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.

ASTM F3125 A325 vs A490 properties comparison

ASTM F3125 consolidated the legacy A325, A490, F1852, and F2280 specifications into a single standard covering both hex and twist-off bolt types. The two primary grades within F3125 differ significantly in material, capacity, and fabrication restrictions:

F3125 also covers F1852 (Group A twist-off) and F2280 (Group B twist-off): These are the TC (tension control) bolt equivalents of A325 and A490 respectively. They have identical mechanical properties but use a twist-off spline tip for installation verification instead of a hex head requiring torque or turn-of-nut inspection.

TC bolts vs hex head bolts

Tension control (TC) bolts, covered under ASTM F3125 Grades F1852 and F2280, have become the predominant bolt type in structural steel construction in the US. They differ from traditional hex head bolts in both the bolt geometry and the installation method:

Advantages of TC bolts: The spline tip is engineered to shear off when the bolt reaches the minimum required pretension from AISC Table J3.1. The visual indicator (missing spline) provides immediate, non-destructive confirmation that the bolt was properly installed. This eliminates the need for torque wrench calibration, turn-of-nut measurement, or DTI compressive washers. For this reason, TC bolts have largely replaced hex head bolts in new US structural steel construction.

Limitation of TC bolts: TC bolts cannot be reused after the spline shears off. If a bolt must be removed and reinstalled (field adjustment, connection revision), a new bolt is required. The sheared spline also means that TC bolts cannot be inspected or retightened after initial installation — if there is concern about relaxation, DTI washers should be specified in addition to the TC bolt.

Pretensioning methods per AISC 360-22

When bolted connections require pretensioning (slip-critical, fatigue, or AISC 360-22 Section J3.1 conditions), the RCSC Specification (referenced by AISC) recognizes four installation methods. Each achieves the same minimum pretension from Table J3.1 but uses a different verification approach:

Turn-of-nut method

The turn-of-nut method is the oldest and most widely recognized pretensioning technique. After the connection is brought to the snug-tight condition (the tightness attained by a few impacts of an impact wrench or the full effort of a worker using an ordinary spud wrench), additional rotation is applied:

The rotation must be applied to the nut while the bolt head is prevented from turning (or vice versa). The faying surfaces must be in firm contact at the snug-tight condition before the additional rotation is applied. The turn-of-nut method is highly reliable when performed correctly, but requires trained ironworkers and visual verification that the specified rotation was achieved. Matchmarking (drawing a line across nut and bolt before turning) is used for inspection.

Direct tension indicator (DTI) method

DTI washers are hardened washers with raised bumps (protrusions) on one face. As the bolt is tightened, the bumps compress. When the gap between the washer and the bolt head (or nut) is small enough to resist a feeler gauge of specified thickness, the bolt has reached the required pretension.

DTIs provide a direct, measurable verification of bolt tension. They are preferred by inspectors because the check is objective (the feeler gauge either passes or does not pass). However, DTIs add material cost and require installation on the correct face (bumps facing the bolt head or nut, not the connected material). Compressible-washer DTIs (e.g., Squirter DTIs) eject orange silicone from the bumps when the bolt reaches pretension, providing a visual indicator visible from the ground.

Tension control (TC) bolt method

As described in the previous section, TC bolts (F1852/F2280) achieve pretension through the calibrated spline shear mechanism. After the connection is snugged, the TC gun engages both the hex head and the spline tip. The gun applies torque to the nut while preventing the bolt from rotating. When the pretension reaches the calibrated value, the spline tip shears off. The missing spline is the visual indicator of proper installation.

RCSC requires: TC bolts must be installed from the nut side (gun drives the nut), with the spline tip accessible for visual inspection after installation. A minimum of two snug passes is recommended for large bolt groups to ensure all bolts are in firm contact before final tightening.

Calibrated wrench method

The calibrated wrench method uses a calibrated torque wrench to achieve the required pretension. The relationship between applied torque and bolt tension is T = k x D x Tb, where k is the nut factor (typically 0.20 for new, lubricated bolts), D is the bolt diameter, and Tb is the target pretension.

This method is the least reliable of the four because the torque-tension relationship (k factor) varies with lubrication, surface condition, thread condition, and number of reuse cycles. RCSC requires daily calibration using a hydraulic bolt tension calibrator for each bolt diameter, grade, and lot. The calibrated wrench method is rarely used in modern practice and is generally reserved for retrofit work where TC bolts, turn-of-nut, or DTIs are not practical.

Bolt grade selection guide

Selecting the correct bolt grade depends on the connection type, loading condition, environment, and code requirements:

Cost optimization tip: When the number of bolts in a connection is governed by geometry (minimum spacing, edge distance, constructability) rather than capacity, A325 bolts are more economical than A490 even though A490 provides more capacity per bolt. The extra capacity goes unused, and A490 costs 30 to 50% more per 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

Regional standards

Reference pages organized by design code jurisdiction:

• Australian bolt-grades
• Canadian bolt-grades
• European bolt-grades
• UK bolt-grades

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.

Design Resources

Calculator tools

• Bolt Torque Calculator
• Bolted Connection Calculator
• Splice Connection Calculator
• Steel Bolted Connection Calculator

Design guides

• Bolt Torque Worked Example
• Bolted Connection Checklist
• Bolted Connection Worked Example
• AISC Shear Tab Example
• EN 1993-1-8 Bolted Connection Worked Example