

This page provides an educational comparison of AISC 360-22 and EN 1993-1-8 bolt design provisions. It is not a substitute for the actual code documents. Always design to the legally adopted building code in your jurisdiction. All design must be independently verified by a licensed Professional Engineer (PE) or Chartered Structural Engineer (C.Eng) before use in any project. Steel Calculator does not reproduce the full text of any copyrighted standard — refer to the official AISC Steel Construction Manual or Eurocode EN 1993-1-8 for the complete provisions. This tool is for preliminary use only; final designs require professional engineering certification.

Codes referenced: AISC 360-22 (Specification for Structural Steel Buildings), EN 1993-1-8:2005 (Eurocode 3: Design of joints), ISO 898-1 (Mechanical properties of fasteners), EN 14399 (High-strength structural bolting).