Bolt Grades — A325, A490, 8.8, 10.9 Strength Reference

Structural bolt grades define the mechanical properties -- tensile strength, proof load, and hardness -- that determine how much shear, tension, and combined load a bolt can carry. Choosing the correct bolt grade is one of the first decisions in any bolted connection design. This page covers ASTM (US), AS/NZS (Australian), and EN/ISO (European) bolt grades used in structural steel connections.

ASTM structural bolt grades (US practice)

AISC 360-22 references ASTM F3125, which consolidates the legacy A325 and A490 designations into a single specification. The two primary grades are:

ASTM F3125 Grade A325 (Group A)

• Minimum tensile strength (Fu): 120 ksi (827 MPa)
• Nominal shear strength, threads included (Fnv): 54 ksi per AISC 360-22 Table J3.2
• Nominal shear strength, threads excluded (Fnv): 68 ksi per AISC 360-22 Table J3.2
• Nominal tensile strength (Fnt): 90 ksi per AISC 360-22 Table J3.2
• Diameter range: 1/2" to 1-1/2"
• Material: Medium-carbon steel, quenched and tempered
• Use: General structural connections -- beam-to-column, splices, bracing. The most common structural bolt grade in US practice.

ASTM F3125 Grade A490 (Group B)

• Minimum tensile strength (Fu): 150 ksi (1034 MPa)
• Nominal shear strength, threads included (Fnv): 68 ksi per AISC 360-22 Table J3.2
• Nominal shear strength, threads excluded (Fnv): 84 ksi per AISC 360-22 Table J3.2
• Nominal tensile strength (Fnt): 113 ksi per AISC 360-22 Table J3.2
• Diameter range: 1/2" to 1-1/2"
• Material: Alloy steel, quenched and tempered
• Use: Heavy connections requiring higher capacity -- moment frames, heavy bracing, transfer girders. Not permitted for galvanized bolts due to hydrogen embrittlement risk.

AISC 360-22 Table J3.2 summary

Note on Fnv values: AISC 360-22 nominal shear strength values already include a 0.80 reduction factor for thread inclusion (the "N" condition). For the "X" condition (threads excluded from the shear plane), the shear strength is Fnv = 0.563 x Fu. For the "N" condition, Fnv = 0.450 x Fu.

Minimum bolt pretension (AISC 360-22 Table J3.1)

For slip-critical connections and connections subject to tension with fatigue:

These pretension values equal approximately 70% of the minimum bolt tensile strength times the tensile stress area.

Metric bolt grades (AS/NZS and EN/ISO practice)

Metric bolt grades use a two-number system (e.g., 8.8) where the first number equals ultimate tensile strength / 100 (in MPa), and the product of both numbers equals yield strength / 10 (in MPa). So Grade 8.8 means: Fu = 800 MPa, Fy = 8 x 8 x 10 = 640 MPa.

Common structural grades

AS 4100-2020 bolt properties

AS 4100 references AS/NZS 1252.1 for high-strength structural bolts. The standard bolt categories are:

• 8.8/S (snug-tight): Installed to snug-tight condition, bearing-type connections.
• 8.8/TB (tension-bearing): Fully tensioned, bearing-type connections. Required for connections subject to vibration or fatigue.
• 8.8/TF (tension-friction): Fully tensioned, friction-type (slip-critical). Used when slip is a serviceability limit state.

EN 1993-1-8 bolt properties

Eurocode 3 uses partial safety factors (gamma_M2 = 1.25 for bolt resistance):

Key Eurocode note: For Grade 10.9, the shear factor alpha_v drops to 0.5 when threads are in the shear plane. This is a significant reduction compared to 8.8 bolts and often catches designers off guard.

Design capacity comparison (single bolt, single shear)

For a common M20 (3/4") bolt in single shear, threads included:

Values assume: Ab = 314 mm^2 (M20 gross area), AISC uses Ab = 0.442 in^2 for 3/4" bolt.

Common mistakes

1. Mixing A325 and A490 in the same joint. While not prohibited, it complicates inspection and can lead to installation errors. Most specifications require all bolts in a joint to be the same grade.

2. Using A490 in galvanized connections. A490 bolts must not be galvanized or coated by hot-dip galvanizing due to hydrogen embrittlement risk. Use A325 (or F3125 Grade A325) for galvanized connections.

3. Ignoring the thread condition. The shear capacity difference between "N" (threads included) and "X" (threads excluded) is approximately 26% for A325 and 24% for A490. Specifying the wrong condition is non-conservative. If in doubt, assume threads are in the shear plane.

4. Applying Grade 10.9 Eurocode alpha_v = 0.6 for threads in plane. EN 1993-1-8 Table 3.4 specifies alpha_v = 0.5 for 10.9 bolts with threads in the shear plane, not 0.6. This is a common calculation error.

5. Confusing proof load with pretension. Proof load is a manufacturing test value. Pretension (from Table J3.1) is the minimum clamping force required for slip-critical or pretensioned connections.

Frequently asked questions

What is the difference between A325 and A490 bolts? A490 bolts have approximately 25% higher tensile and shear strength than A325 bolts (150 ksi vs 120 ksi). A490 bolts use alloy steel and cannot be galvanized. In most routine connections, A325 bolts provide adequate capacity; A490 bolts are used when connection geometry is constrained and higher capacity per bolt is needed.

Is Grade 8.8 the same as A325? Approximately. Grade 8.8 has Fu = 800 MPa (116 ksi) vs A325 Fu = 120 ksi. The capacity differences are small, but the design methods, resistance factors, and pretension values differ between AISC and AS 4100/EN 1993. Never directly substitute one for the other without checking the applicable code.

When must bolts be pretensioned? Per AISC 360-22 Section J3.1, pretensioned installation is required for: slip-critical connections, connections subject to fatigue, column splices in multi-story frames (4+ stories), connections to bracing in braced frames, and connections subject to significant vibration or load reversal.

Run this calculation

• Bolted Connections Calculator -- check bolt shear, tension, and combined load per AISC 360, AS 4100, EN 1993, and CSA S16
• Bolt Torque Calculator

Related references

• Bolt Capacity Table -- shear and tension capacity for A325 and A490
• Bolt Hole Sizes -- standard, oversize, and slotted dimensions
• Bolt Spacing Requirements -- minimum edge distance and pitch
• Bolt Torque Chart -- installation torque by bolt grade
• Steel Connection Design -- overview of connection types
• How to Verify Calculations
• Bolt Bearing Tearout
• Bolt Group Capacity
• Bolt Pattern
• Bolted Connections
• Steel Fy and Fu reference
• Steel tension rods reference

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable standard (ASTM F3125, AS/NZS 1252.1, or EN ISO 898-1) and project specification before use. The site operator disclaims liability for any loss arising from the use of this information.