Bolt Grade Designation System — EN ISO 898-1

EN 1993-1-8 Table 3.3 defines the following bolt grades for structural steel connections. The grade designation X.Y encodes the mechanical properties numerically:

Grade decoding: The first number (X) = f_ub / 100 (e.g., 8 = 800 MPa class). The second number (Y) = 10 × f_yb / f_ub (e.g., 0.8 = f_yb = 0.8 × f_ub). So 8.8 means f_ub = 800 MPa class and f_yb = 0.8 × 800 = 640 MPa.

Bolt Grade Properties — EN 1993-1-8 Table 3.3

Note on k_2: The k_2 value of 0.9 applies to all bolt grades per EN 1993-1-8 Table 3.3 and accounts for the notch effect at the bolt head-to-shank transition. This factor is used in the tension resistance formula: F_t,Rd = k_2 × f_ub × A_s / γ_M2.

Bolt Shear Resistance — EN 1993-1-8 Table 3.4

The design shear resistance per shear plane:

F_v,Rd = (α_v × f_ub × A) / γ_M2

Where:

• α_v = 0.6 for Grades 4.6, 5.6, 8.8 (threads in shear plane)
• α_v = 0.5 for Grade 10.9 (threads in shear plane)
• α_v = 0.6 for all grades when the shear plane passes through the unthreaded shank
• A is the nominal shank area (unthreaded) or tensile stress area A_s (threaded)
• γ_M2 = 1.25

The lower α_v for Grade 10.9 reflects the reduced ductility of higher-strength bolts in shear.

Design Shear Capacities — Single Shear, Threads in Plane (kN)

Formula: F_v,Rd = α_v × f_ub × A_s / 1.25, with α_v = 0.6 (4.6, 5.6, 8.8) or 0.5 (10.9).

For double shear: multiply all values by 2. For threads excluded from shear plane: use nominal shank area A (π × d² / 4) — approximately 25% higher than the tabulated values.

Bolt Tension Resistance — EN 1993-1-8 Table 3.4

The design tension resistance:

F_t,Rd = (k_2 × f_ub × A_s) / γ_M2

Where k_2 = 0.9 (all grades), A_s is the tensile stress area, and γ_M2 = 1.25.

Design Tension Capacities (kN)

Formula: F_t,Rd = 0.9 × f_ub × A_s / 1.25

Combined Shear and Tension — EN 1993-1-8 Table 3.4

When a bolt is simultaneously loaded in shear and tension, the following interaction must be satisfied:

(F_v,Ed / F_v,Rd) + (F_t,Ed / 1.4 F_t,Rd) ≤ 1.0

The 1.4 factor on tension resistance accounts for the beneficial effect of tension on the shear capacity reduction — the interaction is linear, which is more conservative than the quadratic interaction used in some other codes. For a bolt with F_v,Ed = 0.5 F_v,Rd and F_t,Ed = 0.5 F_t,Rd:

Utilisation = 0.5 + (0.5 × F_t,Rd) / (1.4 × F_t,Rd) = 0.5 + 0.357 = 0.857 < 1.0 — OK

Worked Example — M20 Class 8.8 in End Plate Connection

Problem: An end plate moment connection transfers tension to the top row of bolts. Each M20 Class 8.8 bolt carries F_t,Ed = 95 kN and a small shear component F_v,Ed = 12 kN. Verify the bolt capacity.

Step 1 — Tension check: F_t,Rd = 0.9 × 800 × 245 / 1.25 = 141.1 kN > 95 kN — OK. Utilisation = 0.67.

Step 2 — Shear check: F_v,Rd = 0.6 × 800 × 245 / 1.25 = 94.1 kN > 12 kN — OK. Utilisation = 0.13.

Step 3 — Combined shear-tension: (F_v,Ed / F_v,Rd) + (F_t,Ed / 1.4 F_t,Rd) = 12/94.1 + 95/(1.4 × 141.1) = 0.128 + 0.481 = 0.609 < 1.0 — OK.

Result: M20 Class 8.8 is adequate. Grade 10.9 is not required.

Worked Example — M24 Class 10.9 for High Tension Demand

Problem: A portal frame eaves connection requires M24 bolts to resist F_t,Ed = 190 kN per bolt (tension from moment couple). The shear per bolt is F_v,Ed = 30 kN. Check if Class 8.8 is adequate.

Step 1 — Try Class 8.8: F_t,Rd = 0.9 × 800 × 353 / 1.25 = 203.3 kN > 190 kN — tension alone OK.

Step 2 — Combined check: (F_v,Ed / F_v,Rd) + (F_t,Ed / 1.4 F_t,Rd) = 30/135.6 + 190/(1.4 × 203.3) = 0.221 + 0.667 = 0.888 < 1.0 — OK for 8.8.

Step 3 — Consider robustness (try Class 10.9): F_v,Rd = 0.5 × 1000 × 353 / 1.25 = 141.2 kN F_t,Rd = 0.9 × 1000 × 353 / 1.25 = 254.2 kN

Combined: 30/141.2 + 190/(1.4 × 254.2) = 0.212 + 0.534 = 0.746 < 0.888.

Result: Class 8.8 is sufficient. Class 10.9 provides additional robustness margin but is not required by code. Unless the fabricator prefers a uniform bolt grade across the structure, specify 8.8 for cost efficiency.

Grade Selection Decision Matrix for European Practice

Decision rule: Use Class 8.8 as the default structural bolt grade in European practice. It is stocked by every steel fabricator and bolt supplier in Europe. Upgrade to Class 10.9 only when 8.8 capacity is demonstrably insufficient and increasing bolt diameter or count is not feasible. Never specify Class 10.9 for seismic-resisting bracing connections without specific ductility testing to EN 1998.

Frequently Asked Questions

What is the difference between Grade 8.8 and Grade 10.9 bolts per EN 1993-1-8?

Grade 10.9 bolts (f_ub = 1000 MPa, f_yb = 900 MPa) provide 25% higher tensile strength than Grade 8.8 (f_ub = 800 MPa, f_yb = 640 MPa). However, Grade 10.9 has a lower shear factor α_v = 0.5 vs 0.6 for 8.8, so the shear capacity gain is only about 4%. Grade 10.9 bolts have reduced ductility and are more susceptible to hydrogen embrittlement — always specify mechanically galvanised or zinc-flake coated 10.9 bolts. Grade 8.8 is the standard for primary structural connections and is suitable for 80-90% of all structural steel connections in European practice.

What is the notch factor k_2 and why does it equal 0.9?

The k_2 factor (notch factor) in EN 1993-1-8 Table 3.3 accounts for stress concentration at the bolt head-to-shank transition under tension loading. The value of 0.9 reflects test data showing that the tensile strength of a bolt is approximately 90% of the product f_ub × A_s, due to the triaxial stress state at the head-shank fillet radius. This factor applies uniformly to all bolt grades (4.6 through 10.9) per EN 1993-1-8.

Why does Grade 10.9 have α_v = 0.5 instead of 0.6?

EN 1993-1-8 Table 3.4 assigns α_v = 0.5 to Grade 10.9 bolts (compared to 0.6 for all lower grades) to account for the reduced ductility and higher notch sensitivity of high-strength quenched and tempered alloy steel in shear. The reduction is based on extensive testing showing that Grade 10.9 bolts reach their ultimate shear capacity at lower relative deformation. The effective shear capacity of Grade 10.9 (0.5 × 1000 = 500 MPa) is only marginally higher than Grade 8.8 (0.6 × 800 = 480 MPa) when threads are in the shear plane.

Can Grade 5.6 bolts be used for structural connections?

Grade 5.6 bolts (f_ub = 500 MPa, f_yb = 300 MPa) are permitted for structural connections under EN 1993-1-8 but are rarely specified in primary steel frames. Their shear capacity is approximately 63% of Grade 8.8 in the same diameter. They are most commonly used for: (1) secondary member connections where the bolt group has significant redundancy; (2) connections designed for nominal loads only (e.g., 10 kN bracing ties); (3) temporary works and erection aids; and (4) applications where a softer bolt is preferred to protect the connected material (e.g., connecting thin cold-formed sections to hot-rolled members).

What EN standard governs bolt manufacturing for structural connections?

Structural bolts for EN 1993-1-8 connections must comply with: EN ISO 898-1 (mechanical properties of bolts — carbon steel and alloy steel), EN 14399 (high-strength structural bolting assemblies for preloading), and EN 15048 (non-preloaded structural bolting assemblies). Preloaded bolts (Category B and C connections) must be supplied as matched assemblies to EN 14399 with documented k-class (K1 or K2). Non-preloaded bolts (Category A) may be supplied to EN 15048. All structural bolts used in the European Union must carry CE marking under the Construction Products Regulation (EU) No 305/2011.

Design Resources

• EN 1993-1-8 Bolt Pretension — Torque & Tension Values
• EN 1993 Bolt Group Capacity — Eccentric Load per Clause 3.6
• EN 1993 Bolt Bearing and Tearout — Table 3.4
• EN 1993 Bolt Spacing and Edge Distance — Table 3.3
• EN 1993 Steel Grades — f_y and f_u Values
• EN 1993 End Plate Connection Design Guide
• All European Reference Guides →

Reference only. Verify all values against the current edition of EN 1993-1-8:2005 Table 3.3 and 3.4, EN ISO 898-1, and the applicable National Annex. Design calculations must be independently verified by a licensed Structural Engineer. This guide is for educational purposes only and does not constitute professional engineering advice.