Australian Bolt Grades — AS 1252 Bolt Classes 4.6, 8.8, 10.9 for Structural Steel
Complete reference for bolt grades used in Australian structural steel design per AS 4100:2020. AS/NZS 1252 high-strength structural bolt grades: mechanical properties, bolt markings, grade identification, thread dimensions M12 to M36, and strength capacity factors.
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AS/NZS 1252.1 Structural Bolt Grades
AS/NZS 1252.1 (High-Strength Structural Bolts — Bolts, Nuts and Washers) is the Australian standard for structural bolts. It specifies mechanical properties for Grade 8.8 and Grade 10.9 bolts used in structural steel connections. AS 4100 also permits lower-strength Grade 4.6 bolts for non-structural or lightly loaded connections.
Grade 4.6
Grade 4.6 is a low-strength bolt used for non-structural applications and temporary connections. The first digit (4) indicates Fu/100 = 400 MPa, and the product of both digits (4 x 6 = 24) indicates Fy/10 = 240 MPa.
- Minimum tensile strength (fuf): 400 MPa
- Minimum yield strength (fyf): 240 MPa
- Use: Non-structural connections, purlin cleats, handrail brackets, temporary works
- AS 4100 status: Permitted for connections where the design does not require high-strength bolts (Clause 9.3.1)
Grade 8.8
Grade 8.8 is the standard high-strength structural bolt grade in Australia. It accounts for the vast majority of bolted connections in building structures.
- Minimum tensile strength (fuf): 830 MPa (AS/NZS 1252.1 requires 830 MPa, slightly above the nominal 800 MPa)
- Minimum yield strength (fyf): 660 MPa (nominal based on 0.8 x 830)
- Core hardness: 22-34 HRC
- Use: Standard structural connections — beam-to-column, column splices, bracing connections, base plates
- AS 4100 application: Grade 8.8/S (snug-tight), 8.8/TB (tension-bearing), and 8.8/TF (tension-friction)
Grade 10.9
Grade 10.9 is the high-strength structural bolt grade used where higher clamping force or tension capacity is required.
- Minimum tensile strength (fuf): 1040 MPa (AS/NZS 1252.1 requires 1040 MPa)
- Minimum yield strength (fyf): 940 MPa (nominal based on 0.9 x 1040)
- Core hardness: 32-39 HRC
- Use: Heavy connections, moment-resisting frames, highly stressed bracing, connections with limited bolt space
- AS 4100 application: Grade 10.9/TB and 10.9/TF
Complete Bolt Grade Properties Table
| Property | Grade 4.6 | Grade 8.8 | Grade 10.9 |
|---|---|---|---|
| fuf (MPa) | 400 | 830 | 1040 |
| fyf (MPa) | 240 | 660 | 940 |
| fuf/fyf ratio | 1.67 | 1.26 | 1.11 |
| Core hardness (HRC) | — | 22-34 | 32-39 |
| Proof load (% of fuf) | — | 88 | 90 |
| AS 4100 capacity factor phi | 0.80 | 0.80 | 0.80 |
Metric Bolt Dimensions — M12 to M36
Structural bolts in Australia use ISO metric threads per AS 1111 (ISO Metric Threads). The following table gives key dimensions for standard structural bolt sizes.
| Bolt Size | Thread Pitch (mm) | Body Diameter (mm) | Tensile Stress Area At (mm2) | Root Area (mm2) |
|---|---|---|---|---|
| M12 | 1.75 | 12.0 | 84.3 | 76.3 |
| M16 | 2.0 | 16.0 | 157 | 144 |
| M20 | 2.5 | 20.0 | 245 | 225 |
| M22 | 2.5 | 22.0 | 303 | 281 |
| M24 | 3.0 | 24.0 | 353 | 324 |
| M27 | 3.0 | 27.0 | 459 | 427 |
| M30 | 3.5 | 30.0 | 561 | 519 |
| M36 | 4.0 | 36.0 | 817 | 761 |
The tensile stress area is used for bolt tension calculations per AS 4100 Clause 9.3.2.1. The body diameter area (Ab = pi x d2/4) is used for bolt shear calculations when the threads are excluded from the shear plane.
Bolt Markings — Grade Identification
Australian structural bolts are marked per AS/NZS 1252.1 for traceability and grade identification.
| Grade | Bolt Head Marking | Nut Marking | Required Identification |
|---|---|---|---|
| 4.6 | 4.6 or three radial lines | Grade mark or three circumferential grooves | Manufacturer's identification mark |
| 8.8 | 8.8 or six radial lines | Grade mark or six circumferential grooves | Manufacturer's identification mark |
| 10.9 | 10.9 or six radial lines (different pattern) | Grade mark or six circumferential grooves | Manufacturer's identification mark |
In practice, most structural bolts in Australia are clearly stamped with the grade designation (8.8 or 10.9) on the bolt head, along with the manufacturer's mark (e.g., "HB", "BUMAX", "PGB"). Twist-off (TC) bolts typically have the grade marked on the spline end.
AS 4100 Bolt Strength Grades
AS 4100:2020 Clause 9.3.2.1 defines the nominal shear and tension capacity of bolts based on the bolt grade and area.
Nominal Shear Capacity (Vfn)
Threads excluded from shear plane (X-grade): Vfn = 0.62 x fuf x kr x At (or Ab if threads excluded)
Threads included in shear plane (N-grade): Vfn = 0.372 x fuf x kr x At (or Ab)
Where:
- fuf = minimum tensile strength of the bolt
- kr = reduction factor for bolt length (kr = 1.0 for bolt length < 12d, reduces to 0.75 for longer bolts)
- At = tensile stress area (threaded)
- Ab = nominal body area (non-threaded)
Nominal Tension Capacity (Ntf)
Ntf = fuf x At x kr
Where At is the tensile stress area of the threaded portion.
Capacity Factor
For all bolt grades under AS 4100: phi = 0.80
Shear and Tension Capacity Summary — M20 Bolt Example
For the most common structural bolt size (M20, Grade 8.8):
| Condition | Capacity (kN) AS 4100 |
|---|---|
| phi Vfn — threads included (N) | 75.7 |
| phi Vfn — threads excluded (X) | 126.1 |
| Phi Ntf — thread tension | 162.7 |
Values assume: fuf = 830 MPa (8.8), At = 245 mm2, Ab = 314 mm2, kr = 1.0, phi = 0.80.
Bolt Grade Selection Guide for Australian Practice
| Application | Recommended Grade | AS 4100 Category | Pretension |
|---|---|---|---|
| Simple beam shear connections | 8.8/S | Snug-tight bearing | No |
| Moment frame connections | 8.8/TB or 10.9/TB | Tension-bearing | Yes |
| Slip-critical connections | 8.8/TF or 10.9/TF | Tension-friction | Yes |
| Bracing connections | 8.8/S or 8.8/TB | Snug-tight or TB | Varies |
| Column splices (multi-storey) | 8.8/TB | Tension-bearing | Yes |
| Base plate connections | 8.8/S | Snug-tight | No |
| Crane girders | 8.8/TF | Tension-friction | Yes |
| Temporary works | 4.6 | Snug-tight | No |
Design Resources
- Australian Steel Design Guide — AS 4100 overview
- Australian Bolt Hole Sizes — AS 4100 hole dimensions
- Australian Bolt Capacity — Shear and tension tables
- Australian Bolt Pretension — Slip-critical requirements
- AS 4100 Base Plate Design — Base plate reference
- Bolted Connections Calculator
- Bolt Torque Calculator
- Beam Capacity Calculator
- Column Capacity Calculator
Frequently Asked Questions
What is the difference between Grade 8.8 and Grade 10.9 bolts in Australian steel design? Grade 10.9 bolts have approximately 25% higher tensile strength than Grade 8.8 (1040 MPa vs 830 MPa per AS/NZS 1252.1). This provides higher shear and tension capacity for the same bolt diameter. However, Grade 10.9 bolts are more expensive and can be more susceptible to hydrogen embrittlement. In Australian practice, Grade 8.8 is the default for most structural connections, with Grade 10.9 specified only when connection geometry is constrained or high tension capacity is required.
How are Australian bolt grades marked on bolt heads? Per AS/NZS 1252.1, Grade 8.8 bolts are marked with "8.8" or six radial lines on the bolt head, plus the manufacturer's identification mark. Grade 10.9 bolts are marked with "10.9" or a different six-line radial pattern. Grade 4.6 bolts use "4.6" or three radial lines. Nuts are marked with the grade and manufacturer's identification. Twist-off (TC) bolts have the grade marked on the spline end.
What bolt grade should I use for structural steel connections in Australia? Grade 8.8 is the standard structural bolt grade for most applications per AS 4100. Use Grade 8.8/S for snug-tight bearing connections (simple shear connections, bracing) and Grade 8.8/TB or 8.8/TF where pretensioning is required. Grade 10.9 is used for heavy connections where higher capacity per bolt is needed. Grade 4.6 is only suitable for non-structural applications.
What is the tensile stress area of an M20 bolt per Australian standards? The tensile stress area of an M20 bolt is 245 mm2 per AS 1111 (ISO metric thread profile). This area is used with the bolt tensile strength (fuf) to calculate the nominal tension capacity: Ntf = fuf x At. For Grade 8.8 (fuf = 830 MPa): Ntf = 830 x 245 / 1000 = 203 kN nominal, or 162.7 kN factored with phi = 0.80.
Can Grade 4.6 bolts be used in AS 4100 structural connections? Grade 4.6 bolts are permitted under AS 4100 Clause 9.3.1 for connections where the design does not specifically require high-strength bolts. In practice, they are limited to secondary and non-structural connections such as purlin cleats, handrails, and temporary works. Grade 4.6 bolts must not be used in primary structural connections, moment connections, or any connection subject to load reversal or fatigue.
Educational reference only. All design values must be verified against the current editions of AS/NZS 1252, AS 4100, and the project specification. This information does not constitute professional engineering advice. Always consult a qualified structural engineer for design decisions.